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In-process dimensional measurement and control of workpiece accuracy
Int. J. Made. Tools Manufact. Vol. 37. No. 10. pp. 1423-1439. 1997 1997 Published by Elsevier Science Ltd. All filets nt~rved Printed in G~st Britain 0890-6955/~7517.G0 + .00
Pergamon
PII: S0890-6955(97)00019-9
IN-PROCESS D I M E N S I O N A L
MEASUREMENT WORKPIECE ACCURACY
AND CONTROL
OF
T. YANDAYANt and M. BURDEKINt$ (Received 26 November 1996)
Abstract--This paper surveys the available technology for "in-process control" of machined products. particularly in relation to turning, and cylindrical grinding, prior to completion of the machining process. The techniques developed for controlling accuracy of the workpiece, whilst in situ, are described using representative examples of current technology. The description of new commercial instruments which perform "in-process control" and new devices which could he developed as "in-process measurement instruments" are also presented. © 1997 Published by Elsevier Science Ltcl
NOMENCLATURE A Ci D [-)m~x
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resolution distance to the workpiece surface diameter maximum diameter diameter of the roller probe movement distance probe distance from centre line of the workpiece number of revolutions number of pulses for one roller revolution number of pulses for n revolutions of the workpiece maximum peak-to-valley height workpiece deviation machining error in X-direction 1. INTRODUCTION
Automated inspection methods are the solutions to the increasing demand by customers for near perfection in the quality of manufactured parts and a desire to decrease the costs involved. These inspection methods should be able to the control accuracy of the part during the machining process and provide the required feedback information for the necessary corrections. Quality control of the parts can be performed using traditional methods, for example, manual inspection methods. This requires parts to be removed from the vicinity of the production machines to a separate inspection area, resulting in delay in the manufacturing schedule. Statistical sampling procedures can also be applied; however, these only guarantee that a certain expected or average fraction defect rate will be generated during the process. In addition, there is a risk that some defective parts will not be detected. Another traditional quality control method is to perform the inspection on completion of the parts. If the parts are defective, they must be scrapped or reworked at a cost. Consequently, the cost of inspection parts by conventional methods in some cases equals or exceeds the machining cost. The strategy for achieving improved product quality, high productivity and reduced lead times, is to bring the control of workpiece quality close to the machining process. This can be achieved "by in-process measurement" which inspects the parts whilst they are on the machine tool.
"i'Department of Mechanical Engineering, UMIST, Manchester, U.K. :l:To whom all correspondence should be addressed. 1423
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T. Yandayan and M. Burdekin
A survey of in-process gauging was previously made by Tillen [1] and by Tipton [2], comparing current methods and stating the necessity of "in-process gauging". In recent years, Shiraishi [3] made a thorough survey of existing methods between 1961 and 1985. This work extends these surveys up to 1996, by reviewing all developed methods, with representative examples. It also describes recent commercially improved techniques and available instruments for building "in-process gauging equipment". 2. IN-PROCESS MEASUREMENT
The meaning of "in-process" (in situ) measurement is the measurement of workpiece size, whilst the component is located on the machine tool and preferably whilst the machining process is in progress. A desirable "in-process measurement system" is defined by Tillen [1] as one which makes measurements with the minimum of skill and of interruption to the machining process, in order to give sizes in the most suitable form, to enable the finishing cut or pass to produce a workpiece within the desirable tolerances. Numerous disturbances during turning operations such as thermal distortions, tool wear, etc., influence the accuracy of workpiece diameters making it difficult to maintain close tolerances. By "in-process measurement", necessary information is provided, so that correction can be made to cope with such problems. For the case of highest accuracy demands, it is necessary to measure the diameter of the workpiece directly prior to establishing the final finishing cut [1]. The final diameter must also be measured so that the bias, i.e. the difference between the cut applied, and the cut achieved, may be obtained by "in-process measurement".
2.1. The requirement for "in-process measurement" In general, there will be errors of size in any machined workpiece. This means that the actual dimension will be different from nominal dimension. However, these errors should be within certain given limits set by tolerances to obtain the necessary quality. If many nominally identical workpieces are made on a machine tool, and any one dimension is checked, the errors will vary from part to part, and can be divided into two kinds of errors [2]. Random errors which cannot be controlled by the operator, and systematic errors for which a definite cause could be identified. If a sufficient number of workpieces are made and measured, the errors will, in general, be found to follow the usual gaussian distribution characteristic of a random process. Random errors comprise the variations within the machine and the application variations introduced during use. They can be considered as a combination of the errors due to the machine structure, such as variation in bearings, friction, backlash, etc., and the errors due to the operator or control system. In general these errors can be reduced by using a better machine tool, or operator or control system. If the mean value of the measured results obtained from a sufficient number of workpieces is observed during a period of time, there will be a drift in one direction which is caused by systematic errors. These errors can be reduced by suitable inspection correction techniques, which may involve either in-process or post-process measurement and control. The most important systematic errors in practice are those caused by [2]: • Thermal distortions in machine tools. These are presented in most machine tools. When they are significant, attempts should be made to minimize them. Thermal expansions occur in both the machine tool and the component, depending on the materials of their construction. If the magnitude of the expansion exceeds the tolerance limits on the components, appropriate temperature control may be necessary. • Tool wear. This always occurs and causes less stock being removed than is desired. Its importance depends on the rate at which wear takes place. • Deflections of the machine/tooling/workpiece system under cutting forces. Deflection of the machine tool structure itself should be negligible in a well-designed machine structure. However, deflections of slender workpieces cannot always be avoided and other workpieces may demand tooling which cannot be made sufficiently stiff.
In-process dimensional measurement and control of workpiece accuracy
1425
• Deflection under the weight of workpieces. This can be a problem, especially with very large machine tools. Significant deflections can occur in both the machine tool and large workpieces themselves under their own weight. • Misalignment of machine tool axes. This could be present due to lack of squareness of axes relative to each other.
In general, the tolerances will be at least several times the random errors, and it may also be wide enough to accommodate systematic errors as well. If this is so, there may not be a need for "in-process" or "post-process" control [2]. "Post-process measurement" can be applied to high-production runs of smaller parts. The inspection process can be made by traditional methods. If the dimensions are not within the given tolerance zone, a correction can be made to the next part through the machine tool. However, if the manufactured parts are big, with higher material cost and longer cycle times, "in-process measurement" is required to improve the productivity and reduce the cost. With this method, accuracy of the manufactured part can be controlled during the machining process. Even after the machining process, corrections on oversized portions of the part can still be made since the parts stay in situ. To sum up, "in-process measurement" and control is necessary whenever the systematic errors vary significantly, during the machining of one workpiece. Furthermore, it is a good combination for automated manufacturing, particularly for computer numerical controlled machine tools. The desired dimensions can be stored for reference, to be checked with actual dimensions measured by "in-process measurement" and then the corrections can be carded out whilst the component is being machined. 3. A REVIEW OF COMMERCIAL AND EXPERIMENTAL TECHNIQUES
The existing "in-process measurement" methods can be considered as direct and indirect methods, depending on the operation principle. In direct methods, the diameter of the workpiece is directly measured using an adequate instrument, whilst the workpiece is located on the machine tool. Thus, the effects of tool wear distortions and machine errors can be taken into account. On the other hand. the workpiece accuracy is indirectly evaluated from radius measurements, by monitoring the motions of the carriage, carrying the cutting tool or by noting the position of the tip of the cutting tool. When applying such an indirect method, a measuring device is generally set on the slide. The methods of measuring the diameter of workpieces by "in-process measurement" can be classified into seven main methods, according to their operating principle. Both direct and indirect methods might be available in these techniques. 3. I . Mechanical methods
A mechanical method of "in-process measurement" can be defined as one in which the measuring transducer, operates in mechanical contact with the workpiece, although the actual signal may be electrical or pneumatic. 3. I . 1. Direct methods. 3.1.l.1. Caliper type A typical caliper type contact gauge is shown in Fig. l(a). It consists of a simple scissors caliper with non-rotating circular contact pads. The instrument can be set to measure over a range of diameters. Various types of caliper gauges, such as scissors and C calipers, and substantial descriptions with a wide range of experiment results have been performed by Tillen [1 ]. In this method, the contact pads or jaws are in continual rubbing contact with the workpiece. It is attached to the machine bed on its own slide so that it can be rapidly withdrawn and returned to the measuring position in a repeatable fashion. The rear gap of the scissors is bridged by a sensing element, which can be a pneumatic or electrical transducer. The caliper is set with respect to a circular setting master. It is possible to derive an electrical signal with both types of transducer, which can be used to control the machining process such as grinding and turning. Recently, caliper type gauges have been improved utilizing advanced techniques. The
1426
T. Yandayan and M. Burdekin Dial gauge
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OD = Outside diameter OD = Insidediameter Fig. 1. (a) Caliper-type turning gauge [2]. (b) Application of a recent model caliper-type gauge into a grinding process [4]. (c) Application areas for recent model caliper-type gauges [4].
In-process dimensional measurement and control of workpiece accuracy
1427
range of applications and measurements have been extended to reduce the drawbacks of the technique, such as range of workpiece sizes. Measured workpiece diameter range is given as 5-190 mm and repeatability is 0.5 p,m [4]. Application of a new caliper type gauge into a grinding process is shown in Fig. l(b) and various applications areas are illustrated in Fig. l(c) [4]. Calipers are a popular method of direct measurement, often used in grinding. This is because grinding is naturally suited to this type of measurement, as it is a multiple point cutting process, removing small amounts of material from a workpiece with a fine surface texture and on a good quality machine tool. The wear of contact heads is the main drawback of this method, particularly if it is applied to a turning process. Furthermore, these must be set with respect to a setting master each time the workpiece size is altered, causing an increase in lead times. 3.1.1.2. Friction-roller type The friction-roller method measures the perimeter of the workpiece by counting the number of revolutions of the measuring roller for one or more complete revolution of the workpiece [5, 6]. The schematic diagram of the friction-roller type instrument is shown in Fig. 2. Each revolution of the roller produces a number of pulses. Reference pulses are also derived once per revolution of the workpiece, and are used to stop and start the counter, so that it reads the number of measuring pulses every revolution, or for example every 10 revolutions of the workpiece. For instance, if the measuring wheel has a diameter of 100 mm and say gives an output of 10 000 pulses per revolution, then each pulse represents 100/10 000 = 0.01 mm of workpiece diameter if one revolution is measured, or 0.001 mm if the pulses are counted over 10 revolutions of the workpiece. A counter is used to record the number of pulses from the roller, during a limited number of whole revolutions of the workpiece. The diameter of the workpiece, D, is then given by the following equation [5]: D = DRoI'P,J(n'P)
( 1)
where DRo~ is the diameter of the roller, P. is the number of pulses counted during n revolutions of the workpiece and P is the number of pulses for one roller revolution. The resolution of the method is given by [5]: (2)
A = DRoJ(n'P)
where A is the change in diameter which gives rise to a difference of 1 in the number of pulses counted. Knowing DRop,n, P and P, values, actual diameter D can be easily found. In this method, accuracy varies with the type of material. Moreover, the application of the friction roller is restricted to rigid workpieces, due to the high pressure applied by the roller. The instrument, in general, gives better results with larger diameter workpieces than smaller ones, although the resolution is unaffected by workpiece diameter [7]. Several attempts in applying this technique have been made in turning and grinding ICaltkilBDilk
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Fig. 2. The schematic diagram of the friction-roller type instrument.
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T. Yandayan and M. Burdekin
and some of them have been successfully introduced into industry [6, 7]. However, it has been found that these techniques basically show measuring errors, due to slip between the workpiece and the friction roller. The type of workpiece material also influences the measurement process, for example, steel and cast iron produce different results. 3.1.2. Indirect methods. 3.1.2.1. Probe type A probe in mechanical contact with the workpiece is used to determine the actual size of workpiece. For the gauging process, the probe is moved towards the workpiece and deflected by the contact. The co-ordinate value of the point of touch makes it possible to determine the workpiece radius provided the position of the axis of rotation is known. The measuring probes are mounted in tool holders and deposited in the tool turret of the machine during cutting. One such existing system is the well-known Renishaw touch probe [8]. The big significant advantage of this system is that the same sensor can be employed to measure both the internal and outer diameter or workpiece lengths. Different types of probes are commercially available. One interesting type called doubletouch trigger probes (DTP) [9] was developed to solve the problems such as the change in the axis of rotation due to thermal deformations. This reduces the measurement processing time, if the single probe is used by touching the workpiece twice, at the points on the opposite sides of the axis of rotation. With this system, however, the accuracy of the measurement is subject to the accuracy of an individual machine's positioning system and also the traceability of measurement cannot generally be achieved. 3.2. Optical methods An optical method of in-process measurement is defined as one in which the transmitter module produces and emits a light, which is collected and photoelectrically sensed through the object to be measured, by a receiver module. This produces the signals which are converted into a convenient form and displayed as dimensional information, by the electronics process. Optical methods can be considered as non-contact measurement systems, and have unique advantages due to the nature of the light compared with the other methods. The principle advantages are: • Direct mechanical contact between the sensor and the object to be measured is not required. • The distance from the object to be measured to the sensor can be large. • The response time is limited only to the electronics used in the sensor. • The light variations can be directly converted into electrical signals. • Possibility of damage to the surface of a part which might result during contact inspection is eliminated. In fact, the chief advantages of optical methods are that no physical equipment is required in the vicinity of the workpiece, and information on the diameter deviation is conveyed by means of light from the workpiece to a suitable detector. The optical system can usually be arranged to enable the desired distance between the source or the detector and the workpiece to be determined. The use of optical methods increases the rate of dimensional data acquisition. Particularly in recent years, modem photoelectric devices and advanced computers have been developed, thus making the optical system the most convenient method. In addition, the availability of He-Ne lasers resulted in the practical use of optical methods. 3.2.1. Direct methods. In direct optical methods, the diameter of the object is generally measured by interrupting the light emitted from a transmitter, and by detecting this light electronically, to obtain electronic signals, so that this basic data can be converted into dimensional readings.
In-process dimensional measurement and control of workpiece accuracy
1429
3.2.1.1. Scanning light beam Since this technique, in general, uses laser beams for the measurement process, it is also called a scanning laser beam. It basically employs a transmitter module which emits a high-speed scanning laser beam, generally by means of a combination of a mirror and a synchronous motor. The object to be measured interrupts this beam, and produces a time-dependent shadow. This shadow is electronically detected by a receiver, and converted into dimensional readings by a control unit. An instrument has been built for measurement of a rod diameter [10] using a scanning laser beam technique. Due to recent developments in optics, submicron resolution can be achieved with this technique [ 11]. However, the requirement of expensive optical components, especially for large diameters, and the alignment of the measured object with the systematic errors are the main drawbacks of the system. 3.2.1.2. Machine vision The use of machine vision systems, for inspection, is an exciting area, which holds the promise of significant improvements in both the productivity of the inspection process, and the quality of the resulting product. First attempts have been made using these type of techniques for "in-process measurement" particularly in Russia. Vikhman [! 2] proposed a system in which a conventional light source is used and an image of the workpiece is focused on the measuring grid on the face of a television tube (see Fig. 3). The measuring grid consists of alternating transparent and opaque stripes, with spacing in the order of 0.05--0. l mm and the whole image is scanned at right angles to the grid, by detecting the cathode ray beam, so that the workpiece diameter can be measured at any position along its axis. Later, a more advanced technique using a charge coupled device (CCD) and a microcomputer has been developed [13]. The diameter of the workpiece is computed in terms of the image application factor, focal distance and the image length on CCD. The results show that the errors increase for larger diameters. However, the resolution of CCD itself is the instrument--it gives prospect of utilization of CCD for measurement of length. As the length of the pixel is getting shorter, the measuring accuracy can be expected to improve. It is stated that the relative accuracy for the measurement is kept within 0.05% for the workpieces from 30 to 50 mm in diameter. The technology of machine vision inspection is one in which advancements and refinements are continually being made. Further improvements in vision technology will allow improved resolution. However, the cost for application of this technique, particularly in "in-process measurement" of very large diameters, is very high. In addition, the small picture area results in a very limited capability of accomplishing precise measurements. 3.2.1.3. Light gauging The commercially available prototype instrument by Novak [14] which uses a light-gauging method is capable of measuring diameters up to 280 mm with a resolution of 0.001 mm. Also, measurement is repeated within 0.003 mm even when operating on a lathe, and evaluated accuracy is 0.01 ram. The instrument can operate on the machine tool or be handled by a robot. Figure 4 illustrates the principle of the instrument. The He-Ne laser (2 mW) emits a beam (diameter 0.6 mm) which is divided into two measuring beams (A and B) and a C a ~ t~e Workpi~,¢
Fig. 3. Television-type projection system by Vikhman [121. NIN 31:lO-C
1430
T. Yandayanand M. Burdekin Detector
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Fig. 4. Light-gaugingmethod for measuringthe workpiecediameter by Novak [14]. reference beam (C) by the beam dividers. The measuring beams are first reflected towards the workpiece and then back, to be detected by a detector. The parallelism of the measuring beams does not depend on the accuracy of the mechanical parts which guide the optical components. The distance between the measuring beam (A, B) corresponding to workpiece diameter is measured by a Sony linear magnescale. The wire tied around the axis of a d.c. motor adjusts the position of beams A, B symmetrically in order to measure the varies diameters. The repeatability of measurement is estimated to be 0.002-0.003 mm and the accuracy is 0.01 mm. Due to its physical size and complicated structure, such as delicate arms, restrictions arise for inconvenient conditions i.e. swarf and chips which could damage the sensor. Moreover, holding and mounting of the instrument requires careful alignment. 3.2.2. Indirect methods. Indirect optical methods are defined as those in which the diameter of the workpiece is indirectly measured by means of features of the light reflected from the workpiece surface. In general, the light beam is projected on to the workpiece, and reflected on to a photodetector or any electronic device. Any change in the diameter of workpiece, results in a change in the location of the image or a change in the location of the focusing point on the workpiece. Hence, the workpiece diameter can be measured by determining the relation between one of these changes. 3.2.2.1. Light focusing The focusing point of an incident beam on the workpiece surface which produces maximum light intensity, is detected by a photodetector. A change of workpiece diameter yields a deflection of the focusing point and leads to less reflected light intensity or, a change in the place of reflected light image on the photodetector. The sensor output is then used to operate a servo-mechanism, which controls the tool position to maintain the diameter within a prescribed level.
In-process dimensional measurement and control of workpiece accuracy
1431
The concept of the light-focusing method, has been studied by Merglcr and Sahajdak [15, 16]. First, they developed a gauging apparatus for controlling the accuracy of the workpiece diameter with a closed loop system shown in Fig. 5(a). Then they improved the optical configuration of the apparatus [16]. Figure 5(b) shows the consolidated configuration of the improved optical gauging head, and Fig. 5(c) shows the principle of measurement. The incident laser beam is focused on the workpiece surface and the image is analysed with a position-sensitive detector which determines whether the focal point is
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1432
T. Yandayan and M. Burdekin
in front, behind or at the workpiece surface. A servo-control loop drives the optical assembly with a precision actuator to maintain the focal point at the workpiece surface, and the distance is determined by geometry from the mechanical position of the optical components. It is asserted that a measurement precision of ± l0 p.m from a distance of 200 mm was achieved in the experiments. Since the position of the reflected laser spot depends on surface irregularities, a nonuniform workpiece surface is the major limitation in the measurement. Moreover, as the optical system does not measure the true diameter of the workpiece (but rather the distance to the workpiece surface using the carriage as a reference), some machine deflections and thermal expansion errors are outside the control loop. Thus, the overall absolute accuracy of the system depends on the stability of the mounting. 3.2.2.2. Light-spot detection The finish size of a workpiece is continuously monitored by detecting a displacement of a laser spot, which is reflected from the workpiece surface. An example of this method given by Shiraishi [l 7, 18] is illustrated in Fig. 6. The workpiece surface is illuminated by a parallel laser beam, having a diameter of 0.5 ram, passing through the optical system 1. The laser spot on the surface (Or) is magnified by the optical system 2, keeping the view angle of the spot at/3 and the image of the spot is made on the photodetector located at the focal plane Pt. In this situation, when the laser spot moves from Om to O2 corresponding to the workpiece deviation AD, the image moves on the photo detector from Pm to PE. By detecting this displacement of the image on the photodetector, the value of AD is determined. During operation, the finished size of a workpiece is continuously monitored by the laser spot. Meanwhile, an air jet system, arranged close to the cutting edge, is used to blow away smoke, cutting fluid and chips which might otherwise interfere with the measurement. The desired change in diameter is compared with the movement of the spot image on the photodetector and the tool post is automatically moved in the direction to reduce the error. Resolution of the system is stated to be 1/xm and workpiece diameter is produced with overall errors within + 10/zm. However, further investigations for higher reliability are recommended for industrial applications. Surface irregularities of the workpiece can again be considered as a major limitation since they might affect the movement of the spot image on the photodiode. 3.2.2.3. Light sectioning The method introduced by Shiraishi and Sato [19] can be considered as a light-sectioning method. This method not only controls the dimension of workpiece, but also it makes a smoother surface, by removing surface irregularities during machining. The dimensional control is implemented with a reference diameter level of Rmaff2 roughness by the regular cutting tool, whilst in the roughness control a compensatory "flat bite" tool is employed to remove surface irregularities. In the author's view, the method is related to the described concept of dimension and roughness control. The desired diameter is considered as the reference diameter given by the following equation:
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Fig. 6. Principle of light-spot detection method [17].
In-process dimensional measurement and control of workpiece accuracy D = D,n,,~ -
1433
(3)
R,..,~
where D , ~ is the maximum diameter and R~,x is the maximum peak-to-valley height. During the cutting process two possible deflections, either negative or positive, might occur, and are compensated for by reducing or increasing depths of cut, respectively. For determining the reference diameter level, an optical arrangement shown in Fig. 7(a) is used. A He-Ne laser light, having a rectangular beam by lens 1 and concave lens 2, is projected to the upper edge of a workpiece. The laser beam passing throughout the edge makes a sharp profile pattern of the workpiece surface on to the photo array sensor, after it is projected by another lens (lens 2) and concave lens 2. A knife edge between concave lens l and the workpiece is used to improve the sharpness of the pattern. As shown in Fig. 7(b), this pattern comprises a shadow image corresponding to surface profile, and a bright part due to passing light, which changes the sensor output. Once a reference diameter level is fixed as shown Fig. 7(b), positive and negative deflection deviation can be determined from the sensor [Fig. 7(c) and (d)]. Knife Co~.~ leas I _ _ . He'Ne Lensl
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1434
T. Yandayan and M. Burdekin
The experimental system is illustrated in Fig. 8. The laser and optical system 1 are mounted on the same tool post, 0.1 mm behind the regular cutting tool. After a 2-3 mm cut from the start, the machining is stopped for a moment to measure Rmax value (i.e. by the photo array sensor). Then the reference diameter level is determined and set onto the photo array sensor. At the same time, the regular cutting tool and the flat bite tool are adjusted on that level, and then the machining is started again, in conjunction with the dimensional and roughness control. It is stated that both dimensional and surface roughness can be implemented by a minimum amount of correction of 1.0/.Lm, the chief advantage being independent of the workpiece material. However, there might still be error sources when the reference level diameter is defined at the beginning of the cut. Further improvement of the system is required before it is applied to a real industrial environment due to its impractical physical arrangement on the machine tool. 3.3. Pneumatic methods A pneumatic method can be defined as one in which the location of surface is measured, by means of an air jet stream directed against the surface to be measured, and by obtaining a signal from the variation in back pressure in the air feed line. In other words, the measuring system measures a pressure drop in the gap between the air gauge and workpiece, and converts it into an electrical signal. The pneumatic gauging gives a convenient and sensitive way of measuring small distances. The chief advantage is that the continual flow of air through the gauge head can be arranged to blow away cutting fluids and swarfs [3]. Fundamental elements [20] such as air gauges, fluid jet gauges and bubble modulation proximity gauges can be used to form "in-process" gauging equipment. A pneumatic-hydraulic system which implements several types of air gauges for controlling a hydraulic ram, to correct the tool position, has been used [20]. This is illustrated in Fig. 9. Pneumatic methods have also been applied to honing by Militzer [21]. The main disadvantage of the system is that the air gap to be measured must not exceed about 0.010 in. (0.254 mm) to obtain the necessary accuracy [e.g. 0.0003 in. (7.6 Izm)]. From the investigations, pneumatic methods have been offered because of their practical and
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In-process dimensional measurement and control of workpiece accuracy
1435
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application, and it is stated that they will be the most reliable technique among the noncontact methods [3]. Despite this, some problems accompanying pneumatic methods exist, such as temperature effects around the air flow and surface irregularities. Recently, a pneumatic--electric gauging (PEL) system has been developed for practical uses [22]. It is stated that it operates even if the detector nozzles are exposed to extremes of temperature. The accuracy for the system is given according to nozzle type and air gap between the sensor and the workpiece (gap distance) (Table 1). The advantages of the PEL system are stated as follows: • Unaffected by the material. The system responds uniformly regardless of the material to which it is applied---be it metallic or non-metallic, magnetic or non-magnetic, transparent or opaque. • High speed gauging and control. Up to 50 control operations or five gauging operations per second. • Application over a wide temperature range. The PEL system operates even in extreme temperatures.
Using these advances, gauging instruments can be built for application to in-process measurement. However, the main disadvantage of the pneumatic method, in the case of "in-process measurement" of workpiece diameters in turning, is the small air gap between the sensors and the workpiece. 3.3.1. Electrical methods. These type of methods can be considered as non-optical non-contact methods. Various types of electrical field techniques, such as reluctance and capacitance, can be applied to "in-process measurement". The reluctance transducers are proximity devices, which indicate the presence and the distance from the probe of a ferromagnetic substance. The obvious limitation is that the workpiece being inspected must be electromagnetic. A capacitance-based transducer can also be used to measure the distance between the workpiece and the probe. The measurement is based on the variable capacitance from the workpiece-probe coupling This capacitance is inversely proportional to the distance Table I. Accuracy values for PEL system [22] Gap distance Concentric nozzle Sapphire nozzle
1.00 m m 0.15 m m
Accuracy 5 ~.m I t~m
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T. Yandayan and M. Burdekin
between the probe and the workpiece, and thus the distance can be calculated. This technique can be used for various materials as long as the material is an electrical conductor. Electrical methods can also be used for application of indirect methods. Tool deflection due to cutting forces, which can be determined by means of an electric transducer, has been considered to determine the relation between this deflection and dimensional error on the workpiece. This has been applied to control the boring of fuel injector nozzles [23]. Fan and Choo [24] proposed a technique which can be considered as a direct method utilizing electrical techniques. It is called the moving three probe technique (MTPT) which compromises three servo-operated units and non-contact displacement probes (proximitor) (Fig. 10). As seen in Fig. 10(a), each unit consists of a non-contact displacement probe (proximitor) for gap sensing, a stepping motor controlled linear carriage, which can carry the probe for surface tracing, and a displacement transducer (LVTD) for real-time recording of the carriage position. After arranging three units as in Fig. 10(b) for the MTPT method, the workpiece diameter can be determined by the measurement principle shown in Fig. 10(c). Probes S~, Sz and $3 are set in their reference position from lines a, b and c, respectively. The reference line of each probe Si, is located away from the workpiece centre by distance L~. This distance can be measured in advance by contacting the probe to a standard point rod of known diameter, and then recording the movement of this probe back to its specified reference line. From the reference line the movement of the probe S~ is recorded as h; by the LDVT and its distance to the workpiece surface is recorded as 6",.. Then, the diameter of the workpiece can be obtained by the given equation: DI = 2LI - /-,2 +
L3 -
2Ct + C2 - Ca - 2hi + h2 - h3
(4)
It is stated that control of workpiece diameter was achieved to within -1- 10/xm accuracy. However, use of three units (i.e. each containing a digital LVTD, step motor, linear carriage and probe) occupies a large space, which is an obstacle in the turning machining environment. Therefore, it would be difficult to apply this technique to industry without further improvement. Additionally, another limitation could be maintaining the gaps between workpiece and sensors within the measurement range of the probes. 3.4. Ultrasonic methods In this method, ultrasound travels to the workpiece, then reflects back to the transducer which also acts as a receiver. The transit time depends on the variation from the specified distance between worksurface and transducer. By determining the transit time, the distance can be calculated. Recently, an ultrasonic method has been used with the lapping process for in-process measurement of silicon wafer thickness [25]. In general, the pulse-echo technique is used. In this technique, the transducer is first excited by a short electrical pulse; the electrical energy is then converted into sound waves. The sound waves are transmitted from the transducer into the test material through a thin film of coupling liquid such as glycerine, oil or water and are then reflected from the back surface of the test material. The reflected sound waves are received and converted into electrical signals by the transducer. The travelling time from the transmission of sound waves to the reception of them is in direct proportion to the thickness of the test materials, as the sound waves travel with constant velocity through the uniform medium. 3.5. Temperature-Detection method It is well known that, in turning, the machining error comes out as a change in workpiece diameter, and arises predominantly from the thermal expansion of the tool. This has been studied by Takeuchi et al. [26]. It is found that 52% of the total cylindrical error of the workpiece is due to the thermal expansion of the tool, when cutting carbon steel with a cutting speed of 150 m min- ~, rate of feed 0.1 mm rev- ~ and depth of cut 1 mm. By knowing the thermal expansion of the tool, the error due to this effect can be determined and compensation is performed by moving the position of the tool.
In-processdimensionalmeasurementand controlof workpieceaccuracy
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1438
T. Yandayanand M. Burdekin
Figure 11 shows the outline of compensation. The following procedure is performed for the compensation. First, cutting conditions and the analysis program of thermal deformation are stored in a data file. Using this information, the thermal expansion of the tool can be estimated by the use of the thermal displacement function. AX (machining error in the X-direction) is found from the finite element method and the tool is moved back AX in the direction of cut (X-direction). The resolution is reported as 5/zm and the machining accuracy is about 7/~m [26]. With a temperature-detection method, errors such as geometrical errors and the static deformation error of a workpiece associated with the cutting force are not taken into account. However, the method has an advantage in that measuring devices are not necessary, and therefore it becomes an economical and practical way to improve accuracy. 4. CONCLUSION Commercial and experimental "in-process measurement techniques" have been described through examples. In the technical literature, a large amount of information is available on a variety of systems proposed for "in-process measurement" of diameter during turning, but most seem to be unsuitable for practical applications in an industrial environment. Therefore, there is still demand for a practical "non-contact method" which could measure workpiece diameter directly, with high resolution and accuracy and considering all errors. Desired features for in-process gauging equipment are listed below. (1) (2) (3) (4) (5) (6) (7) (8)
High measuring resolution and accuracy. Rapid gauging. Quick change-over to different gauging operations. Low-cost construction. Flexibility--large measuring range of diameters. No need for manual adjustments. Being independent of ambient temperature. Being independent of workpiece surface texture and material.
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In-process dimensional measurement and control of workpiece accuracy
1439
(9) No equipment in the vicinity of workpiece to prevent interfering measuring process. (! 0) During measurement process, all errors should be taken into account. This seems that it can only be implemented by direct measurement of diameter.
REFERENCES [ll Tillen, R., Controlling workpiece accuracy. Prod. Engr, 1964, 43, 499. [2] Tipton, H., In-process measurement and control of workpiece size. Mach. Tool Res., 1966, 5, 93. 131 Shiraishi0 M., Scope of in-process measurement, monitoring and control techniques in machining process-part 2. In-process techniques for workpieces. Precis. Engng, 1989, ll(I), 27. 14l Anon., in-process grinder gauges. Commercial 7-page brochure, Control Gaging lnc., UK (1996). [5] Nokikov, N. I. and Makarevich, B. K., Automatic measuring of dimensions in turning. Meas. Techn., 1961. 6, 431. [6] Ivanov, B. N. and Mel'nichuk, V. A., Measuring the diameter of cylindrical articles by the method of running a roller over their surfaces. Meas. Techn., 1964, 9, 772. [7] Lee, R. L., Turning railway wheel sets. MachineD', 1963, 102, 144. [8] Anon., 3-Dimensional touch trigger probes for machining centres and lathes. Commercial 16-page brochure. Renishaw Electrical Ltd, New Mills, Wotton-under-Edge, UK (1984). [9] Szafarczyk, M. and Misiewski, M., Automatic measurement and correction of workpiece diameter on NC center lathe. Ann. CIRP, 1983, 32(1), 305. [10] Binks, S. D., The development of a laser-operated scanning rod gauge. Meas. Control, 1971, 4(April), T49. [I 11 Anon., World's smallest laser scan micrometer. Commercial 6-page brochure, Keyence Corporation, UK (1990). [121 Vikhman, V. S., Application of the television computing technique in automatic control of dimensions. Meas. Techn., 1963, 11,904. 1131 Takesa, K., Sato, H. and Tani, Y., Measurement of diameter using charge coupled device (CCD). Ann. CIRP, 1984, 33(1), 377. LI41 Novak, A., Sensing of workpiece diameter, vibration, and out-off-roundness by laser-way to automate quality control. Ann. CIRP, 1981, 30(1), 473. [151 H. W. Mergler and S. Sahajdak, In-process optical gauging for numerical machine tool control and automated process. Proc. 3rd NSF/RANN Grantee's Conf. on Production Research and Industrial Automation. Cleveland, OH, p. 57 (1975). [16] H. W. Mergler and S. Sahajdak, In-process optical gauging for numerical machine tool control. Proc. 3rd Int. Conf. on Automated Inspection and Product Control, University of Nottingham, April, p. 57 (19781. [17] Shiraishi, M., In-process control of workpiece dimension in turning. Ann. CIRP, 1979, 28(1), 333. [18] Shiraishi, M., A measuring method of diameter deviation for the turned workpiece with curved profle using an edge of the image. Bull. Jap. Soc. Precis. Engng, 1979, 13, 133. [191 Shiraishi, M. and Sato, S., Dimensional and surface roughness controls in a turning operation. Trans. ASME: J. Engng Ind., 1990, 112, 78. [20] R. Sharp and H. Bath, Non-contact measurement for machine tool control. Proc. Ist Int. Conf. on Fluid l.z)gic and Amplification. Cranfield, Paper GI, September (1965). [21] Militzer, R. W., Automatic sizing techniques for production honing. Tool. Prod., 1965, 31(July), 45. [22] Anon.. The system for pneumo-electric gauging and controI-PEL, problems and solutions. Commercial 34page brochure, Chad Controls, Pneumatic Control Systems, Oldham, UK (1996). [23] Hick, J. R., Boring goes adaptive. Am. Mach., 1967, Ill(August), 93. [241 Fan, K. C. and Chc~, Y. H., In-process dimensional control of the workpiece during turning. Precis. Engng, 1991, 13(I), 27. [25] Tsutsumi, M., Ito, Y. and Masuko, M., Ultrasonic in-process measurement of silicon wafer thickness. Precis. Engng, 1982, 4(4), 195. [261 Takeuchi, Y., Sakamato, M. and Sara, T., Improvement in the working accuracy of an NC lathe by compensating for thermal expansions. Precis. Engng, 1982, 4(1), 19.
Pergamon
PII: S0890-6955(97)00019-9
IN-PROCESS D I M E N S I O N A L
MEASUREMENT WORKPIECE ACCURACY
AND CONTROL
OF
T. YANDAYANt and M. BURDEKINt$ (Received 26 November 1996)
Abstract--This paper surveys the available technology for "in-process control" of machined products. particularly in relation to turning, and cylindrical grinding, prior to completion of the machining process. The techniques developed for controlling accuracy of the workpiece, whilst in situ, are described using representative examples of current technology. The description of new commercial instruments which perform "in-process control" and new devices which could he developed as "in-process measurement instruments" are also presented. © 1997 Published by Elsevier Science Ltcl
NOMENCLATURE A Ci D [-)m~x
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resolution distance to the workpiece surface diameter maximum diameter diameter of the roller probe movement distance probe distance from centre line of the workpiece number of revolutions number of pulses for one roller revolution number of pulses for n revolutions of the workpiece maximum peak-to-valley height workpiece deviation machining error in X-direction 1. INTRODUCTION
Automated inspection methods are the solutions to the increasing demand by customers for near perfection in the quality of manufactured parts and a desire to decrease the costs involved. These inspection methods should be able to the control accuracy of the part during the machining process and provide the required feedback information for the necessary corrections. Quality control of the parts can be performed using traditional methods, for example, manual inspection methods. This requires parts to be removed from the vicinity of the production machines to a separate inspection area, resulting in delay in the manufacturing schedule. Statistical sampling procedures can also be applied; however, these only guarantee that a certain expected or average fraction defect rate will be generated during the process. In addition, there is a risk that some defective parts will not be detected. Another traditional quality control method is to perform the inspection on completion of the parts. If the parts are defective, they must be scrapped or reworked at a cost. Consequently, the cost of inspection parts by conventional methods in some cases equals or exceeds the machining cost. The strategy for achieving improved product quality, high productivity and reduced lead times, is to bring the control of workpiece quality close to the machining process. This can be achieved "by in-process measurement" which inspects the parts whilst they are on the machine tool.
"i'Department of Mechanical Engineering, UMIST, Manchester, U.K. :l:To whom all correspondence should be addressed. 1423
1424
T. Yandayan and M. Burdekin
A survey of in-process gauging was previously made by Tillen [1] and by Tipton [2], comparing current methods and stating the necessity of "in-process gauging". In recent years, Shiraishi [3] made a thorough survey of existing methods between 1961 and 1985. This work extends these surveys up to 1996, by reviewing all developed methods, with representative examples. It also describes recent commercially improved techniques and available instruments for building "in-process gauging equipment". 2. IN-PROCESS MEASUREMENT
The meaning of "in-process" (in situ) measurement is the measurement of workpiece size, whilst the component is located on the machine tool and preferably whilst the machining process is in progress. A desirable "in-process measurement system" is defined by Tillen [1] as one which makes measurements with the minimum of skill and of interruption to the machining process, in order to give sizes in the most suitable form, to enable the finishing cut or pass to produce a workpiece within the desirable tolerances. Numerous disturbances during turning operations such as thermal distortions, tool wear, etc., influence the accuracy of workpiece diameters making it difficult to maintain close tolerances. By "in-process measurement", necessary information is provided, so that correction can be made to cope with such problems. For the case of highest accuracy demands, it is necessary to measure the diameter of the workpiece directly prior to establishing the final finishing cut [1]. The final diameter must also be measured so that the bias, i.e. the difference between the cut applied, and the cut achieved, may be obtained by "in-process measurement".
2.1. The requirement for "in-process measurement" In general, there will be errors of size in any machined workpiece. This means that the actual dimension will be different from nominal dimension. However, these errors should be within certain given limits set by tolerances to obtain the necessary quality. If many nominally identical workpieces are made on a machine tool, and any one dimension is checked, the errors will vary from part to part, and can be divided into two kinds of errors [2]. Random errors which cannot be controlled by the operator, and systematic errors for which a definite cause could be identified. If a sufficient number of workpieces are made and measured, the errors will, in general, be found to follow the usual gaussian distribution characteristic of a random process. Random errors comprise the variations within the machine and the application variations introduced during use. They can be considered as a combination of the errors due to the machine structure, such as variation in bearings, friction, backlash, etc., and the errors due to the operator or control system. In general these errors can be reduced by using a better machine tool, or operator or control system. If the mean value of the measured results obtained from a sufficient number of workpieces is observed during a period of time, there will be a drift in one direction which is caused by systematic errors. These errors can be reduced by suitable inspection correction techniques, which may involve either in-process or post-process measurement and control. The most important systematic errors in practice are those caused by [2]: • Thermal distortions in machine tools. These are presented in most machine tools. When they are significant, attempts should be made to minimize them. Thermal expansions occur in both the machine tool and the component, depending on the materials of their construction. If the magnitude of the expansion exceeds the tolerance limits on the components, appropriate temperature control may be necessary. • Tool wear. This always occurs and causes less stock being removed than is desired. Its importance depends on the rate at which wear takes place. • Deflections of the machine/tooling/workpiece system under cutting forces. Deflection of the machine tool structure itself should be negligible in a well-designed machine structure. However, deflections of slender workpieces cannot always be avoided and other workpieces may demand tooling which cannot be made sufficiently stiff.
In-process dimensional measurement and control of workpiece accuracy
1425
• Deflection under the weight of workpieces. This can be a problem, especially with very large machine tools. Significant deflections can occur in both the machine tool and large workpieces themselves under their own weight. • Misalignment of machine tool axes. This could be present due to lack of squareness of axes relative to each other.
In general, the tolerances will be at least several times the random errors, and it may also be wide enough to accommodate systematic errors as well. If this is so, there may not be a need for "in-process" or "post-process" control [2]. "Post-process measurement" can be applied to high-production runs of smaller parts. The inspection process can be made by traditional methods. If the dimensions are not within the given tolerance zone, a correction can be made to the next part through the machine tool. However, if the manufactured parts are big, with higher material cost and longer cycle times, "in-process measurement" is required to improve the productivity and reduce the cost. With this method, accuracy of the manufactured part can be controlled during the machining process. Even after the machining process, corrections on oversized portions of the part can still be made since the parts stay in situ. To sum up, "in-process measurement" and control is necessary whenever the systematic errors vary significantly, during the machining of one workpiece. Furthermore, it is a good combination for automated manufacturing, particularly for computer numerical controlled machine tools. The desired dimensions can be stored for reference, to be checked with actual dimensions measured by "in-process measurement" and then the corrections can be carded out whilst the component is being machined. 3. A REVIEW OF COMMERCIAL AND EXPERIMENTAL TECHNIQUES
The existing "in-process measurement" methods can be considered as direct and indirect methods, depending on the operation principle. In direct methods, the diameter of the workpiece is directly measured using an adequate instrument, whilst the workpiece is located on the machine tool. Thus, the effects of tool wear distortions and machine errors can be taken into account. On the other hand. the workpiece accuracy is indirectly evaluated from radius measurements, by monitoring the motions of the carriage, carrying the cutting tool or by noting the position of the tip of the cutting tool. When applying such an indirect method, a measuring device is generally set on the slide. The methods of measuring the diameter of workpieces by "in-process measurement" can be classified into seven main methods, according to their operating principle. Both direct and indirect methods might be available in these techniques. 3. I . Mechanical methods
A mechanical method of "in-process measurement" can be defined as one in which the measuring transducer, operates in mechanical contact with the workpiece, although the actual signal may be electrical or pneumatic. 3. I . 1. Direct methods. 3.1.l.1. Caliper type A typical caliper type contact gauge is shown in Fig. l(a). It consists of a simple scissors caliper with non-rotating circular contact pads. The instrument can be set to measure over a range of diameters. Various types of caliper gauges, such as scissors and C calipers, and substantial descriptions with a wide range of experiment results have been performed by Tillen [1 ]. In this method, the contact pads or jaws are in continual rubbing contact with the workpiece. It is attached to the machine bed on its own slide so that it can be rapidly withdrawn and returned to the measuring position in a repeatable fashion. The rear gap of the scissors is bridged by a sensing element, which can be a pneumatic or electrical transducer. The caliper is set with respect to a circular setting master. It is possible to derive an electrical signal with both types of transducer, which can be used to control the machining process such as grinding and turning. Recently, caliper type gauges have been improved utilizing advanced techniques. The
1426
T. Yandayan and M. Burdekin Dial gauge
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In-process dimensional measurement and control of workpiece accuracy
1427
range of applications and measurements have been extended to reduce the drawbacks of the technique, such as range of workpiece sizes. Measured workpiece diameter range is given as 5-190 mm and repeatability is 0.5 p,m [4]. Application of a new caliper type gauge into a grinding process is shown in Fig. l(b) and various applications areas are illustrated in Fig. l(c) [4]. Calipers are a popular method of direct measurement, often used in grinding. This is because grinding is naturally suited to this type of measurement, as it is a multiple point cutting process, removing small amounts of material from a workpiece with a fine surface texture and on a good quality machine tool. The wear of contact heads is the main drawback of this method, particularly if it is applied to a turning process. Furthermore, these must be set with respect to a setting master each time the workpiece size is altered, causing an increase in lead times. 3.1.1.2. Friction-roller type The friction-roller method measures the perimeter of the workpiece by counting the number of revolutions of the measuring roller for one or more complete revolution of the workpiece [5, 6]. The schematic diagram of the friction-roller type instrument is shown in Fig. 2. Each revolution of the roller produces a number of pulses. Reference pulses are also derived once per revolution of the workpiece, and are used to stop and start the counter, so that it reads the number of measuring pulses every revolution, or for example every 10 revolutions of the workpiece. For instance, if the measuring wheel has a diameter of 100 mm and say gives an output of 10 000 pulses per revolution, then each pulse represents 100/10 000 = 0.01 mm of workpiece diameter if one revolution is measured, or 0.001 mm if the pulses are counted over 10 revolutions of the workpiece. A counter is used to record the number of pulses from the roller, during a limited number of whole revolutions of the workpiece. The diameter of the workpiece, D, is then given by the following equation [5]: D = DRoI'P,J(n'P)
( 1)
where DRo~ is the diameter of the roller, P. is the number of pulses counted during n revolutions of the workpiece and P is the number of pulses for one roller revolution. The resolution of the method is given by [5]: (2)
A = DRoJ(n'P)
where A is the change in diameter which gives rise to a difference of 1 in the number of pulses counted. Knowing DRop,n, P and P, values, actual diameter D can be easily found. In this method, accuracy varies with the type of material. Moreover, the application of the friction roller is restricted to rigid workpieces, due to the high pressure applied by the roller. The instrument, in general, gives better results with larger diameter workpieces than smaller ones, although the resolution is unaffected by workpiece diameter [7]. Several attempts in applying this technique have been made in turning and grinding ICaltkilBDilk
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Fig. 2. The schematic diagram of the friction-roller type instrument.
1428
T. Yandayan and M. Burdekin
and some of them have been successfully introduced into industry [6, 7]. However, it has been found that these techniques basically show measuring errors, due to slip between the workpiece and the friction roller. The type of workpiece material also influences the measurement process, for example, steel and cast iron produce different results. 3.1.2. Indirect methods. 3.1.2.1. Probe type A probe in mechanical contact with the workpiece is used to determine the actual size of workpiece. For the gauging process, the probe is moved towards the workpiece and deflected by the contact. The co-ordinate value of the point of touch makes it possible to determine the workpiece radius provided the position of the axis of rotation is known. The measuring probes are mounted in tool holders and deposited in the tool turret of the machine during cutting. One such existing system is the well-known Renishaw touch probe [8]. The big significant advantage of this system is that the same sensor can be employed to measure both the internal and outer diameter or workpiece lengths. Different types of probes are commercially available. One interesting type called doubletouch trigger probes (DTP) [9] was developed to solve the problems such as the change in the axis of rotation due to thermal deformations. This reduces the measurement processing time, if the single probe is used by touching the workpiece twice, at the points on the opposite sides of the axis of rotation. With this system, however, the accuracy of the measurement is subject to the accuracy of an individual machine's positioning system and also the traceability of measurement cannot generally be achieved. 3.2. Optical methods An optical method of in-process measurement is defined as one in which the transmitter module produces and emits a light, which is collected and photoelectrically sensed through the object to be measured, by a receiver module. This produces the signals which are converted into a convenient form and displayed as dimensional information, by the electronics process. Optical methods can be considered as non-contact measurement systems, and have unique advantages due to the nature of the light compared with the other methods. The principle advantages are: • Direct mechanical contact between the sensor and the object to be measured is not required. • The distance from the object to be measured to the sensor can be large. • The response time is limited only to the electronics used in the sensor. • The light variations can be directly converted into electrical signals. • Possibility of damage to the surface of a part which might result during contact inspection is eliminated. In fact, the chief advantages of optical methods are that no physical equipment is required in the vicinity of the workpiece, and information on the diameter deviation is conveyed by means of light from the workpiece to a suitable detector. The optical system can usually be arranged to enable the desired distance between the source or the detector and the workpiece to be determined. The use of optical methods increases the rate of dimensional data acquisition. Particularly in recent years, modem photoelectric devices and advanced computers have been developed, thus making the optical system the most convenient method. In addition, the availability of He-Ne lasers resulted in the practical use of optical methods. 3.2.1. Direct methods. In direct optical methods, the diameter of the object is generally measured by interrupting the light emitted from a transmitter, and by detecting this light electronically, to obtain electronic signals, so that this basic data can be converted into dimensional readings.
In-process dimensional measurement and control of workpiece accuracy
1429
3.2.1.1. Scanning light beam Since this technique, in general, uses laser beams for the measurement process, it is also called a scanning laser beam. It basically employs a transmitter module which emits a high-speed scanning laser beam, generally by means of a combination of a mirror and a synchronous motor. The object to be measured interrupts this beam, and produces a time-dependent shadow. This shadow is electronically detected by a receiver, and converted into dimensional readings by a control unit. An instrument has been built for measurement of a rod diameter [10] using a scanning laser beam technique. Due to recent developments in optics, submicron resolution can be achieved with this technique [ 11]. However, the requirement of expensive optical components, especially for large diameters, and the alignment of the measured object with the systematic errors are the main drawbacks of the system. 3.2.1.2. Machine vision The use of machine vision systems, for inspection, is an exciting area, which holds the promise of significant improvements in both the productivity of the inspection process, and the quality of the resulting product. First attempts have been made using these type of techniques for "in-process measurement" particularly in Russia. Vikhman [! 2] proposed a system in which a conventional light source is used and an image of the workpiece is focused on the measuring grid on the face of a television tube (see Fig. 3). The measuring grid consists of alternating transparent and opaque stripes, with spacing in the order of 0.05--0. l mm and the whole image is scanned at right angles to the grid, by detecting the cathode ray beam, so that the workpiece diameter can be measured at any position along its axis. Later, a more advanced technique using a charge coupled device (CCD) and a microcomputer has been developed [13]. The diameter of the workpiece is computed in terms of the image application factor, focal distance and the image length on CCD. The results show that the errors increase for larger diameters. However, the resolution of CCD itself is the instrument--it gives prospect of utilization of CCD for measurement of length. As the length of the pixel is getting shorter, the measuring accuracy can be expected to improve. It is stated that the relative accuracy for the measurement is kept within 0.05% for the workpieces from 30 to 50 mm in diameter. The technology of machine vision inspection is one in which advancements and refinements are continually being made. Further improvements in vision technology will allow improved resolution. However, the cost for application of this technique, particularly in "in-process measurement" of very large diameters, is very high. In addition, the small picture area results in a very limited capability of accomplishing precise measurements. 3.2.1.3. Light gauging The commercially available prototype instrument by Novak [14] which uses a light-gauging method is capable of measuring diameters up to 280 mm with a resolution of 0.001 mm. Also, measurement is repeated within 0.003 mm even when operating on a lathe, and evaluated accuracy is 0.01 ram. The instrument can operate on the machine tool or be handled by a robot. Figure 4 illustrates the principle of the instrument. The He-Ne laser (2 mW) emits a beam (diameter 0.6 mm) which is divided into two measuring beams (A and B) and a C a ~ t~e Workpi~,¢
Fig. 3. Television-type projection system by Vikhman [121. NIN 31:lO-C
1430
T. Yandayanand M. Burdekin Detector
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Fig. 4. Light-gaugingmethod for measuringthe workpiecediameter by Novak [14]. reference beam (C) by the beam dividers. The measuring beams are first reflected towards the workpiece and then back, to be detected by a detector. The parallelism of the measuring beams does not depend on the accuracy of the mechanical parts which guide the optical components. The distance between the measuring beam (A, B) corresponding to workpiece diameter is measured by a Sony linear magnescale. The wire tied around the axis of a d.c. motor adjusts the position of beams A, B symmetrically in order to measure the varies diameters. The repeatability of measurement is estimated to be 0.002-0.003 mm and the accuracy is 0.01 mm. Due to its physical size and complicated structure, such as delicate arms, restrictions arise for inconvenient conditions i.e. swarf and chips which could damage the sensor. Moreover, holding and mounting of the instrument requires careful alignment. 3.2.2. Indirect methods. Indirect optical methods are defined as those in which the diameter of the workpiece is indirectly measured by means of features of the light reflected from the workpiece surface. In general, the light beam is projected on to the workpiece, and reflected on to a photodetector or any electronic device. Any change in the diameter of workpiece, results in a change in the location of the image or a change in the location of the focusing point on the workpiece. Hence, the workpiece diameter can be measured by determining the relation between one of these changes. 3.2.2.1. Light focusing The focusing point of an incident beam on the workpiece surface which produces maximum light intensity, is detected by a photodetector. A change of workpiece diameter yields a deflection of the focusing point and leads to less reflected light intensity or, a change in the place of reflected light image on the photodetector. The sensor output is then used to operate a servo-mechanism, which controls the tool position to maintain the diameter within a prescribed level.
In-process dimensional measurement and control of workpiece accuracy
1431
The concept of the light-focusing method, has been studied by Merglcr and Sahajdak [15, 16]. First, they developed a gauging apparatus for controlling the accuracy of the workpiece diameter with a closed loop system shown in Fig. 5(a). Then they improved the optical configuration of the apparatus [16]. Figure 5(b) shows the consolidated configuration of the improved optical gauging head, and Fig. 5(c) shows the principle of measurement. The incident laser beam is focused on the workpiece surface and the image is analysed with a position-sensitive detector which determines whether the focal point is
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1432
T. Yandayan and M. Burdekin
in front, behind or at the workpiece surface. A servo-control loop drives the optical assembly with a precision actuator to maintain the focal point at the workpiece surface, and the distance is determined by geometry from the mechanical position of the optical components. It is asserted that a measurement precision of ± l0 p.m from a distance of 200 mm was achieved in the experiments. Since the position of the reflected laser spot depends on surface irregularities, a nonuniform workpiece surface is the major limitation in the measurement. Moreover, as the optical system does not measure the true diameter of the workpiece (but rather the distance to the workpiece surface using the carriage as a reference), some machine deflections and thermal expansion errors are outside the control loop. Thus, the overall absolute accuracy of the system depends on the stability of the mounting. 3.2.2.2. Light-spot detection The finish size of a workpiece is continuously monitored by detecting a displacement of a laser spot, which is reflected from the workpiece surface. An example of this method given by Shiraishi [l 7, 18] is illustrated in Fig. 6. The workpiece surface is illuminated by a parallel laser beam, having a diameter of 0.5 ram, passing through the optical system 1. The laser spot on the surface (Or) is magnified by the optical system 2, keeping the view angle of the spot at/3 and the image of the spot is made on the photodetector located at the focal plane Pt. In this situation, when the laser spot moves from Om to O2 corresponding to the workpiece deviation AD, the image moves on the photo detector from Pm to PE. By detecting this displacement of the image on the photodetector, the value of AD is determined. During operation, the finished size of a workpiece is continuously monitored by the laser spot. Meanwhile, an air jet system, arranged close to the cutting edge, is used to blow away smoke, cutting fluid and chips which might otherwise interfere with the measurement. The desired change in diameter is compared with the movement of the spot image on the photodetector and the tool post is automatically moved in the direction to reduce the error. Resolution of the system is stated to be 1/xm and workpiece diameter is produced with overall errors within + 10/zm. However, further investigations for higher reliability are recommended for industrial applications. Surface irregularities of the workpiece can again be considered as a major limitation since they might affect the movement of the spot image on the photodiode. 3.2.2.3. Light sectioning The method introduced by Shiraishi and Sato [19] can be considered as a light-sectioning method. This method not only controls the dimension of workpiece, but also it makes a smoother surface, by removing surface irregularities during machining. The dimensional control is implemented with a reference diameter level of Rmaff2 roughness by the regular cutting tool, whilst in the roughness control a compensatory "flat bite" tool is employed to remove surface irregularities. In the author's view, the method is related to the described concept of dimension and roughness control. The desired diameter is considered as the reference diameter given by the following equation:
Workpiee¢ Optical
System I
He-NeI.,,sser
Ot
Optical
.....
". . . . .
P2 P~
Image on the photodetector
Fig. 6. Principle of light-spot detection method [17].
In-process dimensional measurement and control of workpiece accuracy D = D,n,,~ -
1433
(3)
R,..,~
where D , ~ is the maximum diameter and R~,x is the maximum peak-to-valley height. During the cutting process two possible deflections, either negative or positive, might occur, and are compensated for by reducing or increasing depths of cut, respectively. For determining the reference diameter level, an optical arrangement shown in Fig. 7(a) is used. A He-Ne laser light, having a rectangular beam by lens 1 and concave lens 2, is projected to the upper edge of a workpiece. The laser beam passing throughout the edge makes a sharp profile pattern of the workpiece surface on to the photo array sensor, after it is projected by another lens (lens 2) and concave lens 2. A knife edge between concave lens l and the workpiece is used to improve the sharpness of the pattern. As shown in Fig. 7(b), this pattern comprises a shadow image corresponding to surface profile, and a bright part due to passing light, which changes the sensor output. Once a reference diameter level is fixed as shown Fig. 7(b), positive and negative deflection deviation can be determined from the sensor [Fig. 7(c) and (d)]. Knife Co~.~ leas I _ _ . He'Ne Lensl
^
, ~
r
~tSe
Concave Leas 2
lens 2
Photo array
(a) Measurin8 system
Brtsht part Shadow ~*Se
Co) Profile inmse on the x m o r
RJ(aewe di,meta tevet
jr//', ...... 4to°, h
(¢) Positive deflection
(d) Nqp~ve deflection
Fig. 7. Light-sectioning method [19]. (a) Measuring system. (b) Profile image on the sensor. (c) Positive deflection. (d) Negative deflection.
1434
T. Yandayan and M. Burdekin
The experimental system is illustrated in Fig. 8. The laser and optical system 1 are mounted on the same tool post, 0.1 mm behind the regular cutting tool. After a 2-3 mm cut from the start, the machining is stopped for a moment to measure Rmax value (i.e. by the photo array sensor). Then the reference diameter level is determined and set onto the photo array sensor. At the same time, the regular cutting tool and the flat bite tool are adjusted on that level, and then the machining is started again, in conjunction with the dimensional and roughness control. It is stated that both dimensional and surface roughness can be implemented by a minimum amount of correction of 1.0/.Lm, the chief advantage being independent of the workpiece material. However, there might still be error sources when the reference level diameter is defined at the beginning of the cut. Further improvement of the system is required before it is applied to a real industrial environment due to its impractical physical arrangement on the machine tool. 3.3. Pneumatic methods A pneumatic method can be defined as one in which the location of surface is measured, by means of an air jet stream directed against the surface to be measured, and by obtaining a signal from the variation in back pressure in the air feed line. In other words, the measuring system measures a pressure drop in the gap between the air gauge and workpiece, and converts it into an electrical signal. The pneumatic gauging gives a convenient and sensitive way of measuring small distances. The chief advantage is that the continual flow of air through the gauge head can be arranged to blow away cutting fluids and swarfs [3]. Fundamental elements [20] such as air gauges, fluid jet gauges and bubble modulation proximity gauges can be used to form "in-process" gauging equipment. A pneumatic-hydraulic system which implements several types of air gauges for controlling a hydraulic ram, to correct the tool position, has been used [20]. This is illustrated in Fig. 9. Pneumatic methods have also been applied to honing by Militzer [21]. The main disadvantage of the system is that the air gap to be measured must not exceed about 0.010 in. (0.254 mm) to obtain the necessary accuracy [e.g. 0.0003 in. (7.6 Izm)]. From the investigations, pneumatic methods have been offered because of their practical and
opfi_~em
Sensing ~ p
1
R
c
u
~
[ B~ehed-type
lathemachine
[
Pr(w.~ion slide unit
Flatbite [ /
tool
Workpiee¢ ~-- . ~ ~ ,' ,'
Regular ~ cutting t o o l m
4
Cutting direction
Fig. 8. Experimental setup for light-sectioning method [191.
Motor
In-process dimensional measurement and control of workpiece accuracy
1435
Gauges
/°/" /" f'tu^..l,-~. ]
Fluid Jet I~CC
~ ......................................
.................................................
Spool Valve -'
I
', Oil
Fig. q. Schematic lay-out for pneumatic-hydraulic in-process control system [20].
application, and it is stated that they will be the most reliable technique among the noncontact methods [3]. Despite this, some problems accompanying pneumatic methods exist, such as temperature effects around the air flow and surface irregularities. Recently, a pneumatic--electric gauging (PEL) system has been developed for practical uses [22]. It is stated that it operates even if the detector nozzles are exposed to extremes of temperature. The accuracy for the system is given according to nozzle type and air gap between the sensor and the workpiece (gap distance) (Table 1). The advantages of the PEL system are stated as follows: • Unaffected by the material. The system responds uniformly regardless of the material to which it is applied---be it metallic or non-metallic, magnetic or non-magnetic, transparent or opaque. • High speed gauging and control. Up to 50 control operations or five gauging operations per second. • Application over a wide temperature range. The PEL system operates even in extreme temperatures.
Using these advances, gauging instruments can be built for application to in-process measurement. However, the main disadvantage of the pneumatic method, in the case of "in-process measurement" of workpiece diameters in turning, is the small air gap between the sensors and the workpiece. 3.3.1. Electrical methods. These type of methods can be considered as non-optical non-contact methods. Various types of electrical field techniques, such as reluctance and capacitance, can be applied to "in-process measurement". The reluctance transducers are proximity devices, which indicate the presence and the distance from the probe of a ferromagnetic substance. The obvious limitation is that the workpiece being inspected must be electromagnetic. A capacitance-based transducer can also be used to measure the distance between the workpiece and the probe. The measurement is based on the variable capacitance from the workpiece-probe coupling This capacitance is inversely proportional to the distance Table I. Accuracy values for PEL system [22] Gap distance Concentric nozzle Sapphire nozzle
1.00 m m 0.15 m m
Accuracy 5 ~.m I t~m
1436
T. Yandayan and M. Burdekin
between the probe and the workpiece, and thus the distance can be calculated. This technique can be used for various materials as long as the material is an electrical conductor. Electrical methods can also be used for application of indirect methods. Tool deflection due to cutting forces, which can be determined by means of an electric transducer, has been considered to determine the relation between this deflection and dimensional error on the workpiece. This has been applied to control the boring of fuel injector nozzles [23]. Fan and Choo [24] proposed a technique which can be considered as a direct method utilizing electrical techniques. It is called the moving three probe technique (MTPT) which compromises three servo-operated units and non-contact displacement probes (proximitor) (Fig. 10). As seen in Fig. 10(a), each unit consists of a non-contact displacement probe (proximitor) for gap sensing, a stepping motor controlled linear carriage, which can carry the probe for surface tracing, and a displacement transducer (LVTD) for real-time recording of the carriage position. After arranging three units as in Fig. 10(b) for the MTPT method, the workpiece diameter can be determined by the measurement principle shown in Fig. 10(c). Probes S~, Sz and $3 are set in their reference position from lines a, b and c, respectively. The reference line of each probe Si, is located away from the workpiece centre by distance L~. This distance can be measured in advance by contacting the probe to a standard point rod of known diameter, and then recording the movement of this probe back to its specified reference line. From the reference line the movement of the probe S~ is recorded as h; by the LDVT and its distance to the workpiece surface is recorded as 6",.. Then, the diameter of the workpiece can be obtained by the given equation: DI = 2LI - /-,2 +
L3 -
2Ct + C2 - Ca - 2hi + h2 - h3
(4)
It is stated that control of workpiece diameter was achieved to within -1- 10/xm accuracy. However, use of three units (i.e. each containing a digital LVTD, step motor, linear carriage and probe) occupies a large space, which is an obstacle in the turning machining environment. Therefore, it would be difficult to apply this technique to industry without further improvement. Additionally, another limitation could be maintaining the gaps between workpiece and sensors within the measurement range of the probes. 3.4. Ultrasonic methods In this method, ultrasound travels to the workpiece, then reflects back to the transducer which also acts as a receiver. The transit time depends on the variation from the specified distance between worksurface and transducer. By determining the transit time, the distance can be calculated. Recently, an ultrasonic method has been used with the lapping process for in-process measurement of silicon wafer thickness [25]. In general, the pulse-echo technique is used. In this technique, the transducer is first excited by a short electrical pulse; the electrical energy is then converted into sound waves. The sound waves are transmitted from the transducer into the test material through a thin film of coupling liquid such as glycerine, oil or water and are then reflected from the back surface of the test material. The reflected sound waves are received and converted into electrical signals by the transducer. The travelling time from the transmission of sound waves to the reception of them is in direct proportion to the thickness of the test materials, as the sound waves travel with constant velocity through the uniform medium. 3.5. Temperature-Detection method It is well known that, in turning, the machining error comes out as a change in workpiece diameter, and arises predominantly from the thermal expansion of the tool. This has been studied by Takeuchi et al. [26]. It is found that 52% of the total cylindrical error of the workpiece is due to the thermal expansion of the tool, when cutting carbon steel with a cutting speed of 150 m min- ~, rate of feed 0.1 mm rev- ~ and depth of cut 1 mm. By knowing the thermal expansion of the tool, the error due to this effect can be determined and compensation is performed by moving the position of the tool.
In-processdimensionalmeasurementand controlof workpieceaccuracy
F
"
/~,~
-t~
. . . .
1437
"
',
. . . .
~, ~,.~
U
. . . . .
/
t
_t
c
~
DigitalLVDT
(a)
Probe
~
@)
(c) Fig. 10. MTlYl" method[24]:(a) measurementprinciple;(b) servomeasurement;and (c) arrangementof three servounits.
1438
T. Yandayanand M. Burdekin
Figure 11 shows the outline of compensation. The following procedure is performed for the compensation. First, cutting conditions and the analysis program of thermal deformation are stored in a data file. Using this information, the thermal expansion of the tool can be estimated by the use of the thermal displacement function. AX (machining error in the X-direction) is found from the finite element method and the tool is moved back AX in the direction of cut (X-direction). The resolution is reported as 5/zm and the machining accuracy is about 7/~m [26]. With a temperature-detection method, errors such as geometrical errors and the static deformation error of a workpiece associated with the cutting force are not taken into account. However, the method has an advantage in that measuring devices are not necessary, and therefore it becomes an economical and practical way to improve accuracy. 4. CONCLUSION Commercial and experimental "in-process measurement techniques" have been described through examples. In the technical literature, a large amount of information is available on a variety of systems proposed for "in-process measurement" of diameter during turning, but most seem to be unsuitable for practical applications in an industrial environment. Therefore, there is still demand for a practical "non-contact method" which could measure workpiece diameter directly, with high resolution and accuracy and considering all errors. Desired features for in-process gauging equipment are listed below. (1) (2) (3) (4) (5) (6) (7) (8)
High measuring resolution and accuracy. Rapid gauging. Quick change-over to different gauging operations. Low-cost construction. Flexibility--large measuring range of diameters. No need for manual adjustments. Being independent of ambient temperature. Being independent of workpiece surface texture and material.
Thermal¢xpaasion~........~ 4~" I. . . . . . . . . . . . . . . . . . . 3 ~ I-. . . . . . . . . . /
/
2AX I- .....
',
; ~~oron
/;
/ I
I
',
I
I
-2&g -3AX
Movementoftool
-4AX Fig. I 1. Outlineof compensation for temperature-detectionmethod [26].
In-process dimensional measurement and control of workpiece accuracy
1439
(9) No equipment in the vicinity of workpiece to prevent interfering measuring process. (! 0) During measurement process, all errors should be taken into account. This seems that it can only be implemented by direct measurement of diameter.
REFERENCES [ll Tillen, R., Controlling workpiece accuracy. Prod. Engr, 1964, 43, 499. [2] Tipton, H., In-process measurement and control of workpiece size. Mach. Tool Res., 1966, 5, 93. 131 Shiraishi0 M., Scope of in-process measurement, monitoring and control techniques in machining process-part 2. In-process techniques for workpieces. Precis. Engng, 1989, ll(I), 27. 14l Anon., in-process grinder gauges. Commercial 7-page brochure, Control Gaging lnc., UK (1996). [5] Nokikov, N. I. and Makarevich, B. K., Automatic measuring of dimensions in turning. Meas. Techn., 1961. 6, 431. [6] Ivanov, B. N. and Mel'nichuk, V. A., Measuring the diameter of cylindrical articles by the method of running a roller over their surfaces. Meas. Techn., 1964, 9, 772. [7] Lee, R. L., Turning railway wheel sets. MachineD', 1963, 102, 144. [8] Anon., 3-Dimensional touch trigger probes for machining centres and lathes. Commercial 16-page brochure. Renishaw Electrical Ltd, New Mills, Wotton-under-Edge, UK (1984). [9] Szafarczyk, M. and Misiewski, M., Automatic measurement and correction of workpiece diameter on NC center lathe. Ann. CIRP, 1983, 32(1), 305. [10] Binks, S. D., The development of a laser-operated scanning rod gauge. Meas. Control, 1971, 4(April), T49. [I 11 Anon., World's smallest laser scan micrometer. Commercial 6-page brochure, Keyence Corporation, UK (1990). [121 Vikhman, V. S., Application of the television computing technique in automatic control of dimensions. Meas. Techn., 1963, 11,904. 1131 Takesa, K., Sato, H. and Tani, Y., Measurement of diameter using charge coupled device (CCD). Ann. CIRP, 1984, 33(1), 377. LI41 Novak, A., Sensing of workpiece diameter, vibration, and out-off-roundness by laser-way to automate quality control. Ann. CIRP, 1981, 30(1), 473. [151 H. W. Mergler and S. Sahajdak, In-process optical gauging for numerical machine tool control and automated process. Proc. 3rd NSF/RANN Grantee's Conf. on Production Research and Industrial Automation. Cleveland, OH, p. 57 (1975). [16] H. W. Mergler and S. Sahajdak, In-process optical gauging for numerical machine tool control. Proc. 3rd Int. Conf. on Automated Inspection and Product Control, University of Nottingham, April, p. 57 (19781. [17] Shiraishi, M., In-process control of workpiece dimension in turning. Ann. CIRP, 1979, 28(1), 333. [18] Shiraishi, M., A measuring method of diameter deviation for the turned workpiece with curved profle using an edge of the image. Bull. Jap. Soc. Precis. Engng, 1979, 13, 133. [191 Shiraishi, M. and Sato, S., Dimensional and surface roughness controls in a turning operation. Trans. ASME: J. Engng Ind., 1990, 112, 78. [20] R. Sharp and H. Bath, Non-contact measurement for machine tool control. Proc. Ist Int. Conf. on Fluid l.z)gic and Amplification. Cranfield, Paper GI, September (1965). [21] Militzer, R. W., Automatic sizing techniques for production honing. Tool. Prod., 1965, 31(July), 45. [22] Anon.. The system for pneumo-electric gauging and controI-PEL, problems and solutions. Commercial 34page brochure, Chad Controls, Pneumatic Control Systems, Oldham, UK (1996). [23] Hick, J. R., Boring goes adaptive. Am. Mach., 1967, Ill(August), 93. [241 Fan, K. C. and Chc~, Y. H., In-process dimensional control of the workpiece during turning. Precis. Engng, 1991, 13(I), 27. [25] Tsutsumi, M., Ito, Y. and Masuko, M., Ultrasonic in-process measurement of silicon wafer thickness. Precis. Engng, 1982, 4(4), 195. [261 Takeuchi, Y., Sakamato, M. and Sara, T., Improvement in the working accuracy of an NC lathe by compensating for thermal expansions. Precis. Engng, 1982, 4(1), 19.