In-proces sensors for surface roughness and their applications

In-process sensors for surface roughness and their applications K. Mitsui* Potential non-contact optical methods for in-process surface roughness measurement are described, including reflected light position detection, focus error detection, using fibre optics and using an optical lever. Results which were compared with those obtained using a mechanical profilometer were found to be in good agreement. Swarf, cutting fluid and machine tool vibration, however, are shown to hamper the accuracy, or feasibility, of inprocess measurement.

Keywords: surface roughness, optical techniques, in-process metrology

Measurement of surface roughness is one of the most important subjects of investigation for the description of machined surfaces. The surface quality so generated by the machining process is important not only for appearance, but also in the study of functional behaviour of parts. The mechanical profilometer which uses a diamond stylus is very sensitive and widely used, but measurement is slow and micro-scratches may be made on soft surfaces with the sharp diamond stylus. Due to the growth of highly automated manufacturing 1, the demand is great for an inprocess sensor for surface roughness measurement for accurate evaluation of generated surfaces during the machining cycle. Furthermore, since surface roughness is closely related to tool wear, tool breakage, existence of self-excited vibration and so on, the identification of cutting condition through surface evaluation can also be expected. By making use of the high speed nature of an in-process surface roughness sensor, it is possible to collect data over a much larger area of the workpiece compared to the ordinary stylus method. Surface roughness evaluation on very accurate parts, such as diamond-turned metal mirrors, for example, is carried out on a very limited area at the moment, but the high speed method for surface roughness measurement will facilitate easy measurement over the whole surface of the mirror. The results so obtained w o u l d give more detailed and better knowledge about the relation between surface roughness and cutting condition or tool wear. In-process measurement of surface roughness needs to be fast. This can be achieved by noncontact optical methods and several techniques along this line have been developed. In this article, * Mechanical Engineering Laboratory of the Ministry of International Trade and Industry, Namiki 1-2, Sakura-mura, Niiharigun, Ibaraki-ken, 305 Japan

the optical method for in-process surface roughness measurement and its applications are described.

Reflected light position detection 2 Fig 1 shows the principle of the measuring method based on the detection of the reflected light position. The laser beam is focused at an angle of 45 ° and, in general, can be chosen arbitrarily. If the surface travels from that of the solid line to that of the broken one, the reflecting point moves from A to B on a screen, so that the trace of the surface by the beam is found by following the movement of the reflected spot on the screen. The behaviour of the reflected beam is magnified through the object lens of a microscope and is projected on to a onedimensional photo-electric diode array set on the screen which enables identification of the position of the spot to be made. The photoelectric diode array used has 256 elements and each element is set 25/~m apart.

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212 0141--6359/86/040212~09/S03.00© 1986 Butterworth 8- Co (Publishers) Ltd

OCTOBER 1986 VOL 8 NO 4

Mitsui--in-process sensors for surface roughness Table 1 Measuring

speed and surface travel

Measuring speed, mm s- 1

100

500

1000

3000

Surface travel,/~m

5.12

25.6

51.2

153.6

a

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0

51.7

and measuring conditions shown in Table 2. These correspond to cutting with a normal tool bit, a broken tool tip and in cases where chatter vibration occurred respectively. The horizontal axis has a logarithmic scale. It can be seen, in normal cutting conditions, that the distribution bandwidth is narrow, but when cutting with a broken tool bit and under self-excited vibration, the distribution curve is spread out. Fig 3 also shows that the characteristic of the distribution forms does not depend on the cutting speed. Beside these, this measuring method was applied to experiments that aim at clear understanding of the relation between machine tool vibration and surface roughness 3'4. Fig 4 shows another measuring apparatus which is also based on detection of the reflected

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Since the machine surface runs fast, the spot on the diode array moves around and the behaviour of a bunch of the video pulses follows the movement. It is necessary for the clock pulse to scan the array fast enough to detect the movement. When the scanning clock pulses return after one cycle to the diodes irradiated by the reflected beam, the movement of the spot should be small in comparison with the wave pattern of the roughness. The response speed of this apparatus depends on the number of elements and the scanning clock pulse frequency of the diode array used. The driving circuit for the array generates 5 MHz clock pulse, so that one scanning cycle for the 256 elements can be taken as about 20 kHz. Table 1 shows the relation between measuring speed and surface travel during one scanning cycle. For example, if the measuring speed is 100 mm s -1, then the surface moves about 5pm during one scanning cycle. Therefore it can be possible to measure those components of the surface roughness in the circumferential direction whose wavelength is greater than 10/~m. Fig 2 shows a measured surface profile obtained in the circumferential direction after cutting had ceased. The workpiece material was brass, 49 mm in diameter and 200 mm long. The result was also compared with that obtained by a roundness meter; they were in good agreement. The sensor was also used for identification of cutting condition by surface roughness measurement. The sensor output signals were transferred to a mini-computer, and histograms of RMS value were obtained. Fig 3 shows the histogram which was obtained under the cutting

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Table 2 Cutting

conditions

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apparatus. The curve shows linearity in the region of -30/~m to 20/~m; this measuring range is considered, in general, to be sufficient for measurement of machined surfaces. As for comparison with the conventional contact method using a diamond stylus, measurement by both methods was conducted for several surfaces which had different surface roughness values. Fig 6 shows the correlation between the two methods. An adequate correlation is estimated from the results of the mea~Jrements.

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light position. This apparatus can also be used for the measurement of ground surfaces 5. A He-Ne laser of 1 mW output power was used as a light source. The laser beam was directed onto the object surface through several mirrors and an objective (NA=0.1, X5). For the detection of the reflected light position, a PIN photodiode position sensor that can measure centre position of the light spot directly as voltage, was used. It had position detection sensitivity of 2.54/~m, and response speed of 3 MHz. Fig 5 shows the characteristic curve of the

214

Focus error detection

30

Judging from sensitivity and compactness, a surface roughness measuring method based on the focus error detection technique is very attractive. Surface roughness profiles are obtained as a small displacement change between objective and lens. Four different focus error detection methods are given in Fig 76'7.

Astigmatic focusing In the astigmatic method, a cylindrical lens is used in the optical system. With the aid of a quadrant diode, a signal can be derived which is approximately proportional to the difference in distance of the detector to the two astigmatic focal lines.

OCTOBER 1986 VOL 8 NO 4

Mitsui--in-process sensors for surface roughness

Critical angle focusing

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In the critical angle focusing method, the steep reflective change around the critical angle of total reflection is used. When the surface is in focus, the amount of incident light flux to two detectors is equal. However, if the surface is out of focus, part of the light flux is lost. Therefore the differential output of two detectors gives the surface displacement.

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The beam reflected back from the surface through the objective is incident on a prism. The converging beam is refracted into two beams, each of which shows the characteristic Foucault intensity distribution as a function of the defocusing of the beam.

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Skew beam focusing In the skew beam method, an auxiliary narrow beam 'b' which passes through the objective off axis is used. The returning beam 'b" hits the detector D1 and D2. The difference in photosignal is a linear measure of the defocus around z=0.

Fig 6 Correlation of surface roughness values obtained by two methods: y = 1.0gx/1.484; correlation coefficient = 0.965

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+ Fig 9 Optical layout of surface roughness sensor using the astigmatic method Astigmatic method 8 Fig 8 shows the principle of the method. The reflected light which returns through the microscope objective makes an image at 'Q'. A cylindrical lens is placed behind the objective to give the astigmatism. The image surface of the cylindrical lens is at "P'. At any cross-section between 'P' and 'Q', the form of the image is, in general, elliptical, but there is one point where the shape becomes circular. The form of the image at the point'S' changes shape according to the target surface position, as shown in Fig 8. Therefore, if a quadrant diode is placed there, an output signal that corresponds to surface displacement can be obtained. Fig 9 shows the arrangement of the optical system of the measuring method. The optical system that has only one quadrant diode is shown in Fig 8. It is sufficient for the displacement measurement of a smooth surface like a mirror, but, for example, in the measurement of roughness of a turned surface, which has rather steep surface features, or a step of thin film that has a very steep slope, this simple

216

optical system does not work properly, because of the diffraction of high light intensity to specific photocells of the quadrant diode, resulting in the surface profile so obtained being greater than the real surface profile. Consequently, the optical system that has two quadrant diodes, as in Fig 9, has been introduced. Here, the influence of diffraction can be eliminated by subtracting the output signal of quadrant diode 'B' from 'A'. The signal corresponding to surface profile is obtained by the following equation: S

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If the reflectivity of the surface changes, the signal, S, is unaffected because of the denominator terms (A1 + A2+ A3+ A4) and ( B I + B 2 + B 3 + B 4 ) . Fig 10 shows the equipment. A diode laser of output power 2 mW and wavelength 790 nm is used as a light source. The objective lens is a standard microscope objective (NA=0.8, X60). The difference signal is linear with respect to object displacement within a range of _+0.5/~m around the focus. The measuring resolution of the equipment approaches 2 nm. Another microscope objective (NA--0.65, X40) was also tried, and it was confirmed that the linear range was increased to + 1.5/~m. The result obtained by this method was compared with that obtained by the diamond stylus method using a specimen with 1 pm deep grooves. As shown in Fig 11, the agreement between the two instruments was excellent, but with the optical method, small crests are observed at both edges of the step. The crests are due to diffraction at the edge of the step, where the surface inclination is rather steep. To reduce this effect two identical channels of the optical system for differential detection were set up as previously described. However, the fact that these crests still appeared in Fig 11 shows that further improvement such as fine positioning of the optical components or quadrant diodes is needed. In the astigmatic method, the focal length of both objective and cylindrical lens, the distance

Fig 10 Surface roughness sensor

OCTOBER 1986 VOL 8 NO 4

Mitsui~in-process sensors for surface roughness Ilpm

Fig 14 shows overlapped curves obtained by both HIPOSS and a precision diamond stylus for a diamond-turned aluminium surface. Fig 14(b) shows the part of the curve showing in (a) with the horizontal axis expanded by a factor of five. The surface profile contains higher frequency components corresponding to the optical measurement. The difference is less than 1 nm in RMS value.

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Fibre optics A surface roughness measurement method using fibre optics is not appropriate for obtaining a surface roughness profile, but it has the advantages that roughness values corresponding to R .... Ra and

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between objective and cylindrical lens and position of the quadrant diode are variable in the optical system design. This allows the equipment to be used for any sensitivity and linearity range.

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Critical angle method ~ The principle of the critical angle method is also based on focus error detection in optical disc technology. Fig 12 shows the principle of the method. After passing through the microscope objective, the polarized laser beam becomes parallel flux if the object surface is in focus (at position 'B'). The critical angle prism is positioned to reflect the beam at the critical angle. Suppose the surface is out of focus, say close to the lens (position 'A'); the light flux diverges slightly and the condition of total reflection is lost at the upper side of the optical axis. The reverse situation (ie convergence) occurs if the surface lies beyond the focus. Therefore the differential output of two photodetectors gives the surface displacement from the focus position. Fig 13 shows the optical system arrangement of the high precision optical surface sensor (HIPOSS). To avoid the influence of surface inclination as well as spatial irregularity of the light source, two identical optical paths are prepared. These will cancel out the deformed component of signals by the following operation:

((A-B)+ (C-D))/(A+B+C+D) The size of the HIPOSS optical head is only 45 mm × 30 mm × 65 mm, and it can be installed side by side with a diamond stylus in a scanning precision instrument.

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Fig 15 Fibre optic sensor. (a) principle of detection and (b) position within process apparatus RMS can be obtained without numerical calculation, miniaturization of equipment is easy and the roughness value obtained represents the average value over a certain two-dimensional area, even though the area is small. There are several reports concerned with the development of equipment and the applications 1°-12. Fig 1 5 shows a fibre optics surface roughness sensor13 that was developed for real-time

218

measurement of surface roughness change during cylindrical grinding. Light from a white light source was applied to the object surface through the emitting fibre and the receiving fibres transmitted the light scattered from the surface asperities to a photo diode. The scattered light that has an incident angle greater than the critical transfer incident angle of the fibre cannot be transmitted to the photo diode. The light that reaches the photo diode depends on the micro-topography of the object surface, corresponding to the surface roughness value. The fibre optics surface roughness sensor was used for real-time measurement of surface roughness change during cylindrical plunge grinding. The influence of grinding fluid and swarf should be eliminated in order to perform stable inprocess measurement. Hence in order to eliminate these disturbances, the measurement was carried out in the grinding fluid. The grinding fluid is supplied from the circumference of the optical probe, and fills the gap between workpiece and the optical probe. Fig 16 shows one of the results of in-process surface roughness measurement. It indicates the influence of the workpiece speed on the surface roughness. In this figure, the change of roughness is shown only for rough grinding. As expected, the final value of the surface roughness increased when the workpiece speed increased.

Optical lever In the field of grinding metal dies and rolling rollers, development of a measuring method for visible chatter patterns has been strongly demanded.

OCTOBER 1986 VOL 8 NO 4

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Fig 18 Measurement results for ground surface. Length of workpiece = l OOmm, grinding wheel speed=2900 rev min-1 and travelling speed of table=27.6 m min -7 gloss of the surface is detected approximately from the output of photo sensor X3, and on subtracting this from X 1 and X 2, the pure waviness component can be measured. Fig 18 shows the surface profile of a ground steel specimen measured with this equipment. Wave-form 'T', 'U' and 'y' shows the output difference of X~ and )(2, the intensity of the reflected beam and the calculated waviness component respectively. The major component in waviness has a spacing of 4.9 mm. Because the table speed of the surface grinding machine was 27.6 m min -1, it is clear that the cause of the waviness component is a vibration component of 94 Hz, close to a natural frequency of the grinding machine. Therefore the waviness component is due to self-excited vibration during grinding. The spacing of the gloss change of the surface is 9.5 mm. The corresponding frequency 48.4 Hz coincides with the rotational speed of the grinding wheel.

Conclusions e=~ x3

Fig 17 Optical layout of apparatus: H M = beam splitter, L = lens, M= mirror and X=photo sensor The measuring method shown in Fig 17 was developed for this purpose 14. The light from white light source'S' is applied to the object surface through several mirrors and lenses. The reflected beam is condensed by lens L3 and then is separated by means of beam splitter HM 3. The one beam is received by photo sensor X~ and X2, and another beam is received by photo sensor X 3. Therefore if the normal vector of object surface and the optical axis are in perfect agreement, the intensity of light at X 1 and X2 is the same. If the object surface has slight waviness, the light intensity at the two sensors are different. The configuration of waviness can be obtained as the output difference of X1 and X 2. But the output of Xl and X 2 a r e also affected by the gloss of the object surface which should be separated. For this purpose, the component due to

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Optical in-process sensors for surface roughness measurement and its applications were introduced. Due to the growing interest in high and ultra precision machining, the automation of measurement and development of more accurate measurement techniques are in urgent need. It is not easy to make in-process measurements, because swart, cutting fluid and vibration of the machine tool may disturb measurement during operation. Therefore further efforts are needed before inprocess measurement will be possible in production. However, with the non-contact optical method of measuring surface roughness, it is possible to develop an on-line or in-process measurement device.

Acknowledgement The author wishes to express his thanks to Mr M. K. Rao, a graduate student of McGill University, Montreal for his help with the English.

219

Mitsui--in-process

sensors f o r surface r o u g h n e s s

References 1 2

3

Sato H. In-process measurement of surface roughness. J. JSME, 1982, 85-761,421 (in Japanese) M i t s u i K. and Sato H. Development of an in-process sensor for surface roughness by laser beam. Proc. 16th Int. MTDR Conf., 1976, 171 M i t s u i K. and S a t o H. A study on the structural vibration of a machine tool to the circumferential surface roughness. Proc. 19th Int. MTDR Conf., 1978, 391

8

M i t s u i K. et aL Development of a high resolution in process sensor for surface roughness by laser beam, Bulletin of the JSPE, 1985, 19(2), 142

9

K o h n o T. et al. Practical non-contact surface measuring instrument with one nanometer resolution. Precision Eng,, 1985, 7(4), 231

10

M i y o s h i T. and S a i t o K. Development of a sensor for measurement of surface gloss. J. JSPE, 1981, 47(3), 338 (in Japanese)

4

U i t s u i K. and Sato H. Frequency characteristic of cutting process identified by an in-process measurement of surface roughness. Ann. CIRP, 1978, 27/1, 67

11

S p u r g e o n D. and Slater R.A.C. In-process indication of surface roughness using a fiber-optics transducer. Proc. 15th Int. MTDR, 1974, 339

5

H a t t o r i a . et al Development of in-process sensor for surface and roundness profile in grinding. Bulletin of Mechanical Engineering Laboratory, 1981, 35(6), 303 (in Japanese)

12

Sekiguchi H. et al. A study of in-process surface roughness measurement. Trans. JSME (part 3), 1977, 43374, 3893 (in Japanese)

6

i a t s u b a y a s h i N. Optics for optical disk. J. JSPE, 1984, 50(12), 1850 (in Japanese)

13

Inasaki I. Development of in-process sensor for surface roughness measurement. Proc. 23rd Int. MTDR, 1982, 109

7

B o u w h u i s G. and Braat J.J.M. Video disk player optics. Applied Optics, 1978, 17-13, 1993

14

Sakai Y, et aL A study on optical chatter sensor. J. JSPE, 1984, 50(9), 1432

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1986 VOL 8 NO 4