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Characterization of precision of a handling system in high performance transfer press for micro forming
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CIRP-1112; No. of Pages 4 CIRP Annals - Manufacturing Technology xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
Characterization of precision of a handling system in high performance transfer press for micro forming Rasoul Mahshid a,*, Hans Nørgaard Hansen (1)a, Mogens Arentoft (2)b a b
Department of Mechanical Engineering, Technical University of Denmark, Lyngby, Denmark IPU Technology Development, Lyngby, Denmark
A R T I C L E I N F O
A B S T R A C T
Keywords: Precision Uncertainty Micro forming
Multi-step micro bulk forming is characterized by complex processes and high precision requirements. In particular the demands regarding handling accuracy between different forming steps are of the order of a few mm. The paper introduces a methodology for the analysis and characterization of this transfer system on component level and system level. Laser interferometry is used in combination with analytical models to predict the positioning ability of the actuator in a static as well as dynamic mode. In combination with an analysis of the grippers, a full description of the transfer precision inside the forming press is obtained. ß 2014 CIRP.
1. Introduction When manufacturing miniaturized metallic mechanical components in mass production by multi-stage micro cold forming one of the big challenges is to transfer the parts with high speed and high precision. This need has been met primarily by conventional transport principles. Micro cold forming allows manufacturing of miniaturized components with diameters down to 150 mm, wall thicknesses as low as 13 mm, the length-to-diameter ratio greater than 55:1 and finished part tolerances as low as 3 mm [1]. Depending on the size and precision of the part, traditional ways of transferring can be costly and inefficient. In order to address these problems, a new high performance transfer press must be developed which allows micro parts to be transferred with tolerances within few microns. Traditionally, in a conventional transfer press an eccentric shaft drives not only the main slide but also the transport device. However due to the reduced overall size of the machine there are several notable challenges for this principle to be adapted in micro forming. These include small tolerances and an increased complexity of the mechanism. Recently a deformation method for manufacturing a micro pin was introduced where it is manufactured from sheet metal [2–4]. This method makes handling easier than in traditional micro bulk forming since the part is connected to a foil when moving in between the different forming stages. Although this type of forming is effective and shows beneficial transferring effects, it also introduces possible quality issues at side surfaces since the deformation process must be ended by a blanking process at the last stage of forming. In this way the final part is punched in order
* Corresponding author. E-mail address: [email protected] (R. Mahshid).
to separate it from the foil. Therefore, if an alternative to conventional transport devices can be developed, there is a potential to significantly improve process efficiency and increase geometrical and surface quality. In the early 2000s, researchers began investigations on process parameters in micro forming. As this research has progressed it has been found that the process parameters can influence significantly accuracy and quality of the forged part. In 2001, Geiger et al. published a paper addressing the scaling effects and solutions for tool manufacturing and explored general aspects and challenges in micro forming [5]. Moreover, they developed a cross transportation system which was able to transfer cylindrical parts with diameter of 0.85 mm with positioning accuracy of 15 mm. The handling device worked with a production speed of 260 strokes per minute. In 2006, Wafios AG manufactured a machine for a multi-stage micro bulk forming which works on the basis of a rotary transfer system and horizontally mounted rotor with eight dies [6]. This machine transfers cylindrical parts with diameters down to 0.5 mm and an output rate of 400 ppm. From studies conducted by Kuhfuss, a concept of linked micro parts was investigated to overcome handling of miniature components [7]. The blanks/parts are interconnected with strap/ wire and are not separated throughout the manufacturing process. The current research involves a new development of a handling system to be integrated into an existing developed micro press in order to maximize the output rate up to 250 strokes per minute without compromising accuracy. The purpose of this research is to explore the effect of the process parameters on the precision of the machine. More specifically, this paper introduces a methodology for determining the precision of handling system of a micro transfer press by combining analyses of positioning accuracy, dynamic behaviour and gripping characteristics.
http://dx.doi.org/10.1016/j.cirp.2014.03.001 0007-8506/ß 2014 CIRP.
Please cite this article in press as: Mahshid R, et al. Characterization of precision of a handling system in high performance transfer press for micro forming. CIRP Annals - Manufacturing Technology (2014), http://dx.doi.org/10.1016/j.cirp.2014.03.001
G Model
CIRP-1112; No. of Pages 4 R. Mahshid et al. / CIRP Annals - Manufacturing Technology xxx (2014) xxx–xxx
2. Challenges and requirements In a standard high performance transfer press all operations are automatic: feeding and transportation of specimens, press force and stroke control, part ejection and stacking of finished parts as well as monitoring and control of the entire transfer press. The functionality of the machine pertains to transferring the workpieces throughout a multi-step former automatically. However, the micro cold forming process implies new challenges to the process and the current machine tools, in particular geometrical accuracies below 10 mm and surface roughness lower than 1 mm. Some attempts have been made to realize such systems by a downscaling approach (e.g. [1,6]). Recently Roehlig investigated small machines to fabricate small workpieces [8]. The principle behind the construction and design of a micro cold forming transfer press is to decrease the size of the machine by using technologies not commonly found in current machines. For a transfer press, the integration of a transport device is specifically of interest to move the parts from one forming operation to the next in such a way as to ensure continuous and uninterrupted motion of the machine at high production speed. The second goal is to increase the maximum achievable production speed. However, it has several potential challenges, such as increasing acceleration and reducing cycle time by using high performance actuators. Precision is influenced by deflections due to mechanical loading (during the forming operation), positioning accuracy and repeatability of the handling system as well as deviations of shape and dimensions of parts throughout the progressive forming process. In order for the progressive forming process to be operational, it is assumed a tolerance of 30 mm of the total handling system is required. A previous study on a micro forming press included a cropping tool for cylindrical billet preparation, and a micro forming press with an integrated transfer system [9]. The gripping principle of transport device was based on surface tension force. The handling device achieved a maximum production rate of 50 strokes per minute. The concept implemented for micro billet cropping machine proved to be successful with respect to the volumetric deviation of the billet within a 1% margin. For the system described herein (see also Fig. 1), a feeder transfers billets (already prepared) continuously and moves them through a channel by means of constant force mechanism into the first station where ejectors insert the workpiece into the grippers. Subsequent workpiece transportation is performed by the grippers. The gripper unit arranges the workpieces from one forming station to the next. The two movements executed by the handling system are performed in longitudinal and transverse direction. The forward motion must have been completed before the main slide starts the forming process (downward direction). The return movement of the grippers is executed when the main slide is at the outmost bottom position. The ejectors complete the cycle when the handling system locates at home position afterwards. The actuation of main slide and transport device is performed by linear motors while the ejectors move by pneumatic cylinders. The transport device consists of a linear motor and a gripper unit. The system relies on a mechanism including 3 pairs of stiff fingers with elastic hinges which support the workpieces during transportation. The grippers work on the basis of self-centring and
Main slide Workpiece Upper tool Lower tool Gripper Ejector
Transfer motion
Ejector motion Approach feed Fig. 1. The elements and their movements in a high performance transfer press.
the friction principle. A linear actuator is used for positioning the gripper. Before analysis of the handling system, the kinematic characteristics of the motions must be available in the form of displacement–time curves. For the linear actuators, the velocity profile was held trapezoidal, as the acceleration was changed with the same value of 20 m/s2 within one movement. The acceleration is directly proportional to the distance between die centres and the production rate squared [10]. The dynamic parameters used in this research were chosen based on preliminary values using kinematic parameter combinations to determine the overall impact that they have on acceleration and force, as listed in Table 1. Therefore, while the parameters were consistently beneficial for the shortest possible cycle time and smooth movement, they do not represent the optimal parameters with respect to real duty cycle. Fig. 2 depicts the nominal motion curve of the transfer system and respective movements of the main slide and ejectors for one cycle. Table 1 Characteristic values and kinematic parameters.
Max. force (N) Displacement (mm) Half cycle time (ms) Max. velocity (mm/s) Acceleration (m/s2) Dwell time (ms) Moving mass (kg)
Stroke (mm)
2
Main slide
Transfer system
5000 8 40 280 20 0 App. 17
12 20 125 460 20 60 App. 0.3
30
20
10
Part transport by transfer
Transfer return without part Main slide movement Lift parts
50
100
150
200
Time (millisecond)
250
Lower ejectors
-10 Fig. 2. Motion curve of the two-axis transfer system.
3. Error analysis The analysis of errors of the handling system is divided into two parts. The first part considers the positioning accuracy and dynamic behaviour of the linear actuator. The second part considers the actuator and the gripper in a combined setup when handling real micro parts. 3.1. Actuator error While transferring the parts in a transfer press, it is important to note that the functionality of the device depends on positioning accuracy due to the fact that the parts need to slide into the dies. As can be seen in Table 1, the output rate of 250 strokes per minute needs 2 g acceleration which potentially is a source of error. When the maximum inertia force and acceleration is applied, the final placement and settle time of the axis was monitored by laser interferometer. The actuator has a linear encoder with 1 mm resolution and encoder count rate of 2 million quadrature counts per second. The test procedure consisted of measuring the position at home and final placement (20 mm displacement) according to ISO 230-2
Please cite this article in press as: Mahshid R, et al. Characterization of precision of a handling system in high performance transfer press for micro forming. CIRP Annals - Manufacturing Technology (2014), http://dx.doi.org/10.1016/j.cirp.2014.03.001
G Model
CIRP-1112; No. of Pages 4 R. Mahshid et al. / CIRP Annals - Manufacturing Technology xxx (2014) xxx–xxx
[11]. The standard foresees a bi-directional approach which is not needed here. Since the horizontal movement of the carrier is between the two points in the experiment, the approach for home position is from the right side and at the final placement from the left side. The measurements were repeated at home and final placements according to the 25 test cycles. Fig. 3 depicts the results. Error bars indicate 2 standard deviation as mentioned in ISO 230-2. The maximum deviation occurred at the final placement and the deviation is lower than 3 mm as can be seen in Fig. 3. This is an acceptable level compared to a total tolerance of 30 mm.
Fig. 3. Static accuracy of transfer system; left: the result of measurements; right: the method of experiment.
However, the above measurements are static, while the transport device must be at final placement before 70 ms from the beginning of the motion assuming a production rate of 250 parts/min (Fig. 2). To observe real time displacement of the carrier, a built-in facility (controller of the linear motor) continuously monitored and captured the position at 1 ms time intervals throughout the half cycle time (125 ms). To verify the repeatability, 25 measurements were tested for the same parameters set of the controller, as listed in Table 2. Also, to provide a means of reference to compare the controller measurements, laser interferometry measurements were conducted for the same parameter sets to verify repeatability between the two measuring methods. Table 2 Parameters of the controller.
3
Fig. 5. Displacement–time curve connected to Parameter Set 2; left: half-cycle time; right: the region of concern. Error bars indicate 1 standard deviations.
Fluctuations with overshoots, undershoots and a steady state error are present at the region of concern which shift the start motion of upper die to the right towards a longer cycle time. When applying the conditions associated with Parameter Set 1 (Fig. 4), an increase of 6% at the time of 55 ms is observed as the highest overshoot. If assuming a tolerance of 10 mm on the target position (20 mm displacement), the carrier enters into the border of the tolerance not before 110 ms. Likewise, when comparing the motion of carrier to the target line, associated with Parameter Set 2, an increase of 1% at the time of 55 ms is observed for the highest overshoot. Fortunately, the improvement of the positioning ability, while moved under Parameter Set 2, is significantly increased in comparison to Parameter Set 1. The results from using Parameter Set 2 yielded an average overshoot of 206 mm from the target line. Consequently, the positioning accuracy fell inside the tolerance window at the time of 70 ms. While not shown in this paper, more tests were carried out for other combinations of the parameters to find an optimal parameter set. However, the settle time is not lowered to the same extent as occurred under Parameter Set 2. To achieve a significantly better result, new equipment would need to be used. The conclusion of this part of the investigation shows that for the machine described herein a balance between the maximum achievable output rate must be made with the choice of linear actuator. The dynamic behaviour of the actuator and gripper must be considered in order to achieve a satisfactory positioning accuracy. 3.2. Gripper error
Parameter set
Proportional gain
Integral gain
Derivative gain
1 2
30 20
75 75
1600 1625
The curves of the motion versus time for the actuator while applying the conditions associated with parameters set 1 and Parameter Set 2 are shown in Figs. 4 and 5, respectively. Note that, the testing region is half cycle time due to the repetition. As can be observed in Fig. 2, the displacement within 50–70 ms is very important since the forward motion must have been completed before the grippers open, i.e. the upper die elements make contact with the transported parts. Good agreement between controller output and interferometer was found between the two measurements.
Fig. 6 shows the schematic of the grippers for 3 consecutive stations of the handling device. For the tests, grippers were manufactured with a curvature of radius 1 mm in the front profile. In order to establish the effect of the gripper’s profile on the positioning accuracy, specimens with diameters of 3, 2.5, 2 and 1.5 mm were examined. To reduce the effect of variability of diameters of the specimens, all of the workpieces came from standard rods holding a diameter tolerance of less than 10 mm and were sectioned into sizes of 5.0 mm length. The measurement instrument used was an optical coordinate measuring machine with a resolution of 0.5 mm and an MPE of 4 mm in all 3 directions (xyz).
Fig. 6. Gripper schematic, contact of the specimens with front profile of grippers and used coordinate system.
Fig. 4. Displacement–time curve connected to Parameter Set 1; left: half-cycle time; right: the region of concern. Error bars indicate 1 standard deviations.
The specimens were loaded into the grippers manually at the home position in order to establish the effect of the self-centring principle and tilt effect on the positioning accuracy. The grippers and therefore the specimens were moved using the linear actuator introduced in Section 3. The measurements concern the coordinate of the centres of specimens at all three positions. The specimens
Please cite this article in press as: Mahshid R, et al. Characterization of precision of a handling system in high performance transfer press for micro forming. CIRP Annals - Manufacturing Technology (2014), http://dx.doi.org/10.1016/j.cirp.2014.03.001
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CIRP-1112; No. of Pages 4 R. Mahshid et al. / CIRP Annals - Manufacturing Technology xxx (2014) xxx–xxx
4
are extracted from the grippers after 5 measurements. The tests were run 5 times and at each test, 3 parts were selected randomly. The tests lead to evaluation of the positioning accuracy of the specimens in the gripper as shown in Fig. 7. It is observed that for specimen diameters above 2 mm, deviations are below 5 mm taking into account 2 standard deviations. However for diameters smaller than 2 mm a considerable increase in the deviations (and thereby in the error) is observed. This means that the geometry of the grippers must be appropriately designed with respect to the workpiece geometry and dimension in order to minimize this error contribution. In particular it was observed that the curvature of radius of the grippers must be slightly lower than the specimen’s radius. Station 1
Station 2
As stated in Section 1 it is assumed that a total tolerance of 30 mm of the handling system is required. In Fig. 8, the conformance zone according to ISO 14253-1 is shown in respect with measurement uncertainty and designer’s specification. It can be observed that with the estimated uncertainties the positioning accuracy falls inside the tolerance zone. This is under the assumption that the dynamic behaviour is satisfactory (as described in Section 3.1). It can also be seen that the nominal position is not met, in particular there is a larger distance between stations 1 and 2 than between stations 2 and 3. This is probably due to misalignment between gripper and actuator and also between the different fingers of the gripper.
Station 3
Deviation (µm)
25 20 15 Fig. 8. Conformance decision for the transfer system.
10 5 0
5. Conclusion Ø 3 mm
Ø 2.5 mm
Ø 2 mm
Ø 1.5 mm
Fig. 7. Comparison of precision and standard deviation for positioning of parts with different diameters with the same gripper geometry. Error bars indicate 1 standard deviation.
4. Uncertainty estimation As previously discussed, the distance between the two consecutive stations is equal to the distance between the die centres (20 mm). Therefore, the correlation of independent variables is expressed by the following mathematical formula: Di ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðxi xiþ1 Þ2 þ ðyi yiþ1 Þ2
(1)
where (xi,yi) is the measurand of the centre coordinate of the specimen grasped in ith station to calculate the distance between the two consecutive stations. The measurements presented in Section 3.2 lead to the calculation of the average distance of D1 and D2 as shown in Fig. 6 and the reproducibility (standard deviation) as listed in Table 3. Table 3 The mean value and standard deviation of D1 and D2 based on 25 measurements. Parameter
Mean value (mm)
Standard deviation (mm)
D1 D2
20.021 19.985
2.0 2.3
The basis of every procedure for calculation of uncertainty is the GUM [12]. When applying the PUMA method (ISO 14253-2) while using the uncertainty table [13], as briefly shown in Table 4, an expanded uncertainty of 9.0 mm was calculated. The systematic error of the measuring system (Bias), the resolution of the measuring instrument and the reproducibility contribute to the estimation of the uncertainty.
Table 4 Calculation of uncertainty with the application of PUMA method.
Variance of y, u2(y) Standard uncertainty of y, u(y) Degrees of freedom of y, n(y) Confidence level Coverage factor (Student t) Expanded uncertainty U(y)
u2j ðyÞ
u4j ðyÞ=n j
2.0E11 4.5E06 52 95% 2.0E+00 9.0E06
7.7E24
m
This paper has introduced an analysis of the precision of a handling system for micro forming. The proposed method includes static and dynamic analysis of the positioning accuracy of the actuator, as well as an integrated analysis of part positioning involving actuator and gripper. The latter also includes the effects of misalignment between part and gripper finger. The analysis of the actuator shows static positioning accuracies of the order of 3 mm and considerably larger errors when used in a dynamic mode. Controller settings of the actuator heavily influence the overshoot of the actuator and the settling time. The integrated system (actuator and gripper) is analyzed using real micro parts, and a considerable effect from geometrical compliance between gripper and part dimension is observed. The overall target of a part positioning with respect to the dies in a tolerance zone of 30 mm is verified with an expanded uncertainty of 9 mm. Acknowledgements This work was supported by the Ministry of Science, Technology and Research of Iran. Furthermore Myhrwolds Foundation, Denmark, is acknowledged for their support. References [1] http://www.deringerney.com/products-and-capabilities/cold-forming/. [2] Merklein M, Stellin T, Engel U (2012) Experimental Study of a Full Forward Extrusion Process From Metal Strip. Key Engineering Materials 504–506:587–592. [3] Tan MJ, Lim SCV, Ghassemali E, Jarfors AEW (2013) Progressive Microforming Process: Towards the Mass Production of Micro-Parts Using Sheet Metal. International Journal of Advanced Manufacturing Technology 66:611–621. [4] Fu MW, Chan WL (2013) Micro-Scaled Progressive Forming of Bulk Micropart via Directly Using Sheet Metals. Materials and Design 49:774–783. [5] Geiger M, Kleiner M, Eckstein R, Tiesler N, Engel U (2001) Microforming. CIRP Annals – Manufacturing Technology 50:445–462. [6] Wafios (2006) Horizontally Mounted Rotor with Eight Dies. Wire 56:14–15. [7] Kuhfuss B (2013) Machines and Handling. in Vollertsen F, (Ed.) Micro Metal Forming, Springer Berlin, Heidelberg311–343. [8] Ro¨hlig B, Wulfsberg JP (2013) Paradigm Change: Small Machine Tools for Small Workpieces. Production Engineering 7:465–468. [9] Arentoft M, Eriksen RS, Hansen HN, Paldan NA (2011) Towards the First Generation Micro Bulk Forming System. CIRP Annals – Manufacturing Technology 60:335–338. [10] Mahshid R, Hansen HN, Arentoft M (2012) A New Approach for Handling of Micro Parts in Bulk Metal Forming. 7th International Conference on Micro Manufacturing (ICOMM), Northwestern University, pp. 568–570. [11] ISO 230-2:2006 (2006) Test Code for Machine Tools. Part 2. Determination of Accuracy and Repeatability of Positioning Numerically Controlled Axes. [12] JCGM 100:2008 (2008) Guide to the Expression of Uncertainty in Measurement (GUM), Joint Committee for Guides in Metrology (JCGM). [13] Mahshid R, Hansen HN, Arentoft M (2013) Accuracy of Transferring Microparts in a Multi Stage Former. Key Engineering Materials 554–557:900–907.
Please cite this article in press as: Mahshid R, et al. Characterization of precision of a handling system in high performance transfer press for micro forming. CIRP Annals - Manufacturing Technology (2014), http://dx.doi.org/10.1016/j.cirp.2014.03.001
CIRP-1112; No. of Pages 4 CIRP Annals - Manufacturing Technology xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
Characterization of precision of a handling system in high performance transfer press for micro forming Rasoul Mahshid a,*, Hans Nørgaard Hansen (1)a, Mogens Arentoft (2)b a b
Department of Mechanical Engineering, Technical University of Denmark, Lyngby, Denmark IPU Technology Development, Lyngby, Denmark
A R T I C L E I N F O
A B S T R A C T
Keywords: Precision Uncertainty Micro forming
Multi-step micro bulk forming is characterized by complex processes and high precision requirements. In particular the demands regarding handling accuracy between different forming steps are of the order of a few mm. The paper introduces a methodology for the analysis and characterization of this transfer system on component level and system level. Laser interferometry is used in combination with analytical models to predict the positioning ability of the actuator in a static as well as dynamic mode. In combination with an analysis of the grippers, a full description of the transfer precision inside the forming press is obtained. ß 2014 CIRP.
1. Introduction When manufacturing miniaturized metallic mechanical components in mass production by multi-stage micro cold forming one of the big challenges is to transfer the parts with high speed and high precision. This need has been met primarily by conventional transport principles. Micro cold forming allows manufacturing of miniaturized components with diameters down to 150 mm, wall thicknesses as low as 13 mm, the length-to-diameter ratio greater than 55:1 and finished part tolerances as low as 3 mm [1]. Depending on the size and precision of the part, traditional ways of transferring can be costly and inefficient. In order to address these problems, a new high performance transfer press must be developed which allows micro parts to be transferred with tolerances within few microns. Traditionally, in a conventional transfer press an eccentric shaft drives not only the main slide but also the transport device. However due to the reduced overall size of the machine there are several notable challenges for this principle to be adapted in micro forming. These include small tolerances and an increased complexity of the mechanism. Recently a deformation method for manufacturing a micro pin was introduced where it is manufactured from sheet metal [2–4]. This method makes handling easier than in traditional micro bulk forming since the part is connected to a foil when moving in between the different forming stages. Although this type of forming is effective and shows beneficial transferring effects, it also introduces possible quality issues at side surfaces since the deformation process must be ended by a blanking process at the last stage of forming. In this way the final part is punched in order
* Corresponding author. E-mail address: [email protected] (R. Mahshid).
to separate it from the foil. Therefore, if an alternative to conventional transport devices can be developed, there is a potential to significantly improve process efficiency and increase geometrical and surface quality. In the early 2000s, researchers began investigations on process parameters in micro forming. As this research has progressed it has been found that the process parameters can influence significantly accuracy and quality of the forged part. In 2001, Geiger et al. published a paper addressing the scaling effects and solutions for tool manufacturing and explored general aspects and challenges in micro forming [5]. Moreover, they developed a cross transportation system which was able to transfer cylindrical parts with diameter of 0.85 mm with positioning accuracy of 15 mm. The handling device worked with a production speed of 260 strokes per minute. In 2006, Wafios AG manufactured a machine for a multi-stage micro bulk forming which works on the basis of a rotary transfer system and horizontally mounted rotor with eight dies [6]. This machine transfers cylindrical parts with diameters down to 0.5 mm and an output rate of 400 ppm. From studies conducted by Kuhfuss, a concept of linked micro parts was investigated to overcome handling of miniature components [7]. The blanks/parts are interconnected with strap/ wire and are not separated throughout the manufacturing process. The current research involves a new development of a handling system to be integrated into an existing developed micro press in order to maximize the output rate up to 250 strokes per minute without compromising accuracy. The purpose of this research is to explore the effect of the process parameters on the precision of the machine. More specifically, this paper introduces a methodology for determining the precision of handling system of a micro transfer press by combining analyses of positioning accuracy, dynamic behaviour and gripping characteristics.
http://dx.doi.org/10.1016/j.cirp.2014.03.001 0007-8506/ß 2014 CIRP.
Please cite this article in press as: Mahshid R, et al. Characterization of precision of a handling system in high performance transfer press for micro forming. CIRP Annals - Manufacturing Technology (2014), http://dx.doi.org/10.1016/j.cirp.2014.03.001
G Model
CIRP-1112; No. of Pages 4 R. Mahshid et al. / CIRP Annals - Manufacturing Technology xxx (2014) xxx–xxx
2. Challenges and requirements In a standard high performance transfer press all operations are automatic: feeding and transportation of specimens, press force and stroke control, part ejection and stacking of finished parts as well as monitoring and control of the entire transfer press. The functionality of the machine pertains to transferring the workpieces throughout a multi-step former automatically. However, the micro cold forming process implies new challenges to the process and the current machine tools, in particular geometrical accuracies below 10 mm and surface roughness lower than 1 mm. Some attempts have been made to realize such systems by a downscaling approach (e.g. [1,6]). Recently Roehlig investigated small machines to fabricate small workpieces [8]. The principle behind the construction and design of a micro cold forming transfer press is to decrease the size of the machine by using technologies not commonly found in current machines. For a transfer press, the integration of a transport device is specifically of interest to move the parts from one forming operation to the next in such a way as to ensure continuous and uninterrupted motion of the machine at high production speed. The second goal is to increase the maximum achievable production speed. However, it has several potential challenges, such as increasing acceleration and reducing cycle time by using high performance actuators. Precision is influenced by deflections due to mechanical loading (during the forming operation), positioning accuracy and repeatability of the handling system as well as deviations of shape and dimensions of parts throughout the progressive forming process. In order for the progressive forming process to be operational, it is assumed a tolerance of 30 mm of the total handling system is required. A previous study on a micro forming press included a cropping tool for cylindrical billet preparation, and a micro forming press with an integrated transfer system [9]. The gripping principle of transport device was based on surface tension force. The handling device achieved a maximum production rate of 50 strokes per minute. The concept implemented for micro billet cropping machine proved to be successful with respect to the volumetric deviation of the billet within a 1% margin. For the system described herein (see also Fig. 1), a feeder transfers billets (already prepared) continuously and moves them through a channel by means of constant force mechanism into the first station where ejectors insert the workpiece into the grippers. Subsequent workpiece transportation is performed by the grippers. The gripper unit arranges the workpieces from one forming station to the next. The two movements executed by the handling system are performed in longitudinal and transverse direction. The forward motion must have been completed before the main slide starts the forming process (downward direction). The return movement of the grippers is executed when the main slide is at the outmost bottom position. The ejectors complete the cycle when the handling system locates at home position afterwards. The actuation of main slide and transport device is performed by linear motors while the ejectors move by pneumatic cylinders. The transport device consists of a linear motor and a gripper unit. The system relies on a mechanism including 3 pairs of stiff fingers with elastic hinges which support the workpieces during transportation. The grippers work on the basis of self-centring and
Main slide Workpiece Upper tool Lower tool Gripper Ejector
Transfer motion
Ejector motion Approach feed Fig. 1. The elements and their movements in a high performance transfer press.
the friction principle. A linear actuator is used for positioning the gripper. Before analysis of the handling system, the kinematic characteristics of the motions must be available in the form of displacement–time curves. For the linear actuators, the velocity profile was held trapezoidal, as the acceleration was changed with the same value of 20 m/s2 within one movement. The acceleration is directly proportional to the distance between die centres and the production rate squared [10]. The dynamic parameters used in this research were chosen based on preliminary values using kinematic parameter combinations to determine the overall impact that they have on acceleration and force, as listed in Table 1. Therefore, while the parameters were consistently beneficial for the shortest possible cycle time and smooth movement, they do not represent the optimal parameters with respect to real duty cycle. Fig. 2 depicts the nominal motion curve of the transfer system and respective movements of the main slide and ejectors for one cycle. Table 1 Characteristic values and kinematic parameters.
Max. force (N) Displacement (mm) Half cycle time (ms) Max. velocity (mm/s) Acceleration (m/s2) Dwell time (ms) Moving mass (kg)
Stroke (mm)
2
Main slide
Transfer system
5000 8 40 280 20 0 App. 17
12 20 125 460 20 60 App. 0.3
30
20
10
Part transport by transfer
Transfer return without part Main slide movement Lift parts
50
100
150
200
Time (millisecond)
250
Lower ejectors
-10 Fig. 2. Motion curve of the two-axis transfer system.
3. Error analysis The analysis of errors of the handling system is divided into two parts. The first part considers the positioning accuracy and dynamic behaviour of the linear actuator. The second part considers the actuator and the gripper in a combined setup when handling real micro parts. 3.1. Actuator error While transferring the parts in a transfer press, it is important to note that the functionality of the device depends on positioning accuracy due to the fact that the parts need to slide into the dies. As can be seen in Table 1, the output rate of 250 strokes per minute needs 2 g acceleration which potentially is a source of error. When the maximum inertia force and acceleration is applied, the final placement and settle time of the axis was monitored by laser interferometer. The actuator has a linear encoder with 1 mm resolution and encoder count rate of 2 million quadrature counts per second. The test procedure consisted of measuring the position at home and final placement (20 mm displacement) according to ISO 230-2
Please cite this article in press as: Mahshid R, et al. Characterization of precision of a handling system in high performance transfer press for micro forming. CIRP Annals - Manufacturing Technology (2014), http://dx.doi.org/10.1016/j.cirp.2014.03.001
G Model
CIRP-1112; No. of Pages 4 R. Mahshid et al. / CIRP Annals - Manufacturing Technology xxx (2014) xxx–xxx
[11]. The standard foresees a bi-directional approach which is not needed here. Since the horizontal movement of the carrier is between the two points in the experiment, the approach for home position is from the right side and at the final placement from the left side. The measurements were repeated at home and final placements according to the 25 test cycles. Fig. 3 depicts the results. Error bars indicate 2 standard deviation as mentioned in ISO 230-2. The maximum deviation occurred at the final placement and the deviation is lower than 3 mm as can be seen in Fig. 3. This is an acceptable level compared to a total tolerance of 30 mm.
Fig. 3. Static accuracy of transfer system; left: the result of measurements; right: the method of experiment.
However, the above measurements are static, while the transport device must be at final placement before 70 ms from the beginning of the motion assuming a production rate of 250 parts/min (Fig. 2). To observe real time displacement of the carrier, a built-in facility (controller of the linear motor) continuously monitored and captured the position at 1 ms time intervals throughout the half cycle time (125 ms). To verify the repeatability, 25 measurements were tested for the same parameters set of the controller, as listed in Table 2. Also, to provide a means of reference to compare the controller measurements, laser interferometry measurements were conducted for the same parameter sets to verify repeatability between the two measuring methods. Table 2 Parameters of the controller.
3
Fig. 5. Displacement–time curve connected to Parameter Set 2; left: half-cycle time; right: the region of concern. Error bars indicate 1 standard deviations.
Fluctuations with overshoots, undershoots and a steady state error are present at the region of concern which shift the start motion of upper die to the right towards a longer cycle time. When applying the conditions associated with Parameter Set 1 (Fig. 4), an increase of 6% at the time of 55 ms is observed as the highest overshoot. If assuming a tolerance of 10 mm on the target position (20 mm displacement), the carrier enters into the border of the tolerance not before 110 ms. Likewise, when comparing the motion of carrier to the target line, associated with Parameter Set 2, an increase of 1% at the time of 55 ms is observed for the highest overshoot. Fortunately, the improvement of the positioning ability, while moved under Parameter Set 2, is significantly increased in comparison to Parameter Set 1. The results from using Parameter Set 2 yielded an average overshoot of 206 mm from the target line. Consequently, the positioning accuracy fell inside the tolerance window at the time of 70 ms. While not shown in this paper, more tests were carried out for other combinations of the parameters to find an optimal parameter set. However, the settle time is not lowered to the same extent as occurred under Parameter Set 2. To achieve a significantly better result, new equipment would need to be used. The conclusion of this part of the investigation shows that for the machine described herein a balance between the maximum achievable output rate must be made with the choice of linear actuator. The dynamic behaviour of the actuator and gripper must be considered in order to achieve a satisfactory positioning accuracy. 3.2. Gripper error
Parameter set
Proportional gain
Integral gain
Derivative gain
1 2
30 20
75 75
1600 1625
The curves of the motion versus time for the actuator while applying the conditions associated with parameters set 1 and Parameter Set 2 are shown in Figs. 4 and 5, respectively. Note that, the testing region is half cycle time due to the repetition. As can be observed in Fig. 2, the displacement within 50–70 ms is very important since the forward motion must have been completed before the grippers open, i.e. the upper die elements make contact with the transported parts. Good agreement between controller output and interferometer was found between the two measurements.
Fig. 6 shows the schematic of the grippers for 3 consecutive stations of the handling device. For the tests, grippers were manufactured with a curvature of radius 1 mm in the front profile. In order to establish the effect of the gripper’s profile on the positioning accuracy, specimens with diameters of 3, 2.5, 2 and 1.5 mm were examined. To reduce the effect of variability of diameters of the specimens, all of the workpieces came from standard rods holding a diameter tolerance of less than 10 mm and were sectioned into sizes of 5.0 mm length. The measurement instrument used was an optical coordinate measuring machine with a resolution of 0.5 mm and an MPE of 4 mm in all 3 directions (xyz).
Fig. 6. Gripper schematic, contact of the specimens with front profile of grippers and used coordinate system.
Fig. 4. Displacement–time curve connected to Parameter Set 1; left: half-cycle time; right: the region of concern. Error bars indicate 1 standard deviations.
The specimens were loaded into the grippers manually at the home position in order to establish the effect of the self-centring principle and tilt effect on the positioning accuracy. The grippers and therefore the specimens were moved using the linear actuator introduced in Section 3. The measurements concern the coordinate of the centres of specimens at all three positions. The specimens
Please cite this article in press as: Mahshid R, et al. Characterization of precision of a handling system in high performance transfer press for micro forming. CIRP Annals - Manufacturing Technology (2014), http://dx.doi.org/10.1016/j.cirp.2014.03.001
G Model
CIRP-1112; No. of Pages 4 R. Mahshid et al. / CIRP Annals - Manufacturing Technology xxx (2014) xxx–xxx
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are extracted from the grippers after 5 measurements. The tests were run 5 times and at each test, 3 parts were selected randomly. The tests lead to evaluation of the positioning accuracy of the specimens in the gripper as shown in Fig. 7. It is observed that for specimen diameters above 2 mm, deviations are below 5 mm taking into account 2 standard deviations. However for diameters smaller than 2 mm a considerable increase in the deviations (and thereby in the error) is observed. This means that the geometry of the grippers must be appropriately designed with respect to the workpiece geometry and dimension in order to minimize this error contribution. In particular it was observed that the curvature of radius of the grippers must be slightly lower than the specimen’s radius. Station 1
Station 2
As stated in Section 1 it is assumed that a total tolerance of 30 mm of the handling system is required. In Fig. 8, the conformance zone according to ISO 14253-1 is shown in respect with measurement uncertainty and designer’s specification. It can be observed that with the estimated uncertainties the positioning accuracy falls inside the tolerance zone. This is under the assumption that the dynamic behaviour is satisfactory (as described in Section 3.1). It can also be seen that the nominal position is not met, in particular there is a larger distance between stations 1 and 2 than between stations 2 and 3. This is probably due to misalignment between gripper and actuator and also between the different fingers of the gripper.
Station 3
Deviation (µm)
25 20 15 Fig. 8. Conformance decision for the transfer system.
10 5 0
5. Conclusion Ø 3 mm
Ø 2.5 mm
Ø 2 mm
Ø 1.5 mm
Fig. 7. Comparison of precision and standard deviation for positioning of parts with different diameters with the same gripper geometry. Error bars indicate 1 standard deviation.
4. Uncertainty estimation As previously discussed, the distance between the two consecutive stations is equal to the distance between the die centres (20 mm). Therefore, the correlation of independent variables is expressed by the following mathematical formula: Di ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðxi xiþ1 Þ2 þ ðyi yiþ1 Þ2
(1)
where (xi,yi) is the measurand of the centre coordinate of the specimen grasped in ith station to calculate the distance between the two consecutive stations. The measurements presented in Section 3.2 lead to the calculation of the average distance of D1 and D2 as shown in Fig. 6 and the reproducibility (standard deviation) as listed in Table 3. Table 3 The mean value and standard deviation of D1 and D2 based on 25 measurements. Parameter
Mean value (mm)
Standard deviation (mm)
D1 D2
20.021 19.985
2.0 2.3
The basis of every procedure for calculation of uncertainty is the GUM [12]. When applying the PUMA method (ISO 14253-2) while using the uncertainty table [13], as briefly shown in Table 4, an expanded uncertainty of 9.0 mm was calculated. The systematic error of the measuring system (Bias), the resolution of the measuring instrument and the reproducibility contribute to the estimation of the uncertainty.
Table 4 Calculation of uncertainty with the application of PUMA method.
Variance of y, u2(y) Standard uncertainty of y, u(y) Degrees of freedom of y, n(y) Confidence level Coverage factor (Student t) Expanded uncertainty U(y)
u2j ðyÞ
u4j ðyÞ=n j
2.0E11 4.5E06 52 95% 2.0E+00 9.0E06
7.7E24
m
This paper has introduced an analysis of the precision of a handling system for micro forming. The proposed method includes static and dynamic analysis of the positioning accuracy of the actuator, as well as an integrated analysis of part positioning involving actuator and gripper. The latter also includes the effects of misalignment between part and gripper finger. The analysis of the actuator shows static positioning accuracies of the order of 3 mm and considerably larger errors when used in a dynamic mode. Controller settings of the actuator heavily influence the overshoot of the actuator and the settling time. The integrated system (actuator and gripper) is analyzed using real micro parts, and a considerable effect from geometrical compliance between gripper and part dimension is observed. The overall target of a part positioning with respect to the dies in a tolerance zone of 30 mm is verified with an expanded uncertainty of 9 mm. Acknowledgements This work was supported by the Ministry of Science, Technology and Research of Iran. Furthermore Myhrwolds Foundation, Denmark, is acknowledged for their support. References [1] http://www.deringerney.com/products-and-capabilities/cold-forming/. [2] Merklein M, Stellin T, Engel U (2012) Experimental Study of a Full Forward Extrusion Process From Metal Strip. Key Engineering Materials 504–506:587–592. [3] Tan MJ, Lim SCV, Ghassemali E, Jarfors AEW (2013) Progressive Microforming Process: Towards the Mass Production of Micro-Parts Using Sheet Metal. International Journal of Advanced Manufacturing Technology 66:611–621. [4] Fu MW, Chan WL (2013) Micro-Scaled Progressive Forming of Bulk Micropart via Directly Using Sheet Metals. Materials and Design 49:774–783. [5] Geiger M, Kleiner M, Eckstein R, Tiesler N, Engel U (2001) Microforming. CIRP Annals – Manufacturing Technology 50:445–462. [6] Wafios (2006) Horizontally Mounted Rotor with Eight Dies. Wire 56:14–15. [7] Kuhfuss B (2013) Machines and Handling. in Vollertsen F, (Ed.) Micro Metal Forming, Springer Berlin, Heidelberg311–343. [8] Ro¨hlig B, Wulfsberg JP (2013) Paradigm Change: Small Machine Tools for Small Workpieces. Production Engineering 7:465–468. [9] Arentoft M, Eriksen RS, Hansen HN, Paldan NA (2011) Towards the First Generation Micro Bulk Forming System. CIRP Annals – Manufacturing Technology 60:335–338. [10] Mahshid R, Hansen HN, Arentoft M (2012) A New Approach for Handling of Micro Parts in Bulk Metal Forming. 7th International Conference on Micro Manufacturing (ICOMM), Northwestern University, pp. 568–570. [11] ISO 230-2:2006 (2006) Test Code for Machine Tools. Part 2. Determination of Accuracy and Repeatability of Positioning Numerically Controlled Axes. [12] JCGM 100:2008 (2008) Guide to the Expression of Uncertainty in Measurement (GUM), Joint Committee for Guides in Metrology (JCGM). [13] Mahshid R, Hansen HN, Arentoft M (2013) Accuracy of Transferring Microparts in a Multi Stage Former. Key Engineering Materials 554–557:900–907.
Please cite this article in press as: Mahshid R, et al. Characterization of precision of a handling system in high performance transfer press for micro forming. CIRP Annals - Manufacturing Technology (2014), http://dx.doi.org/10.1016/j.cirp.2014.03.001