Precision punching of hole on composite panels

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1 Indian Journal of Engineering & Materials Sciences Vol. 22, December 2015, pp Precision punching of hole on composite panels H Y Chan, A B Abdullah* & Z Samad School of Mechanical Engineering, Universiti Sains Malaysia, Nibong Tebal, Pulau Pinang, Malaysia Received 10 November 2014; accepted 2 July 2015 This study aims to investigate the effect of die clearance on the punched hole quality of composite panels. In practice, industries produce holes on composite panels by drilling. However, this process is time consuming, which lowers productivity. The process may improve by implementing punching instead. In this study, few die sets with different die clearances are fabricated, and an experiment is performed using a Universal Testing Machine (UTM). The images of the punched holes are scanned using a high-resolution machine vision camera, and then the holes are analyzed using the KLONK image measurement software. Results show that a smaller die clearance increases the accuracy of the punched hole size, and the effect of two-step punching on the quality of the punched hole sizes are also depicted. Keywords: Punching, Die clearance, Image processing, Composite, Laminates Composite materials are recently used to replace certain metals in manufacturing applications, such as in the aerospace industry 1,2. These materials are used because of the weight reduction, reliability, and strength of the materials on composite parts especially for complex and high-precision applications. Many composite parts need to be machined or cut to meet the finished dimensions. Some parts require holes and other features to accommodate hardware such as fasteners. Drilling is the main method to produce holes. However, in some cases, different features are required aside from just a hole profile. For metals, punching is one of the methods to produce holes and other shapes. Thus, the main objective of this research is to study the feasibility of replacing drilling with punching by focusing on the effect of die clearance on the quality of punched holes, while considering that the parameters are studied elsewhere and the influence is recognized. Hence, the findings can potentially improve the quality and reduce the cost of hole production on composite panels. Previous research has identified die clearance as one of the major influences on punching. Many methods exist to study drilling and achieve an optimal hole quality on composite panels. These methods include analytical approaches 3, finite element method 4-6, neural networks modeling 7, and design of experiments 8,9. The mechanical characteristics of punching and the geometrical aspect of sheared edges *Corresponding author ( mebaha@usm.my) are generally affected by different parameters. These parameters include clearance, tool geometry, speed or stroke rate, blank holder force, sheet thickness, blank layout, material type, punch-die alignment, and friction 10. Regarding drilling of composites, 11 Desai et al. find spindle speed as the most influential parameter for circularity, whereas feed rate is the most influential for hole size. In another study, Ali et al. 12 find that milling is better than drilling to produce holes on composites. Experiments are performed to investigate the impact response of composite materials 13. Direct punching on composite materials is hardly studied. Most of the punching processes used in the sheet metal industry can produce complex profiles and high-precision holes. In addition, the amount of practical work done is relatively insufficient, thus, further experiment-based investigations are needed. Methodology The methodology of this study consisted of five distinct steps. First, the study began with the identification of the parameters. Next was the experimental set-up, where puncher and die material, i.e., D2 and composites panel were procured. Accordingly, the design, fabrication, and assembly of the die set and test rig were performed. Subsequently, the results were experimented and measured. Meanwhile, the images of the holes were captured using a machine vision camera. Finally, measurements were performed using the KLONK

642 INDIAN J. ENG. MATER. SCI., DECEMBER 2015 image measurement software. The puncher design was improved, and the effects on the surface quality of the holes were quantified.

Although the equation was developed specifically for metals, the die clearance was set based on this equation because of limited resources.

2 642 INDIAN J. ENG. MATER. SCI., DECEMBER 2015 image measurement software. The puncher design was improved, and the effects on the surface quality of the holes were quantified. The measurement result was verified using Alicona. Experimental design The die clearance was determined based on Eq. (1), as suggested in reference 14. Although the equation was developed specifically for metals, the die clearance was set based on this equation because of limited resources., (1) where c is the single die clearance; t is the strip thickness; S is the material shearing strength; and K is the clearance coefficient, whose scale is K = The shearing strength can be estimated using S = 0.7 UTS. Hence, a total of 10 die sets (Fig. 1) with different punch diameters (3, 5, and 10 mm) and die clearances (25%, 30%, and 35%) were selected. The selected punch Fig. 1 Isometric view of bottom die diameters were based on the hole size that were most commonly required in composite panel assemblies, whereas the selected die clearances were according to actual industrial applications. At least three specimens were tested for every cutting condition for repeatability. The experimental designs are given in Table 1. Experimental set-up A laboratory die rig was specially designed for this systematic investigation (Fig. 2). The die was placed on an Instron 3367 UTM with a punch travel speed of 5 mm/s. As the punch traveled downward, the initial action was clamping the composite panel. The bottom die was pressed by the top die to generate a holding pressure on the panel. The punch proceeded further to cut the composite panel successively. Three samples were collected for each parameters set in the experimental design. No. Table 1 Experimental design Punch diameter Clearance (%) Die diameter Fig. 2 (a) laboratory die rig and (b) die set0

CHAN et al.: PRECISION PUNCHING OF HOLE ON COMPOSITE PANELS 643 Sample analysis The cut surface quality was evaluated based on three criteria: top surface diameter (Fig.

Upon completion of the punching, images of the samples were captured using a machine vision camera, which created high resolution digital images of the samples.

Measurement of the top and bottom surface diameters First, calibration was performed using a ruler along the sample, where the images were captured.

Finally, the value of the top or bottom surface diameter was generated Incomplete shearing ratio First, calibration was performed using a ruler along the image samples collected.

3 CHAN et al.: PRECISION PUNCHING OF HOLE ON COMPOSITE PANELS 643 Sample analysis The cut surface quality was evaluated based on three criteria: top surface diameter (Fig. 3), bottom surface diameter (Fig. 3), and incomplete shearing ratio (Fig. 4). Meanwhile, the result of the maximum load required for the punching and other related graphs were recorded. Upon completion of the punching, images of the samples were captured using a machine vision camera, which created high resolution digital images of the samples. In addition, the camera also ensured that the images were parallel and noise such as shadows were avoided. The images obtained were then analyzed using the KLONK image measurement software. Measurement of the top and bottom surface diameters First, calibration was performed using a ruler along the sample, where the images were captured. Next, the perimeter of the surface diameter of the holes was illustrated manually. Finally, the value of the top or bottom surface diameter was generated Incomplete shearing ratio First, calibration was performed using a ruler along the image samples collected. Next, the perimeter of the surface diameter of the hole was illustrated manually. Subsequently, the value of the area illustrated was generated automatically by the operating software. This area value was regarded as hole area (A) (Fig. 5). Afterwards, the perimeter of the clean hole area was illustrated manually. The value of the illustrated clean hole area was then generated automatically through the operating software. This area value was regarded as clean hole area (A c ) (Fig. 6). Finally, the incomplete shearing ratio was calculated through the following equation: A Ac Ratio of incomplete shearing = 100% A Validation Validation was important to determine the effectiveness and accuracy of the new measurement method proposed. In this project, a new method that involved image scanning through a machine vision camera was employed for the measurement, followed by the measurement of the hole surface diameter and area using the KLONK image measurement software. A total of two samples (samples 1 and 2 of the Ø10 mm puncher with 10% die clearance) were measured using Alicona InfiniteFocus (Alicona IFM) 15. Alicona IFM is 3D surface measurement technique where profile of the scanned image will be based on the focus variation. Therefore, the amplitude of each scanned points will be recorded. The results generated were then compared with the results of the new measurement method proposed. This proposed Fig. 3 A schematic cross-sectional view of sheared material Fig. 5 Hole area (A) Fig. 4 Example of incomplete shearing Fig. 6 Clean hole area (A c )

4 644 INDIAN J. ENG. MATER. SCI., DECEMBER 2015 method can be acceptable if the value difference between the two samples was less than 10%. Results and Discussion The result can be described in terms of top and bottom diameter of the hole, incomplete shearing and load required for the punching. Figure 7 shows the effect of die clearance on the top surface diameter. The graph shows three almost straight lines for all three puncher diameters. These lines suggest that die clearance does not affect the top surface diameter significantly. The values of the top surface diameters with different puncher diameters minimally changed regardless of the difference in the die clearance values. For the Ø 10 and Ø5 mm puncher diameters, the top surface diameter values are very close to the puncher diameters of between 0.08 and +0.07, and 0.12 mm, respectively. However, for the Ø3 mm puncher, the deviation reaches up to 0.50 mm. Figure 8 shows the deviation of the top surface diameter from the puncher diameter. The trends of the graph suggest that the values of the top surface Fig. 7 Effect of die clearance on top surface diameter Fig. 8 Effect of die clearance on the deviation of top surface diameter from punch diameter

5 CHAN et al.: PRECISION PUNCHING OF HOLE ON COMPOSITE PANELS 645 diameter are close to the Ø10 and Ø5 mm puncher diameters. The deviations of the Ø10 mm puncher are between 0.813% and 0.686%. Meanwhile, for the Ø5 mm puncher, the deviations are between 2.368% and 0.579%. However, the same phenomenon is not observed on the Ø3 mm puncher because of the great deviations of between 8.817% and %. This phenomenon is inapplicable on the Ø3 mm puncher mostly because of the puncher damage after several operations. Hence, the top surface diameters of all the samples (except for the Ø3 mm puncher) seem to follow the corresponding puncher diameter values. The Ø3 mm puncher is bent after several punching. This bending occurs because the ratio of the puncher diameter d to the material thickness t does not satisfy Eq. (2), as recommended by Suchy 16. (2) Eq. (2) is used widely when dealing with materials that consist of metals or metal alloys. However, this equation also seems to work for composite panels. Smaller ratios are generally not recommended in shearing operations because of the more complicated manufacturing method required, and not because smaller holes cannot be produced. The effect of die clearance on the bottom surface diameter is further studied, as shown in Fig. 9. The trends of the graph suggest that the bottom surface diameter of all the specimens increase as the die clearance (die diameter) expands. Based on Fig. 10, the trends show that the deviations between the bottom surface diameter and the die diameter are minimal. The deviations of the Ø10 mm puncher lie between 0.179% and 1.034%. For the Ø3 mm puncher, the deviations are between 0.291% and 1.263%. Meanwhile, for the Ø3 mm puncher, the deviations are between 0.296% and 0.492%. Hence, the bottom surface diameter seems to follow the corresponding die diameter value. Figure 11 shows the effect of die clearance on the incomplete shearing area ratio. The trends of the graph suggest that the incomplete shearing area ratio increases as the die clearance expands. However, this trend is inapplicable on the Ø10 mm puncher with 30% clearance. Figure 12 shows the effect of die clearance on the maximum load, and Fig. 13 shows the effect of die clearance on the compressive stress. For the same clearance, the puncher with a larger diameter has higher compressive stress and maximum load. In addition, for the same puncher diameter, the compressive stress and maximum load increase as the die clearance (die diameter) decreases. Improving the cut surface quality Based on the preliminary cut surface quality analysis, several conclusions are drawn: (i) The top surface diameter of a hole tends to be close to the puncher diameter; (ii) the bottom surface diameter of a hole tends to be close to the bottom die diameter; (iii) the amount of incomplete shearing decreases as the die clearance reduces; (iv) for the same puncher diameter, the compressive stress and maximum load increase as the die clearance (die diameter) reduces; Fig. 9 Effect of die clearance on bottom surface diameter

6 646 INDIAN J. ENG. MATER. SCI., DECEMBER 2015 Fig. 10 Effect of die clearance on the deviation of bottom surface diameter from die diameter Fig. 11 Ratio of incomplete shearing area and (v) for the same clearance, a larger puncher diameter generates higher compressive stress and maximum load. Therefore, the die geometry and punching mechanism require modification to improve the blanking hole quality. Modification of the punching mechanism To produce Ø10 and Ø5 mm holes with favorable cut surface quality and dimensional accuracy, the bottom die clearance requires a minimal value, given that the bottom surface diameter of a hole tends to be close to the bottom die diameter. However, for the same puncher diameter, the compressive stress and maximum load increase as the die clearance (die diameter) decreases. Hence, a two-step punching mechanism is proposed to reduce the effect of die clearance on the maximum load.

7 CHAN et al.: PRECISION PUNCHING OF HOLE ON COMPOSITE PANELS 647 Fig. 12 Effect of die clearance on maximum compressive load For the Ø10 mm hole: (i) Three 10 mm holes are created using the Ø5.00 Ø10 mm puncher, as shown in Fig. 14; (ii) the first punching step is performed using the Ø5 mm punch section, followed by the second punching step performed using the Ø10 mm punch section; (iii) the material is expected to fracture at 2.5 mm Fig. 13 Effect of die clearance on compressive stress of the material thickness. Hence, the penetration of the second puncher (Ø10 mm puncher) is stopped when the puncher reached 2.5 mm deep from the top surface of the composite panel; (iv) the bottom die clearance is 1% (Ø mm); (v) given the limitations of the machine, the penetration depths vary between 12.5 and 14.0 mm.

8 648 INDIAN J. ENG. MATER. SCI., DECEMBER 2015 For the Ø5 mm hole: (i) Three 5 mm holes are created using the Ø4 Ø5 mm puncher, as shown in Fig. 15; (ii) the first punching step is performed using the Ø4 mm punch section, followed by the second punching step performed using the Ø5 mm punch section; (iii) The material is expected to fracture at 2.5 mm of the material thickness. Hence, the penetration of the second puncher (Ø5 mm Fig. 14 Detail drawing of Ø5 mm and Ø10 mm puncher Fig. 15 Detail drawing of Ø4 mm and Ø5 mm dual stage puncher puncher) is stopped when the puncher reached 2.5 mm deep from the top surface of the composite panel; (iv) the bottom die clearance is 1% (Ø5.068 mm); and (v) given the limitations of the machine, the penetration depths vary between 12.5 and 14.0 mm. The blank holder force, punch velocity, and measurement method are similar to those of the previous experiment. The experimental design which comprised the two-step punching mechanism is shown in Table 2. The experimental results are shown in Tables 3-5. Table 3 shows the experimental results of the Ø5 Ø10 mm puncher with 1% die clearance (Ø mm), while Table 4 shows the results of the incomplete shearing percentages of the Ø5 Ø10 mm puncher with 1% die clearance (Ø mm). Table 5 shows the experimental results of the Ø4 Ø5 mm puncher with 1% die clearance (Ø5.068 mm). Based on Table 6, the hole produced by the Ø5- Ø10 mm puncher has the lowest difference value (0.084 mm) between the top and bottom diameters compared with the other holes produced by the previous Ø10 mm puncher. In addition, this hole has the lowest amount of incomplete shearing with a value of 12.25%. Through the two-step or dual stage punching, the die clearance value can be reduced greatly without increasing the maximum load. The hole produced by the Ø5 Ø10 mm puncher only has a maximum load of N but with minimal die clearance (1%). The Ø4 Ø5 mm puncher produces the same outcome. Based on Table 7, the hole produced by the Ø4 Ø5 mm puncher has the lowest difference value No. Table 2 Experimental design for dual stage puncher Punch diameter (1 st stage punch) (2 nd stage punch) (1 st stage punch)- 5.00(2 nd stage punch) Clearance (%) Die diameter Ø5 mm-10 mm Punch 1% Die clearance (Ø mm) Table 3 Experimental results of Ø5-10mm punch with 1% die clearance (Ø mm) Top surface diameter Bottom surface diameter Deviation of bottom surface diameter from bottom die diameter (%) Deviation of top surface diameter from punch die diameter (%) Difference between top and bottom diameter Sample Sample Sample Average

9 CHAN et al.: PRECISION PUNCHING OF HOLE ON COMPOSITE PANELS 649 Table 4 Incomplete shearing percentages of Ø5-10 mm punch with 1% die clearance (Ø mm) Ø5 mm-10 mm Punch 1% Die clearance (Ø mm) Hole area, A(mm 2 ) Clean hole area, A c (mm 2 ) Amount of incomplete shearing percentage, (A- A c )/A (%) Comment Sample Very Low Sample Very Low Sample Very Low Average Very Low Table 5 Experimental results of Ø4-5mm punch with 1% die clearance (Ø5.068 mm). Ø4 mm-5 mm Punch 1% die clearance (Ø mm) Top diameter Bottom diameter Deviation of bottom surface diameter from bottom die diameter (%) Deviation of top surface diameter from punch diameter (%) Difference between top and bottom diameter Sample Sample Sample Average Table 6 Incomplete shearing percentages of dual phase Ø4-5mm puncher and 1% die clearance (Ø5.068mm). Ø4 mm -5 mm Punch 1% die clearance (Ø mm) Hole area, A (mm 2 ) Clean Hole area, A c (mm 2 ) Incomplete shearing Percentage, (A- A c )/A (%) Comment Sample Low Sample Low Sample Low Average Low (0.08 mm) between the top and bottom diameters compared with the other hole produced by the previous Ø5 mm puncher. Moreover, this hole has the lowest amount of incomplete shearing with a value of 31.52%, as given in Table 8. Furthermore, this hole also has the lowest maximum load value required is 8744 N and the lowest die clearance value (1%). For the dual phase Ø4- Ø5 mm puncher, the maximum load occurred when the first puncher penetrated the composite panel. The first puncher produced a higher compressive load compared with the second puncher because the first puncher sheared off a higher proportion of material than the second puncher. However, for the dual-phase Ø5- Ø10 mm puncher, Table 7 Comparison of experimental results between dual phase Ø5- Ø10 mm puncher and present Ø10mm puncher Ø5 mm-10 mm Punch (Die clearance 1%) Ø10 mm Punch (Die clearance 10%) Ø10mm Punch (Die clearance 25%) Ø10mm Punch (Die clearance 30%) Ø10mm Punch (Die clearance 35%) Difference between top and bottom diameter, Amount of incomplete shearing, percentage (%) Maximum compressiv e load,(n) Table 8 Comparison of experimental results between dual phase Ø4- Ø5 mm puncher and present Ø5 mm puncher Ø4mm-5mm Punch (Die clearance 1%) Ø5mm Punch (Die clearance 25%) Ø5mm Punch (Die clearance 30%) Ø5mm Punch (Die clearance 35%) Difference between top and bottom diameter, Amount of incomplete shearing, percentage (%) Maximum compressive load, (N) Fig. 16 The load changes for the dual phase puncher the maximum load occurred when the second puncher penetrated the composite panel. Figure 16 shows the load pattern of the dual phase puncher described previously. Based on Table 9, the results generated using Alicona IFM is slightly different with those generated

image measurement Difference Ø10 mm Punch 1% Die clearance (Ø10.07 mm) Sample 1 Top Ø = 10.16 mm Top Ø = 10.

23 mm Bottom Ø = 10.25 mm Ø10 mm Punch 1% Die clearance (Ø10.07 mm) Sample 2 Top Ø = 10.16 mm Top Ø = 10.

96 mm Bottom Ø = 9.80 mm through the machine vision camera and KLONK image measurement software.

Moreover, the percentage difference of the other values are between 1% and +1%.

10 650 INDIAN J. ENG. MATER. SCI., DECEMBER 2015 Table 9 Comparison of experimental data obtained with Alicona IFM Specimen Alicona KLONK image measurement Difference Ø10 mm Punch 1% Die clearance (Ø10.07 mm) Sample 1 Top Ø = mm Top Ø = mm diameter = 0.08 mm percentage = 0.83% diameter = mm percentage = -0.19% Bottom Ø = mm Bottom Ø = mm Ø10 mm Punch 1% Die clearance (Ø10.07 mm) Sample 2 Top Ø = mm Top Ø = mm diameter = mm percentage = 0.81% diameter = -0.16mm percentage = 1.62 % Bottom Ø = 9.96 mm Bottom Ø = 9.80 mm through the machine vision camera and KLONK image measurement software. The largest difference in percentage is in the bottom surface diameter of sample 2, that is, 1.62%. Moreover, the percentage difference of the other values are between 1% and +1%. This result suggests that the new measurement method is acceptable because the value difference between these methods is less than 10%. Conclusions Understanding and studying the behavior of punching a composite sandwich structured panel remained as one of the rarely researched areas in this particular industry because producing holes on composite panels were typically done by drilling. Therefore, this study was performed to evaluate the cut surface quality of holes through punching. The

11 CHAN et al.: PRECISION PUNCHING OF HOLE ON COMPOSITE PANELS 651 designed bottom die and punch were fabricated, and the following are drawn from this study: (i) The top surface diameter of a hole tended to be close to the puncher diameter. (ii) The die clearance did not significantly affect the top surface diameter. (iii) The bottom surface diameter of a hole tended to be close to the bottom die diameter. (iv) The amount of incomplete shearing decreased as the die clearance reduced. (v) For the same punch diameter, the compressive stress and maximum load increased as the die clearance (die diameter) decreased. (vi) For the same clearance, the puncher with a larger diameter had higher compressive stress and maximum load. (vii) To produce Ø10 and Ø5 mm holes with favorable cut surface quality and dimensional accuracy, the bottom die clearance value needed to be minimal because the bottom surface diameter of a hole tended to be close to the bottom die diameter. (viii) The two-step (dual phase) punching mechanism reduced the effect of die clearance on the maximum load. Acknowledgement The authors would like to acknowledge Universiti Sains Malaysia for their sponsorship through Research University Grant Scheme (#1001/PMEKANIK/814270). References 1 Botelho E C, Silva R A, Pardini L C and Rezende M C, Mater Res, 9(3) (2006) Edward T, Ingenia, 36 (2008) Rahmé P, Landon Y, Lachaud F, Piquet R & Lagarrigue P, Int J Adv Manuf Technol, 52(5-8) (2011) Isbilir O & Ghassemieh E, Appl Compos Mater, 19(3-4) (2012) Phadnis V A, Roy A & Silberschmidt V V, J Phys: Conf Ser, doi: / /382/1/ Zhang H L & Chen J, Adv Mater Res, 188 (2011) Mayyas A, Qasaimeh A, Alzoubi K, Lu S, Hayajneh M & Hassan A, J Miner Mater Charact Eng, 11(10) (2012) Mohd Ariffin M K A, Mohd Ali M I, Sapuan S M & Ismail N, Sci Res Essay, 4 (10) (2009) Vankanti V K & Ganta V, J Mater Res Technol, 3(1) (2014) Al-Momani E & Rawabdeh I, JJMIE, 2 (2008) Desai B, Rana J & Gajera H, J Info, Knowledge Res Mech Eng, 2(2) (2013) Ali H M, Iqbal A & Liang L, Sadhana, 38 (4) (2013) Evci C & Gülgeç M, Int J Impact Eng, 43 (2012) Lin Z C & Chang H, J Intellt Manuf, 5 (1994) Instron Corporation, Instron Series 3300 Load Frames Including Series 3340, 3360, 3380, in Reference Manual- Equipment, I. Corporation, ed., Ivana Suchy, Handbook of die design, 2 nd ed, , 2006.

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