# Hole Quality Observation in Single-Shot Drilling of CFRP/Al7075-T6 Composite Metal Stacks Using Customized Twist Drill Design

^{1}

^{2}

^{3}

^{4}

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Materials and Methods

#### 2.1. Worpiece Materials

_{2}/0/90/0/90/0/135/45

_{2}/135]

_{s}. The CFRP laminate was then covered with a 0.08 mm thin layer of glass/epoxy woven fiber at the top and bottom to prevent delamination at both the entrance and exit of the hole during drilling. As a result, the final thickness of the entire composite panel, including the paint application, was 3.587 mm. The CFRP was compressed using a vacuum pump and controlled atmospheric conditions during the curing process. The autoclave was equipped with a prepared mold to keep the laminate. The temperature was raised to 180 °C during the curing cycle at a rate of 3 °C/min and maintained for 120 min. The temperature was then gradually brought back to normal temperature. The entire cycle was carried out in an autoclave at a pressure of 700 kPa and the laminate was packed in a vacuum bag that was depressurized to 70 kPa. Because of the curing recipe’s application, the nominal fiber volume was 60%. The mechanical and physical characteristics of the stack materials employed in this work are compiled in Table 1.

#### 2.2. Cutting Tool Fabrication

^{3}, both of which are much higher than those of the workpiece, it was selected as the drill bit material. Helitronic Tool Studio version 1.9.216.0 software (Walter Maschinenbau GmbH, Garbsen, Germany) was used to design the drills with special custom drill geometry. A particular wheel must perform numerous consecutive operations while grinding with a cutting tool. These operations include pointing, gashing, and clearing. In the program, a chisel edge angle of 30° to 45° was set for the gashing process and a primary clearance angle of 6° to 8° was selected for the clearance phase. The point angle was finally established from 130° to 140° during the pointing phase. Figure 1a–c demonstrates the manufacturing procedure and the wheel type used to modify the twist drill design using a CNC grinding machine (Walter Maschinenbau GmbH, Garbsen, Germany).

#### 2.3. Drilling Process

#### 2.4. Hole Edge Defect Measurement

#### 2.4.1. Exit Delamination

_{nom}is the nominal area of the drilled hole and A

_{max}is the damaged area of the composite laminate after the drilling process.

#### 2.4.2. Burr Height

#### 2.5. Hole Integrity

#### 2.5.1. Hole Diameter Error

#### 2.5.2. Hole Circularity

#### 2.6. Response Surface Methodology (RSM)

^{k}+ 2k + 6, was used to produce the number of experiment runs, where k was the number of components with replications at the design center. A quadratic model was applied to the optimization to fit and estimate the minimal point. The mathematical model for each answer was created using these data points, as illustrated in Equation (5) [40,41].

_{i}and X

_{j}are the input variables; β

_{0}is an offset term; β

_{i}, β

_{ii}, and β

_{ij}are the interaction coefficients of linear, quadratic, and second-order terms.

## 3. Results and Discussion

#### 3.1. Exit Delamination Analysis

_{d-exit}values for the entire run. The values acquired for each trial appeared to be nearly identical on the graph. The typical F

_{d-exit}value ranged from 1.0038 to 1.0196. Since every value was below the tolerance level advised by aerospace manufacturers in accordance with OEM standards, it is evident that the range of the drill geometry in this study had no impact on the F

_{d-exit}value. However, ref. [44] reported that twist drills are less efficient than core drills since the thrust force is not much focused on the middle of the drill bit and cutting edges, but usually distributed over the periphery of the bit.

#### 3.1.1. Regression Model and ANOVA

_{1}) is shown in Equation (6).

_{1}), the F-value in the ANOVA analysis was 8.66 and the probability value (p-value) was less than 0.05, as shown in Table 4. Furthermore, the p-value of 0.9214 indicated that the lack of fit was related to pure error and was not significant. The model’s significant value and the lack of fit’s non-significant value supported the validity of the log-transformed model. The point angle was insignificant, despite the fact that the percentage of contribution (PC) for each model term, A, B, BC, and B

^{2}, had a considerable impact on exit delamination, with values of 43.7%, 10.5%, 8.4%, and 20.4%, respectively. The values of the R

^{2}, adjusted R

^{2}(Adj R

^{2}), and predicted R

^{2}(Pred R

^{2}) coefficients were used to assess the model’s goodness of fit. The Adj R

^{2}value of 0.7053 and Pred R

^{2}value of 0.6885 were in reasonable accord as the discrepancy was less than 0.2.

^{2}should be at least 0.80 for a model to fit the data well. The correlation coefficient (R

^{2}) and adjusted coefficient (Adj. R

^{2}) values in this instance were 0.7974 and 0.7053, respectively, demonstrating the significance of the fit of the RSM model and its potential for response prediction.

#### 3.1.2. Effect of Geometric Parameters on Exit Delamination

_{1}) panel, the perturbation plot shown in Figure 11a was used to determine the sensitivity of each factor. The exit delamination was significantly affected by the chisel edge angle (A). The exit delamination of the CFRP was decreased by increasing the chisel edge angle. With these drill geometries, a lower exit delamination was consequently produced. In this parameter analysis, the primary clearance angle (B) had a significantly greater effect than the point angle (C) on the exit delamination (Y

_{1}) of the CFRP. According to the quadratic model that was fitted, a curvilinear profile was observed, as shown in Figure 11b. By maintaining the third parameter i.e., chisel edge angle constant at the middle level (37.5°), the graph indicated delamination with regard to two alternative parameters. When the point angle was set at 130° and the primary clearance angle was set at 8°, exit delamination was decreased.

#### 3.2. Burr Height Analysis

_{bmax}value was minimal, measuring between 40.2 and 271.2 µm, as shown in Figure 13. A lower H

_{bmax}value was found in R16 with a 45° chisel edge angle, 6° primary clearance angle, and 130° point angle, whereas R3 yielded a higher H

_{bmax}value with a 30° chisel edge angle, 7° primary clearance angle, and 135° point angle [35]. These results are in line with ref. [49] in which the burr height ranged from 133.62 to 211.45 μm when they used a 130° point angle and from 1036.25 to 2066.85 μm when they used a tool with a 110° point angle. Further, these authors mentioned that the drill with a 130° point angle produced a uniform burr type and a 110° point angle produced transient and crown burrs during single-shot drilling of CFRP/Al7075-T6 material [49].

_{bmax}value rose, resulting in a significant rolled-up phenomenon at the Al707-T6 panel’s exit. This is because there was less space for the chip to flow during evacuation when the bit first contacted the material during the cutting operation, because the chisel edge angle of 30° was less than 45°, as shown in Figure 14. The ineffective chip flow increased the shear and decreased the cutting efficiency during the drilling process. The cutting heat also makes the material more ductile and uses more energy [32]. As a result, burrs along the hole’s edge are easily produced. The replicated tools yielded consistent results, as shown in Figure 13, proving that they were properly manufactured.

#### 3.2.1. Regression Model and ANOVA

_{bmax}, the regression model for the response was enhanced by log transformation. The final empirical model for the actual causes of burr formation at the Al7075-T6 panel’s exit (Y

_{2}) was

_{bmax}in the ANOVA analysis was 9.718 and the p-value was lower than 0.05, as shown in Table 5. Furthermore, the p-value of 0.683 indicated that the lack of fit was related to pure error and was not significant. The model’s significant value and the lack of fit’s non-significant value supported the validity of the log-transformed model. As indicated in Table 5, the factors that significantly influenced the results had a confidence level above 95% and a p-value lower than 0.05. The second term, B, was insignificant, despite the fact that the p-values for the other model terms (A, C, BC, A

^{2}, and B

^{2}) had a considerable impact on H

_{bmax}. The adjusted R

^{2}value of 0.733 and the predicted R

^{2}value of 0.602 were in reasonable accord as the discrepancy was less than 0.2. Since a ratio greater than 4 is preferred when measuring the signal-to-noise ratio [45], as mentioned in Section 3.1.1, and a strong signal of 12.85 was obtained here, this model was utilized to navigate the design space. The correlation coefficient (R

^{2}) and adjusted coefficient (Adj R

^{2}) values in this instance were 0.818 and 0.733, respectively, demonstrating the significance of the fit of the RSM model and its potential for response prediction.

_{bmax}log

_{10}was 0.0732, according to Figure 15b. For example, the actual number fell between 1.9068 and 2.0532, and the anticipated value was 1.98. For a dataset with a normal linear relationship, the RSM model can be used to estimate the value if two-thirds of the residual data points (Figure 15b) are within SEE i.e., above or below the least squares line [50].

#### 3.2.2. Effect of Geometric Parameters on Burr Height Formation

_{bmax}of the Al7075-T6 panel (Y

_{2}), the perturbation plot shown in Figure 16a was used to determine the sensitivity of each factor. The variables had a significant impact on the specific responses in the drilling of stack-up material. The H

_{bmax}value was significantly affected by the chisel edge angle (A). The H

_{bmax}value at the exit of Al7075-T6 was decreased by increasing the chisel edge angle. An extreme chisel edge angle made clearance possible and made the shearing of materials by the cutting edges more effective (Figure 14). With these drill geometries, less burr formation was consequently produced. In this parameter analysis, the primary clearance angle (B) and point angle (C) had a moderate impact on the response of (Y

_{2}).

_{bmax}with regard to two alternative parameters. When minimum point angle and primary clearance angle were set, i.e., primary clearance angle of 6° and point angle of 130°, the H

_{bmax}was decreased.

#### 3.3. Multiple Response Optimization

_{1}and Y

_{2}are tabulated in Table 7. For the suggested optimal drill bit shape, the discrepancies between the predicted and actual trial results were 0.11% and 9.72% respectively, hence validating that the proposed optimized cutter geometry was confirmed in the optimization model.

#### 3.4. Hole Diameter Error

#### 3.5. Hole Circularity

## 4. Conclusions

- Although the point angle of the twist drill had to be raised from 130° to 140°, the delamination at the CFRP exit hole had a favorable effect on hole integrity.
- The average burr height was minimal, measuring between 40.2 and 271.2 µm. The lower burr height was found with a 45° chisel edge angle, 6° primary clearance angle, and 130° point angle. When the chisel edge angle was reduced to 30°, the burr height rose, resulting in a significant rolled-up phenomenon at the Al707-T6 panel’s exit hole due to less available space for chip evacuation.
- The lowest hole diameter error values were obtained with values of 0.96 µm, 2.36 mm, and -1.4 µm for the stack-up diameter error, CFRP diameter error, and Al7075-T6 diameter error, respectively. At the same time, the hole circularity error was less than 30 µm in all runs, which was within OEM standards.
- Multiple response optimization was employed to optimize drill geometric parameters and the best drill geometry for a customized twist drill was proposed. To obtain minimal hole edge defects, it was discovered that the combination of 45° chisel edge angle, 8° primary clearance angle, and 130° point angle is the ideal drill geometry for a twist drill design, with a desirability index level of 0.773.

## Author Contributions

## Funding

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## References

- Geng, D.; Liu, Y.; Shao, Z.; Lu, Z.; Cai, J.; Li, X.; Jiang, X.; Zhang, D. Delamination formation, evaluation and suppression during drilling of composite laminates: A review. Compos. Struct.
**2019**, 216, 168–186. [Google Scholar] [CrossRef] - Slamani, M.; Chatelain, J.-F. Assessment of the suitability of industrial robots for the machining of carbon-fiber reinforced polymers (CFRPs). J. Manuf. Process.
**2019**, 37, 177–195. [Google Scholar] [CrossRef] - Xu, J.; Zhou, L.; Chen, M.; Ren, F. Experimental study on mechanical drilling of carbon/epoxy composite-Ti6Al4V stacks. Mater. Manuf. Process.
**2019**, 34, 715–725. [Google Scholar] [CrossRef] - Li, C.; Xu, J.; Chen, M.; An, Q.; El Mansori, M.; Ren, F. Tool wear processes in low frequency vibration assisted drilling of CFRP/Ti6Al4V stacks with forced air-cooling. Wear
**2019**, 426–427, 1616–1623. [Google Scholar] [CrossRef] - Xu, J.; El Mansori, M.; Voisin, J.; Chen, M.; Ren, F. On the interpretation of drilling CFRP/Ti6Al4V stacks using the orthogonal cutting method: Chip removal mode and subsurface damage formation. J. Manuf. Process.
**2019**, 44, 435–447. [Google Scholar] [CrossRef] - Pecat, O.; Brinksmeier, E. Low damage drilling of CFRP/titanium compound materials for fastening. Procedia CIRP
**2014**, 13, 1–7. [Google Scholar] [CrossRef] [Green Version] - Pecat, O.; Brinksmeier, E. Tool wear analyses in low frequency vibration assisted drilling of CFRP/Ti6Al4V stack material. Procedia CIRP
**2014**, 14, 142–147. [Google Scholar] [CrossRef] [Green Version] - Zitoune, R.; Krishnaraj, V.; Collombet, F. Study of drilling of composite material and aluminium stack. Compos. Struct.
**2010**, 92, 1246–1255. [Google Scholar] [CrossRef] - Abdelhafeez, A.M.; Soo, S.L.; Aspinwall, D.K.; Dowson, A.; Arnold, D. Burr formation and hole quality when drilling titanium and aluminium alloys. Procedia CIRP
**2015**, 37, 230–235. [Google Scholar] [CrossRef] - Pilný, L.; De Chiffre, L.; Píška, M.; Villumsen, M.F. Hole quality and burr reduction in drilling aluminium sheets. CIRP J. Manuf. Sci. Technol.
**2012**, 5, 102–107. [Google Scholar] [CrossRef] - Liu, D.; Tang, Y.; Cong, W.L. A review of mechanical drilling for composite laminates. Compos. Struct.
**2012**, 94, 1265–1279. [Google Scholar] [CrossRef] - Vilches, F.J.T.; Hurtado, L.S.; Fernández, F.M.; Gamboa, C.B. Analysis of the chip geometry in dry machining of aeronautical aluminum alloys. Appl. Sci.
**2017**, 7, 132. [Google Scholar] [CrossRef] [Green Version] - Abhishek, K.; Datta, S.; Mahapatra, S.S. Optimization of thrust, torque, entry, and exist delamination factor during drilling of CFRP composites. Int. J. Adv. Manuf. Technol.
**2015**, 76, 401–416. [Google Scholar] [CrossRef] - Pawar, O.A.; Gaikhe, Y.S.; Tewari, A.; Sundaram, R.; Joshi, S.S. Analysis of hole quality in drilling GLARE fiber metal laminates. Compos. Struct.
**2015**, 123, 350–365. [Google Scholar] [CrossRef] - Luo, B.; Li, Y.; Zhang, K.; Cheng, H.; Liu, S. Effect of workpiece stiffness on thrust force and delamination in drilling thin composite laminates. J. Compos. Mater.
**2016**, 50, 617–625. [Google Scholar] [CrossRef] - Khashaba, U.A. Improvement of toughness and shear properties of multiwalled carbon nanotubes/epoxy composites. Polym. Compos.
**2018**, 39, 815–825. [Google Scholar] [CrossRef] - Geier, N.; Xu, J.; Pereszlai, C.; Poór, D.I.; Davim, J.P. Drilling of carbon fibre reinforced polymer (CFRP) composites: Difficulties, challenges and expectations. Procedia Manuf.
**2021**, 54, 284–289. [Google Scholar] [CrossRef] - Gaugel, S.; Sripathy, P.; Haeger, A.; Meinhard, D.; Bernthaler, T.; Lissek, F.; Kaufeld, M.; Knoblauch, V.; Schneider, G. A comparative study on tool wear and laminate damage in drilling of carbon-fiber reinforced polymers (CFRP). Compos. Struct.
**2016**, 155, 173–183. [Google Scholar] [CrossRef] - Khashaba, U.A.; El-Sonbaty, I.; Selmy, A.I.; Megahed, A.A. Machinability analysis in drilling woven GFR/epoxy composites: Part I—Effect of machining parameters. Compos.-A Appl. Sci. Manuf.
**2010**, 41, 391–400. [Google Scholar] [CrossRef] - Ashrafi, S.A.; Sharif, S.; Farid, A.A.; Yahya, M.Y. Performance evaluation of carbide tools in drilling CFRP-Al stacks. J. Compos. Mater.
**2014**, 48, 2071–2084. [Google Scholar] [CrossRef] - Wang, C.-Y.; Chen, Y.-H.; An, Q.-L.; Cai, X.-J.; Ming, W.-W.; Chen, M. Drilling temperature and hole quality in drilling of CFRP/aluminum stacks using diamond coated drill. Int. J. Precis. Eng.
**2015**, 16, 1689–1697. [Google Scholar] [CrossRef] - Heberger, L.; Kirsch, B.; Donhauser, T.; Nissle, S.; Gurka, M.; Schmeer, S.; Aurich, J.C. Influence of the quality of rivet holes in carbon-fiber-reinforced-polymer (CFRP) on the connection stability. Procedia Manuf.
**2016**, 6, 140–147. [Google Scholar] [CrossRef] [Green Version] - Lambiase, F.; Durante, M. Mechanical behavior of punched holes produced on thin glass fiber reinforced plastic laminates. Compos. Struct.
**2017**, 173, 25–34. [Google Scholar] [CrossRef] - Giasin, K. Machining Fibre Metal Laminates and Al2024-T3 Aluminium Alloy. Ph.D. Thesis, The University of Sheffield, Sheffield, UK, 2016. [Google Scholar]
- Biermann, D.; Heilmann, M. Burr minimization strategies in machining operations. In Burrs—Analysis, Control and Removal; Aurich, J., Dornfeld, D., Eds.; Springer: Berlin/Heidelberg, Germany, 2010; pp. 13–20. [Google Scholar] [CrossRef]
- Pilny, B.L. High speed drilling of aluminium plates. In Faculty of Mechanical Engineering—Instituteof Manufacturing Technology; BRNO Univresity of Technology: Brno-střed, Czech Republic, 2011; p. 151. [Google Scholar]
- Ramulu, M.; Branson, T.; Kim, D. A study on the drilling of composite and titanium stacks. Compos. Struct.
**2001**, 54, 67–77. [Google Scholar] [CrossRef] - Ko, S.-L.; Lee, J.-K. Analysis of burr formation in drilling with a new-concept drill. J. Mater. Process. Technol.
**2001**, 113, 392–398. [Google Scholar] [CrossRef] - Sakurai, K.; Adachi, K.; Kawai, G.; Sawai, T.; Ogawa, K. High feed rate drilling of aluminum alloy. Mater. Sci. Forum
**2000**, 331–337, 625–630. [Google Scholar] [CrossRef] - Hamade, R.F.; Ismail, F. A case for aggressive drilling of aluminum. J. Mater. Process. Technol.
**2005**, 166, 86–97. [Google Scholar] [CrossRef] - Soo, S.L.; Abdelhafeez, A.M.; Li, M.; Hood, R.; Lim, C.M. The drilling of carbon fibre composite-aluminium stacks and its effect on hole quality and integrity. Proc. Inst. Mech. Eng. B Manag. Eng. Manufact.
**2019**, 233, 1323–1331. [Google Scholar] [CrossRef] - Mahdi, A.; Turki, Y.; Habak, M.; Salem, M.; Bouaziz, Z. Experimental study of thrust force and surface quality when drilling hybrid stacks. Int. J. Adv. Manuf. Technol.
**2020**, 107, 3981–3994. [Google Scholar] [CrossRef] - Benezech, L.; Landon, Y.; Rubio, W. Study of manufacturing defects and tool geometry optimisation for multi-material stack drilling. Adv. Mat. Res.
**2011**, 423, 1–11. [Google Scholar] [CrossRef] - Kuo, C.L.; Soo, S.L.; Aspinwall, D.K.; Thomas, W.; Bradley, S.; Pearson, D.; M’Saoubi, R.; Leahy, W. The effect of cutting speed and feed rate on hole surface integrity in single-shot drilling of metallic-composite stacks. Procedia CIRP
**2014**, 13, 405–410. [Google Scholar] [CrossRef] [Green Version] - Hassan, M.H.; Abdullah, J.; Franz, G. Multi-objective optimization in single-shot drilling of CFRP/Al stacks using customized twist drill. Materials
**2022**, 15, 1981. [Google Scholar] [CrossRef] - Rivero, A.; Aramendi, G.; Herranz, S.; Lopez de Lacalle, L.N. An experimental investigation of the effect of coatings and cutting parameters on the dry drilling performance of aluminium alloys. Int. J. Adv. Manuf. Technol.
**2006**, 28, 1–11. [Google Scholar] [CrossRef] - Kim, D.; Ramulu, M. Drilling process optimization for graphite/bismaleimide–titanium alloy stacks. Compos. Struct.
**2004**, 63, 101–114. [Google Scholar] [CrossRef] - Freddi, A.; Salmon, M. Introduction to the Taguchi method. In Design Principles and Methodologies; Springer Tracts in Mechanical Engineering; Springer: Cham, Switzerland, 2019; pp. 159–180. [Google Scholar] [CrossRef]
- Behera, S.K.; Meena, H.; Chakraborty, S.; Meikap, B.C. Application of response surface methodology (RSM) for Optimization of leaching parameters for ash reduction from low-grade coal. Int. J. Min. Sci. Technol.
**2018**, 28, 621–629. [Google Scholar] [CrossRef] - Humbird, D.; Fei, Q. Chapter 20—Scale-Up Considerations for Biofuels. In Biotechnologogy for Biofuel Production and Optimization; Eckert, C.A., Trinh, C.T., Eds.; Elsevier Science: Amsterdam, The Netherlands, 2016; pp. 513–537. [Google Scholar] [CrossRef]
- Şenaras, A.E. Parameter Optimization Using the Surface Response Technique in Automated Guided Vehicles. In Sustainable Engineering Products and Manufacturing Technologies; Elsevier: Amsterdam, The Netherlands, 2019; pp. 187–197. [Google Scholar] [CrossRef]
- Sivaiah, P.; Chakradhar, D. Analysis and modeling of cryogenic turning operation using response surface methodology. Silicon
**2018**, 10, 2751–2768. [Google Scholar] [CrossRef] - Senthilkumar, M.; Prabukarthi, A.; Krishnaraj, V. Machining of CFRP/Ti6Al4V stacks under minimal quantity lubricating condition. J. Mech. Sci. Technol.
**2018**, 32, 3787–3796. [Google Scholar] [CrossRef] - Tsao, C.C.; Hocheng, H. Parametric study on thrust force of core drill. J. Mater. Process. Technol.
**2007**, 192–193, 37–40. [Google Scholar] [CrossRef] - Cai, B.-Y.; Ge, J.-P.; Ling, H.-Z.; Cheng, K.-K.; Ping, W.-X. Statistical optimization of dilute sulfuric acid pretreatment of corncob for xylose recovery and ethanol production. Biomass Bioenergy
**2012**, 36, 250–257. [Google Scholar] [CrossRef] - Guan, X.; Yao, H. Optimization of Viscozyme L-assisted extraction of oat bran protein using response surface methodology. Food Chem.
**2008**, 106, 345–351. [Google Scholar] [CrossRef] - Sui, S.; Song, G.; Sun, C.; Zhu, Z.; Guo, K.; Sun, J. Experimental investigation on the performance of novel double cone integrated tool in one-shot drilling of metal stacks. Int. J. Adv. Manuf. Technol.
**2020**, 109, 523–534. [Google Scholar] [CrossRef] - Gaitonde, V.N.; Karnik, S.R.; Achyutha, B.T.; Siddeswarappa, B. Taguchi optimization in drilling of AISI 316L stainless steel to minimize burr size using multi-performance objective based on membership function. J. Mater. Process. Technol.
**2008**, 202, 374–379. [Google Scholar] [CrossRef] - Hassan, M.H.; Abdullah, J.; Mahmud, A.S.; Supran, A. Burr height as quality indicator in single shot drilling of stacked CFRP/Aluminium composite. Key Eng. Mater.
**2017**, 744, 327–331. [Google Scholar] [CrossRef] - Siegel, A.F. (Ed.) Chapter 11—Correlation and Regression: Measuring and predicting relationships. In Practical Business Statistics, 7th ed.; Academic Press: Cambridge, MA, USA, 2016; pp. 299–354. [Google Scholar]
- Giasin, K.; Hawxwell, J.; Sinke, J.; Dhakal, H.; Köklü, U.; Brousseau, E. The effect of cutting tool coating on the form and dimensional errors of machined holes in GLARE fibre metal laminates. Int. J. Adv. Manuf. Technol.
**2020**, 107, 2817–2832. [Google Scholar] [CrossRef] [Green Version] - Fernández-Pérez, J.; Cantero, J.L.; Díaz-Álvarez, J.; Miguélez, M.H. Hybrid composite-metal stack drilling with different minimum quantity lubrication levels. Materials
**2019**, 12, 448. [Google Scholar] [CrossRef] - Hassan, M.H.; Abdullah, J.; Mahmud, A.S.; Supran, A. Effect of drill geometry and drilling parameters on the formation of adhesion layer in drilling composite-metal stack- up material. J. Mech. Eng.
**2018**, 5, 90–98. [Google Scholar]

**Figure 1.**Location of grinding wheel for the tool fabrication: (

**a**) fluting wheel, (

**b**) gashing wheel, (

**c**) clearance/point angle wheel.

**Figure 2.**Observation of (

**a**) exit (

**b**) close-up view of delamination at the exit using an Alicona InfinateFocus optical microscope.

**Figure 3.**Type of burr formation observed under the Alicona Infinite Focus optical microscope: (

**a**) uniform burr formation, (

**b**) rolled-back burr formation.

**Figure 4.**Measurement process of maximum burr formation: (

**a**) 3D observation of burr formation, (

**b**) maximum burr formation identification, and (

**c**) maximum burr formation measurement.

**Figure 5.**(

**a**) Position of probe during measurement of each laminate CFRP and Al7075-T6 (

**b**) point of contact for hole diameter measurement and hole circularity measurement.

**Figure 10.**CFRP exit delamination analysis for (

**a**) actual and predicted plot and (

**b**) predicted and residual plot.

**Figure 11.**(

**a**) Perturbation plot and (

**b**) 3D Response surface for exit delamination of CFRP. A, chisel edge angle; B, primary clearance angle; C, point angle.

**Figure 14.**Space for chip flow showing two geometries with same primary clearance angle and point angle where (

**a**) drill with 30° of chisel edge angle (

**b**) drill with 45° of chisel edge angle.

**Figure 15.**Analysis of maximum burr formation on Al7075−T6 for (

**a**) actual and predicted plot and (

**b**) predicted and residual plot.

**Figure 16.**(

**a**) Perturbation plot and (

**b**) 3D response surface for exit burr height at Al7075−T6. A, chisel edge angle; B, primary clearance angle; C, point angle.

Properties | Tensile Strength [MPa] | Elasticity Module [GPa] | Elongation [%] | Flexural Strength [MPa] | Density [g/cm^{3}] | Thickness [mm] |
---|---|---|---|---|---|---|

CFRP | 2723 | 164 | 1.62 | 1500 | 1.601 | 3.587 |

Al7075-T6 | 558 | 71.7 | 13 | - | 2.597 | 3.317 |

**Table 2.**Experimental factors at different levels of chisel edge angle (A), primary clearance angle (B), and point angle (C).

Level | Chisel Edge Angle [°] | Primary Clearance Angle [°] | Point Angle [°] | Spindle Speed [rev/min] | Feed Rate, [mm/rev] |
---|---|---|---|---|---|

Minimum | 30 | 6 | 130 | ||

Midpoint | 37.5 | 7 | 135 | 2600 | 0.05 |

Maximum | 45 | 8 | 140 |

Input Variables | Lower Level (−1) | Coded Level (0) | Higher Level (+1) |
---|---|---|---|

Chisel Edge Angle [A°] | 30 | 37.5 | 45 |

Primary Clearance Angle [B°] | 6 | 7 | 8 |

Point Angle [C°] | 130 | 135 | 140 |

Source | Sum of Squares | df | Mean Square | F Value | p-Value | PC (%) | |
---|---|---|---|---|---|---|---|

Model (Y_{1}) | 0.0032506 | 5 | 0.0004911 | 8.66 | 0.0015 | Significant | |

Chisel edge angle (A) | 0.001695 | 1 | 0.001695 | 29.87 | 0.0002 | 43.7% | |

Primary clearance angle (B) | 0.0004053 | 1 | 0.0004053 | 7.15 | 0.0217 | 10.5% | |

Point angle (C) | 3.393 × 10^{−5} | 1 | 3.393 × 10^{−5} | 0.6 | 0.4556 | 0.9% | |

BC | 0.0003246 | 1 | 0.0003246 | 5.72 | 0.0357 | 8.4% | |

B^{2} | 0.0007918 | 1 | 0.0007918 | 13.96 | 0.0033 | 20.4% | |

Residual | 0.000624 | 11 | 5.673 × 10^{−5} | 16.1% | |||

Lack of Fit | 0.0001586 | 6 | 2.643 × 10^{−5} | 0.28 | 0.9214 | not significant | |

Pure Error | 0.0004654 | 5 | 9.308 × 10^{−5} | ||||

Cor Total | 0.0038746 | 16 | |||||

Std. Dev. | 7.53 × 10^{−3} | R^{2} | 0.7974 | ||||

Mean | 1.04 | Adj R^{2} | 0.7053 | ||||

C.V. % | 0.73 | Pred R^{2} | 0.6885 | ||||

PRESS | 9.59 × 10^{−4} | Adeq Precision | 10.446 |

Source | Sum of Squares | df | Mean Square | F Value | p-Value Prob > F | PC (%) | |
---|---|---|---|---|---|---|---|

Model (Y_{2}) | 0.55648 | 6 | 0.08024 | 9.71841 | 0.0004 | significant | |

Chisel edge angle (A) | 0.16492 | 1 | 0.16492 | 19.97565 | 0.0006 | 24.8% | |

Primary clearance angle (B) | 0.0077 | 1 | 0.0077 | 0.93271 | 0.3518 | 1.2% | |

Point angle (C) | 0.0569 | 1 | 0.0569 | 6.89196 | 0.021 | 8.6% | |

BC | 0.08154 | 1 | 0.08154 | 9.87583 | 0.0078 | 12.3% | |

A^{2} | 0.07645 | 1 | 0.07645 | 9.25995 | 0.0094 | 11.5% | |

B^{2} | 0.16897 | 1 | 0.16897 | 20.46549 | 0.0006 | 25.5% | |

Residual | 0.10733 | 13 | 0.00826 | 16.2% | |||

Lack of Fit | 0.05707 | 8 | 0.00713 | 0.70959 | 0.6832 | not significant | |

Pure Error | 0.05026 | 5 | 0.01005 | ||||

Cor Total | 0.66381 | 19 | |||||

Std. Dev. | 0.09086 | R^{2} | 0.817699 | ||||

Mean | 2.02779 | Adj R^{2} | 0.733559 | ||||

C.V. % | 4.48093 | Pred R^{2} | 0.601704 | ||||

PRESS | 0.2345 | Adeq Precision | 12.8502 |

Contraints | |||
---|---|---|---|

Factor/Response | Goal | Lower Limit | Upper Limit |

Chisel edge angle (A) | Within range | 30° | 45° |

Primary clearance angle (B) | Within range | 6° | 8° |

Point angle (C) | Within range | 130° | 140° |

Burr Height (H_{bmax}) | Minimize | 40.2 µm | 271.2 µm |

Delamination (F_{d-exit}) | Minimize | 1.0046 | 1.0196 |

**Table 7.**Prediction of the optimized model of twist drill bit for edge defect analysis when drilling CFRP/Al7075-T6 stack-up material.

Responses (Y) | Y_{1}, [µm] | Y_{2}, [µm] |
---|---|---|

Model response | 1.00528 | 82.2307 |

Experimental | 1.00635 | 74.234 |

Error (%) | 0.11 | 9.72 |

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. |

© 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Joy Mathavan, J.; Hassan, M.H.; Xu, J.; Franz, G.
Hole Quality Observation in Single-Shot Drilling of CFRP/Al7075-T6 Composite Metal Stacks Using Customized Twist Drill Design. *J. Compos. Sci.* **2022**, *6*, 378.
https://doi.org/10.3390/jcs6120378

**AMA Style**

Joy Mathavan J, Hassan MH, Xu J, Franz G.
Hole Quality Observation in Single-Shot Drilling of CFRP/Al7075-T6 Composite Metal Stacks Using Customized Twist Drill Design. *Journal of Composites Science*. 2022; 6(12):378.
https://doi.org/10.3390/jcs6120378

**Chicago/Turabian Style**

Joy Mathavan, Jebaratnam, Muhammad Hafiz Hassan, Jinyang Xu, and Gérald Franz.
2022. "Hole Quality Observation in Single-Shot Drilling of CFRP/Al7075-T6 Composite Metal Stacks Using Customized Twist Drill Design" *Journal of Composites Science* 6, no. 12: 378.
https://doi.org/10.3390/jcs6120378