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Drill bit with a helical groove edge for clean drilling of carbon fiber-reinforced plastic
Journal of Materials Processing Tech. 274 (2019) 116291
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Drill bit with a helical groove edge for clean drilling of carbon fiberreinforced plastic ⁎
Zhen Yua, Changping Lib, , Rendi Kurniawana, Ki Moon Parka, Tae Jo Koa, a b
T
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School of Mechanical Engineering, Yeungnam University, 214-1 Dae-dong, Gyeongsan-si, Gyeongsangbuk-do, 712-749, South Korea College of Mechanical and Electrical Engineering, Hunan University of Science and Technology, Xiangtan, 411201, China
A R T I C LE I N FO
A B S T R A C T
Associate Editor: E. Budak
This paper presents a new drill bit with a helical groove edge for drilling carbon fiber reinforced plastic (CFRP). The drill has double point angles, a helical flute, and a helical groove edge away from the drill point angle. The helical groove edge has the same rotational direction as the flute angle direction of the drill bit. With the help of the additional cutting action of the helical groove edge, the drilled holes showed eliminated burrs, reduced delamination, and decreased tearing. An analysis was conducted for the burr removal mechanism and thrust force model acting on the chip area. The experimental verification assures that the drill can produce good quality of the hole’s surface, entrance, and exit, even after long service life.
Keywords: CFRP Drilling Burr Delamination Helical groove
1. Introduction Carbon fiber-reinforced plastic (CFRP) is utilized for its unique properties in various applications, which include aerospace, automotive, and sports equipment. Bolts are generally used for joining composites in the aviation industry. Therefore, drilling holes in CFRP is inevitable for the assembly process. There are various processes for making holes in CFRP composites, such as laser machining (Voisey et al., 2006 or Herzog et al., 2008) or water jet cutting (Azmir and Ahsan, 2009 or Shanmugam et al., 2008). However, conventional drilling is still the most common processing technique (López de Lacalle et al., 2011; Pujana et al., 2009). Since the bonding strength between the fiber and the resin is lower than the fiber strength, the fiber is more difficult to cut than the resin. This causes some problems around the cutting area. Sorrentino et al. (2017, 2018) and An et al. (2018) investigated the damage on the surface around the hole and inside the hole wall surfaces (burrs, tearing, delamination, spalling, and matrix thermal damage). According to Voß et al. (2016), these problems seriously affect the mechanical properties and service life of CFRP. Delamination and burrs usually occur at the entrance and exit of the hole, but the mechanisms are greatly different. Gaugel et al. (2016) reported that the delamination that occurs at the hole entrance is peel-up delamination, while push-out delamination occurs at the hole exit. Fig. 1 shows the burr formation at the entrance and exit of CFRP composites while drilling, which was obtained with a high-speed camera (Phantom Miro C110). As shown in
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the photos, the main problem is the entrance and exit burr defects on the CFRP. If there are some defects around the hole, an adequate remedy is to remove the burr. Many different kinds of processing methods have been reported for deburring in CFRP drilling. Recently, Kurniawan et al. (2017) investigated the deburring of CFRP using ultrasonic electrical discharged machining (USEDM). The results showed that a high burr removal rate (BRR) and deburring quality can be achieved. Park et al. (2019) presented an evaluation of hybrid cryogenic processes for deburring CFRP. A frozen ice layer provided support to the CFRP exit hole, and their method was effective for removing burrs. However, the expensive and complex drilling processes are not desirable for mass production. Therefore, a new drill bit that does not generate burrs or removes burrs while drilling is necessary. Generally, the drill bit geometry has a significant effect on the tool wear and delamination. Moreover, it can suppress or reduce the exit burr. Therefore, many researchers have studied the tool geometry for drilling of CFRP, as reported by Abrão et al. (2007). The suggested drill types mainly include compound twist drills (Kumaran et al., 2017), candle stick drills (Tsao and Hocheng, 2005), double point drills (Karpat et al., 2012); step drills (Feito et al., 2018) and new structure drill (Su et al., 2018, 2019). Tsao and Hocheng (2011) proposed compound core drills for secondary machining of the CFRP. They generate low thrust force, low delamination, low chip blockage, and a high chip removal rate. Jia et al. (2016) studied a novel intermittent saw-tooth drill with a
Corresponding authors. E-mail addresses: [email protected] (C. Li), [email protected] (T.J. Ko).
https://doi.org/10.1016/j.jmatprotec.2019.116291 Received 1 March 2019; Received in revised form 25 May 2019; Accepted 1 July 2019 Available online 05 July 2019 0924-0136/ © 2019 Elsevier B.V. All rights reserved.
Journal of Materials Processing Tech. 274 (2019) 116291
Z. Yu, et al.
Fig. 1. Burr production while drilling.
Fig. 2. Definition of fiber cutting angle: (a) cutting edge location in CFRP drilling, (b) relation between the cutting edge and fiber orientation.
delamination. The helical grooved cutting edge is used for removing generated burrs during in-feeding and retracting movements of the drill. Furthermore, the tool geometry is relatively easier to fabricate than the previous drill tool design (Jia et al., 2016; Karpat et al., 2012). Before investigating the performance, the burr generation and removal mechanism of this new tool is discussed. Also, the model of the thrust force acting on this tool is explained. The helical groove drill can produce a good quality of the hole’s surface, entrance, and exit, even lower wear when compared to a general drill.
counter-rotating blade. The cutting lip of the structure can reverse the cutting direction from downward to upward, which reduces the damage to the exit of the drilled hole. Butler-Smith et al. (2015) presented the cutting mechanism of diamond micro-core drills. The cleanliness of fiber breaking was improved, and the tendency of material layering was reduced. Like in these studies, modification of the drill bit geometry is a better strategy to minimize or prevent the occurrence of burrs. Regarding this purpose, many researchers have proposed appropriate geometrical drill bits to reduce or eliminate uncut fiber or the delamination of CFRP. However, satisfied results with respect to removing burrs, minimizing delamination and tearing has not been proposed. Therefore it is necessary for developing a new geometry of the drill bit to achieve excellent burr removal. In this research, a new type of drill geometry is proposed. It consists of double point angles and an additional helical grooved cutting edge, which has the same direction as the drill flute angle. The double point angle is used to reduce the thrust force, which is related to the
2. Drilling mechanism of CFRP 2.1. Relation between cutting edge and fiber orientation As shown in Fig. 2, the cutting force and generated burrs are affected by the relation of the fiber orientation and cutting edge location. An et al. (2015) studied the effects of fiber orientation angle on cutting 2
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Fig. 3. The morphological change of burrs in the drilling during.
A change in the angle means that the fiber lacks subsequent radial support. When the matrix stress is less than the thrust force, the fibers are torn and change direction. This makes it easy to cause serious damage around the hole surface or the inner hole wall, such as tearing, burrs, and delamination.
force characteristics of UD-CFRP laminates using the orthogonal cutting test. In the forward fiber direction of α < 90°, the main cutting force (Fc) and radial thrust force (Fz) linearly increase as the angle α increase. On the other hand, Fc and Fz are relatively low when in reverse fiber direction of α is large than 90° (α > 90°). Wang et al. (2017) studied the effect of the fiber cutting angle on burr formation. The results showed that when a drill is used for drilling CFRP composites, burrs are easily formed within the range of fiber cutting angles of 0° < α < 90°. Fig. 2(a) shows the angle of the cutting edge location during drilling, in which the multi-directional fabric CFRP composite has an orientation of each layer between 0° and 90° with respect to the fiber cross weave. This affects the delamination of the inlet and outlet of the hole and the formation of burrs. The fiber cutting angle (α) is measured between the cutting direction of the drill bit and the fiber angle. The angle (α) changes are given by the following equation:
αn = 180° − [αn − 1 + βn − 1 + (−1)n*90°]
2.2. Burr analysis of general drilling During CFRP drilling, there are a varying stress conditions at the hole entrance and exit. At the entrance, the bonding strength of the epoxy between the fiber layers is the only force that resists delamination. Ferret et al. (2000) found that the local peel force is greater than the allowable peeling force of the fiber layer, so the thrust force exceeds the interlayer strength and causes delamination. At the exit of the hole, the CFRP is subjected to the maximum thrust force before the main cutting edge of the drill bit enters the final layer. If the thrust force exceeds the bonding stress between the fibers layers, an initial delamination is formed. If there is a lack of support on the exit side of the hole, the downward thrust warps and tears the fiber outward. In this case, the fibers cannot be cut anymore during the subsequent edge trimming processes (Hintze and Hartmann, 2013). Fig. 3 shows the deburring process by the cutting edge of a general drill bit with a complex three-dimensional cutting process. In the model, the feed direction of the tool is perpendicular to the workpiece
(1)
where n is the changes between the cutting edge and fiber angle, and β is the rotation angle of the fiber. The damage of the hole inner surface is affected by the fiber orientation and cutting direction. As shown in Fig. 2(b), when the cutting edge cuts the joint (e.g., points a, b, c, and d) with fiber orientations of 0° and 90°, the instantaneous large cutting angle (α) changes the inner wall’s contour shape and surface roughness. 3
Journal of Materials Processing Tech. 274 (2019) 116291
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Fig. 4. Detailed parameters of helical groove drill bit.
flute angle. Therefore, there are additional lower and upper helical groove cutting edges as well in the drill body. The detailed parameters of the drill bit are shown in Fig. 4.
surface. After the drill bit penetrates the workpiece, the burr at the exit of the hole is subjected to three forces: the cutting force Fc, the radial cutting force Fcr, and the thrust Fz. There are three cases of burrs during drilling: ❶ the original position of the burr, ❷ the burr moving along the edge of the drill bit, and ❸ the burr on the side of the drill bit. When the drill bit moves downward, the drill bit’s edge cuts the burr. The burr is subjected to the cutting force Fc1, the radial force Fcr1, and the thrust force Fz1 at the edge of the drill bit, and the resultant downward force is F1. If the cutting force of Fc2 is greater than the lateral bending force at the burr break position, the burr bends toward Fcr2. The effective force of the cutting force Fc2 gradually decreases, and the burr moves from position ❷ to position ❸. This causes the burr to deform vertically due to the weak support of the drill bit. If the cutting force Fc3 for the burr is zero at the cutting edge of the drill bit, the burr is only subjected to the radial shear force Fcr3. Therefore, it is downward due to thrust force Fz3. In this process, the number of fiber breaks in the burr root is increased. Therefore, the stress at the end of the burr decreases with the decrease of the bonding force between fibers.
2.3.2. Burr removal mechanism in the helical groove drill The force model and the deburring mechanism are shown in Fig. 5. Fig. 5(a) shows the analysis of the cutting process of the margin edge of the helical groove drill during downward feed. Before the helical groove edge reaches the hole exit, the helical groove drill bit is the same as a general drill bit that cuts burrs by the margin edge. The burr is tilted up along the edge of the drill bit and reaches the lower cutting edge of the helical groove, and then the lower cutting edge of the helical groove begins to remove the uncut burrs. In this case, the cutting operation of the groove can be described by a three-dimensional cutting model, as shown in Fig. 5(b). The cutting force F3 is the resultant force of the groove’s lower-edge shear force Fc3 and the radial force Fcr3. Consequently, the slot actually pushes up to cut the burr. Therefore, the wall of the machined hole becomes a support to cut the uncut burrs, which must be squeezed into the hole for shear. Fig. 5(c) shows the deburring process at the lower edge of the helical groove when the tool returns. The cutting force has a large tangential component on the helical groove’s lower edge, which helps to cut the fiber since the helix angle of the groove is large. A scissor-like structure is formed between the helical groove’s lower edge and the wall of the machined hole, which can cut off burrs very easily. The grooves shear the burrs at the drill exit throughout the drilling process. Consequently, the proposed drill bit can effectively remove exit burrs.
2.3. Analysis of helical groove drill 2.3.1. Design of new drill The reason why general drill bits cannot effectively cut burrs is that there is no effective supporting force at the bottom fiber layer. Therefore, a helical grooved cutting edge was added to a double point angle drill bit, which provides an upward vertical cutting force to remove exit burrs. The drill is shown in Fig. 4. It has diameter of 6 mm with 6 grooves, point angle 1 of 90°, and point angle 2 of 80°. The groove has an axial width of 1 mm and radial height of 1.5 mm. The angles between the cylindrical surface and helical groove’s upper edge and lower edge are 50° and 105°, respectively. The proposed drill bit has two point angles to reduce the thrust force, which has already been investigated by other researchers. However, the drill has additional helical grooves that have the same rotational direction as the
2.3.3. Burr removal procedure The process of drilling CFRP with the new drill bit can be divided into 8 steps, as shown in Fig. 6. In step 1, the main cutting edge enters and begins to cut the CFRP. Moreover, peel-up delamination occurs at the hole entry. In step 2, the drill bit completely enters the CFRP, and the burrs produced by the peel up and cannot be removed effectively. In step 3, the drill bit moves down near the final layers, delamination 4
Journal of Materials Processing Tech. 274 (2019) 116291
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Fig. 5. Deburring mechanism on helical groove drill bit: (a) the process of the drill margin edge cutting a burr when drilling downward; (b) the lower edge of the groove removing the burr when drilling downward; (c) the process of the helical groove’s lower edge removing the burr when drilling upward.
If Vc − Vf > 0, Cutting of burrElse, Nocuttingofburr
begins to appear because there is no corresponding support force at the hole exit, and the thrust force is greater than the bonding force between the fiber layers. In step 4, the drill bit continues to move downward, and the entrance burr is held by the helical groove. The burr is bent down under the action of the thrust and radial cutting force and is removed by the helical groove’s upper edge. In step 5, the drill bit continues to move down, the main cutting edge cannot effectively cut off the delaminated fibers at the exit, and burrs are generated. In step 6, the exit burr is drawn into the helical groove. Because the direction of the tool rotation is the same as that of the helical groove, the groove makes the burr move upward, and the burr is removed by the helical groove’s lower edge. The cutting situation of the burr removal at stage 6 is expressed as follows when the drill bit moves downward (feed rate Vf ↓):
where Vc is the cutting speed of the groove’s lower edge defined by vc = πdntanθ , Vf is the feed rate, d is the diameter, n is the spindle speed, and θ is the helical groove angle. In step 7, the helical groove’s lower edge cuts the exit burr again when the drill bit returns. The cutting situation of the burr removal at stage 7 is expressed as follows when the drill bit moves upward (feed rate Vf ↑):
Always Vc + Vf > 0, and cutting of burr In step 8, the drilling process finishes. 2.3.4. Thrust force analysis Karpat et al. (2012) analyzed the thrust force acting on a double5
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Fig. 6. Deburring mechanism of CFRP using the new drill bit design. Table 1 Material properties of CFRP. Longitudinal (σ1t) and traverse tensile strength (σ2t) Longitudinal (σ1c) and traverse compression strength (σ2c) Young's modulus Shear modulus Poisson’s ratio Density Rockwell hardness
840 MPa 570 MPa 61.5 GPa 3.7 GPa 0.3 155 kg/m3 70–75 HRB
Fig. 7. (a) Thrust forces acting on the drill bit, (b) deburring force.
Fig. 9. Experimental setup.
point drill bit with respect to the cutting edge, as shown in Fig.7(a). The total thrust force acting on the drill bit is the sum of the thrusts acting on the three different edges. The thrust force in the primary region (l1) is the sum of the chiseling force (Fch) and the primary edge force (Fc1). The secondary edge force (Fc2) is from the secondary region (l2). The thrust cutting force of each part is determined by Eqs. (2) and (3). Therefore, the point angles of the drill bit, θ1 and θ2, and the primary (l1) and secondary (l2) cutting edge lengths are important design
Fig. 8. Illustration of the weave orientation of CFRP.
6
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Fig. 10. Picture of the helical groove on the drill. Table 2 Drill and drilling conditions of fabric woven CFRP. Acronym
Diameter (mm)
Drill edge angles 1, 2 (degree)
Flute angle
Spindle speed (rpm)
Feed rate (mm/rev)
Helical groove drill General drill
6 6
90, 80 90, 0
20 20
800, 1500, 3000 800, 1500, 3000
0.025, 0.05, 0.1 0.025, 0.05, 0.1
Fig. 11. The delamination and burrs of the exit: (a) delamination by a general drill; (b) burr inner surface obtained with a general drill; (c) delamination by helical groove drill; (d) burr at inner surface obtained with a helical groove drill.
force is subjected to the radial cutting force and upward cutting force when the helical lower edge removes a burr. The deburring force Fdb is formulated below:
variables.
Fl1 = Fch + Fc1
(2)
Fl2 = Fc2
(3)
Fz = Fch + Fc1 + Fc 2
(4)
Fdb = Fdz + Fdc
(5)
where Fdz is the radial force of the helical groove, and Fdc is its upward cutting force.
Fz is the thrust force, Fch is the chisel force, Fl1 is the force of primary region, Fl2 is the force of secondary region, Fc1 is the main edge force, and Fc2 is the secondary edge force. As shown in Fig. 7(b), the deburring 7
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Table 3 Burr morphology at the entrance and exit of the hole (spindle speed 800 rpm, feed rate 0.025 mm/rev).
3. Experimental conditions In this study, a multi-directional fabric CFRP composite with a fiber volume fraction of 0.6 was used. The angle of the carbon fibers and the orientation of each layer are 0° and 90°, respectively, as shown in Fig. 8. The mechanical characteristics of the CFRP composite are shown in Table 1. The Korea Carbon Convergence Technology Institute provided the CFRP composites. The manufacturing includes compression molding followed by curing at elevated temperatures. Then, the fabricated composite was cut to obtain the desired dimensions of 90 mm × 90 mm × 5 mm. An experiment was carried out using a CNC machine (DMC model SS-600), as shown in Fig. 9. A general drill and helical groove drill were used in the experiment, and both drills had a diameter of 6 mm. Both drills were prepared by SJ Tools (S. Korea). Fig. 10 shows the drill geometry information, and Table 2 shows the experimental conditions. The recommended optimal cutting conditions for drilling composites are high cutting speed with a low feed rate or low cutting speed with high feed rate, as demonstrated by Shyha et al. (2009, 2011). Thus, the cutting parameters in Table 2 were adopted for the drilling trials. The delamination factor experiment was repeated for three times of each cutting parameter for 100 drilling times. The cutting force measurement during drilling was repeated for five times for each cutting parameter.
Fig. 12. Wear of drill flank face: (a) general drill; (b) helical groove drill.
4. Results and discussion 4.1. Effect of burr removal and tearing reduction Fig. 11 shows SEM images of exit delamination and the cut burrs after drilling using a general drill bit and a helical groove drill bit. When drilling with the general drill bit, the exit burr is torn away and causes severe tearing damage, as shown in Fig. 11(a). Fig. 11(b) shows that the contour of the burr is irregular, which means that the tearing of the hole outlet increased. However, the tearing contour obtained using the new drill bit is almost identical to the contour of the edge of the hole, and the damage is much smaller, as shown in Fig. 11(c). In addition, the contour of the burr is very similar to the contour of the edge of the hole, as shown in Fig. 11(d). This shows that the burr is effectively removed by the helical groove’s lower edge, which reduces the generation of tearing.
Fig. 13. Diagram of measured diameters.
8
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Fig. 14. Delamination factor of hole exit under different processing conditions.
Fig. 15. Thrust force measurement in CFRP drilling process (spindle speed: 800 rpm; feed rate: 0.025 mm/rev).
Fig. 16. Thrust force of general drill and helical groove drill.
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Fig. 17. Inner surface profiles of hole: (a) the wall surface of the drilled hole, (b) the surface profiles of the hole surface measured for 9 areas along with the hole depth direction (left: proposed drill; right: general drill).
0.025 mm/rev, the delamination factor increases as the spindle speed increases. As the number of drill holes increases, the delamination factor increases. However, the delamination factor of the helical groove drill bit is always lower than that of the general drill bit. The reason is that the burr is removed by the edge of the helical groove’s lower edge, which reduces the effect of burrs on delamination. However, the general drill bits tore the burrs by the margin edge, which results in an increase in delamination. The delamination factor of the first hole is quite different from that of the 100th hole. However, with the increase of the feed speed and spindle speed, the difference is gradually reduced. Also, the delamination factor of the 100th hole drilled by the general drill bit is obviously larger than that of the proposed drill bit. In particular, the delamination factor of the 100th hole is almost unchanged compared with that of the first hole at 1500 rpm and 0.025 mm/rev.
An optical microscope (Sometech SV-55) was used to observe the delamination or burrs at the hole exit. After drilling 100 holes, the delamination at the entrance by the helical groove drillbits were small, and there are few burrs. However, the delamination area of the hole entrance increased significantly with the increase of the number of drill holes made by general drill bit. As shown in Table 3, for the exit of the first hole, both drills showed good drilling performance with almost no burrs. However, due to the wear of the general drill bit, the fiber cannot be cut effectively, and the burrs increase as the number of drill holes increases. In contrast, the hole rarely forms burrs even after drilling 100 holes in the case of the helical groove drill. Fig. 12 shows the wear of the flank face of the tool after drilling 100 holes. The wear on the flank face of the two drills is almost the same. However, the quality of holes processed with new drill bits is obviously better than that of the general drill. In addition, burrs and delamination are obviously reduced. This can be explained by the removal of the burr mainly depending on the subsequent cutting of the helical groove lower edge. As shown in Fig. 13, the maximum diameter of the delamination was measured with an optical microscope. Chen (1997) evaluated the delamination of the hole exit by the delamination factor, which is based on the maximum diameter or delamination of the damaged area. The equation is given as follows:
Fd =
Dmax Dnom
4.2. Thrust force variations In the drilling experiment, the thrust force was measured using a Kistler tool dynamometer (9256C2). Fig. 15 shows a comparison of the thrust force Fz of both drill bits at 800 rpm/min and 0.025 mm/rev. The total thrust force can be classified into four cutting steps for both tools: a→b: the main edge enters the workpiece; b→c: the secondary edge enters the workpiece; c→d: the cutting edge passes through the workpiece; and d→f : the cutting edge penetrates the workpiece. The entrance cutting region has two different forces. The primary region force (a→b) is denoted as Fl1, and the secondary region force (b→c) is denoted as Fl2, as shown in Fig. 7. When the main cutting edge moves from c to d, the drilling force is maximized. A sudden drop in thrust force occurs at the moment when the drill bit penetrates the CFRP from d to f. The point e is when the
(6)
The variations of the delamination factor (Fd) under different processing conditions are shown in Fig. 14. The comparing between the delamination factors obtained at a constant spindle speed of 800 rpm/ min, the delamination factor increases as the feed rate increases. Comparing the delamination factors obtained at a constant feed rate of 10
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2 Additional milling action is applied inside of the hole in the case of the helical groove edge, and the hole’s inner surface has good surface integrity.
helical groove drill bit contacts the surface of the workpiece. The thrust force is affected by the helical groove for a very short time and then gradually decreases. This is due to the helical groove edge, which is subjected to resistance caused by directional thermal expansion of the CFRP during the drilling process. From e to f, the radial cutting force and thrust force increase due to the action of the helical groove edge to cut the inner surface of the hole. At point g, the general drill returns to the original location, but the proposed drill is still drilling due to the additional helical groove edges. At the moment of point h, exit burrs are removed by the helical groove’s lower edge for the first time, as shown in Fig. 6 (step 6). The thrust Fdb at point h is the resultant force of the radial cutting force for the burr Fdc and the upward cutting force Fdz, as shown in Eq. (5). Point i is when the helical groove drill returns, the lower edge of the groove again mills the inner wall of the hole, and the burrs are removed for a second time. Upward thrust force is generated, resulting in a larger negative thrust force than that of the general drill. Also, as shown in Fig. 16, the change in thrust force under different processing parameters. It is clear that the effect of the feed rate is more pronounced than the effect of the spindle speed. Generally, the thrust force is observed to increase as the feed rate increases. The increase in spindle speed has the opposite effect when the feed per revolution is the same. It is observed that the thrust slightly decreases with the increase of the spindle speed, but the influence of the spindle speed is much lower than the influence of the feed rate. The thrust force of the helical groove drill bit is much smaller than that of the general drill bit. This means that the thrust force has a great influence on the occurrence of delamination or burrs.
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A comparative study on tool wear and laminate damage in drilling of carbon-fiber reinforced polymers (CFRP). Compos. Struct. 155, 173–183. Herzog, D., Jaeschke, P., Meier, O., Haferkamp, H., 2008. Investigations on the thermal effect caused by laser cutting with respect to static strength of CFRP. Int. J. Mach. Tools Manuf. 48, 1464–1473. Hintze, W., Hartmann, D., 2013. Modeling of delamination during milling of unidirectional CFRP. Procedia CIRP 8, 444–449. Jia, Z.Y., Fu, R., Niu, B., Qian, B.W., Bai, Y., Wang, F.J., 2016. Novel drill structure for damage reduction in drilling CFRP composites. Int. J. Mach. Tools Manuf. 110, 55–65. Karpat, Y., Değer, B., Bahtiyar, O., 2012. Drilling thick fabric woven CFRP laminates with double point angle drills. J. Mater. Process. Tech. 212, 2117–2127. Kumaran, S.T., Ko, T.J., Li, C.P., Yu, Z., Uthayakumar, M., 2017. Rotary ultrasonic machining of woven CFRP composite in a cryogenic environment. J. Alloys Compd. 698, 984–993. Kurniawan, R., Kumaran, S.T., Prabu, V.A., Yu, Z., Park, K.M., Kwak, Y.I., Ko, T.J., 2017. Measurement of burr removal rate and analysis of machining parameters in ultrasonic assisted dry EDM (US-EDM) for deburring drilled holes in CFRP composite. Measurement 1110, 98–115. López de Lacalle, L.N., Fernández, A., Olvera, D., Lamikiz, A., Olvera, D., Rodríguez, C., 2011. Monitoring deep twist drilling for a rapid manufacturing of light high strength parts. Mech. Syst. Sig. Process. 25, 2745–2752. Park, K.M., Kurniawan, R., Yu, Z., Ko, T.J., et al., 2019. Evaluation of a hybrid cryogenic deburring method to remove uncut fibers on carbon fiber-reinforced plastic composites. Int. Adv. Manuf. Technol. 101, 1509–1523. Pujana, J., Rivero, A., Celaya, A., López de Lacalle, L.N., 2009. Analysis of ultrasonicassisted drilling of Ti6Al4V. Int. J. Mach. Tools. Manuf. 49, 500–508. Shanmugam, D.K., Nguyen, T., Wang, J., 2008. A study of delamination on graphite/ epoxy composites in abrasive waterjet machining. Compos. Part. A-Appl. Sci. 39, 923–929. Shyha, I.S., Aspinwall, D.K., Soo, S.L., Bradley, S., 2009. Drill geometry and operating effects when cutting small diameter holes in CFRP. Int. J. Mach. Tools Manuf. 49 (12), 1008–1014. Shyha, I.S., Soo, S.L., Aspinwall, D.K., Bradley, S., Perry, R., Harden, P., et al., 2011. Hole quality assessment following drilling of metallic-composite stacks. Int. J. Mach. Tools. Manuf. 51 (7–8), 569–578. Sorrentino, L., Turchetta, S., Bellini, C., 2017. In process monitoring of cutting temperature during the drilling of FRP laminate. Compos. Struct. 168, 549–561. Sorrentino, L., Turchetta, S., Bellini, C., 2018. A new method to reduce delaminations during drilling of FRP laminates by feed rate control. Compos. Struct. 186, 154–164. Su, F., Zheng, L., Sun, F.J., Wang, Z.H., Deng, Z.H., Qiu, X.Y., 2018. Novel drill bit based on the step-control scheme for reducing the CFRP delamination. J. Mater. Process. Tech. 262, 157–167. Su, F., Deng, Z.H., Sun, F.J., Li, S.C., Wu, Q.P., Jiang, X.J., 2019. Comparative analyses of damages formation mechanisms for novel drills based on a new drill-induced
4.3. Surface roughness comparison Fig. 17 shows 3-D images of the hole’s inner surface taken by a noncontact camera (Nano system NV-2000). Fig. 17(b) shows the results of longitudinal arithmetic average roughness Ra. The measuring position was added in Fig. 17(a) by a red dot-line mark. There are pits on the inner surface and epoxy resin residues on the hole wall after drilling with the general drill bit. The surface roughness becomes worse as the spindle speed and feed rate increase. A high feed rate and spindle speed produce great impact force, which causes more fiber bundles to peel off from the hole wall and forms deeper pits. In addition, the cutting temperature increases with the increase of cutting speed. The high temperature and bending fracture of the material cause the fibers to deform, and the epoxy resin adheres to the inner wall of the hole. Also, a large deviation between the peak and valleys of surface roughness can be observed. On the other hand, the helical groove lower edge can cut the inner wall of the hole while drilling with the new drill bit. Therefore, fibers attached to the inner surface of the hole can be cut and removed. The surface profiles were compared, as shown in Fig. 17(b). The surface roughness is lowest when the spindle speed is 800 rpm and the feed rate is 0.0025 mm/rev for the helical groove drill bit. In summary, the helical groove drill can produce a better inner surface. 5. Conclusion This study investigated the delamination and burr removal of CFRP when drilling with a helical groove drill. The proposed drill has double point angles and an additional helical groove cutting edge with the same rotational direction as the flute angle. Experiments were carried out to verify the performance by comparison with a general drill. The following conclusions were obtained: 1 The lower cutting edge of the helical groove drill removes burrs generated at the hole entrance and exit effectively during the downward and upward feed. Due to this action, burrs were not identified at the hole exit even after more than 100 drilling operations. 11
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Voß, R., Henerichs, M., Rupp, S., Kuster, F., Wegener, K., 2016. Evaluation of bore exit quality for fibre reinforced plastics including delamination and uncut fibres. CIRP J. Manuf. Sci. Technol. 12, 56–66. Wang, F.J., Yin, J.W., Ma, J.W., Jia, Z.Y., Yang, F., Niu, B., 2017. Effects of cutting edge radius and fiber cutting angle on the cutting-induced surface damage in machining of unidirectional CFRP composite laminates. Int. J. Adv. Manuf. Technol. 91, 3107–3120.
damages analytical model. J. Mater. Process. Tech. 271, 111–125. Tsao, C.C., Hocheng, H., 2005. Effect of eccentricity of twist drill and candle stick drill on delamination in drilling composite materials. Int. J. Mach. Tools Manuf. 45, 125–130. Tsao, C.C., Hocheng, H., 2011. Evaluation of drilling parameters on thrust force in drilling carbon fiber reinforced plastic (CFRP) composite laminates using compound corespecial drills. Int. J. Mach. Tools Manuf. 51, 740–744. Voisey, K.T., Fouquet, S., Roy, D., Clyne, T.W., 2006. Fiber swelling during laser drilling of carbon fiber composites. Opt. Laser Eng. 44, 1185–1197.
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Journal of Materials Processing Tech. journal homepage: www.elsevier.com/locate/jmatprotec
Drill bit with a helical groove edge for clean drilling of carbon fiberreinforced plastic ⁎
Zhen Yua, Changping Lib, , Rendi Kurniawana, Ki Moon Parka, Tae Jo Koa, a b
T
⁎
School of Mechanical Engineering, Yeungnam University, 214-1 Dae-dong, Gyeongsan-si, Gyeongsangbuk-do, 712-749, South Korea College of Mechanical and Electrical Engineering, Hunan University of Science and Technology, Xiangtan, 411201, China
A R T I C LE I N FO
A B S T R A C T
Associate Editor: E. Budak
This paper presents a new drill bit with a helical groove edge for drilling carbon fiber reinforced plastic (CFRP). The drill has double point angles, a helical flute, and a helical groove edge away from the drill point angle. The helical groove edge has the same rotational direction as the flute angle direction of the drill bit. With the help of the additional cutting action of the helical groove edge, the drilled holes showed eliminated burrs, reduced delamination, and decreased tearing. An analysis was conducted for the burr removal mechanism and thrust force model acting on the chip area. The experimental verification assures that the drill can produce good quality of the hole’s surface, entrance, and exit, even after long service life.
Keywords: CFRP Drilling Burr Delamination Helical groove
1. Introduction Carbon fiber-reinforced plastic (CFRP) is utilized for its unique properties in various applications, which include aerospace, automotive, and sports equipment. Bolts are generally used for joining composites in the aviation industry. Therefore, drilling holes in CFRP is inevitable for the assembly process. There are various processes for making holes in CFRP composites, such as laser machining (Voisey et al., 2006 or Herzog et al., 2008) or water jet cutting (Azmir and Ahsan, 2009 or Shanmugam et al., 2008). However, conventional drilling is still the most common processing technique (López de Lacalle et al., 2011; Pujana et al., 2009). Since the bonding strength between the fiber and the resin is lower than the fiber strength, the fiber is more difficult to cut than the resin. This causes some problems around the cutting area. Sorrentino et al. (2017, 2018) and An et al. (2018) investigated the damage on the surface around the hole and inside the hole wall surfaces (burrs, tearing, delamination, spalling, and matrix thermal damage). According to Voß et al. (2016), these problems seriously affect the mechanical properties and service life of CFRP. Delamination and burrs usually occur at the entrance and exit of the hole, but the mechanisms are greatly different. Gaugel et al. (2016) reported that the delamination that occurs at the hole entrance is peel-up delamination, while push-out delamination occurs at the hole exit. Fig. 1 shows the burr formation at the entrance and exit of CFRP composites while drilling, which was obtained with a high-speed camera (Phantom Miro C110). As shown in
⁎
the photos, the main problem is the entrance and exit burr defects on the CFRP. If there are some defects around the hole, an adequate remedy is to remove the burr. Many different kinds of processing methods have been reported for deburring in CFRP drilling. Recently, Kurniawan et al. (2017) investigated the deburring of CFRP using ultrasonic electrical discharged machining (USEDM). The results showed that a high burr removal rate (BRR) and deburring quality can be achieved. Park et al. (2019) presented an evaluation of hybrid cryogenic processes for deburring CFRP. A frozen ice layer provided support to the CFRP exit hole, and their method was effective for removing burrs. However, the expensive and complex drilling processes are not desirable for mass production. Therefore, a new drill bit that does not generate burrs or removes burrs while drilling is necessary. Generally, the drill bit geometry has a significant effect on the tool wear and delamination. Moreover, it can suppress or reduce the exit burr. Therefore, many researchers have studied the tool geometry for drilling of CFRP, as reported by Abrão et al. (2007). The suggested drill types mainly include compound twist drills (Kumaran et al., 2017), candle stick drills (Tsao and Hocheng, 2005), double point drills (Karpat et al., 2012); step drills (Feito et al., 2018) and new structure drill (Su et al., 2018, 2019). Tsao and Hocheng (2011) proposed compound core drills for secondary machining of the CFRP. They generate low thrust force, low delamination, low chip blockage, and a high chip removal rate. Jia et al. (2016) studied a novel intermittent saw-tooth drill with a
Corresponding authors. E-mail addresses: [email protected] (C. Li), [email protected] (T.J. Ko).
https://doi.org/10.1016/j.jmatprotec.2019.116291 Received 1 March 2019; Received in revised form 25 May 2019; Accepted 1 July 2019 Available online 05 July 2019 0924-0136/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Burr production while drilling.
Fig. 2. Definition of fiber cutting angle: (a) cutting edge location in CFRP drilling, (b) relation between the cutting edge and fiber orientation.
delamination. The helical grooved cutting edge is used for removing generated burrs during in-feeding and retracting movements of the drill. Furthermore, the tool geometry is relatively easier to fabricate than the previous drill tool design (Jia et al., 2016; Karpat et al., 2012). Before investigating the performance, the burr generation and removal mechanism of this new tool is discussed. Also, the model of the thrust force acting on this tool is explained. The helical groove drill can produce a good quality of the hole’s surface, entrance, and exit, even lower wear when compared to a general drill.
counter-rotating blade. The cutting lip of the structure can reverse the cutting direction from downward to upward, which reduces the damage to the exit of the drilled hole. Butler-Smith et al. (2015) presented the cutting mechanism of diamond micro-core drills. The cleanliness of fiber breaking was improved, and the tendency of material layering was reduced. Like in these studies, modification of the drill bit geometry is a better strategy to minimize or prevent the occurrence of burrs. Regarding this purpose, many researchers have proposed appropriate geometrical drill bits to reduce or eliminate uncut fiber or the delamination of CFRP. However, satisfied results with respect to removing burrs, minimizing delamination and tearing has not been proposed. Therefore it is necessary for developing a new geometry of the drill bit to achieve excellent burr removal. In this research, a new type of drill geometry is proposed. It consists of double point angles and an additional helical grooved cutting edge, which has the same direction as the drill flute angle. The double point angle is used to reduce the thrust force, which is related to the
2. Drilling mechanism of CFRP 2.1. Relation between cutting edge and fiber orientation As shown in Fig. 2, the cutting force and generated burrs are affected by the relation of the fiber orientation and cutting edge location. An et al. (2015) studied the effects of fiber orientation angle on cutting 2
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Fig. 3. The morphological change of burrs in the drilling during.
A change in the angle means that the fiber lacks subsequent radial support. When the matrix stress is less than the thrust force, the fibers are torn and change direction. This makes it easy to cause serious damage around the hole surface or the inner hole wall, such as tearing, burrs, and delamination.
force characteristics of UD-CFRP laminates using the orthogonal cutting test. In the forward fiber direction of α < 90°, the main cutting force (Fc) and radial thrust force (Fz) linearly increase as the angle α increase. On the other hand, Fc and Fz are relatively low when in reverse fiber direction of α is large than 90° (α > 90°). Wang et al. (2017) studied the effect of the fiber cutting angle on burr formation. The results showed that when a drill is used for drilling CFRP composites, burrs are easily formed within the range of fiber cutting angles of 0° < α < 90°. Fig. 2(a) shows the angle of the cutting edge location during drilling, in which the multi-directional fabric CFRP composite has an orientation of each layer between 0° and 90° with respect to the fiber cross weave. This affects the delamination of the inlet and outlet of the hole and the formation of burrs. The fiber cutting angle (α) is measured between the cutting direction of the drill bit and the fiber angle. The angle (α) changes are given by the following equation:
αn = 180° − [αn − 1 + βn − 1 + (−1)n*90°]
2.2. Burr analysis of general drilling During CFRP drilling, there are a varying stress conditions at the hole entrance and exit. At the entrance, the bonding strength of the epoxy between the fiber layers is the only force that resists delamination. Ferret et al. (2000) found that the local peel force is greater than the allowable peeling force of the fiber layer, so the thrust force exceeds the interlayer strength and causes delamination. At the exit of the hole, the CFRP is subjected to the maximum thrust force before the main cutting edge of the drill bit enters the final layer. If the thrust force exceeds the bonding stress between the fibers layers, an initial delamination is formed. If there is a lack of support on the exit side of the hole, the downward thrust warps and tears the fiber outward. In this case, the fibers cannot be cut anymore during the subsequent edge trimming processes (Hintze and Hartmann, 2013). Fig. 3 shows the deburring process by the cutting edge of a general drill bit with a complex three-dimensional cutting process. In the model, the feed direction of the tool is perpendicular to the workpiece
(1)
where n is the changes between the cutting edge and fiber angle, and β is the rotation angle of the fiber. The damage of the hole inner surface is affected by the fiber orientation and cutting direction. As shown in Fig. 2(b), when the cutting edge cuts the joint (e.g., points a, b, c, and d) with fiber orientations of 0° and 90°, the instantaneous large cutting angle (α) changes the inner wall’s contour shape and surface roughness. 3
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Fig. 4. Detailed parameters of helical groove drill bit.
flute angle. Therefore, there are additional lower and upper helical groove cutting edges as well in the drill body. The detailed parameters of the drill bit are shown in Fig. 4.
surface. After the drill bit penetrates the workpiece, the burr at the exit of the hole is subjected to three forces: the cutting force Fc, the radial cutting force Fcr, and the thrust Fz. There are three cases of burrs during drilling: ❶ the original position of the burr, ❷ the burr moving along the edge of the drill bit, and ❸ the burr on the side of the drill bit. When the drill bit moves downward, the drill bit’s edge cuts the burr. The burr is subjected to the cutting force Fc1, the radial force Fcr1, and the thrust force Fz1 at the edge of the drill bit, and the resultant downward force is F1. If the cutting force of Fc2 is greater than the lateral bending force at the burr break position, the burr bends toward Fcr2. The effective force of the cutting force Fc2 gradually decreases, and the burr moves from position ❷ to position ❸. This causes the burr to deform vertically due to the weak support of the drill bit. If the cutting force Fc3 for the burr is zero at the cutting edge of the drill bit, the burr is only subjected to the radial shear force Fcr3. Therefore, it is downward due to thrust force Fz3. In this process, the number of fiber breaks in the burr root is increased. Therefore, the stress at the end of the burr decreases with the decrease of the bonding force between fibers.
2.3.2. Burr removal mechanism in the helical groove drill The force model and the deburring mechanism are shown in Fig. 5. Fig. 5(a) shows the analysis of the cutting process of the margin edge of the helical groove drill during downward feed. Before the helical groove edge reaches the hole exit, the helical groove drill bit is the same as a general drill bit that cuts burrs by the margin edge. The burr is tilted up along the edge of the drill bit and reaches the lower cutting edge of the helical groove, and then the lower cutting edge of the helical groove begins to remove the uncut burrs. In this case, the cutting operation of the groove can be described by a three-dimensional cutting model, as shown in Fig. 5(b). The cutting force F3 is the resultant force of the groove’s lower-edge shear force Fc3 and the radial force Fcr3. Consequently, the slot actually pushes up to cut the burr. Therefore, the wall of the machined hole becomes a support to cut the uncut burrs, which must be squeezed into the hole for shear. Fig. 5(c) shows the deburring process at the lower edge of the helical groove when the tool returns. The cutting force has a large tangential component on the helical groove’s lower edge, which helps to cut the fiber since the helix angle of the groove is large. A scissor-like structure is formed between the helical groove’s lower edge and the wall of the machined hole, which can cut off burrs very easily. The grooves shear the burrs at the drill exit throughout the drilling process. Consequently, the proposed drill bit can effectively remove exit burrs.
2.3. Analysis of helical groove drill 2.3.1. Design of new drill The reason why general drill bits cannot effectively cut burrs is that there is no effective supporting force at the bottom fiber layer. Therefore, a helical grooved cutting edge was added to a double point angle drill bit, which provides an upward vertical cutting force to remove exit burrs. The drill is shown in Fig. 4. It has diameter of 6 mm with 6 grooves, point angle 1 of 90°, and point angle 2 of 80°. The groove has an axial width of 1 mm and radial height of 1.5 mm. The angles between the cylindrical surface and helical groove’s upper edge and lower edge are 50° and 105°, respectively. The proposed drill bit has two point angles to reduce the thrust force, which has already been investigated by other researchers. However, the drill has additional helical grooves that have the same rotational direction as the
2.3.3. Burr removal procedure The process of drilling CFRP with the new drill bit can be divided into 8 steps, as shown in Fig. 6. In step 1, the main cutting edge enters and begins to cut the CFRP. Moreover, peel-up delamination occurs at the hole entry. In step 2, the drill bit completely enters the CFRP, and the burrs produced by the peel up and cannot be removed effectively. In step 3, the drill bit moves down near the final layers, delamination 4
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Fig. 5. Deburring mechanism on helical groove drill bit: (a) the process of the drill margin edge cutting a burr when drilling downward; (b) the lower edge of the groove removing the burr when drilling downward; (c) the process of the helical groove’s lower edge removing the burr when drilling upward.
If Vc − Vf > 0, Cutting of burrElse, Nocuttingofburr
begins to appear because there is no corresponding support force at the hole exit, and the thrust force is greater than the bonding force between the fiber layers. In step 4, the drill bit continues to move downward, and the entrance burr is held by the helical groove. The burr is bent down under the action of the thrust and radial cutting force and is removed by the helical groove’s upper edge. In step 5, the drill bit continues to move down, the main cutting edge cannot effectively cut off the delaminated fibers at the exit, and burrs are generated. In step 6, the exit burr is drawn into the helical groove. Because the direction of the tool rotation is the same as that of the helical groove, the groove makes the burr move upward, and the burr is removed by the helical groove’s lower edge. The cutting situation of the burr removal at stage 6 is expressed as follows when the drill bit moves downward (feed rate Vf ↓):
where Vc is the cutting speed of the groove’s lower edge defined by vc = πdntanθ , Vf is the feed rate, d is the diameter, n is the spindle speed, and θ is the helical groove angle. In step 7, the helical groove’s lower edge cuts the exit burr again when the drill bit returns. The cutting situation of the burr removal at stage 7 is expressed as follows when the drill bit moves upward (feed rate Vf ↑):
Always Vc + Vf > 0, and cutting of burr In step 8, the drilling process finishes. 2.3.4. Thrust force analysis Karpat et al. (2012) analyzed the thrust force acting on a double5
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Fig. 6. Deburring mechanism of CFRP using the new drill bit design. Table 1 Material properties of CFRP. Longitudinal (σ1t) and traverse tensile strength (σ2t) Longitudinal (σ1c) and traverse compression strength (σ2c) Young's modulus Shear modulus Poisson’s ratio Density Rockwell hardness
840 MPa 570 MPa 61.5 GPa 3.7 GPa 0.3 155 kg/m3 70–75 HRB
Fig. 7. (a) Thrust forces acting on the drill bit, (b) deburring force.
Fig. 9. Experimental setup.
point drill bit with respect to the cutting edge, as shown in Fig.7(a). The total thrust force acting on the drill bit is the sum of the thrusts acting on the three different edges. The thrust force in the primary region (l1) is the sum of the chiseling force (Fch) and the primary edge force (Fc1). The secondary edge force (Fc2) is from the secondary region (l2). The thrust cutting force of each part is determined by Eqs. (2) and (3). Therefore, the point angles of the drill bit, θ1 and θ2, and the primary (l1) and secondary (l2) cutting edge lengths are important design
Fig. 8. Illustration of the weave orientation of CFRP.
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Fig. 10. Picture of the helical groove on the drill. Table 2 Drill and drilling conditions of fabric woven CFRP. Acronym
Diameter (mm)
Drill edge angles 1, 2 (degree)
Flute angle
Spindle speed (rpm)
Feed rate (mm/rev)
Helical groove drill General drill
6 6
90, 80 90, 0
20 20
800, 1500, 3000 800, 1500, 3000
0.025, 0.05, 0.1 0.025, 0.05, 0.1
Fig. 11. The delamination and burrs of the exit: (a) delamination by a general drill; (b) burr inner surface obtained with a general drill; (c) delamination by helical groove drill; (d) burr at inner surface obtained with a helical groove drill.
force is subjected to the radial cutting force and upward cutting force when the helical lower edge removes a burr. The deburring force Fdb is formulated below:
variables.
Fl1 = Fch + Fc1
(2)
Fl2 = Fc2
(3)
Fz = Fch + Fc1 + Fc 2
(4)
Fdb = Fdz + Fdc
(5)
where Fdz is the radial force of the helical groove, and Fdc is its upward cutting force.
Fz is the thrust force, Fch is the chisel force, Fl1 is the force of primary region, Fl2 is the force of secondary region, Fc1 is the main edge force, and Fc2 is the secondary edge force. As shown in Fig. 7(b), the deburring 7
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Table 3 Burr morphology at the entrance and exit of the hole (spindle speed 800 rpm, feed rate 0.025 mm/rev).
3. Experimental conditions In this study, a multi-directional fabric CFRP composite with a fiber volume fraction of 0.6 was used. The angle of the carbon fibers and the orientation of each layer are 0° and 90°, respectively, as shown in Fig. 8. The mechanical characteristics of the CFRP composite are shown in Table 1. The Korea Carbon Convergence Technology Institute provided the CFRP composites. The manufacturing includes compression molding followed by curing at elevated temperatures. Then, the fabricated composite was cut to obtain the desired dimensions of 90 mm × 90 mm × 5 mm. An experiment was carried out using a CNC machine (DMC model SS-600), as shown in Fig. 9. A general drill and helical groove drill were used in the experiment, and both drills had a diameter of 6 mm. Both drills were prepared by SJ Tools (S. Korea). Fig. 10 shows the drill geometry information, and Table 2 shows the experimental conditions. The recommended optimal cutting conditions for drilling composites are high cutting speed with a low feed rate or low cutting speed with high feed rate, as demonstrated by Shyha et al. (2009, 2011). Thus, the cutting parameters in Table 2 were adopted for the drilling trials. The delamination factor experiment was repeated for three times of each cutting parameter for 100 drilling times. The cutting force measurement during drilling was repeated for five times for each cutting parameter.
Fig. 12. Wear of drill flank face: (a) general drill; (b) helical groove drill.
4. Results and discussion 4.1. Effect of burr removal and tearing reduction Fig. 11 shows SEM images of exit delamination and the cut burrs after drilling using a general drill bit and a helical groove drill bit. When drilling with the general drill bit, the exit burr is torn away and causes severe tearing damage, as shown in Fig. 11(a). Fig. 11(b) shows that the contour of the burr is irregular, which means that the tearing of the hole outlet increased. However, the tearing contour obtained using the new drill bit is almost identical to the contour of the edge of the hole, and the damage is much smaller, as shown in Fig. 11(c). In addition, the contour of the burr is very similar to the contour of the edge of the hole, as shown in Fig. 11(d). This shows that the burr is effectively removed by the helical groove’s lower edge, which reduces the generation of tearing.
Fig. 13. Diagram of measured diameters.
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Fig. 14. Delamination factor of hole exit under different processing conditions.
Fig. 15. Thrust force measurement in CFRP drilling process (spindle speed: 800 rpm; feed rate: 0.025 mm/rev).
Fig. 16. Thrust force of general drill and helical groove drill.
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Fig. 17. Inner surface profiles of hole: (a) the wall surface of the drilled hole, (b) the surface profiles of the hole surface measured for 9 areas along with the hole depth direction (left: proposed drill; right: general drill).
0.025 mm/rev, the delamination factor increases as the spindle speed increases. As the number of drill holes increases, the delamination factor increases. However, the delamination factor of the helical groove drill bit is always lower than that of the general drill bit. The reason is that the burr is removed by the edge of the helical groove’s lower edge, which reduces the effect of burrs on delamination. However, the general drill bits tore the burrs by the margin edge, which results in an increase in delamination. The delamination factor of the first hole is quite different from that of the 100th hole. However, with the increase of the feed speed and spindle speed, the difference is gradually reduced. Also, the delamination factor of the 100th hole drilled by the general drill bit is obviously larger than that of the proposed drill bit. In particular, the delamination factor of the 100th hole is almost unchanged compared with that of the first hole at 1500 rpm and 0.025 mm/rev.
An optical microscope (Sometech SV-55) was used to observe the delamination or burrs at the hole exit. After drilling 100 holes, the delamination at the entrance by the helical groove drillbits were small, and there are few burrs. However, the delamination area of the hole entrance increased significantly with the increase of the number of drill holes made by general drill bit. As shown in Table 3, for the exit of the first hole, both drills showed good drilling performance with almost no burrs. However, due to the wear of the general drill bit, the fiber cannot be cut effectively, and the burrs increase as the number of drill holes increases. In contrast, the hole rarely forms burrs even after drilling 100 holes in the case of the helical groove drill. Fig. 12 shows the wear of the flank face of the tool after drilling 100 holes. The wear on the flank face of the two drills is almost the same. However, the quality of holes processed with new drill bits is obviously better than that of the general drill. In addition, burrs and delamination are obviously reduced. This can be explained by the removal of the burr mainly depending on the subsequent cutting of the helical groove lower edge. As shown in Fig. 13, the maximum diameter of the delamination was measured with an optical microscope. Chen (1997) evaluated the delamination of the hole exit by the delamination factor, which is based on the maximum diameter or delamination of the damaged area. The equation is given as follows:
Fd =
Dmax Dnom
4.2. Thrust force variations In the drilling experiment, the thrust force was measured using a Kistler tool dynamometer (9256C2). Fig. 15 shows a comparison of the thrust force Fz of both drill bits at 800 rpm/min and 0.025 mm/rev. The total thrust force can be classified into four cutting steps for both tools: a→b: the main edge enters the workpiece; b→c: the secondary edge enters the workpiece; c→d: the cutting edge passes through the workpiece; and d→f : the cutting edge penetrates the workpiece. The entrance cutting region has two different forces. The primary region force (a→b) is denoted as Fl1, and the secondary region force (b→c) is denoted as Fl2, as shown in Fig. 7. When the main cutting edge moves from c to d, the drilling force is maximized. A sudden drop in thrust force occurs at the moment when the drill bit penetrates the CFRP from d to f. The point e is when the
(6)
The variations of the delamination factor (Fd) under different processing conditions are shown in Fig. 14. The comparing between the delamination factors obtained at a constant spindle speed of 800 rpm/ min, the delamination factor increases as the feed rate increases. Comparing the delamination factors obtained at a constant feed rate of 10
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2 Additional milling action is applied inside of the hole in the case of the helical groove edge, and the hole’s inner surface has good surface integrity.
helical groove drill bit contacts the surface of the workpiece. The thrust force is affected by the helical groove for a very short time and then gradually decreases. This is due to the helical groove edge, which is subjected to resistance caused by directional thermal expansion of the CFRP during the drilling process. From e to f, the radial cutting force and thrust force increase due to the action of the helical groove edge to cut the inner surface of the hole. At point g, the general drill returns to the original location, but the proposed drill is still drilling due to the additional helical groove edges. At the moment of point h, exit burrs are removed by the helical groove’s lower edge for the first time, as shown in Fig. 6 (step 6). The thrust Fdb at point h is the resultant force of the radial cutting force for the burr Fdc and the upward cutting force Fdz, as shown in Eq. (5). Point i is when the helical groove drill returns, the lower edge of the groove again mills the inner wall of the hole, and the burrs are removed for a second time. Upward thrust force is generated, resulting in a larger negative thrust force than that of the general drill. Also, as shown in Fig. 16, the change in thrust force under different processing parameters. It is clear that the effect of the feed rate is more pronounced than the effect of the spindle speed. Generally, the thrust force is observed to increase as the feed rate increases. The increase in spindle speed has the opposite effect when the feed per revolution is the same. It is observed that the thrust slightly decreases with the increase of the spindle speed, but the influence of the spindle speed is much lower than the influence of the feed rate. The thrust force of the helical groove drill bit is much smaller than that of the general drill bit. This means that the thrust force has a great influence on the occurrence of delamination or burrs.
Acknowledgments This work was supported by the Technology Innovation Program (10053248, Development of Manufacturing System for CFRP (Carbon Fiber Reinforced Plastics) Machining), which is funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea). References Abrão, A.M., Faria, P.E., Campos Rubio, J.C., Reis, P., Paulo Davim, J., 2007. Drilling of fiber reinforced plastics: a review. J. Mater. Process. Tech. 186, 1–7. An, Q.L., Ming, W.W., Cai, X.J., Chen, M., 2015. Study on the cutting mechanics characteristics of high-strength UD-CFRP laminates based on orthogonal cutting method. Compos. Struct. 131, 374–383. An, Q.L., Chen, J., Cai, X.J., P, T.T., Chen, M., 2018. Thermal characteristics of unidirectional carbon fiber reinforced polymer laminates during orthogonal cutting. J. Reinf. Plast. Comp. 37 (13), 905–916. Azmir, M.A., Ahsan, A., 2009. A study of abrasive water jet machining process on glass/ epoxy composite laminate. J. Mater. Process. Technol. 209, 6168–6173. 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4.3. Surface roughness comparison Fig. 17 shows 3-D images of the hole’s inner surface taken by a noncontact camera (Nano system NV-2000). Fig. 17(b) shows the results of longitudinal arithmetic average roughness Ra. The measuring position was added in Fig. 17(a) by a red dot-line mark. There are pits on the inner surface and epoxy resin residues on the hole wall after drilling with the general drill bit. The surface roughness becomes worse as the spindle speed and feed rate increase. A high feed rate and spindle speed produce great impact force, which causes more fiber bundles to peel off from the hole wall and forms deeper pits. In addition, the cutting temperature increases with the increase of cutting speed. The high temperature and bending fracture of the material cause the fibers to deform, and the epoxy resin adheres to the inner wall of the hole. Also, a large deviation between the peak and valleys of surface roughness can be observed. On the other hand, the helical groove lower edge can cut the inner wall of the hole while drilling with the new drill bit. Therefore, fibers attached to the inner surface of the hole can be cut and removed. The surface profiles were compared, as shown in Fig. 17(b). The surface roughness is lowest when the spindle speed is 800 rpm and the feed rate is 0.0025 mm/rev for the helical groove drill bit. In summary, the helical groove drill can produce a better inner surface. 5. Conclusion This study investigated the delamination and burr removal of CFRP when drilling with a helical groove drill. The proposed drill has double point angles and an additional helical groove cutting edge with the same rotational direction as the flute angle. Experiments were carried out to verify the performance by comparison with a general drill. The following conclusions were obtained: 1 The lower cutting edge of the helical groove drill removes burrs generated at the hole entrance and exit effectively during the downward and upward feed. Due to this action, burrs were not identified at the hole exit even after more than 100 drilling operations. 11
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