Twist drill for advanced materials

Abstract

The present invention is concerned with twist drills for drilling of composite materials such as carbon fibre reinforced plastic (CFRP) and glass fibre reinforced plastic (GFRP). The present invention proposes that a twist drill (2) is provided with a variable helix having a defined start and finish helix angle, in combination with primary and secondary relief angles such that the drill (2) is adapted to minimise thrust force, particularly when used for drilling fibre-containing composite materials and especially for hand drilling. Start and finish helix angles of 50 and 10; 50 and 30; and 30 and 10 have been shown to provide excellent cutting performance and exit hole quality. A large secondary chisel edge angle (24) has also been found to contribute to excellent performance with composite materials, including stack machining.

Claims

1. A twist drill comprising: a shank; a drill body; a drill tip including a pilot, a cutting edge, a primary facet extending behind the cutting edge and a secondary facet extending behind the primary facet, wherein a relief angle of the primary facet is 5 to 40 and a relief angle of the secondary facet is 10 to 40; a progressive transition portion between the pilot and the drill body, the progressive transition portion being at least a double chamfer having a first chamfer portion extending from and behind the pilot and a second chamfer portion extending from and behind the first chamfer portion, the first chamfer having a first chamfer portion angle of 12 to 18 and the second chamfer having a second chamfer portion angle of 5 to 15; and at least one flute extending from the drill tip to the drill body, wherein a helix angle of each flute decreases from a start helix angle of 25 to 60 at the drill tip to a finish helix angle of 0 to 35 in the drill body.

2. The twist drill according to claim 1, wherein the helix angle decreases by at least 10 from the start angle to the finish angle.

3. The twist drill according to claim 1, wherein the start helix angle is 40 to 60.

4. The twist drill according to claim 1, wherein the finish helix angle is 0 to 20.

5. The twist drill according to claim 1, wherein the start helix angle is 48 to 52 and the finish helix angle is 8 to 12.

6. The twist drill according to claim 1, wherein the start helix angle is 28 to 32 and the finish helix angle is 8 to 12.

7. The twist drill according to claim 1, wherein the helix angle decreases smoothly and continuously from the start helix angle to the finish helix angle.

8. The twist drill according to claim 1, wherein the relief angle of the primary facet is 10 to 40 and the relief angle of the secondary facet is 15 to 30.

9. The twist drill according to claim 1, wherein the drill has only two flutes.

10. The twist drill according to claim 1, wherein the pilot has a length of at least 2 mm.

11. The twist drill according to claim 1, wherein the transition portion includes a third chamfer portion extending behind the second chamfer portion.

12. The twist drill according to claim 1, wherein the drill tip includes a chisel edge.

13. The twist drill according to claim 12, wherein a chisel edge angle of the chisel edge is 105 to 115.

14. The twist drill according to claim 12, wherein a length of the chisel edge is 0.03 to 0.15 mm.

15. The twist drill according to claim 12, wherein the drill tip includes a secondary chisel edge.

16. The twist drill according to claim 15, wherein a secondary chisel edge angle of the secondary chisel edge is 145 to 155.

17. The twist drill according to claim 12, wherein a length of the chisel edge is 0.05 to 0.15 mm.

18. The twist drill according to claim 1, wherein the drill tip has a point angle of 80 to 140.

19. The twist drill according to claim 18, wherein the drill tip has a point angle of 85 to 95.

20. The twist drill according to claim 1, wherein the drill tip has an axial rake angle of 6 to 15.

21. The twist drill according to claim 1, wherein each flute has a right hand helix.

22. The twist drill according to claim 1, further comprising a back edge relief associated with each cutting edge.

23. The twist drill according to claim 1, wherein the relief angle of the primary facet is 15 to 30.

24. The twist drill according to claim 1, wherein the drill has three flutes.

25. The twist drill according to claim 1, wherein the second chamfer portion has an angle of no more than 80.

26. A method of drilling a composite material containing fibres, wherein the method includes the step of drilling the composite material with a twist drill comprising: a shank; a drill body; a drill tip including a pilot, a cutting edge, a primary facet extending behind the cutting edge and a secondary facet extending behind the primary facet along a length of the drill tip, wherein a relief angle of the primary facet is 5 to 40 and a relief angle of the secondary facet is 10 to 40; a progressive transition portion between the pilot and the drill body, the progressive transition portion being at least a double chamfer having a first chamfer portion extending from and behind the pilot and a second chamfer portion extending from and behind the first chamfer portion, the first chamfer having a first chamfer portion angle of 12 to 18 and the second chamfer having a second chamfer portion angle of 5 to 15; and at least one flute extending from the drill tip to the drill body, wherein a helix angle of the flute decreases from a start helix angle at the drill tip to a finish helix angle in the drill body.

27. The method according to claim 26, wherein the composite material is carbon fibre reinforced plastic (CFRP) or glass reinforced plastic (GFRP).

28. The method according to claim 27, wherein the composite material is an aircraft component, wind turbine component, boat component or vehicle panel.

29. The method according to claim 27, wherein the step of drilling comprises hand drilling.

30. The method according to claim 27, wherein the method is a method of stack drilling.

31. The method according to claim 27, further comprising the step of regrinding a twist drill.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention and experiments illustrating the advantages and/or implementation of the invention are described below, by way of example only, with respect to the accompanying drawings, in which:

(2) FIG. 1 shows a side view of a variable helix twist drill, being a first embodiment of the present invention;

(3) FIG. 2 shows an end-on axial view of the twist drill of FIG. 1;

(4) FIG. 3 shows an expanded view of the drill tip of the twist drill of FIG. 1;

(5) FIGS. 4A and 4B show the results of an exit hole quality test in CFRP material for an embodiment of the present invention (4A), and a commercially available drill (4B);

(6) FIG. 5 shows a side view of a variable helix twist drill having a constant helix portion, being a second embodiment of the present invention;

(7) FIG. 6 shows a side view of a variable helix twist drill, being a further embodiment of the present invention; and

(8) FIGS. 7A and 7B shows the results of an exit hole quality test in CFRP for the embodiment of FIG. 5 (7A) and the embodiment of FIG. 6 (7B).

DETAILED DESCRIPTION OF EMBODIMENTS AND EXPERIMENTS

(9) FIG. 1 shows a twist drill 2 of the present invention. The drill comprises a shank (not shown), drill body 4 and drill tip 6. Two helical flutes 8 extend from the drill tip to the drill body. The helix angle is comparatively large at the start of the helix, being 50 and comparatively small at the end of the helix, being 10. Other angles are possible, for example 25 to 60, suitably 40 to 60, for the start angle, and 0 to 35 for the finish angle.

(10) The helix is formed using a spline function. The spline function is selected so that the change in helix angle as a function of the axial distance from the start of the helix at the drill tip is smooth and continuous. This is in contrast to conventional variable helix drills wherein the change in helix angle is stepped, such that there are transitions or steps between helix angles. In contrast, this embodiment, with its smooth and continuous variation in helix angle does not have any such steps. This has the significant advantage that material is removed and evacuated more efficiently along the flutes.

(11) The width of the flutes is substantially constant along the length of the flutes.

(12) Drill 2 also comprises an extended pilot 10. The primary and secondary cutting edges (cutting lips) at the pilot tip form the point, which has a point angle 12 of 90. Other point angles are possible, for example 80 to 140.

(13) The drill 2 comprises a progressive transition between the comparatively narrow pilot 10 and the wider drill body 4. A double chamfer serves to reduce the thrust force and to increase the cutting resistance in order to counteract for the pushing effect that would occur during cutting at the transition point between the chamfer and the drill body. Specifically, the drill 2 comprises a first chamfer 14 having an angle of 15 and a length of 5 mm, and a second chamfer 16 having an angle of 10 and a length of 4 mm, Other chamfer angles and lengths are possible as described herein. Indeed, additional chamfers (i.e. third, fourth, etc chamfers) are possible.

(14) The diameter of the pilot is 47% of the drill diameter. As explained herein, the drill diameter is measured at the widest point of the drill, which in this case is at the forwardmost part of the drill body 4 immediately adjacent the second chamfer 16.

(15) The extended pilot is 3 mm long, measured from a point immediately adjacent the forwardmost part of the first chamfer 14 to a point immediately adjacent the point (i.e. not including the point). Other pilot lengths are possible, for example 2 mm to 6 mm.

(16) The extended pilot can be reground, thereby permitting multiple uses of the same drill. Indeed, up to three regrinds are possible, which represents a considerable cost and material saving for the end user as compared to purchasing new drills.

(17) FIG. 2 shows an axial view of the drill 2. Chisel edge 20 has a length of 0.1 mm and a chisel edge angle 22 of 110. Other chisel lengths and chisel angles are possible, as described herein.

(18) A characteristic of the drill 2 that makes it particularly effective at drilling composite material containing fibres is a second chisel edge. Furthermore, secondary chisel edge angle 24 is large, being 150. Other secondary chisel edge angles are possible, for example 120 to 170.

(19) FIG. 3 shows an enlarged view of the drill tip 6 and especially the pilot 10. The primary and secondary cutting edges of the point are provided with a primary relief 30 (also known as primary facet or flank face clearance) and secondary relief 32 (also known as secondary facet or flank face clearance). The respective relief angles (also known as clearance) are 10- and 20 respectively. In another embodiment having otherwise identical geometry, the respective relief angles are 25 and 20. Other primary and secondary clearance angles are possible, for example 5 to 40 (preferably 10 to)40 and 10 to 40 respectively.

(20) The drill 2 has an axial rake angle of 8. In another embodiment having otherwise identical geometry, the axial rake angle is 10. However, other rake angles are possible, for example 3 to 15.

(21) As described above, the combination of the variable helix, the primary and secondary reliefs and the secondary chisel edge in particular impart the drill with unexpectedly good performance when cutting composite materials such as CFRP. Indeed, a highly desirable combination of low thrust force and excellent exit hole quality (little or no fraying of the material) has been achieved. Furthermore, little or no pushing occurs, which makes the drill especially suited for hand drilling.

(22) Testing of Drill Performance

(23) The performance of an embodiment of the present invention was compared with a commercially available hand drill that is marketed for use with CFRP. The drill performance was quantified by measuring thrust force and hole quality.

(24) Drill Geometry

(25) A twist drill was manufactured in accordance with the methods described herein. Specifically, the following steps were undertaken:

(26) 1. Rods are cut into desired length which is the length of the drill

(27) 2. A first chamfer and a pilot are formed in the blank.

(28) 3. Blanks are back tapered.

(29) Using an CNC machine, the following steps were performed:

(30) 4. Fluting to form two flutes with variable helix. The variable helix is formed using an exponential spline function. In this way, the helix is smooth and is free of the break points that characterise conventional helices.

(31) 5. Fluting land is produced and body clearance is generated along the flute

(32) 6. Body clearance is created at the pilot

(33) 7. A second chamfer is formed and the body clearance is created both for first and second chamfers.

(34) 8. Pointing to create the primary facet and secondary facet. The primary facet is created to have a chisel edge angle of 110 and a primary clearance of 10. The secondary facet is created to have a secondary clearance of 20.

(35) 9. Gashing is carried out to create a rake angle of 8 and a chisel length of 0.1 mm.

(36) The flute was formed using a spline function, to provide a smooth continuous transition along the drill, from start angle to finish angle.

(37) The completed drill had the following geometry: Helix length=38 mm Start helix angle=50 Finish helix angle=10 Pilot length=3 mm Diameter of pilot=47.24% of drill diameter Point angle=90 Axial rake angle=8 Chisel edge angle=110 Chisel length=0.1 mm. Secondary chisel angle=150. Primary clearance=10. Secondary clearance=20. Chamfer 1 angle=15 Chamfer 1 length=5 mm Chamfer 2 angle=10 Chamfer 2 length=4 mm

(38) This drill is referred to as drill #1 for the purposes of the tests.

(39) Note that, as mentioned above, a further embodiment has identical geometry except for an axial rake angle of 10 and a primary clearance of 25.

(40) A commercially available hand drill was also tested: Drill #2: Carbide hand tool.

(41) Test Procedures

(42) In order to measure the thrust and the hole quality, two tests were carried out:

(43) (1) Automated drilling for thrust force measurement

(44) (2) Hand drilling for hole quality examination

(45) Even though drill #1 is particularly adapted for hand drilling operation, testing using a CNC 4 axis machine was needed for thrust force measurement.

(46) However, for hole quality measurements, hand drilling was carried out.

(47) The test workpiece in each test was an epoxy based CFRP of 10 mm thickness.

(48) For the hole quality test the exit face of the workpiece was provided with a glass scrim. This configuration, which is encountered for example in the aerospace industry, represents a particularly difficult challenge.

(49) The materials, tools and machine employed in the tests are summarised in Table 1 (automated drilling for force measurement) and Table 2 (hand drilling for exit hole quality measurement).

(50) TABLE-US-00001 TABLE 1 Materials, tools and machine used in the thrust force test Machine Type CNC 4 axis machine Coolant Dry Materials Type Epoxy based CFRP (Material 1) Thickness 10 mm Drills Diameter 6.35 Coating Bright Type Drill #1, Drill #2

(51) TABLE-US-00002 TABLE 2 Materials, tools and machine used in the hole quality test Hand Tool Type Power hand drill Materials Type Epoxy based CFRP with a glass cloth at the exit face (Material 2) Thickness 10 mm Drills Diameter 6.35 Coating Bright Type Drill #1, Drill #2
Test (1): Thrust Force Measurement

(52) For each drill, ten holes were drilled in Material 1 using a CNC machine. The thrust force was measured and recorded using a Kistler Dynamometer.

(53) Test (2): Hole Quality Determination

(54) For each drill, a number of holes were drilled in Material 2 using a power hand drill.

(55) The hole quality at the exit face was observed and captured using an optical microscope.

(56) Results

(57) The average computed thrust force and the exit hole captured images are summarised in Table 3. FIGS. 4A and 4B show the captured images for each of the two drills: FIG. 4A is drill #1; and FIG. 4B is drill #2.

(58) Drill #1 achieved not only a low thrust force but also excellent exit hole quality. In contrast, drill #2 demonstrated poor or very poor exit hole quality.

(59) TABLE-US-00003 TABLE 3 Test results Drill #1 Drill #2 Thrust force on Material 1 96N 146N Hole quality on Material 2 Excellent Poor

(60) An additional advantage of drill #1 is that it can be reground. This is attractive to end users because the cost of re-grinding is normally much lower than the cost of a new drill.

(61) FIG. 5 shows a further embodiment. In this embodiment, drill 100 comprises a helical flute having a linear section 102 in which the helix angle is constant, and a variable section 104 in which the helix angle decreases. In this embodiment, the linear section is about 35-45% of the flute length.

(62) The helix angle in the linear section is 50, although other start angles are possible, for example 25 to 60, suitably 40 to 60.

(63) The helix angle in the variable section decreases from 50 (the start angle of the variable helix section) to 30 (the finish angle of the variable helix section). Other finish angles are possible, for example 0 to 35.

(64) The cutting tip 106 includes a secondary chisel angle of 140, although other angles are possible, for example 120 to 170. The drill tip also includes primary and secondary clearances, being 15 and 20 respectively.

(65) The point angle is 85, and the chisel length is 0.1 mm. The drill tip has an axial rake angle of 5.

(66) In testing with CFRP materials, this drill was found to provide very good hole quality. An example of excellent exit hole quality is shown in FIG. 7A. In particular, excellent results in stack machining (e.g. with a stack of 40 mm thickness) was achieved. The drill 100 is particularly suited to automated drilling.

(67) A further embodiment is shown in FIG. 6.

(68) FIG. 6 shows a twist drill 200 of the present invention. The drill comprises a shank, drill body 204 and drill tip 206. Three helical flutes 208 (only two are visible) extend from the drill tip to the drill body. The helix angle is comparatively large at the start of the helix, being 30 and comparatively small at the end of the helix, being 10. Other angles are possible, for example 25 to 60 for the start angle, and 0 to 35 for the finish angle.

(69) In the same way as for the example shown in FIG. 1, the helix is formed using a spline function. The spline function is selected so that the change in helix angle as a function of the axial distance from the start of the helix at the drill tip is smooth and continuous, i.e. without steps. This has been found by testing to permit material to be removed and evacuated more efficiently along the flutes.

(70) The width of the flutes is substantially constant along the length of the flutes.

(71) Drill 200 also comprises an extended pilot 210. The primary, secondary and tertiary cutting edges (cutting lips) at the pilot tip form the point, which has a point angle of 90. Other point angles are possible, for example 80 to 140.

(72) The drill 200 comprises a progressive transition portion 212 between the comparatively narrow pilot 210 and the wider drill body 204. A triple chamfer 212a, 212b, 212c serves to reduce the thrust force and to increase the cutting resistance in order to counteract for the pushing effect that would occur during cutting at the transition point between the chamfer and the drill body.

(73) The extended pilot is 3 mm long, measured from a point immediately adjacent the forwardmost part of the first chamfer to a point immediately adjacent the point (i.e. not including the point). Other pilot lengths are possible, for example 2 mm to 6 mm.

(74) In the same way as for the example shown in FIG. 1, the extended pilot can be reground, thereby permitting multiple uses of the same drill. Indeed, up to three regrinds are possible, which represents a considerable cost and material saving for the end user as compared to purchasing new drills.

(75) Drill 200 has a diameter of 13 mm. The speed and feed are normally altered to compensate for diameter change in automated drilling. The speed will be reduced for large diameters in order to achieve the same surface speed used in smaller diameters. Higher or lower feed will be used to compensate for speed changes, However, in hand drilling operation, the speed is alterable but the feed is subjected to individual operators. It is difficult to instruct the operators whether to push harder or not to push harder. In order to address this problem associated with larger diameter hand drilling, drill 200 has been provided with an additional cutting edge as compared to the example of FIG. 1 (and hence an additional flute). Experiments have shown that the additional cutting edge helps the tool to engage into the workpiece and cut with little impact on feed.

(76) Drill 200 has also been provided with back edge relief 220. This has been found in testing to significantly reduce the problem of overheating and hence melting of the workpiece. The problem of heat build up has been found to be particularly acute with large diameter drills (especially diameters above 7.8 mm) and the provision of a back edge relief is particularly effective for those large diameter drills. Drill 200 has a back edge relief on each of its 3 lands, i.e. a back edge relief associated with each cutting edge.

(77) A further characteristic of the drill 200 that makes it particularly effective at drilling composite material containing fibres is a second chisel edge (not shown). Furthermore, the secondary chisel edge angle is large, being about 150. Other secondary chisel edge angles are possible, for example 120 to 170.

(78) Similar tests to those carried out on the drill of FIG. 1 demonstrated that drill 200 produces very good entry and exit hole quality on carbon fibre composite material (CFRP), including CFRP with twill fibre layout as well as those with uni-directional (UD) fibre layout. It also performed well on CFRP with a glass scrim on the exit face.

(79) As can be seen from FIG. 7B, exit hole quality is excellent.