Twist drill and method of drilling composite materials, use and method regrinding and manufacturing thereof

Abstract

A twist drill for drilling composite materials includes a shank; a drill body; a drill tip having a cutting edge, a chisel edge, and a secondary chisel edge, wherein the secondary chisel angle of the drill tip is 145 to 165 and the point angle is 70 to 100; and a flute extending from the drill tip to the drill body. The flute has a constant helix, and the helix angle of the flute being selected from the range 45 to 55. A method of drilling a composite material comprising fibers uses the twist drill of the present embodiment. Suitably, the composite material is carbon fiber reinforced plastic or glass fiber reinforced plastic, and optionally being a laminate material such that the method comprises stack drilling. Embodiments achieve a combination of good hole quality, good tool life and good hole size spread.

Claims

1. A twist drill for drilling composite materials, the twist drill comprising: a shank; a drill body; a drill tip including a cutting edge and at least two chisel edges, the at least two chisel edges comprising a primary chisel edge and a secondary chisel edge, wherein the secondary chisel edge has a secondary chisel angle of 145 to 165, the drill tip also having a point angle of 70 to 100 and 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 15 to 25, and a relief angle of the secondary facet is 15 to 30; and a flute extending from the drill tip to the drill body, the flute having a constant helix, and a helix angle of the flute being selected from the range 45 to 55.

2. The twist drill according to claim 1, wherein the drill tip has an axial rake angle of 5 to 8.

3. The twist drill according to claim 1, wherein the drill tip has a point angle of 85 to 88.

4. The twist drill according to claim 1, wherein the secondary chisel angle is 145 to 155.

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

6. A method of drilling composite material comprising fibres, wherein the method includes the step of drilling the composite material using a twist drill comprising a shank; a drill body; a drill tip including a cutting edge and at least two chisel edges, the at least two chisel edges comprising a primary chisel edge and a secondary chisel edge, wherein the secondary chisel edge has a secondary chisel angle of 145 to 165, the drill tip also having a point angle of 70 to 100 and 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 15 to 25, and a relief angle of the secondary facet is 15 to 30; and a flute extending from the drill tip to the drill body, the flute having a constant helix, and a helix angle of the flute being selected from the range 45 to 55.

7. The method according to claim 6, wherein the composite material is carbon fibre reinforced plastic (CFRP) or glass fibre reinforced plastic (GFRP).

8. The method according to claim 7, wherein the step of drilling comprises automated drilling.

9. The method according to claim 7, wherein the method is a method of stack drilling.

10. The method according to claim 9, wherein the stack comprises CFRP and GFRP.

11. The method according to claim 9, wherein the stack comprises CFRP and Al.

12. A regrinding method comprising the step of regrinding a twist drill so as to form a twist drill having a shank; a drill body; a drill tip including a cutting edge and at least two chisel edges, the at least two chisel edges comprising a primary chisel edge and a secondary chisel edge, wherein the secondary chisel edge has a secondary chisel angle of 145 to 165, the drill tip also having a point angle of 70 to 100 and 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 15 to 25, and a relief angle of the secondary facet is 15 to 30; and a flute extending from the drill tip to the drill body, the flute having a constant helix, and a helix angle of the flute being selected from the range 45 to 55.

13. A method of manufacturing a twist drill for drilling composite materials, the method comprising the steps of: (i) fluting a drill blank to produce a helical flute with a constant helix angle in the range of 45 to 55 from the start to the end of the flute; (ii) forming a cutting edge at the end of the flute; (iii) forming a drill tip to form a point angle of 70 to 100 and 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 15 to 25, and a relief angle of the secondary facet is 15 to 30; (iv) forming a primary chisel angle; and (v) forming a secondary chisel angle in the range 140 to 165.

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 constant helix twist drill being an embodiment of the present invention;

(3) FIG. 2 shows a side view of a constant helix twist drill of FIG. 1 the first aspect of the present invention;

(4) FIGS. 3A and 3B show end-on axial views of the twist drill of FIG. 1;

(5) FIGS. 4A and 4B show the results of an exit hole quality test in a 10 mm thick epoxy based CFRP with a glass scrim for the embodiment of FIG. 1 of the present invention (4A), and a commercially available drill (4B);

(6) FIG. 5 is a graph showing the hole size spread produced by a 20 constant helix drill for drill speeds and feeds shown in Table 1; and

(7) FIG. 6 is a graph showing the hole size spread produced by a 50 constant helix drill for drill speeds and feeds shown in Table 1.

DETAILED DESCRIPTION OF EMBODIMENTS AND EXPERIMENTS

(8) 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 a constant helix angle of 50, although other constant helix angles are possible, for example 45 to 55.

(9) The width of the flute is substantially constant along the length of the flute.

(10) The primary and secondary cutting edges 10 (cutting lips) at the drill tip form the point, which has a point angle 12 of 85. Other point angles are possible, for example 70 to 100.

(11) The cutting edge 10 of drill 2 was edge corrected to produce a straight cutting edge 10, an artefact of this edge correction can be seen in FIG. 1 as feature 14.

(12) Drill 2 has body clearance 16 along the flutes 8.

(13) FIG. 2 shows a rotated side view of drill 2. The cutting edges of the point are provided with a primary relief 21 (also known as primary facet or flank face clearance) and secondary relief 22 (also known as secondary facet or flank face clearance). The respective relief angles (also known as clearance) are 10 and 20 respectively.

(14) FIGS. 3A and 3B show axial views of drill 2. Chisel edge 31 has a length of 0.5 mm and a chisel angle 34 of 115. Other chisel lengths and chisel angles are possible, as described herein.

(15) The secondary chisel edge 32 is provided with a large secondary chisel angle 33 of 146.

(16) A characteristic of the drill 2 that makes it particularly effective at drilling composite material containing fibres is a secondary chisel edge. Furthermore, secondary chisel edge angle 33 is large, being 146. Other secondary chisel edge angles are possible, for example 140 to 165.

(17) As described above, the combination of the constant large (quick) helix, specified point angle 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 good hole quality (little or no fraying of the material), good tool life, and hole size spread within H7 tolerance is achieved. The twist drill is also comparatively easy to manufacture (for example, as compared to a variable helix).

(18) Testing of Drill Performance

(19) The performance of an embodiment of the present invention was compared with a commercially available automated drill that is marketed for use with CFRP. The drill performance was quantified by measuring tool life, exit hole quality and hole size spread.

(20) Test (1): Exit Hole Quality

(21) In order to measure exit hole quality automated drilling was carried out on a test workpiece. The test workpiece for each test was a 10 mm thick epoxy based CFRP, with uni-directional fibres and a glass scrim. This configuration, which is encountered for example in the aerospace industry, represents a particularly difficult challenge.

(22) Drill Geometry

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

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

(25) 2. Blanks are back tapered.

(26) Using a CNC machine, the following steps were performed:

(27) 3. Fluting to form two flutes 8 with a constant helix.

(28) 4. Fluting land is produced and body clearance 16 is generated along the flute.

(29) 5. Pointing to create the primary facet 21, the secondary facet 22 and the point with a point angle 12 of 85. The primary facet is created to have a primary clearance of 10. The secondary facet is created to have a secondary clearance of 20 and a secondary chisel angle of 146.

(30) 6. Gashing is carried out to create a rake angle of 5.

(31) The completed drill had the following geometry: Helix angle=50 Point angle=85 Axial rake angle=5 Secondary chisel angle=146. Primary clearance=10. Secondary clearance=20.

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

(33) A commercially available automated drill was also tested: Drill #2.

(34) As can be seen from FIG. 4A the exit hole quality of drill #1 is excellent, far better than that produced by drill #2 (see FIG. 4B).

(35) Test (2): Tool Life

(36) Due to high strength and fibre reinforcement CFRP is extremely abrasive. The tool life of uncoated Drill #1 (as described above) was found to be significantly longer than that of uncoated Drill #2 (Drill #1 drilled 360 holes, compared to 140 for Drill #2).

(37) Test (3): Spread in Hole Size

(38) The twist drills used in this comparative test are identical except for their helix angle. Drill #A has a constant helix angle of 50, Drill #B has a constant helix angle of 20.

(39) The performance of 6.35 mm diameter (the drill diameter is shown on FIGS. 5 and 6 by lines 51 and 61 respectively) drills with 20 constant helix (slow) and 50 constant helix (quick) were tested.

(40) In order to measure the spread exit hole quality automated drilling was carried out a test workpiece using three speeds and feeds. The test workpiece for each test was an 8 mm thick MTM 46 resin based CFRP, with unidirectional fibres. This configuration, which is encountered for example in the aerospace industry, represents a particularly difficult challenge.

(41) Table 1 identifies the drill used for each test by helix angle and specifies the speed and feed used for each test.

(42) TABLE-US-00001 TABLE 1 hole size spread test conditions Helix Speed Feed Test angle (m/min) (mm/rev) 1 50 80 0.04 2 20 200 0.04 3 50 80 0.13 4 50 200 0.04 5 20 130 0.04 6 50 80 0.08 7 20 80 0.08 8 50 200 0.13 9 50 130 0.08 10 50 130 0.04 11 20 200 0.08 12 20 130 0.13 13 20 130 0.08 14 20 80 0.13 15 20 200 0.13 16 50 200 0.08 17 50 130 0.13 18 20 80 0.04

(43) FIGS. 5 and 6 show the spread of hole size obtained at different speeds and feeds for 20 helix and 50 helix drills respectively. FIG. 6 shows that across all test conditions the 50 constant helix drill formed holes with size within tolerance identified by line 62, corresponding to H7. FIG. 5 shows that in same conditions, the 20 helix drill did not produce holes with size within tolerance specified by either line 52 or 53, corresponding to H7 and H8 respectively, across the range of speed and feed tested. These results show that a large helix angle, as specified in the claims consistently show hole size within tolerance and this is not the case for drills having a smaller helix angle. This experiment illustrates an advantage of the large helix angle.