GB2029288A - Tapered hole cutter - Google Patents

Abstract

A rotary cutter for cutting tapered holes in metal workpieces has an elongate flat cutter blade 12 tapered in the dimension between longitudinal cutting edges 38 for applying the taper to the hole, and tapered in the dimension between the blade sidefaces 30 so that the cutting edges along the length of the blade will lead the cutting edges proximate the point and thereby minimize point drift. The blade has a drill type leading cutting edge 50. All cutting edges of the blade are provided with a substantial negative rake for chip control, and the chips are blown out of the hole by jet streams of cutting fluid directed along the longitudinal cutting edges of the blade from radial slots or holes in a collar 16 and shank 14. The blade terminates in a countersinking cutting edge profile 18. The shank has slots 24 for bayonet mounting. <IMAGE>

Description

SPECIFICATION Improvements relating to hole cutters The conventional practice for preparing tapered holes in metal structures to receive tapered interference fasteners is to first drill a tapered hole with a tapered twist drill, and then to employ a second, reaming operation utilizing a tapered twist-type reamer to produce the final tapered hole dimensions and finish. This duplication of effort required in adding the reaming step after the drilling step has generally been found necessary where twist-type drills and reamers were employed to achieve the reliability of hole quality necessary to assure reliability of the interference preloading in the structures which are being fastened. In addition to this problem of duplication of effort, the twist-type drills and reamers are characteristically complex and expensive to make, and have a relatively limited operational life.

Accordingly, with the conventional practice of using twist drills and twist-type reamers to produce tapered holes of the required dimensional reliabilty and bore finish for receiving tapered interference fasteners, both the tools and the drilling and reaming operations were much more difficult and expensive than for the preparation of straight, cylindrical holes.

One of the most difficult problems to cope with in the use of both prior art blade-type cutters and twist-type drills and reamers is a tendency for the point to drift or wander as it travels through the workpiece, due primarily to non-homogeniety of the workpiece material. This problem is compounded in the case of twist drills and reamers because of their tendency to unwind. This is one of the reasons why successive drilling and reaming operations have heretofore generally been required to produce tapered holes of sufficient reliability for use in a high performance tapered interference fastening system.

Another serious problem in the use of prior art cutters, drills and reamers for producing high quality tapered holes is that such prior art devices conventionally produce chip strings which tend to be difficult to eject from the hole, and which have a tendency to gall and thereby spoil the surface finish of the hole.

A particular example of a hole cutter in accordance with this invention will now be described with reference to the accompanying drawings; in which:~ Figure 1 is a perspective view of one form of blade-type cutter according to the invention; Figure 2 is a side elevational view of the bladetype cutter shown in Figure 1; Figure 3 is an edge elevational view of the cutter shown in Figures 1 and 2; Figure 4 is a front end elevational view of the cutter shown in Figures 1-3; Figure 5 is a rear end elevational view, with a portion broken away, of the cutter shown in Figures 1-4;; Figure 6 is a diagrammatic front end elevational view of the cutter shown in Figures 1-5 particularly illustrating the negative rake angle of each of the blade cutting edges, the bearing and relief surface portions of the edge surfaces of the blade, and the relief angle at each of the blade edges; Figure 7 is a diagrammatic end elevational view similar to Figure 6, but illustrating another form of the invention wherein the side surfaces of the cutter blade are concave in transverse section; and Figure 8 is an elevational view, partly in section, illustrating a biade-type cutter according to the invention engaged through a laminar workpiece at the completion of the cutting of a tapered hole.

Figures 1-6 and 8 of the drawing illustrate a presently preferred form of blade type cutter according to the invention for cutting tapered holes in metal workpieces. This blade type cutter 10 includes a forward blade portion 12, a rearward shank or mounting portion 14, and an enlarged intermediate portion including an annular collar 16 and an countersink-forming portion 18.

The rearward shank or mounting portion 1 4 of cutter 10 is adapted to be axially engaged within a suitable holder sleeve 20 shown in phantom lines, the holder sleeve 20 adapting the cutter 10 to the rotary driving spindle of a suitable conventional power driving tool. The rearwardly facing shoulder 22 of collar 16 engages the forward annular edge of holder sleeve 20 to locate the cutter 10 in holder sleeve 20, and a ganerally transverse slot 24 is provided in the shank or mounting portion 14 of cutter 10 for bayonet-type locking engagement thereof in the holder sleeve 20. A driving flat 26 is provided on the shank or mounting portion 14 of cutter 1 0 and is drivingly engageable with a corresponding flat section within the holder sleeve 20.

The blade 12, countersink-forming portion 18, collar 16 and rearward shank or mounting portion 14 are all axially aligned with each other and with the holder sleeve 20, so that the blade 12 and countersink-forming portion 1 8 of the cutter 10 will be rotated about their longitudinal axial centers by the driving tool.

A pair of cutting fluid channels 28 is arranged in the cutter 10 to supply a continuous jet stream of cutting fluid along each of the cutting edges of the blade 12 during operation of the cutter 1 0.

These cutting fluid channels 28 are preferably provided as illustrated in the form of a pair of diametrically opposite, radially outwardly opening, straight grooves that are oriented parallel to the longitudinal axis of the cutter 1 0 and substantially in alignment with the respective leading, cutting edges of blade 12. The holder sleeve 20 surrounds the rearward shank portion of cutter 10 in close fitting relationship and thereby provides outer wall means for these longitudinal cutting fluid channels 28. Although the cutting fluid channels 28 are shown as open grooves in the region of annular collar 1 6, it is to be understood that they may alternatively include hole portions extending through the collar 16.

Cutting fluid is supplied under pressure from a suitable source through the power driving tool and associated holder sleeve 20, the fluid being confined and directed as it passes forwardly through the channels 28 into a pair of jet streams that are aimed along the cutting edges of the blade. Any conventional cutting fluid may be employed with the cutter 10. Preferably air or a cutting mist is used, but if desired a liquid cutting fluid may be employed. The manner in which the two jet streams of cutting fluid cooperate with the cutter blade 12 during rotation thereof for the efficient ejection of all cutting chips from the tapered workpiece bore will be described in detail hereinafter.

Referring particularly to Figures 1-4 and 6 of the drawing, in the form of the invention there illustrated the forward blade portion 12 of the cutter may be broadly described as being elongated and generally flat, with a pair relative#ly wide, oppositely facing flat side surfaces 30 and a pair of relatively narrow, oppositely facing edge surfaces 34. The direction from one side 30 to the other side 30 of blade portion 12 will sometimes hereinafter be referred to as the "thickness" direction, while the direction from one edge 34 to the other edge 34 of blade portion 12 will sometimes hereinafter be referred to as the "width" direction. The blade 12 is tapered in both the thickness direction and the width direction.

Thus, in the thickness direction the blade 12 has a uniform taper between the side surfaces 30 of an included angle designated 32 in Figure 3, the blade 12 being thrnner proximate the forward, point end than it is proximate the rearward end adjacent the countersink-forming portion 18.

Similarly, the blade 12 has a uniform taper in the width direction between edge surfaces 34 of an included angle designated 36 in Figure 2, the width of blade 12 being less proximate the forward point end of blade 12 than it is proximate the rearward end adjacent the countersinkforming portion 18.

The thickness taper is preferably symmetrical on opposite sides of an axial plane 33 extending in the width direction as shown in Figure 3; i.e., the included thickness taper angle 32 is preferably equally distributed on opposite sides of the central axis of the cutter 10. Similarly, the width taper is preferably symmetrical on#opposite sides of an axial plane 37 extending in the thickness direction as shown in Figure 2; i.e., the included width taper angle 36 is preferably equally distributed on opposite sides of the central axis of the cutter 10.

The amount of the width taper angle 36 will be determined by the purpose for which the tapered hole that is being cut by the cutter 10 is to be used. For example, if the tapered hole is to be used as a part of a high performance tapered fastening system for aircraft and space vehicles, then the width taper will preferably be approximately 1/4 inch per foot, or a width taper angle 36 of approximately 1 11' 38".

The thickness taper of the blade 12 as represented by taper angle 32 in Figure 3 is an important aspect of the present invention, substantially eliminating cutter point drift or wander, which is otherwise a very serious problem in both blade type cutters and twist tvpe drills and reamers. Any workpiece is to some extent nonhomogeneous, which tends to make the point of a drill or reamer drift or wander as it travels through the workpiece. Without the thickness taper of the present invention, it has been found that with blade type cutters for cutting tapered holes this problem of point drift or wander is serious, and that many of the holes become to some extent "triangulated" due to point drift or wander.In the use of conventional twist drills and reamers, the point leads the cutting edge and thereby controls the direction of drilling all of the way through the hole, and a combination of the effect of the point striking non-homogeneous material in a workpiece and the drill tending to unwind in a non-uniform manner results in a serious drift or wander problem.

However, with the thickness taper of the present invention, once the point of the blade has gone past the surface through which the cutter is entering, then the thickness taper will cause the cutting edge proximate the point of the blade 12 always to trail the cutting edge of the blade above the point, so that all of the way through the hole the upper part of the cutting edge will lead the part of the cutting edge proximate the point and the point will never control the direction of drilling and reaming regardless of the point encountering non-homogeneous workpiece material. The result is the substantially complete elimination of the usual point drift or wander, with tapered holes of improved reliability and quality being produced.

The preferred range for the included thickness taper angle 32 is from about 40 to about 61/20, while the maximum range for the included thickness taper angle 32 is from about-30 to about 80. The optimum thickness taper angle 32 for the cutting of any particular tapered hole depends upon the material of which the workpiece is composed. For example, tests indicate that in a blade type cutter 10 employed for cutting a tapered hole in aircraft aluminum or titanium, the best included thickness taper angle 32 is approximately 41/20. For a cutter 10 employed to cut a tapered hole in Inconel, or steel such as 9 Nickel 4 cobalt steel or PH13-8 age hardenable stainless steel, the best included thickness taper angle 32 is approximately 60.

An included thickness taper angle 32 of at least about 30 is required to minimize or substantially eliminate point drift or wander and resulting hole "triangulation" so that accurate point tracking will occur throughout the length of each hole, and the cutting of substantially "true" tapered holes will be reliable. As the thickness taper angle 32 is increased above the minimum angle of about 30, there is, of course, even less tendency for the point to drift, and the greater thickness taper angle gives the forward blade portion 12 of the cutter 1 0 greater structural strength, which is desirable for cutting through relatively tough material such as Inconel or steel.However, as the thickness taper angle 32 is increased, the increased lead of the upper part of the cutter edge over the part proximate the point results in the cutter not cutting as freely, and greater torque and axial thrust are therefore required. Also, as the thickness taper angle 32 is increased, the resulting increased thickness of the upper or rearward part of the blade 1 2 begins to block the chip exit so that the chips do not flow out as freely; and at the same time the lateral spacing between the cutting and trailing edges at each of the side surfaces 30 of the blade 12 decreases so that there is a less discrete definition between the cutting fluid entrance passage and the chip exit flow space, with a resulting tendency of the cutting fluid inflow to interfere with the free exiting of the chips.Above about an 80 thickness taper angle 32 the accumulation of these adverse factors tends to cause galling and questionable hole quality.

The cutter 10 is adapted to be driven in a clockwise direction (looking forward from the rearward shank or mounting portion 14) for cutting a tapered hole. Thus, as viewed in Figure 5 the cutter 10 will be rotated clockwise, while as viewed in Figures 4 and 6 the cutter 10 will be rotated anti-clockwise. Thus, each of the side surfaces 30 has a leading edge and a trailing edge, and these leading edges of the side surfaces 30 where the side surfaces 30 intersect the respective edge surface 34 form a pair of elongated blade cutting edges 38 which are best seen in Figures 4 and 6, but are also seen in Figures 1 and 2. In the present invention these cutting edges 38 are each provided with a substantial negative rake angle, and this is another important aspect of the invention.This negative rake angle is the angle 40 illustrated in Figure 6, and is the angle included between a radius line which is shown as a dotted line from the axial center of the blade to one of the cutting edges 38 and the direction of the side surface 30 where it terminates at the cutting edge 38. The preferred range for the cutting edge negative rake angle 40 proximate the point end of the blade 1 2 is from about 100 to about 160. The purpose of this relatively large negative rake angle is to cause the blade 12 to reliably break up the chips that are formed to a small enough size so that they can be continuously blown out of the hole by the cutting fluid throughout each hole cutting operation.

The conventional twist drill or reamer has a positive rake angle, and tends to cut the chips in the form of strings which are jacked out of the hole by the helical flutes of the twist drill or reamer. However, a blade type of cutter cannot jack the chips out, and the chips must therefore be blown out, which causes the chip control problem to become a critical one. As the blade 12 of the present cutter is cutting a hole through a workpiece, the two jet streams of cutting fluid which are directed from channels 28 along the leading, cutting edges 38 of blade 12 pick up the chips and blow them back out of the hole proximate the trailing edges of the blade.It will be apparent from the drawing that there is only a very limited amount of space available between the side surfaces 30 of blade and the wall of the hole for both entry of the cutting fluid jet streams and ejection thereby of the chips. It has been found that if the negative rake angle is too small, or if there is a zero or positive rake angle, then the chips tend to form in strings, which prevents the chips from being cleanly blown out of the hole.

Chip control is then lost, the chips tending to clog up the fluid and causing galling and degeneration of the finish in the hole. On the other hand, if the negative rake angle is too great, then the cutting shear planes appear to get confused and tearing of the workpiece material tends to occur rather than cutting which again results in galling in the hole and degeneration of the finish in the hole.

The configuration of each of the edge surfaces 34 is also an important factor in achieving reliable hole surface quality. Each of the edge surfaces 34 includes a leading bearing surface portion 42 immediately adjacent to the respective blade cutting edge 38, and a trailing relief surface portion 44 which occupies the remainder of the respective edge surface 34. Each of the bearing surface portions 42 is arcuate relative to the central longitudinal axis of the blade 12 as best seen in Figure 6, while each of the relief surface portions 44 is preferably flat. As shown in Figure 6, each of the relief surface portions 44 trails its respective bearing surface portion 42 at a relief angle 46 relative to a line 47 perpendicular to the side surfaces 30 and to the axial plane 33.For convenience in visualizing this relief angle 46, a circle 48 has been shown in phantom, which is coaxial with the longitudinal axis of the blade 1 2 and is coincident with the arcsof the bearing surface portions 42.

The width of each of the bearing surface portions 42, between the respective cutting edge 38 and relief surface portion 44, is preferably within the range of from about .003 inch to about .010 inch. If the bearing surface portions are too wide, then the edge surfaces 34 tend to "clad up" with material from the workpiece. This is a critical problem when cutters according to the present invention are employed for cutting tapered holes in aluminum workpieces. On the other hand, if the bearing surface portions 42 are too narrow, then the cutter will tend to lose its axial stability and there will be a tendency to produce out-of-round holes.

The preferred range for the edge margin relief angle 46 is from about 3 to about 80, with a preferred relief angle 46 of about 50 30t. If the, relief angle 46 is too small, then more axial push is required on the tool, and the blade does not cut freely, with a result that rubbing and galling occur proximate the trailing edges of the edge surfaces 34 and hole quality becomes questionable. On the other hand, if the relief angle 46 is too great, then the blade 1 2 tends to "chatter" in the hole, and stability tends to be lost so that the holes are not as consistent.

For most workpiece materials it is preferred to employ a point 50 of the type commonly referred to as a helical drill point, and this is the type of point 50 that is illustrated in the drawing. Thus, for aircraft aluminum or titanium workpieces, the point 50 will normally be of the helical type. For some workpiece materials, such as steels, a "split crankshaft" type point suchas the standard NAS 907-P3 point may altematively used.

Point 50 includes center or crown 52 and forwardly facing, generally helically formed point surfaces 54 which intersect at their leading edges with side surfaces 30 to form point cutting edges 56. The point angle 58, which is the angle between the point cutting edges 56 forwardly of the point as shown in Figure 2, is preferably on the order of about 1350.

The point cutting edges 56, like the blade cutting edges 38, are provided with a negative rake angle to assure good chip breakup at the point cutting edges 56 as the point is advancing first through the initial surface of the workpiece and then through the body of the workpiece. This negative rake angle for the point cutting edges 56 is provided by extending the flat side surfaces 30 straight down to their intersections with the point surfaces 54. Accordingly, as will be apparent from Figure 3 of the drawing, the negative rake angle for the point cutting edges 56 will be equal to onehalf of the included thickness taper angle 32, and will be in a preferred range of from about 20 to about 31/40, and a maximum range of from about 11/20 to about 40.The negative rake angle for each of the point cutting edges 56 is that angle included between the axial plane 33 and the respective side surface 30.

A further factor of importance regarding the point 50 of blade 12 is the ratio of blade thickness to blade width proximate the point 50. Having the blade thickness/width ratio proximate the point within the range of from about 1/3 to about 1/5, and preferably about 1/4, is important to assure accurate tracking of the point, and avoidance of point drift or wander, from first penetration of the point into the surface of the workpiece until the point 50 has completed penetration through the surface of the workpiece and the edge bearing surface portions 42 commence to enter the hole.

The countersink-forming portion 18 includes flat side surfaces 60 which are simply rearward continuations of the respective blade side surfaces 30, arcuate edge surfaces 62 which are forward continuations of the annular collar 16, and inclined forwardly facing surfaces 64 which are helically formed similarly to the forward point surfaces 54. The leading edges of the inclined forwardly facing surfaces 64 intersect the respective side surfaces 60 at countersink cutting edges 66. The countersink cutting angle 68 between countersink cutting edges 66 as shown in Figure 2 will typically be about 1000. It is to be noted that the countersink cutting edges 66 also have a negative rake angle which is the same negative rake angle as that of the point cutting edges 56.

Figure 7 diagrammatically illustråtes another embodiment of the present invention wherein the cutter 1 0a has a concave-sided blade 12a wherein the side surfaces are concave in transverse cross section, while nevertheless being straight in longitudinal section.

The elongated blade cutting edges 38a of concave-sided blade 12a have a negative rake angle 40a which is still within the preferred range of from about 100 to about 1 60, despite the concavity of the side surfaces 30a.

The other ranges set forth hereinabove for the form of the invention shown in Figures 1-6 also apply to the form of the invention shown in Figure 7. Thus, the edge bearing surface width for the concave-sided blade 12a is preferably within the range of from about .003 inch to about .010 inch; the edge margin relief angle for the blade 1 2a is preferably from about 30 to about 80 relative to an axial plane corresponding to the axial plane 33 of Figure 3; and the ratio of the cutter thickness at the center of the point to the cutter width proximate the point is preferably from about 1/3 to about 1/5.Further the blade thickness taper angle is preferably from about 30 to about 80, and the point cutting edges preferably each have a negative rake angle of one-half of the blade thickness taper angle, or from about 1 1/20 to about 40. The blade 12a will also have a width taper angle similar to that of the blade 12 o cutter 10, as for example a taper of about 1/4 inch per foot, or about 101 '38".

Figure 8 illustrates the manner in which a blade type cutter 10 according to the present invention is employed for cutting a tapered, countersunk hole in a metal workpiece. The workpiece is generally designated 70, and in the illustration comprises a pair of overlapping structural members 72 and 74 forming parts of a highly stressed structure which are to be fastened together by a tapered interference fastener.

In Figure 8 the point 50 of blade 12 has progressed completely through the workpiece members 72 and 74 and the elongated blade cutting edges 38 have reamed the tapered bore 76 out to its final dimensions. Similarly, the cutting edges 66 of countersink-forming portion 18 of the cutter 10 have reamed out a countersink 78, which is optional to the fastening system, in the upper end of bore 76. With the various angles and dimensions of the cutter blade 12 maintained within the ranges set forth hereinabove, the tapered hole 76 will be reliably made as to both dimensional control and surface finish control.

Accordingly, the required hole configuration accuracy and surface finish necessary for tapered interference fastner installations are reliably provided by the present invention.

Claims (27)

1. Atapered hole cutter which comprises: An elongated, generally flat cutter blade having front and rear ends, a pair of opposite sides and a pair of opposite edges, said front end of said blade comprising a drill point, said edges of said blade comprising a pair of elongated cutting edges, said blade being tapered in the width direction between said cutting edges from a relatively wider rear end to a relatively narrower front end, and said blade being tapered in the thickness direction between said sides from a relatively thicker rear end to a relatively thinner front end.

2. A tapered hole cutter as defined in claim 1, wherein said width taper is substantially uniform along the length of said blade.

3. A tapered hole cutter as defined in claim 1, wherein said thickness taper is substantially uniform along the length of said blade.

4. A tapered hole cutter as defined in claim 3, wherein said thickness taper has an included angle within the range of from about 30 to about 80.

5. A tapered hole cutter as defined in claim 3, wherein said thickness has an included angle within the range of from about 40 to about 6 1/20.

6. A tapered hole cutter as defined in claim 1, wherein said elongated cutting edges each have a negative rake angle.

7. A tapered hole cutter as defined in claim 6, wherein said negative rake angle is within the range of from about 100 to about 160.

8. A tapered hole cutter as defined in claim 1, wherein said edges of said blade each further comprise a leading bearing surface portion next to the respective said cutting edge and a trailing relief surface portion next to said bearing surface portion.

9. A tapered hole cutter as defined in claim 8, wherein each of said bearing surface portions is arcuate in transverse section.

10. A tapered hole cutter as defined in claim 9, wherein the width of each of said bearing surface portions is within the range of from about .003 inch to about .010 inch.

11. A tapered hole cutter as defined in claim 9, wherein each of said relief surface portions breaks from its respective said bearing surface portion at a relief angle within the range of from about 30 to about 80.

1 2. A tapered hole cutter as defined in claim 11, wherein said relief angle is about 5 1/20.

13. A tapered hole cutter as defined in claim 9, wherein each of said relief surface portions is substantially straight in transverse section.

1 4. A tapered hole cutter as defined in claim 1, wherein said drill point comprises a helical drill point.

15. A tapered hole cutter as defined in claim 1, wherein said drilhpoint comprises a pair of cutting edges each having a negative rake angle.

1 6. A tapered hole cutter as defined in claim 15, wherein said negative rake angle of each of said point cutting edges is within the range of from about 1 1/2 to about 40.

17. A tapered hole cutter as defined in claim 15, wherein said negative rake angle of each of said point cutting edges is within the range of from about 20 to about 3 1/40.

18. A tapered hole cutter as defined in claim 1, wherein said blade has a thickness/width ratio proximate said drill point within the range from about 1/3 to about 1/5.

19. A tapered hole cutter as defined in claim 1, wherein said blade has å thickness/width ratio proximate said drill point of about 1/4.

20. A tapered hole cutter as defined in claim 1, wherein said sides of said blade are substantially flat.

21. A tapered hole cutter as defined in claim 1, wherein said sides of said blade are concave in transverse section.

22. A tapered hole cutter as defined in claim 21, wherein said sides of said blade are substantially straight in longitudinal section.

23. A tapered hole cutter as defined in claim 1, which comprises countersink-forming means integrally formed on said rear end of said blade.

24. A tapered hole cutter as defined in claim 23, wherein said countersink-forming means comprises a widened rear end portion of said blade having sides which are extensions of said blade sides.

25. A tapered hole cutter as defined in claim 1, which comprises mounting means extending rearwardly from said blade'adapted for connection to a rotary driving tool, said mounting means having cutting fluid passage means therein with a rearward fluid input portion adapted to receive cutting fluid from said driving tool and a forward jet stream-forming portion comprising a pair of passages each of which is substantially aligned with a respective one of said blade edges and is adapted to direct a jet stream of cutting fluid along the respective said cutting edge.

26. A tapered hole cutter as defined in claim 25, wherein said mounting means comprises a rearwardly extending# shank integral with said blade and a holder sleeve engageable over said shank and connectable to said driving tool, said cutting fluid passage means comprising a pair of open grooves in said shank which are at least partly covered by said sleeve.

27. A tapered hole cutter substantially as described with reference to the accompanying drawings.