DRILL BIT FOR FIBER REINFORCED PLASTICS

Abstract: A rotary drill bit (100) suitable for drilling fiber reinforced plastic (6), which drill bit comprises a spade drill head (2) comprising a primary cutting edge (9); a disc-shaped base (3) intergral with and axially aligned with said drill head, having a diameter substantially equal to the width of said drill head wherein the radially outer edge (5) of the rearward face of said base is of sufficiently small radius to form a fiber-cutting reverse cutting edge; a collar (4) integral with and axially aligned with said base, having diameter substantially less than that of said base; and, preferably, a shank (14) integral with and axially aligned with said collar, having diameter less than or equal to that of said base.

Description: DRILL BIT FOR FIBER REINFORCED PLASTICS TECHNICAL FIELD The present invention relates to a rotary drill bit, and more particularly to a rotary drill bit adapted to drill a sharply defined hole throught fiber reinforced plastic with a single pass of the drill bit. BACKGROUND ART Fiber reinforced plastic materials are well known to offer many physical properties advantageous in the fabrication of devices such as, for example, automotive vehicle body panels and aircraft body panels and the like. Both thermoplastics and thermoset plastics are used in such fiber reinforced plastic materials. Known fibers include, for example, nylon, Kevlar (trademark, DuPont de Nemours and Company, Wilmington, Delaware), graphite and most commonly glass fibers. it is often necessary to drill holes through fiber reinforced plastic panels to provide a means of attachment of the panel, for example, to a second such panel, or to a device to be mounted thereon or for various other reasons. While drill bits are known which are •' suitable to provide a drilled hole through fiber reinforced plastic, a drill bit is needed which produces a sharply defined hole without leaving fiber-overhang and without causing so-called "volcanoing". Fiber-overhang is the presence of short lengths of reinforcing fibers extending from the surface of the fiber reinforced material into or about the drilled hole. When a drill bit of known design is passed through a fiber reinforced plastic section to provide a drilled hole, some reinforcing fibers within the drilled hole are not cut exactly at the surface of the drilled hole. Rather, short lengths of such fibers remain, which extend from the plastic at the surface of the drilled hole. So-called "volcanoing" is the bulging of the surface of the fiber reinforced plastic at the edge of the drilled hole, typically on the exit side of the hole. This is due, at least in part, to fracturing of the reinforced plastic as the drill bit approaches and breaks through the surface of the fiber-reinforced plastic material at the exit side of the drilled hole. Both fiber-overhang and volcanoing present aesthetic disadvantages. In addition, both fiber-overhang and volcanoing present serious disadvantages in subsequent fabrication steps employing the fiber reinforced plastic section. For example, if a reinforced plastic panel is intended to be joined to another panel through the drilled hole, with planar surface contact between them, volcanoing can, in effect, increase the thickness of the fiber reinforced panel at the attachment location and thus prevent close contact between the two joined panels. The result can be decreased structural rigidity, decreased strength, a greater likelihood of attachment failure, and increased fabricating costs. While drilled holes of poor quality " can be improved by additional processing steps- subsequent to drilling, such additional processing steps entail increased fabrication costs and decreased production efficiency. The principle object of the present invention is to provide a new and improved drill bit for drilling holes through fiber reinforced plastic. More particularly, it is an object of this invention to provide a drill bit which provides, at conventional machining speeds, a hole of improved quality, specifically a sharply defined hole without volcanoing and without fiber-overhang. In addition, it is an object of this invention to provide such a drill bit which can be easily fabricated at OMP relatively low cost. It is yet another object to provide a drill bit which has high strength and durability and low fragility. Various drill bits designs are known to the 5 skilled of the art. U.S. Patent No. 3,170,224 discloses a rotary cutting tool in which the backface of the drill head is provided with a cutting edge so as to either deburr or chamfer the work piece. U.S. Patent No. 2,575,239 discloses a drill bit shank having a transversly 10 extending slot 11 and a reversable bit. U.S. Patent No. 4,040,764 discloses a tool holder 10 and Fig. 1 therein illustrates the tool holder holding a cutting tool 13, which cutting tool is a spade drill. U.S. Patent No. 4,120,601 discloses a spade drill in which the drill head 15 is inserted into a slotted blade holder. A cutting edge 17 is shown. The blade extension 11 also appears to disclose a cutting edge. The width of the spade drill is apparently wider then the circumference of the drill holder. All such known drill bits, however, fail to 20 provide a sharply defined drilled hole by a single pass through and back out of fiber reinforced plastic. Finally, U.S. Patent No. 3,824,027 shows a reamer attached to a reduced diameter rod portion 18, which connects the reamer to a rear shank portion 21 which is apparently 25 smaller in dia ter than the reamer. U.S. Patent No. ' • • .. 2,746,722 discloses a reversible drill bit in which a washer bit 12 is employed in combination with a standard drill head. The washer bit is provided with cutting projections 38 which are adapted to engage the inner 30 surface of the drill hold and perform chipping operations so as to facilitate the removal of the entire dirll bit assembly. The above described and still further objects of the present invention will be apparent from the following 5 detailed specification, appended claims and attached drawings. DISCLOSURE OF INVENTION The objects of the invention are realized in accordance with the present invention by providing a rotary drill bit for drilling fiber reinforced plastics, which drill bit comprises: a spade drill head comprising a leading point and a primary cutting edge extending axially rearward from said leading point; a disc-shaped base integral with and axially aligned with the drill head, having a diameter substantially equal to the width of the drill head wherein the radially outward edge of the rearward face of the base, that is, the face adjacent the collar, is sufficiently sharp to form a fiber cutting edge; and a collar integral with and axially aligned with the base, having diameter substantially less than that of the base. The drill bit of the invention preferably is used in conjunction with a suitable drill bit shank. The drill bit is formed integrally with such shank or is formed separately and fitted into the end of such shank to be integral therewith. The shank is axially aligned with the drill bit, having diameter less than or equal to that of the base. BRIEF DESCRIPTION OF THE DRAWING Referring to the drawings, wherein like characters indicate like parts throughout the several views: Fig. 1 is an elevation view of a drill bit according to a preferred embodiment of the present invention fitted in a suitable drill bit shank. Fig. 2 is a plan view of the drill bit of Fig. 1 shown protruding through a drilled hole prior to withdrawal of the drill bit through the drilled hole. Fig. 3 is a view taken from line 3-3 along the axis of the drill bit of Fig. 1. DETAILED DESCRIPTION OF THE INVENTION Referring now to the drawings. Figs. 1-3 illustrate a rotary drill bit 100 according to a preferred embodiment of the present invention. The drill bit comprises a spade drill head 2 integral with and axially aligned with disc-shaped base 3. The disc-shaped base has a substantially uniform diameter substantially equal to the width of the spade drill head measured at the widest point of the spade drill head. A collar 4, preferably cylindrical, is integral with and axially aligned with the aforesaid base. The collar has a diameter substantially less than that of the base. The radially outward edge 5 of the base forms a sharp corner. That is, it forms a corner of non-radiused edge. In the manufacture of a drill bit, for example, in machining it, the rearward surface of the base proximate to edge 5 and the outside surface of- the base are made to form a non-broken edge. In this way, edge 5 forms a cutting- edge sufficiently sharp to cut off any fiber-overhang in the drilled hole upon withdrawal of the drill bit through the fiber reinforced plastic. That is, when the drill bit is withdrawn back through the fiber reinforced plastic after first passing through to form the drilled hole, the edge 5 forms a reverse cutting edge which is highly effective in removing fiber over-hang from the drilled hole. The base 3 preferably has sufficient axial height to provide strength and wear-resistance for the reverse cutting edge. Thus, for example, for a drill bit to provide a 9/64th inch (.36 cm) hole, it is preferred that the base have an OM axial height of about l/16th inch (.16 cm). Since the fibers used in fiber reinforced plastic can have a diameter as small as .001 inch (.003 cm) or less, it will be evident that to provide removal of substantially all fiber over-hang it will be necessary that the reverse cutting edge, and therefore also the base, at edge 5, must have a diameter substantially equal to the diameter of the drilled hole. Accordingly the diameter of the base must be substantially equal to the width of the spade drill head measured at its widest point. For many typical applications the preferred diameter for the drilled hole, and hence the preferred diameter for the spade drill head and the base is from about .19 inch (.48 cm) to about .26 inch (.66 cm) . It will be apparent from the foregoing description that the collar portion of the drill bit is recessed to allow the uncut fibers remaining on the surface of the drilled hole (after the drill bit has passed into and through but not yet back through the fiber reinforced plastic) to spring or fall away from the wall of the drilled hole into the open space surrounding the collar. As the drill bit is being withdrawn back through the fiber reinforced plastic to complete a single pass of the drill bit, the uncut fibers are thus presented to be cut by the reverse cutting edge. The collar is preferably cylindrical and in drill bits according to the invention wherein the diameter of the spade drill head and base is, for example, from about .19 inch (.48 cm) to about .26 inch (.66 cm), it is generally most preferred that the collar portion have a diameter of about .09 inch (.23 cm) to about .13 inch (.33 cm), respectively. Likewise, the collar should be long enough, i.e., have sufficient axial length, to permit the overhanging fibers to spring or fall into the gap surrounding the collar as the drill bit is being withdrawn at routine machining speeds out of the fiber reinforced plastic. In general, it is not necessary or even desirable that the collar be equal- in length to the thickness of the fiber reinforced plastic. For many typical applications, wherein the diameter of the spade drill head and base is from about .19 inch (.48 cm) to about .26 inch (.66 cm), it is preferred that the collar have an axial height of about .135 inch (.34 cm). Opposite the drill head and . base, the collar preferably is integrally attached to a suitable stem or shank 14 of substantially uniform diameter. The shank diameter is preferably less than or equal to that of the base of the drill bit. More preferably the shank has a diameter only minimally less than that of the base. Thus, for example, the shank may typically be .001 inch (.003 cm) less in diameter than the base. This prevents binding when the shank passes through the drilled hole. Referring to Fig. 2, upon withdrawal of the drill bit back through the fiber reinforced plastic. 6, the reverse cutting edge 5 will encounter the fiber over-hang 7 which is in the open space surrounding the collar 4, and will cut off such fiber over-hang substantially at the surface of the drilled hole. The spade drill head can comprise any suitable design of which numerous variations are known . to the skilled of the art. From the leading point 8 of the drill head, that is, from the point which first contacts the fiber reinforced plastic, a primary cutting edge 9 extends axially rearward towards the base and collar. The angle of the primary cutting edge, that is the primary entry angle should be sufficiently acute to minimize or avoid any -break-out effects causing the aforementioned volcanoing which often occurs in the drilling of reinforced plastic. Preferably the primary entry angle, measured as the acute angle of intersection between the axis of- the drill bit and the primary cutting edge, is about 30° to about 60°, most preferably about 45 ;'o OMPI At greater primary entry angle, for example, near 60°, the spade drill head will be less fragile, but in using the drill bit, since there is a shorter cutting edge, greater axial drilling pressure will be reguired and 5 greater wear can result. In addition, a greater tendency to cause volcanoing can result. At a lesser primary entry angle, for example, at or near 30°, the spade drill head will tend to be somewhat more fragile near the leading point, but will require less axial drilling pressure and 10 will suffer less wear. The entire cutting edge of the drill head can be presented at the above described- primary entry angle. Alternately, a portion of the cutting edge can be provided at the primary cutting angle and a portion extending 5 axially rearward of the primary cutting edge can be provided at a secondary entry angle. Accordingly, as illustrated by the preferred embodiment of Figs. 1-3, a portion of the cutting edge, secondary cutting edge, is provided at a r more acute angle than the primary entry 0 angle. Preferably, the cutting edge of the spade drill head provides a secondary entry angle of from about 15° to about 45°, most preferably about 22°. One or more additional portions of the cutting edge can be extended axially -rearward from the secondary 5 cutting edge at greater or lesser entry angle(s). • * .- Preferably the cutting edge does not extend to the base. Rather, the cutting edge preferably reaches its maximum width and terminates somewhat above, that is, axially forward of the base. The edge of the spade drill head can 0 then extend axially rearward to the base through a tertiary portion 11 parallel or ' substantially parallel to the axis of the drill bit. In this case, the spade drill head can be said to provide a tertiary entry angle more acute than the secondary entry angle, more specifically 5 this preferred" embodiment provides a tertiary entry angle of 0° or approximately 0 β . According to a preferred embodiment, and as shown in Figs. 1-3, the narrow sides of the spade drill head, that is the thin strip of surface 12 adjacent each cutting edge preferably has a rake or back-draft of about 10°. Providing such rake has been found to provide greater ease of machining and a more sharply defined drill hole. Such rake is especially advantageous where the thickness of the spade drill head, that is the distance between the opposing faces 13 of the spade drill head, where the spade drill head meets the base, approaches or equals the full diameter of the drill bit. It should be understood that there need not be a sharp line of demarcation between the base and the spade drill head. Rather the two opposing faces of the spade drill head can fair gradually into the full diameter of the base. Alternatively, the thickness of the spade drill head where it meets the base can be substantially less than the diameter of the drill bit. Where the thickness of the spade drill head is substantially less than the diameter of the drill bit, the two faces of the spade drill head can be flat and either parallel or convergent toward the leading point such that the spade drill head is more narrow toward the leading point. Preferably they are convergent. Most preferably, the two faces of the spade drill head .are flat, that is planar, or substantially flat, and convergent toward the. leading point, such that the two faces form an angle of intersection with the axis of the drill bit of from about 1° to about 15°. From many typical applications such angle of intersection is most preferably about 10°. The thickness of the spade drill head at the leading point can be from approximately zero, that is, in ' a practical sense, minimally greater than zero, to any suitable width. For many typical applications, a preferred ' thickness, at the leading point is from minimally greater than zero to about .02 inch (.05 cm), more preferably about .016 inch (.04 cm). Some thickness more than minimally greater than zero is preferred to make the drill bit less fragile at the leading point. This is especially true where extremely hard but somewhat brittle materials are used, for example, carbide steels. For many typical applications, the spade drill head of the drill bit of the invention can have an axial length approximately .25 inch (.64 cm), that is, it can be .25 inch (.64 cm) in length from the leading point to the beginning .of the base. Where the spade drill head provides a compound angle of entry such as, for example, in the preferred embodiment of Figs. 1-3, the primary cutting edge providing the primary angle of entry can extend along a portion of the edge of the spade drill head equivalent to an axial length of about .035 inch (.09 cm). The secondary cutting edge which provides a secondary angle of entry can extend along the remainder of the edge of the spade drill head to the. base. More preferably, however, the secondary cutting edge of the spade drill head can extend along a portion of the edge of the spade drill head equivalent to an axial length of about .1 inch (.25 cm). It is generally preferred that the ratio of the axial length of the primary cutting edge to that of the secondary cutting edge, if any, for the preferred angles of entry disclosed above, be approximately .35 : 1, respectively. The novel drill bit of the present invention can be fabricated according to methods well known to the skilled of the art. Thus, for example, the spade drill head, base and collar portions can be fashioned together - from a single piece of suitable drill bit material. Such materials include many well known to the skilled 'of the art such as, for example, carbide steels, and the like. A preferred material is grade C3 carbide steel. The unitary spade drill head, base and collar can be provided separately or in combination with a suitable shank 14. The end face of the shank can be provided with a suitable receiving void or socket. A portion of the collar or an extension therefrom is then inserted into such socket and fixed by suitable means, for example, by braising. The shank can comprise any suitable material, of which many are known to the skilled of the art, for example, high speed steel. The above-described materals and additional alternative materials and the above-described methods of fabrication and alternative methods will be within the skill of the art in view of the present disclosure. Modifications of the novel drill bit described herein, falling within the scope of the invention, will also be apparent in view of the present disclosure. Thus, for example, the entry angle of the cutting edge of the spade drill head need not be constant nor change abruptly. Rather, it can change in continuous fashion over all or a portion of its length. It is expected, however, that this modification would entail increased fabrication costs and would be less preferred. The novel drill bit of the present invention provides a significant advance in the fabrication of fiber reinforced plastics. In particular, the novel drill bit of the invention provides a drilled hole of improved quality, that is, a drilled hole more sharply defined than was previously possible at conventional machining speeds and cycle times. Most significantly, the novel drill bit provides drilled holes with substantially no fiber-overhang and substantially no volcanoing. Accordingly, drilled holes can now be provided in fiber reinforced plastics suitable for fabrication uses requiring close tolerances, without additional processing steps subsequent to the drilling to improve the quality of the drilled hole. In addition, the drill bit of the invention can be fabricated to exact specifications at relatively low cost. It has high strength and durability and low fragility and is thus suitable for use under demanding conditions for high productivity. OMPI While several specific embodiments of the novel drill bit of the invention have been described and illustrated in detail, the present invention is not limited to these specific embodiments. Rather, it will be understood and obvious to those skilled in the art, given the present disclosure, that the invention can be embodied otherwise than as described and illustrated, without departing from the scope of the invention. INDUSTRIAL APPLICABILITY It will be apparent in view of the foregoing disclosure that the present drill bit invention has industrial applicability to the manufacture and assembly of fiber-reinforced plastic components including, for example, automotive vehicle body panels and airplane body panels and the .like, and that it provides a drill bit for drilling sharply defined holes through fiber-reinforced plastic.

Claims: I claim: 1. A novel rotary drill bit for drilling fiber reinforced plastics, comprising: a spade drill head comprising a central axis, a leading point at its frontend and a primary cutting edge sloped rearward and radially outward from said leading point; a disc-shaped base integral with and axially aligned with said drill head, having a diameter substantially equal to the width of said spade drill head, wherein the radially outer edge of the rearward face of said base ' .is sufficiently sharp to form a fiber cutting edge; and a collar integral with and axially aligned with said base, having diameter substantially less than that of said base. 2. The drill bit of claim 1 wherein said drill head, base and collar comprise carbide steel. 3. The drill bit of claim 1 wherein spade drill head provides a primary entry angle of from about 30° to about 60°. 4. The drill bit of claim 3 wherein said primary entry angle is about 45°. OMPI 5. The drill bit of claim 3 wherein a secondary cutting edge of said spade drill head extends rearward and radially outward of said primary cutting edge at a secondary entry angle more acute than said primary entry angle. 6. The drill bit of claim 5 wherein said secondary entry angle is from about 15° to about 45°. 7. The drill bit of claim 6 wherein said secondary entry angle is about 22°. 8. The drill bit of claim 5 wherein a tertiary portion of said spade drill head extends rearward of. said secondary cutting edge and provides a tertiary entry angle more acute than said secondary entry angle. 9. The drill bit of claim 8 wherein said tertiary entry angle is approximately 0°. 10. The drill bit of claim 1 wherein the cutting edge of said spade drill head has a rake of about 10°. 11. The drill bit of claim 1 wherein the thickness of said spade drill head is substantially less than the diameter of said drill bit and the two faces of said spade drill head are substantially flat and are convergent toward said leading point. 12. The drill bit of claim 11 wherein the two faces of said spade drill head form an angle of intersection with the axis of said drill bit of from about 1° to about 15°. 13. The drill bit of claim 12.wherein said angle of intersection is about 10°. 14. The drill bit of claim 1 wherein the two opposing faces of said spade drill head are- substantially planar. 15. The drill bit of claim 1 wherein the thickness between the two opposing faces of said spade drill head at the leading point of said spade drill head is from minimally greater than 0 to about .02 inch. 16. The drill bit of claim 15 wherein the thickness of said spade drill head at said leading point is about .016 inch. .17. The- drill bit of claim 1 wherein the thickness between the two opposing faces of said spade drill head, where said spade drill head meets said base, is substantially equal to the diameter of said base. 18. The drill bit of claim 1 wherein the axial height e-f said spade drill head is about .25 inches. 19. The drill bit of claim 1 wherein the axial height of said primary cutting edge is about .035 inch. 20. The drill bit of claim 1 wherein the axial height of secondary cutting edge is about .1 inch. 21. The drill bit of claim 1 wherein said base diameter is from about .19 inch to about .26 inch. 22. The drill bit of claim 1 wherein said base has an axial height of about .06 inch. 23. The drill bit ' of claim 1 wherein said collar is cylindrical. 24. The drill bit of claim 22 wherein said collar has a diameter of about .09 to about .13 inch. 25. The drill bit of claim 1 " wherein said collar has an axial height of about .135 inch. 26. The drill bit of claim 1 further comprising a drill bit shank integral with said drill fait, which shank is axially aligned with said drill • bit and of diameter less than or equal to that of said base. 27. The drill bit of claim 26 wherein said shank has a diameter minimally less than that of said base. 28. The drill bit of claim 26 wherein said shank has a diameter about .001 inch less than that of said base.

Inventors: SKIDMORE MICHAEL L, SKIDMORE, MICHAEL, L.
Applicants: FORD FRANCE, FORD MOTOR CANADA, FORD MOTOR CO, FORD WERKE AG, FORD FRANCE S.A., FORD MOTOR COMPANY, FORD MOTOR COMPANY LIMITED, FORD MOTOR COMPANY OF CANADA, LIMITED, FORD-WERKE AKTIENGESELSCHAFT
Application year: 1982
Main classification: B23B51
IPC codes: B23B51