GB2074061A - Countersinking holes in a sheet of glass fibre or carbon fibre reinforced material - Google Patents

Abstract

Material is removed from the sheet by a rotatable tool provided with at least one cutting edge (8) formed in a diamond tool bit (7), the said cutting edge being disposed along the generatrix of a conical surface the axis of which coincides with the axis of rotation of the tool. The bit is sintered to a tungsten carbide plate (10) welded in a recess of the tool. The tool is guided by a shank (13). <IMAGE>

Description

SPECIFICATION A method for countersinking holes formed in a sheet of glass fibre or carbon fibre reinforced material The present invention relates to a method of countersinking holes formed in a sheet of carbon fibre of glass fibre reinforced material, by means of which it is possible to obtain countersunk holes such as are required in aeronautical applications, substantially free from defects and with very reduced working times.

As is known, in aeronautical applications, use is made of structures consituted by several sheets of carbon fibre and/or glass fibre reinforeced materials and of other metal materials, which are joined together by means of connection members inserted in suitable holes formed in the said sheets; for receiving the heads of these members the associated holes must be suitable countersunk, substantially with a conical surface, in such a way as to form a seat for the head of the member itself.

Particular difficulties are encountered in countersinking holes formed in glass fibre or carbon fibre reinforced materials because of the characteristics of these materials which, as is known, tend to wear tools made of conventional materials quite significantly, and to break up into the form of short fibres at the edges of the countersink (delaminations).

Countersinking is usually performed by means of a process consisting in removal of material from the sheet by means of a rotatable tool provided with suitable cutting edges; the said tool, which is mounted in a hand-held drilling machine or on the movable head of a pillar drill, is usually made of tungsten carbide or is provided with bits or inserts of this material on which the said cutting edges are formed.

In another method the tool is constituted by a suitably shaped grinding wheel the surface of which is covered with diamond powder.

Countersunk holes obtained with one of the above described methods have several disadvantages.

Above all, when the first above described method is used, short fibres of the material of the sheet in which the hole is formed tend to become detached so that this edge is entirely irregular; when, on the other hand, the second method is used, the surfaces of the countersink obtained tend to be rather irregular and somewhat rough, and, in any case, have significant scoring. The wear of the tools, in particular in the first above indicated method, is very high.

The object of the present invention is that of providing a method of countersinking a hole formed in a sheet of carbon fibre or glass fibre reinforced material which will be free from the disadvantages indicated above.

The method of the invention is characterised by the fact that it comprises an operation for removal of the material from the said sheet by means of a rotatable tool provided with at least one cutting edge formed in a diamond bit, the said cutting edge being disposed substantially along a generatrix of a conical surface the axis of which coincides with the axis of rotation of the said tool.

For a better understanding of the method of the invention an example of its use will now be described with reference to the attached drawings, in which: Figures land 2 are two orthogonal side views of a tool used in the method of the invention; Figure 3 is a plan view from below of the tool of Figure 1; Figure 4 is a section of a part of the tool of Figure 1, taken on the line IV-IV of Figure 1; Figure 5 is a schematic side view of the performance of the method with the tool of the preceding Figures; and Figure 6 illustrates the countersunk hole obtained.

The method of the invention is usable for countersinking a hole, such as that indicated with the reference numeral 1 in Figure 5, formed in a sheet of any kind of glass fibre or carbon fibre reinforced material. The said sheets, as has been mentioned, consitute aeronautical structural elements, and the connection between the various sheets is obtained by means of suitable connection members inserted in holes formed in the sheets themselves.

The method comprises substantially one operation with which material from the said sheet is removed by means of a rotatable tool, illustrated in Figures 1 to 4, which is provided with at least one cutting edge formed in a diamond bit and disposed substantially along the generatrix of a conical surface the axis of which coincides with the axis of rotation of the tool. It has been found, in fact, that if the removal of the of the material is performed in the manner indicated, not only is a countersink formed which has very good characteristics of shape and dimensions, but the tool utilised for the performance of this operation is worn significantly less, thereby allowing high productivity to be obtained.

The speed of rotation of the said tool can lie within a very wide range of values, nevertheless obtaining, in each case, entirely satisfactory results; it has been found that this speed can lie between 500 and 18000 rpm.

The said operation can be performed by using a manually controlled drill on which the tool is mounted, with the advantage of giving rise to a very reduced working time, not requiring complex machines or equipment, and allowing a significant flexibility on the work floor.

During the first mentioned operation, the said tool is guided by a shank which can be inserted in the hole which is to be countersunk.

Atool suitable for performing the first described operation of the method is shown in Figures 1 to 4. It comprises a substantially cylindrical body 3 provided with a spigot 4 which can be fixed, for example by means of a threaded coupling, to the spindle of a hand held drilling machine; the lower part of the body 3 of the tool is delimited by a substantially conical surface 5 and has a helical groove 6 of the type formed on conventional countersinking tools.

The said tool is provided with a pair of diamond bits 7 each of which has a substantially rectilinear cutting edge 8 orientated with respect to the axis of the tool in such a way as to lie on a generatrix of a conical surface coaxial with the said axis. The lie of this edge is clearly visiable in the side view of Figure 1, whilst the cross sectional shape of it can be seen clearly in Figure 4.

The diamond of the bit 7 is conveniently a normal polycrystalline industrial diamond grit.

Each of the two tool bits 7, in the illustrated embodiment of the tool, is conveniently supported by a corresponding tungsten carbide plate 10 which operates to support the forces which are transmitted to the diamond bit during working, and to transmit them to the body of the tool 3. For this purpose each of the these tool bits and the associated tungsten carbide plate 10 are joined together by sintering the assembly thus obtained is lodged in a corresponding recess 11 (Figure 4) of the tool body and the fixing of the first to the second can be obtained in any convenient manner, for example by welding the plate 10 to the body itself.

The illustrated embodiment has two diamond tool bits offset by substantially 180 C; it is, however, apparent that the number of tool bits can be different from this. The body 3 of the tool is moreover provided with a central hole 12 able to receive a substantially cylindrical shank 13 having a diameter slightly less than that of the hole 1 which is to be countersunk, and the connection of the said shank to the body can be effected, for example by means of a clamping grub screw inserted in a corresponding threaded hole 14 in the tool body, as can be clearly seen in Figure 2.

In Figure 5 the tool is shown in the operation of the method first described. During this, the operator first inserts the shank 13 into the hole 1, thereby perfectly guiding the tool with respect to the hole, and subsequently moves the cutting edges 8 of the diamond tool bits 7 into contact with the material of the sheet 2, whilst the tool is driven to rotate at a speed lying within the range first indicated; in this way there is obtained a conical countersink 15 which, at the end of the operation, has the form shown in Figure 6. It has been found that the said countersink has a conical surface 16 with little roughness and entirely devoid of scoring and damage; moreover, at the edge 17 of the countersink itself there is no separation of the fibres of the material of the sheet 2, but this edge is entirely smooth and continuous. The tolerances on the form and on the dimensions of the countersink thus obtained are very strict.

It has also been established that the same tool can be used to perform several thousands of countersinks before requiring sharpening of the cutting edges 8; consequently the productivity obtainable with the method described is very high.

It will be apparent that it is possible to modify both the operation of the method described and the tool used in the method, within the scope of the invention.

Claims (9)

1. A method of countersinking a hole formed in a carbon fibre or glass fibre reinforced material, characterised by the fact that it comprises an operation for removal of material from the said sheet by means of a rotatable tool provided with at least one cutting edge formed on at least one diamond tool bit, the said cutting edge being located substantially along a generatrix of a conical surface the axis of which coincides with the axis of rotation of the said tool.

2. A method according to Claim 1, characterised by the fact that the speed of rotation of the said tool lies between 50.0 and 18000 revolutions per minute.

3. A method according to Claim 1, or Claim 2, characterised by the fact that the said operation is performed by means of a manually controlled drill on which the said tool is mounted.

4. A method according to any preceding Claim, characterised by the fact that the or each said diamond tool bit is supported by a tungsten carbide plate.

5. A method according to any preceding Claim, characterised by the fact that the said tool is provided with a cylindrical shank coaxial with the said axis of rotation of the said tool and able to be inserted, with a predetermined radial play, in the said hole to be countersunk.

6. A method according to any preceding Claim, characterised by the fact that the said tool includes two said diamond tool bits provided with respective cutting edges, disposed on the said tool and displaced by substantially 1800 from one another.

7. A method according to Claim 6, characterised by the fact that the said tool is a conical countersinking tool provided with recesses able to receive the said tool bits, there being provided means for fixing the said tool bits in the said recesses.

8. A method according to Claim 7, characterised by the fact that each of the said tool bits is fixed to the associated tungsten carbide plate by sintering, the said plate being fixed to the associated recess by welding.

9. A method for countersinking a hole, substantially as described and as illustrated in the attached drawings.