GB2074059A - Making holes of a predetermined diameter in an assembly of superimposed sheets - Google Patents

Abstract

The sheets include at least one glass fibre or carbon fibre reinforced sheet, a sheet of steel and a sheet of titanium. Initially a manually controlled twist drill produces an undersized hole. Enlargement of the hole is performed with an automatic drill provided with a twist drill bit (7) having two sections of different diameters to produce a hole slightly undersized. This hole is then reamed out, the reaming operation being performed with an automatic reaming tool (15) having two sections of different diameters one of which serves as a guide. <IMAGE>

Description

SPECIFICATION A method of making holes of a predetermined diameter in an assembly of superimposed sheets including at least one sheet of a glass fibre or carbon fibre reinforced material, a sheet of steel and a sheet of titanium The present invention relates to a method of making holes of a predetermined diameter in an assembly of superimposed sheets comprising at least one sheet of a glass fibre or carbon fibre reinforced material, a sheet of steel, and a sheet of titanium.

Composite sheets of this type are utilised in the aeronautical field for forming structures with a very high mechanical strength and very low weight. In order to join the sheets of the assembly rigidly together and/or to connect together the various elements of a structure, the above mentioned sheets are connected by means of connection elements inserted in corresponding holes formed in them.

The subject of the present invention is a method of making such holes in composite sheets of the type indicated.

Such holes are usually made by means of a manually controlled drill or by means of a proper drilling machine provided with an operating head, movable with a predetermined speed of advance on suitable guides; in both cases the tool used is a tungsten carbide twist drill bit.

Holes made using one of the above indicated methods have serious disadvantages which make them poorly suited for aeronautical constructions for which construction elements entirely free from defects are required.

In fact, holes obtained in the above indicated manner usually have numerous defects because they are not produced with shape and dimensions corresponding to those intended and, above all, because the surfaces of the holes and the regions in immediate proximity to these are damaged, thus prejudicing the mechanical strength of connections formed thereby, which connections must resist, as is known, very high working stresses.

In such holes there are sometimes found small cracks and broken pieces of fibre in the region of the edge of the hole formed in the sheet of glass fibre or carbon fibre reinforced material, or else damage to the surface of these produced by steel drilling chips; the roughness of the surfaces of the holes thus obtained is always very high. Moreover, it is only possible to obtain very wide tolerances on the diameters of the holes, and these, very often, have an undesirable ovalisation or conicity.

Finally a significant wear of the tools is encountered, this being due above all to the action produced on their cutting edges by the carbon fibres; these must therefore be replaced very often.

The object of the present invention is that of providing a method of the type indicated with which the disadvantages indicated above will be eliminated so that holes devoid of defects can be formed in an economical manner with shape and dimensions corresponding to those intended, with very strict tolerances and with a high finish on the associated surfaces.

According to one aspect of the present invention there is provided a method of making holes of a predetermined final diameter in an assembly of superimposed sheets comprising at least one sheet of glass fibre or carbon fibre reinforced material, a sheet of steel, and a sheet of titanium, the thickness of the said sheets lying, respectively between 2 and 10 mm, between 0 and 1.5 mm and between 2 and 5 mm, characterised by the fact that for each hole there is performed a first drilling operation using a manually advanced drill provided with a twist drill bit of a first diameter less than the said predetermined final diameter in such a way as to obtain, at the end of the said first operation, a hole with a diameter substantially equal to the said first diameter; a second operation for enlargement of the said hole obtained at the end of the said first operation, performed with an automatically advanced drill provided with a twist drill bit having two sections of different diameters, an upper section of diameter sightly less than the said predetermined final diameter, e.g. about 1/32" less, and a lower section of diameter slightly less than that of the upper section, e.g. about 1/32" less, in such a way as to obtain, at the end of the said operation, a hole with a diameter substantially equal to that of the said upper section diameter; a third operation for reaming the said hole obtained at the end of the second operation, performed with an automatically advanced drill provided with a reaming tool having two sections of different diameters, an upper section of diameter equal to the said final dimeter and a lower section of diameter equal to the said diameter obtained at the end of the said second operation, so as to guide the said reaming tool by means of the said lower section of the reaming tool during the said reaming operation.

According to another aspect of the invention, there is provided a method of making holes of a predetermined final diameter in an assembly of superimposed sheets including at least one sheet of a glass fibre or carbon fibre reinforced material, a sheet of steel and a sheet of tianium wherein each hole is formed in a plurality of operations comprising a pre-reaming operation performed with twist drill bit having an upper section of diameter slightly less than said predetermined final diameter and a lower, leading section having a diameter slightly less than that of the upper section to obtain a hole with a diameter substantially equal to that of the upper section and less than said final diameter; and a second reaming operation for said hole obtained from the preceding operation performed with a reaming tool having an upper reaming section of diameter equal to said final diameter and a lower guiding section closely fitting said hole obtained from the preceding operation so as to guide the reaming tool during the reaming operation.

To provide a better understanding of the method of the present invention an example of the operations will now be described with reference to the attached drawings, in which: Figures 1 and 2 schematically represent the assembly of sheets at the beginning and the end respectively of a first operation of the method; Figures 3 and 4 schematically represent the assembly of sheets respectively at the beginning and end of a second operation; and Figures 5 and 6 schematically represent the assembly of sheets respectively at the beginning and end of a third operation which completes the method.

The method of the invention is used for drilling an assembly of superimposed sheets such as that shown in Figure 1 and constituted by at least a first sheet 1 of glass fibre or carbon fibre reinforced material, a second sheet 2 of steel and a third sheet 3 of titanium; the sheet 2 is conveniently made from a steel of the type which can be rolled thinly, that is one which is able to attain a thickness adapted for location between the other two sheets. Composite sheets of this type are largely used in aeronautical constructions for obtaining structural elements with very high mechanical strength and very low weight; the connection between the sheets of the assembly or between different structural elements is obtained by means of connection members inserted in holes which can be formed with the method of the invention.

The best results of the method are obtained when the thickness of the said sheets 1,2,3 lies, respectively, between 2 and 10 mm, 0 and 1.5 mm and 2 and 5 mm. The diameter of the holes is conveniently between 2.5 and 9 mm.

To obtain each of these holes there is performed a first piercing operation by means of a manually advanced drill (not shown) provided with a tungsten carbide twist drill bit 4; the relative position of this drill bit with respect to the composite sheet to be pierced, at the beginning of this first operation, is shown in Figure 1.

The said twist drill bit is a normal right hand twist bit, of the type used in normal drilling operations, for example with a helical flute forming an angle of 30 with the axis of the drill bit; the cone angle of the tip, indicated a in Figure 1 may be 135 . The diameter d, of the drill bit lies between 1/2 and 2 of the predetermined final diameter of the hole which it is desired to form, whilst the speed of rotation of the drill bit itself, which is constant during the whole of the first operation, may lie between 450 and 550 revolutions per minute.

The operation is performed by inserting the twist drill bit 4 into a guide bush 5 which is suitably supported, for example by means of a plate 6, and having a height h lying between 25 and 30 mm, in such a way that the drill bit itself is firmly guided whilst it penetrates the composite sheet 1, 2, 3. For this purpose a small diametral play is left between the drill bit and the surface of the hole of the bush, which play may be of the order of 0.01 mm.

To perform this operation, the operator, after having positioned the bush 5 so that its axis coincides with the axis of the hole to be formed, introduces the twist drill into the bush itself, applying to the drill a manual force sufficient to cause advance of the drill bit.

During the course of this operation the central web of the hole which is to be formed is substantially removed, thereby obtaining with notable speed, as can be seen in Figure 2, a hole substantially of diameter d, even if it is not very regular both dimensionally and from the point of view of the surface finish.

If the diameter of the final hole which is to be obtained should be very small, the first operation described above could even be omitted, and the second operation now to be described would be the first drilling step.

The second operation of the method consists in enlarging the hole thus obtained: in Figure 3 there is schematically shown the tool, also constituted by a twist drill 7, positioned with respect to the assembly of plates 1,2,3 at the beginning of the said operation.

The twist drill 7 comprises two sections, one a lower section 9 and the other an upper section 8, of different diameters; the diameter of the upper section, indicated dc in Figure 3 is tied to the predetermined final diameter df of the hole which is to be obtained (Figure 6) by the relation dc = df - 1/32(0.8 mm) (1) the diameter of the lower section, indicated dd, is tied to the diameter dc (and therefore on the basis of (1 ) to the diameter df) by the relation: dd = dc - 1/32" = df - 2/32" (1.6 mm) (2) Apart from the presence of the two sections of different diameter, the twist drill 7 is a normal right hand tungsten carbide twist drill, for example with helical flutes inclined by 30 with respect to the axis of the drill.

The tip angle ss (Figure 3), which is the same as the angle of the junction region between the lower section 9 and the upper section 8 of the drill, is 1350.

The twist drill 7 is mounted on the movable head of a drilling machine of the portable type having automatic advance (not shown).

The speed of rotation of the drill 7 may be chosen in the range 250-350 rpm, whilst the speed of advance may be equal to 2/10 mm per rotation.

During the course of the drilling performed in this operation, the twist drill 7 is guided by a bush 10, suitably supported, for example by means of a plate 11; in this case, too, a diametral play of substantially 0.01 mm is left between the drill and the inner surface of the bush, and the height h of the bush lies between 25 and 30 mm.

During the operation, the hole of diameter d, obtained at the end of the preceding operation is enlarged, first by the lower section and then by the upper section 8 of the drill, thus obtaining at the end of the operation, as can be seen in Figure 4, a hole of diameter dc which, on the basis of the relation (1), is slightly less than the final diameter df which it is desired to obtain.

In the third operation of the method the hole thus obtained is reamed to bring it to the final desired diameter and to impart to the surface a very high finish.

The reaming tool 15 used during this operation is shown in Figure 5 in the relative position which it occupies with respect to the assembly of plates 1,2,3 at the beginning of the reaming operation.

This tool, which is a normal tungsten carbide hole reaming tool, comprises two sections of different diameter, a lower section 16 and an upper section 17, with diameters of d,c and df. The junction angle y and the tip angle a (Figure 5) are respectively 135 and 90 . This tool is mounted, as in the preceding operation, on a drilling machine or a reamer with a tool carrier head having automatic advance, and during the course of the operation is guided by a bush 18 of height h of 25-30 mm; between the tool and the surface of the hole of the bush there is left a diametral play of substantially 0.01 mm.

The speed of rotation of the reamer tool may lie between 250 and 350 rpm and the speed of advance is equal to 2/10 mm per rotation.

During this third operation the lower section 16 of the tool 15 is inserted into the hole the diameter of which is equal to that of the section itself, such that the upper section 18 is guided in a very strict manner by the said coupling, and therefore during its advance is able to perform a perfect reaming of the hole of diameter dc, bringing it to the final diameter df (Figure 6).

Holes obtained with the method described above are perfectly cylindrical and the tolerance on the diameter, with respect to the nominal diameter, is of the order of 0.002" (0.05 mm); the surface finish is also significantly raised and is almost specular; average roughnesses obtained with the method of the invention are equal to or less than 80 RHR. There is no damage to the edges of the holes of the upper sheet 1 and the lower sheet 3, and these edges are devoid of delaminations or other defects.

If the holes are to be formed with the same characteristics as just described, with wider tolerances on the diameter, but still lying between those first indicated (that is 0.002" = 0.05 mm), and 0.003" (equal to 0.076 mm), the speed of rotation of the twist drill in the second operation and of the reaming tool in the third operation can be increased with respect to the values first indicated, with the advantage of reducing the time necessary for the formation of the holes. If the tolerance on the diameter of the holes lies in the above indicated range, the said speed may be in the range lying between 450 - 550 rpm. These operations can, in these conditions, be performed with a manually advanced drill.

It will be apparent that modifications and variations can be made to the operations of the method of the invention described, and the operating conditions first explained, within the scope of the invention.

Claims (10)

1. A method of making holes of a predetermined final diameter in an assembly of superimposed sheets including at least one sheet of a glass fibre or carbon fibre reinforced material, a sheet of steel and a sheet of titanium, the thickness of the said sheets lying respectively between 2 and 10 mm, between 0 and 1.5 mm and between 2 and 5 mm, characterised by the fact that for each hole there is performed a first drilling operation by means of a manually advanced drill provided with a twist drill bit having a first diameter ess than the said predetermined final diameter, in such a way as to obtain, at the end of the said first operation, a hole with a diameter substantially equal to the said first diameter; a second operation for enlargement of the said hole obtained at the end of the said first operation, performed with an automatically advanced drill having a twist drill bit with two sections of different diameters, an upper section of diameter slightly less than said predetermined final diameter, and a lower section of diameter slightly less than that of the upper section, in such a way as to obtain, at the end of the said operation, a hole with a diameter substantially equal to that of the upper section and less than said final diameter; a third operation for reaming the said hole obtained at the end of the second operation, performed with an automatically advanced drill provided with a reaming tool having two sections of different diameters, an upper section of diameter equal to the said final diameter and a lower section of diameter equal to the said diameter obtained at the end of the said second operation, so as to guide the said reaming tool by means of the said lower section of the reaming tool during the said reaming operation.

2. A method according to Claim 1, characterised by the fact that during the said operations the said twist drill bits and the said reaming tool are guided by means of respective guide bushes in which the said tools are inserted with a diametral play of the order of 0.01 mm, the axial length of the said bushes lying between 25 and 30 mm.

3. A method according to Claim 1 or Claim 2, characterised by the fact that in the said first operation the speed of rotation of the twist drill is selected from the range 450-550 rpm and in the said second and third operations the speed of rotation of the twist drill and of the said reamer tool is selected from the range 250-350 rpm.

4. A method according to Claim 3, characterised by the fact that the speed of advance of the said twist drill and of the said reamer tool in the said second and third operation, respectively, are chosen to be substantially 0.2 mm per revolution.

5. A method according to Claim 1 or Claim 2 characterised by the fact that in the said operations the speed of rotation of the said twist drill in the pre-reaming operation and of the said reaming tool is chosen to lie in the range 450-550 rpm, the said operations being performed by means of a manually advanced drill.

6. A method according to any preceding claim, characterised by the fact that the angle of the tip of the said twist drill used in the said first operation is 135 .

7. A method according to any preceding claim, characterised by the fact that the angle of the junction between the said lower and upper sections of the said reaming tool is 135 .

8. A method of making holes of a predetermined final diameter in an assembly of superimposed sheets including at least one sheet of a glass fibre or carbon fibre reinforced material, a sheet of steel and a sheet of titanium wherein each hole is formed in a plurality of operations comprising a pre-reaming operation performed with twist drill bit having an upper section of diameter slightly less than said predetermined final diameter and a lower, leading section having a diameter slightly less than that of the upper section to obtain a hole with a diameter substantially equal to that of the upper section and less than said final diameter; and a second reaming operation for said hole obtained from the preceding operation performed with a reaming tool having an upper reaming section of diameter equal to said final diameter and a lower guiding section closely fitting said hole obtained from the preceding operation so as to guide the reaming tool during the reaming operation.

9. A method of making holes of a predetermined diameter substantially as described and illustrated in the attached drawings.

10. An assembly of superimposed sheets having a plurality of holes made in accordance with the method of any one of the preceding claims.