GB2074060A - Forming holes of a predetermined diameter in a set of superimposed sheets - Google Patents

Abstract

The sheets include at least one glass fibre or carbon reinforced sheet, and a sheet of aluminium. Initially a manually controlled twist drill produces an undersized hole. Enlargement of the hole is performed with a manually controlled 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 by a reaming tool (15) having two sections of different diameters, one of which serves as a guide. <IMAGE>

Description

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

Composite sheets of this type are used in the aeronautical field to form structures with very high mechanical strength and extremely low weight. To join the two sheets of the assembly rigidly together and/or to connect the various elements of a structure together, the said sheets are connected by means of connection members inserted in corresponding holes formed in them.

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

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

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

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

Sometimes small cracks and broken pieces of fibre are found in the edge region of holes formed in the sheet of glass fibre or carbon fibre reinforced material, or else damage to the surface of these produced by drill chippings of aluminium are found; also the surfaces of the holes thus formed are very rough. Moreover, it is only possible to obtain very wide tolerances on the diameter of the holes and these very often have undesired ovalisation or conicity.

Finally, a significant wear of the tools is encountered, this being generated above all by the action on the cutting edges of the tools by the carbon fibres; tools 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 above mentioned disadvantages can be eliminated, so that holes devoid of defects can be obtained in an economical manner with shape and dimensions corresponding to those intended, with very strict tolerances and with a big finish of the associated surfaces.

According to the present invention there is provided a method of forming holes of a predetermined diameter in an assembly of superimposed sheets including at least one sheet of glass fibre or carbon fibre reinforced material and a sheet of aluminium, the thickness of the said sheets lying between 2 and 10 mm and between 2 and 8 mm respectively, 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, less than that of the said predetermined final diameter, in such away 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 by means of a manually advanced drill having a twist drill bit with two sections of different diameters, an upper section of diameter slightly 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 of substantially said upper section diameter; a third phase of reaming the said hole obtained at the end of the second operation, performed with a manually 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 end of the said second operation, in such a way that the said reaming tool is guided by means of the said lowe; section of the tool itself during the said reaming phase.

According to the another aspect of the invention, there is provided a method of making at least one hole of a predetermined diameter in an assembly of superimposed sheets including at least one sheet of glass fibre of carbon fibre reinforced material and a sheet of aluminium, wherein the or each hole is formed in a plurality of operations comprising a pre-reaming operation performed with a 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.

For a better understanding of the method of the present invention examples of the operations will now be described with reference to the attached drawings in which: Figures 1 and 2 schematically represent the assembly of two sheets respectively at the beginning and at the end of a first operation of the method; Figures 3 and 4 schematically represent the assembly of two sheets respectively at the beginning and the end of a second operation; Figures 5 and 6 schematically represent the assembly of two sheets respectively at the beginning and at the end of a third operation with the method is completed; and Figures 6 and 7 represent the composite sheet respectively at the beginning and at the end of an alternative operation to that of Figure 1.

The method of the invention is used for drilling an assembly of superimposed sheets such as that illustrated in Figure 1 and constituted by at least a first sheet of a glass fibre or carbon fibre reinforced material 1 and a second sheet of aluminium. Composite sheets of this type are largely utilised in aeronautical constructions to obtain structural elements with very high mechanical strength and extremely 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 obtained with the method of the invention.

The best results of the method are obtained when the thickness of the said two sheets 1 and 2 lies respectively between 2 and 10 mm and between 2 and between 2 and 8 mm. The diameter of the holes is conveniently between 4 and 6.5 mm.

To obtain each of these holes there is performed a first drilling 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 drill of the type utilised in usual drilling operations, for example with helical flutes forming an angle of 300 with the axis of the drill; the core angle of the drill tip, indicated a in Figure 1 must be equal to 1350.

The diameter do of this drill bit lies between 1/2 and 2/3 of the predetermined final diameter of the hole which it is desired to obtain, whilst the speed of rotation of the drill itself, which is constant during the whole of the first operation, may lie between 2800 and 9000 rpm.

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 the axis of which coincides with that of the hole to be formed, in such a way that the drill itself is rigorously guided whilst it penetrates the assembly of plates 1 and 2. For this purpose, these is left a diametral play between the drill bit and the surface of the hole of the bush, which may be of the order of 0.01 mm; the height h of this bush is conveniently between 25 and 30 mm.

To perform this operation the operator, after having positioned the bush 4 in a way that axis of this latter coincides with that of the hole to be formed, introduces the twist drill bit into the bush itself manually applying to the drill a force sufficient to determine the drill feed.

During the course of this operation the central web of the hole is substantially removed, thereby obtaining with significant speed, as can be seen in Figure 2, a hole substantially of diameter da which may not be very regular both dimensionally and from the point of view of the suface finish. If the diameter of the final hole which is to be obtained should be very small this first described operation could 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 formed; in Figure 3 there is schematically shown the tool used in this operation, again constituted by a twist drill bit 7, positioned with respect to the assembly of plates 1 and 2 at the beginning of the said operation.

The twist drill bit 7 includes 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 it is desired to obtain (Figure 1 ) 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 according to (1) to the diameter df) by the relation: dd = dc -1/32" = df2/32" (1.6 mm) -- (2) Apart from the presence of two sections of different diameters the twist drill bit 7 is a normal tungsten carbide right hand twist drill, for example with helical flutes inclined at 300 with respect to the axis of the drill.

The angles P (Figure 3) of the tip and of the junction region between the lower section 9 and the upper section 8 of the drill are equal to 1350.

In this operation too the twist drill 7 is mounted on a manually controlled drill (not shown).

The speed of rotation of the drill 7 may be selected in the range 2800 - 9000 rpm.

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

During the operation the hole of diameter da obtained at the end of the preceding operation is enlarged, first by the lower section 9 and then by the upper section 8 of the drill, thereby obtaining, as can be seen in Figure 4, at the end of the operation a hole of diameter dc which, according to equation (1) is slightly less than the final diameter df which it is desired to obtain.

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

The reaming tool 15 utilised during the operation is shown in Figure Sin the relative position which it occupies with respect to the assembly of plates 1 and 2 at the beginning of the operation.

This tool, which is a normal tungsten carbide hole reaming tool, includes two sections of different diameter, one a lower section 16 and the other an upper section 17 with diameters respectively equal to dc and df. The junction angle a and the tip angle 5 (Figure 5) are respectively 1350 and 900. This tool is mounted as in the preceding operations on a manually advanced drill, and in the case of this operation is guided by a bush 18; 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 reaming tool may lie between 2300 and 2800 rpm.

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, and therefore the upper section 18 is guided in a rigourous manner by the said coupling; during the advancing movement of the tool this is able to perform a perfect reaming of the hole of diameter dc to take 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); also the surface finish is significantly high and almost specular; average roughness obtained with the method of the invention is equal to or less than 80 RHR. No damage to the edges of the holes in the sheets 1 and 2 occurs; these edges are devoid of delaminations or of other defects.

If it was desired to form holes with the characteristics just described above, but with wider tolerances on the diameter, still lying between that first indicated (0.002" = 0.05 mm) and 0.003" (= 0.076 mm), the guidance of the tool (twist drill and reamer) can be simplified in the three operations; these, rather than being guided by bushes such as 10 and 18, can be guided simply, and in a less precise manner, by means of normal drilling jigs such as the one shown for example in Figure 7. This comprises, conveniently, a plate 20 on which there are formed holes 21 the axes of which coincide with those of the holes to be obtained, and a bush 22 which is insertable in one of the said holes and to which there is rigidly connected a washer 23 into the hole of which the twist drill bit or the reamer tool can be introduced.

It will be apparent that the operations of the method described and the operating conditions first explained can be modified and varied, within the scope of the present invention.

Claims (9)

1. A method of making holes of a predetermined diameter in an assembly of superimposed sheets including at least one sheet of glass fibre or carbon fibre reinforced material and a sheet of aluminium, the thickness of the said sheets lying between 2 and 10 mm and between 2 and 8 mm respectively, characterised by the fact that to form 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, less than that of 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 by means of a manually advanced drill provided with a twist drill bit having two sections of different diameters, an upper section of diameter slightly less than the said predetermined final diameter and a lower section of diameter slightly less 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 said upper section and less than the said final diameter; a third operation of reaming the said hole obtained at the end of the second operation, performed with a manually 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, in such a way that the said reaming tool is guided by means of the said lower section of the tool itself during the said reaming operation.

2. A method according to Claim 1, characterised by the fact that in the said first and second operations the speed of rotation of the said twist drill is selected to lie within the range 2800 - 9000 rpm, and in the said third operation the speed of rotation of the said reaming tool is selected to lie in the range 2300 - 2800 rrrn.

3. A method according to Claim 1 or Claim 2 characterised by the fact that the said twist drills and the said reaming tool are guided during the said three operations by means of guide bushes the length of which lies between 25 and 30 mm in each of which bushes the said tool is inserted with a diametral play of the order of 0.01 mm.

4. A method according to claim 1 or claim 2, characterised by the fact that, the said twist drills and the said reaming tool are guided during each of the said three operations by means of a device including a plate which can be superimposed over the said composite sheet to be drilled and provided with a plurality of holes, and for each said operation at least one bush insertable in each of the said holes of the plate and provided with a guide hole operable to guide the respective twist drill or the said reaming tool.

5. A method according to any preceding claim, characterised by the fact that the angle of the tip of the twist drill utilised in the said first operation is 1350.

6. A method according to any preceding claim, characterised by the fact that the angle of the tip of the said reaming tool is 900 and the angle of the junction between the said lower section and the said upper section of the reaming tool is 1350.

7. A method of making at least one hole of a predetermined diameter in an assembly of superimposed sheets including at least one sheet of glass fibre or carbon fibre reinforced material and a sheet of aluminium, wherein the or each hole is formed in a plurality of operations comprising a pre-reaming operation performed with a 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.

8. A method of forming holes of predetermined diameter substantially as described and as illustrated in the attached drawings.

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