GB2082951A - Coining die - Google Patents

Abstract

In conventional cold working of a fastener hole in materials to be joined (15, 16) by drawing an oversized mandrel (19) through the hole, the unworked hole edge can act later as a centre for fatigue damage in loaded structures. Therefore, a die (12) having a raised coining annulus (14) is provided to deformably contact the hole edge when it is urged into the hole, as for example in reaction to drawing the mandrel (19). Non- deforming contact between a material surface (11) and a die surface (18) limits the extent of deformation. <IMAGE>

Description

SPECIFICATION Improvements in or relating to tools for the preparation of engineering materials This invention relates to the preparation of engineering materials and in particular to the cold working of the surface of holes in such materials.

A common method of fixing one structural integer to another is by a substantially cylindrical element passing through concentric cylindrical holes in the integers. In the fabrication of metal structures the element may be a rivet, the fixing being achieved between the rivet head and a deformable portion of the rivet shank. For maximum security the rivet shank is an interference fit with the hole surfaces. Aiternative fixings include a cylindrical pin and nut or deformable collar.

The method described above is commonly employed, for example, in the manufactur of aircraft structure and in this and other fields close attention must be paid to the placement and formation of holes to ensure that the materials are not unduly weakened. A known technique for preparing materials such that the likelihood of failure at a hole is reduced is that of cold working the hole surface, wherein a mandrel having a portion of effective diameter slightly in excess of the diameter of the hole is drawn through the hole to deform the hole surface. It has been established in the art that this technique improves the fatigue performance of material in which a hole has been made and in particular increases resistance to the formation of fatigue cracks at the hole. The technique is for example often specified as part of the assembly procedure for modern aircraft structures.

In many applications countersunk fixing is advantageous. Unfortunately the countersunk portion of a hole cannot be cold-worked. In tests performed on a number of engineering material specimens it has been established that the edge formed where the countersunk surface meets the cylindrical hole surface is particularly prone to act as a centre for fatigue cracking.

According to the present invention there is provided a die for insertion into a countersunk portion of a hole in an engineering material, the die having a raised coining annulus arranged to deform the material at an edge of the countersunk surface as, in use, the die is urged into the hole and having a surface arranged to non-deformably contact the material to limit the extent of deformation.

A die in accordance with the present invention may advantageously be formed as a portion of a tool including means for urging the die portion into a hole.

The surface arranged to non-deformably contact the material preferably lies on a cone of the same slope as the countersunk surface of the hole. Alternatively the surface may be formed on a tool which includes a die in accordance with the present invention, contact with the material being made on a surface surrounding a hole.

In a preferred form of the present invention a tool including a die portion in accordance with the present invention has a cold working mandrel and means for pulling the mandrel through a hole such that the die is urged into the hole as a reaction to the pulling force. Advantageously the die portion is made detachable for conventional cold working.

For convenience in this specification the deformation of at an edge of a countersunk surface is referred to as coining and in order that features and advantages of the present invention may be fully appreciated embodiments will now be described with reference to the accompanying diagrammatic drawings of which: Figure 1 represents a tool for cold working and coining a hole having a countersunk portion.

Figure 2, (a) and (b), represent different views of a die for coining a hole having a countersunk portion.

A tool 10 (Fig. 1) for coining a countersunk portion 11 of a hole in an engineering material has a die portion 12 having a raised coining annulus 14. As an example, two metal plates 15 and 16 are to be joined by passing a rivet through concentric cylindrical holes. The raised coining annulus 14 is arranged to coin the edge of the countersunk surface where it meets the hole wall as the die 12 is urged into the hole.

The die portion 12 is urged into the hole by a force in direction X produced by means well known in the art (not shown in the figure) and transmitted via the spacer 17. The coining is achieved by the coining annulus 14 producing a deformation at the edge. The extent of deformation is limited by a surface 1 8 of the die portion 12 contacting the countersunk surface 11 of the hole non-deformably.

The tool 10 includes a cold working mandrel 1 9 arranged to cold work the cylindrical surface of the hole. The cold working occurs as the mandrel 1 9 is drawn through the hole by pulling (by means not shown in the figure) in the direction Y. For convenience in use the mandrel cooperates with a deformable hollow cylindrical sleeve 20 held in the hole by a shoulder 21 on the spacer 1 7 so that the mandrel may be inserted from the countersunk side of the hole. Alternatively the cold working mandrel has a portion of diameter in excess of the diameter of the hole.

It will be appreciated that the die 1 2 may be urged into the hole as a reaction to pulling the mandrel 1 9 through the hole and that cold working of the hole, including coining the edge of the countersunk surface 11 may be accomplished in a single and straightforward operation. This arrangement is particularly advantageous when used as part of the maintenance procedure on existing aircraft, to improve fatigue performance and thereby increase the aviation safety of such aircraft.

The die 12 may be of unitary construction with the spacer 1 7 or advantageously separately formed. An additional view of the die 12 is shown at Figure 2 wherein common reference numerals have been used.

Claims (8)

1. A die for insertion into a countersunk portion of a hole in an engineering material, the die having a raised coining annulus arranged to deform the material at an edge of the countersunk surface as, in use, the die.is urged into the hole and having a surface to non-deformably contact the material to limit the extent of deformation.

2. A die as claimed in claim 1 formed as a portion of a tool including means for urging the die into the hole.

3. A die as claimed in claim 1 or claim 2 and wherein limiting surface contact is made on a surface surrounding the hole.

4. A die as claimed in claim 1, claim 2 or claim 3 and wherein the surface arranged to nondeformably contact the material lies on a cone frustrum of the same slope as the countersunk surface of the hole.

5. A tool including a die as claimed in any preceding claim and a cold working mandrel.

6. A tool including a die as claimed in claims 1 to 4 wherein the die is urged into the hole as a reaction to pulling a mandrel.

7. A die substantially as herein described with reference to the drawings.

8. A tool substantially as herein described with reference to Fig. 1 of the drawings.