GB2122937A - Method and mandrel for blind riveting - Google Patents

Abstract

A pull-through mandrel (21, 22) for installing tubular blind rivets (11, 12) of the type which buckle outwardly to form a large blind head, has a relatively steep shoulder at the leading end of its head (22) so as to enable the transmission of sufficient axial force to the rivet to cause buckling. When the mandrel head passes through the bore of the tubular rivet it scrapes off material from the bore wall which forms a ring around the mandrel stem in front of the head. The mandrel head is provided with a number of relieved zones (33) around its periphery, which ensure that the scraped-off material forms a number of separate pieces, which fall off the mandrel stem, instead of a complete ring. <IMAGE>

Description

SPECIFICATION Method and mandrel for blind riveting This invention relates to pull-through blind riveting, that is to say a method of riveting in which a tubular rivet with a head at one end is inserted into a hole in a workpiece, and the rivet is installed by pulling completely through the tubular rivet bore, in the direction towards the rivet head from the other end (the tail end) thereof, the enlarged head of a mandrel, without access to the remote side of the workpiece. The mandrel head is too large to pass through at least part of the rivet bore without deforming the surrounding rivet shank, with the result that at least part of the rivet shank is radially enlarged to form a blind head. During this operation the preformed head of the rivet is supported on a nosepiece of a tool having means for retracting the mandrel into a bore through the nosepiece.

Commonly a number of rivets (for example, between twenty-five and one hundred) are provided on the mandrel, successive rivets being installed by reciprocation of the mandrel with respect to the nosepiece and forward feeding of the next rivet through the nose-piece which is split to allow such feeding and closes behind the fed rivet.

A blind head of relatively large radial extent can be obtained by arranging that part of the rivet shank buckles outwardly, to form a fold which provides the blind head. However this requires the transmission a substantial axial force from the mandrel head to the rivet shank.

One problem which has been found to occur is that, when the design and operation is such that sufficient axial force is transmitted by the mandrel head to the rivet shank, the mandrel head on its passage through the rivet bore will remove from the wall of the rivet bore a small amount of material which forms a ring which remains trapped on the mandrel stem between the mandrel head and the next rivet. When the mandrel head is drawn into the next rivet the ring abuts the tail end of the next rivet, and is expanded by the mandrel head and falls off outside the tool. However, when the last rivet of the number loaded on the mandrel is installed, there is no next rivet to abut the ring, which is then drawn inside the tool and often jams the mechanism of the tool, particularly upon release of the mandrel from the tool for reloading with a further set of rivets.

The present invention seeks to overcome this problem.

The invention provides, in one of its aspects, a method of pull-through blind riveting in which the mandrel head on its passage through the rivet bore removes a quantity of material from the wall of the rivet bore, the configuration of the mandrel head being such that the removed material does not encircle the mandrel stem sufficiently to be retained thereon but is free to fall off the mandrel stem.

Preferably the configuration of the mandrel head is such that material is removed from the wall of the rivet bore at a plurality of removal zones spaced apart circumferentially of the mandrel head by a plurality of relief zones at which no material is removed.

Preferably each removal zone includes a position of local maximum radius of at least the landing end of the mandrel head and each relief zone includes a position of local minimum radius of at least the leading end of the mandrel head.

In a preferred form of the invention, the crosssectional configuration of the periphery of the mandrel head at least at its leading end comprises a plurality of zones of circular radius each joined to the next by a zone in which the radius is less than the aforesaid circular radius.

Preferably in each zone of reduced radius the radius progresF ely decreases away from each end of the zone.

Preferably the zone of reduced radius comprises a straight line chord.

The invention also provides a mandrel for use in a method of pull-through blind riveting as aforesaid, in which the cross-sectional configuration of the periphery of the mandrel head at least at its leading end comprises a plurality of zones of circular radius each joined to the next by a zone in which the radius is less than the aforesaid circular radius.

Preferably in each zone of reduced radius the radius progressively decreases away from each end of the zone.

Preferably the zone of reduced radius comprises a straight line chord.

One embodiment of the invention, in the form of a method of pull-through blind riveting and a mandrel for use therein, will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is an axial section on the line I-I of Figure 2 through a rivet and mandrel (the mandrel being shown not in section); Figure 2 is an end elevation in the direction of the arrow II in Figure 1; Figure 3 is a cross-section through the mandrel on the line Ill-Ill of Figure 1; Figure 4 is similar to Figure 1 but shows the rivet just after the completion of installation; Figure 5 is a detail section on an enlarged scale showing the leading face of the mandrel head; and Figure 6 corresponds to Figure 3 but shows a modified form of mandrel head.

In this example the invention is applied to the installation of a blind rivet of a type in which the blind head is formed by outward buckling of the rivet shank due to axial compression thereof, rather than by simple radial expansion directly by the mandrel head. This type of rivet requires the transmission of substantial axial force from the mandrel head to the rivet shank with a consequent ring formation problem. This problem is overcome by the application of the present invention.

The tubular rivet is made of aluminium alloy containing about 2% magnesium. It comprises an elongated shank 11 with an enlarged head 12 at one end, a bore 13 extending completely through the shank and head. The tail-most part (about one third of the length of the rivet) is provided with a thickened wall 14 due to this part of the bore 13 being of slightly reduced diameter. The remainder of the length of the rivet shank is formed with three longitudinal slots such as 1 5 in Figure 4, which separate the shank into three legs.

The mandrel is made of steel and comprises an elongated shank 21 (only part of which is shown in the Figures) having at one end a radially enlarged head 22. The shank 21 just passes through the reduced part of the rivet bore in the tail-most part 14, but the head is too large to do so. In use, the rivet is loaded on the mandrel with a number of other rivets, the mandrel forming part of a riveting tool inculding a nosepiece 23. The shank of the rivet is inserted through aligned holes 24 in two sheets 25, 26 to be riveted together until the rivet head 12 abuts the nearer sheet 26. The tool is then operated to retract the mandrel into the nosepiece.The increasing axial force exerted on the tail-end of the rivet shank by the mandrel head causes the shank to buckle, so that the three legs of the shank fold outwards to form a blind head comprising three folds 27 which contact the back sheet 25. The sheets 25, 26 are clamped together between the preformed rivet head 12 and the blind head 27. As the mandrel tension increases, the tail-most part 14 of the shank radially enlarges to allow the mandrel head to pass through it and completely out of the rivet.

In order that the mandrel head 22 will transmit sufficient axial force to the rivet shank to cause the latter to buckle, before the tail-most part of the shank allows the mandrel head to enter it, the leading end face 28 of the mandrel head has to have a relatively steep inclination to the mandrel's longitudinal axis, instead of a shallow taper. In this particular example, the form of this leading face, illustrated in Figure 5, is such that its axial section comprises a first quadrant 29 merging the wall of the mandrel stem 21 to a direction perpendicular to the mandrel axis, and second quandrant 31 meeting the first at a common tangent perpendicular to the mandrel axis, the second quadrant merging to the circumferential wall of the mandrel head 22. However, any other suitable configuration of leading end face for the mandrel could be employed.

As previously mentioned it is found that when a mandrel head of circular symmetry and such a configuration is pulled through the tail-most part 14 of the rivet shank, the degree of interference between the leading end of the mandrel and the rivet shank wall results in material being removed from the shank bore wall which forms a ring of metal encircling the shank and trapped thereon between the mandrel head and the tail-end of the following rivet.

In order to overcome this problem, the mandrel head of this example is provided with a periphery of cross-sectional configuration which comprises four zones 32 of circular radius (i.e. the full outside radius of the head), each zone 32 being joined to the next by one of four zones 33 each of which comprises a straight line chord of the circle.

In other words, the mandrel head is of generally cylindrical form but is provided with four flats 33 spaced apart equally about the axis. When the mandrel head is eventually pulled through the tailmost part 14 of the mandrel head, the action of the leading end of the mandrel head on the bore wall is modified with respect to that of a mandrel head of simple cylindrical form, i.e. without the flats. Each zone 32 of full circular radius removes metal from the bore wall of the rivet shank tail 14 (and may therefore be referred to as a removal zone), leaving a scar as illustrated at 34 in Figure 4. However over each zone 32 of chord form (and which may be referred to as a relief zone) metal is not removed from the bore wall, which is left intact along four corresponding axial strips such as 35.The result is that the removed metal does not completely encircle the mandrel stem sufficiently to be retained thereon, but forms four separate segments such as illustrated at 36 in Figure 4, which are free to fall off the mandrel outside the tool. As illustrated in Figure 4, a typical metal segment 36 tapers in thickness towards each end, and is of rather longer extent peripherally of the mandrel than the peripheral extent of a full radius zone 32 of the mandrel head. This result, together with the fact that, as also illustrated in Figure 4, the peripheral extent of each scar 34 on the rivet shank bore wall is rather greater than the corresponding peripheral extent of the mandrel head full radius zone 32, may mean that metal is also removed to a certain extent by the side edge portions of each mandrel head flat 33.This seems possible, since the chord configuration of the cross-section of each flat face means that the radius at the face of the flat decreases progressively away from each edge of the flat (and is a local minimum at the mid-line of the flat). Consequently, to be strictly accurate, the peripheral extent of each removal zone, over which metal is removed by the mandrel head from the rivet shank bore wall, is greater than the peripheral extent of the full radius part 32 of the head, whilst correspondingly the peripheral extent of each relief zone, over which no material is thus removed, is less than the peripheral extent of each flat 33. However, the important thing is that there are zones across which no metal is removed, thereby resulting in separate pieces of metal such as 36 which are free to fall off the mandrel stem.

This occurs even with the last rivet of a number loaded on the mandrel, since removal of the metal from the mandrel does not depend upon the action of the next rivet.

In the foregoing example, the most relevant dimensions of the rivet and mandrel are as follows:internal diameter of tail-most part of rivet shank bore 14 =2.93 to 2.98 mm.

full diameter of mandrel head over zones 32 =3.56 to 3.61 mm.

minimum diameter of mandrel head across flats 33 =3.20 to 3.25 mm.

diameter of mandrel shank 14 =2.87 to 2.92 mm.

The invention is not restricted to the details of the foregoing example. For instance, the precise configuration of the relief of the mandrel head may be modified, provided that (a) the mandrel will successfully install the rivet, and (b) any material removed from the shank bore does not encircle the mandrel stem sufficiently to be retained on it, but falls off. Thus the number of removal zones and relief zones could be less than, or greater than four; e.g. three, or six.

Furthermore, since it is the leading end of the mandrel head which removes the material from the rivet shank, it would be possible to provides the full relief only at the leading end of the mandrel head, with the relief zones progressively tapering, to a larger diameter, may be to the full diameter of the mandrel head.

Figure 6 illustrates one possible alternative form of mandrel head relief zones. It has only three zones 133, spaced apart evenly around the periphery of the head. Each zone 133 is of less peripheral extent than each of the four zones 33 of the mandrel of Figures 1 to 6, and is in the form of a flat-bottomed groove with sidewalls at right angles to the bottom face. Thus the radius to the bottom of each groove is substantially constant across each groove. The radial depth of each groove extends down to level with the outside of the mandrel shank 121, and is thus deeper than the maximum "depth" of each flat 33 at its centre. It is found that such an arrangement of relief zones is also effective in ensuring that a complete ring is not formed.

Claims (13)

Claims

1. A method of pull-through blind riveting in which the mandrel head on its passage through the rivet bore removes a quantity of material from the wall of the rivet bore, the configuration of the mandrel head being such that the removed material does not encircle the mandrel stem sufficiently to be retained thereon but is free to fall off the mandrel stem.

2. A method of riveting as claimed in Claim 1, in which the configuration of the mandrel head is such that material is removed from the wall of the rivet bore at a plurality of removal zones spaced apart circumferentially of the mandrel head by a plurality of relief zones at which no material is removed.

3. A method as claimed in Claim 2, in which each removal zone includes a position of local maximum radius of at least the leading end of the mandrel head and each relief zone includes a position of local minimum radius of at least the leading end of the mandrel head.

4. A method as claimed in any of the preceding claims, in which the cross-sectional configuration of the periphery of the mandrel head at least at its leading end comprises a plurality of zones of circular radius each joined to the next by a zone in which the radius is less than that the aforesaid circular radius.

5. A method as claimed in Claim 4, in which in each zone of reduced radius the radius progressively decreases away from each end of the zone.

6. A method as claimed in Claim 5, in which the zone of reduced radius comprises a straight line chord.

7. A method as claimed in Claim 4, in which in each zone of reduced radius the radius is substantially constant across each zone.

8. A method of blind riveting substantially as hereinbefore described with reference to, and illustrated in, the accompanying drawings.

9. A mandrel for use in a method of pullthrough blind riveting as claimed in any of the preceding claims, in which the cross-sectional configuration of the periphery of the mandrel head at least at its leading end comprises a plurality of zones of circular radius each joined to the next by a zone in which the radius is less than the aforesaid circular radius.

10. A mandrel as claimed in Claim 8, in which in each zone of reduced radius the radius progressively decreases away from each end of the zone.

11. A mandrel as claimed in Claim 9 in which the zone of reduced radius comprises a straight line chord.

12. A mandrel as claimed in Claim 9, in which in each zone of reduced radius the radius is substantially constant across each zone.

13. A mandrel substantially as hereinbefore described with reference to, and illustrated in, the accompanying drawings.