Rivet formation

Details Rivet formation Rivet formation

1998-01-06

1999-07-06

Larson, Lowell A.

Metal deforming

By use of closed-die and coacting work-forcer

Forcing work into or within closed die; e.g., forging

723792, 413 14, B21D 5138

Patent

active

059184990

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

This invention relates to a process for forming a rivet. In particular, it relates to a process for forming a rivet in a thin metal sheet such as is used, for example for a metal container end to which a tab or ring pull is to be fixed, typically a so-called "easy open end".
It is a particular aim of this invention to provide a process which is suitable for high production rates. It is further desirable to enable rivets of small diameter to be formed in hard materials which are not very malleable such as double reduced steel. The use of double reduced steel is becoming increasingly popular in the metal packaging industry.
A process for forming a hollow rivet is described in FR-2660220 (EP-0451013). In that document, two operations are used to form the rivet. Firstly, a completely circular boss is formed by drawing the metal in a punch, the boss having annular corrugations. The rivet is then formed from this boss and having the same centre as the boss.
This process results in considerable necking of the metal in the region of the corrugations. The main force is exerted in the central part, which can lead to excessive and badly controlled thinning of the metal. This is particularly the case during the second stage, when the corrugations are ironed until they completely disappear. This consequently increases the risk of breaking off the rivet when the rivet is staked or flattened for securing a tab onto a container end. Moreover, the metal blanks are generally lacquered prior to rivet formation, so that the process frequently results in damage to this coating. Finally, the punch operation must be fairly slow to avoid splitting of the metal.
As a result of these problems, the process of FR-2660220 is in practice only suitable for use with very malleable metals, such as aluminium. This process is therefore incompatible with high production rates, unsuitable for the formation of rivets of small diameter, and difficult to use with high carbon steel sheet, such as double reduced steel.
The present invention seeks to overcome the above problems.


SUMMARY OF THE INVENTION

According to the present invention, there is provided a process for making a rivet in a thin metal sheet for a metal container end, comprising forming a boss from the sheet metal and then converting the boss into an upstanding hollow rivet; characterised by the process comprising at least two operations to form a boss, the operations including: substantially circular configuration and creating at least one annular corrugation; and the corrugation; and flattening the corrugation to push metal back towards the centre.
Advantageously, during the transformation of the blister into the boss, the sheet is clamped gently outside the corrugation so as to allow sliding of the metal during the flattening of the corrugation. This limits thinning of the metal and avoids damage to any coating.
A preferred embodiment of the invention will now be described, by way of example only, with reference to the drawings, in which:


BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a to 1e are half side sections of a first station for forming a blister;
FIGS. 2a to 2e are half sections of a second station for forming a boss from the blister of figure le; and
FIGS. 3 and 4 are half side sections of third and fourth stations respectively for conversion of the boss into a rivet.


DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to figure 1a to 1c, there is shown a blank of thin sheet metal 11, typically of double reduced steel, for subsequent conversion into an easy open end for a metal container. This blank is advantageously prelacquered and/or decorated.
An easy open end is provided with a conventional tab or ring pull (not shown) which is fixed to the end by riveting. The present invention concerns the formation of the rivet in the blank (FIG. 4) by successive operations on a transfer press.
The first station 12 of the transfer press comprises a clamping member 14 which is axially movable so as to clamp the blank on a flat upper face of an

REFERENCES:
patent: 3191564 (1965-06-01), Fraze
patent: 3638597 (1972-02-01), Brown
patent: 4783985 (1988-11-01), LeBarge et al.

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