Metal treatment – Compositions
Patent
1995-10-30
1998-11-17
Andrews, Melvyn
Metal treatment
Compositions
29525, 228115, B23K 2012
Patent
active
058370668
DESCRIPTION:
BRIEF SUMMARY
This invention is concerned with ways of making joints, and relates in particular to improving the joint-producing method described in International Patent Publication WO 91/19,589 (the Specification of our International Patent Application PCT/GB 91/00,950) by overcoming the problem of making reliable rubbing contacts between loose or ill-fitting parts with a clear gap between them.
In the aforementioned Patent Publication there is described and claimed a method of securing against lateral motion two bodies held in face to face asperity contact, thereby to make a join between the two bodies, in which method there is inserted into the interface between the two bodies a material--referred to hereinafter variously as an anti-lubricant, a gall enhancer, a gall-enhancing agent, a gall-promoting agent or a gall promoter--that on minimal initial lateral relative motion of the two surfaces promotes rapid but controllable "galling" between the two surfaces, this galling binding the surfaces against further such motion.
Operating experience has shown there are many uses for the abovementioned gall-enhanced friction joints, but in some applications it is either difficult, impractical or uneconomic to provide a close touching fit between the parts to be joined. This is unfortunate, for to make a satisfactory mechanical joint using the aforementioned galling method it is essential to ensure regular and predictable contacts between the two surfaces being joined, together with sufficient interfacial forces (pressure) to cause galling in the presence of a gall enhancing-agent, as the surfaces are rubbed one against the other to form the joint (for galling to happen it is believed that some plastic deformation must occur simultaneously at the contact point on at least one of the rubbing surfaces). However, there may well be considerable size--or "fit"--variation between the parts (typically made of a metal such as either a hard or a soft steel) being joined by the method. This variation commonly occurs in the size of metal parts manufactured on conventional metal-working machines with tools such as cutting bits, boring tools, drills and reamers on machines such as automatic lathes and machining centres. It is principally due to tool wear, but is also influenced by bearing wear within the machines, and by the general machine condition. It is therefore a wide-spread problem particularly in large scale mass manufacture of low cost parts on ageing equipment.
A predictable and reliable means of making good rubbing contact between the surfaces to be joined is fundamental to making good gall-enhanced mechanical friction joints between metal parts. To achieve this it is necessary to cause considerable plastic deformation of contacting asperities to reduce their height and so allow many more minor asperities to come into good rubbing contact. The problem is most acute in joining hard parts, where it is very difficult to achieve significant surface plastic deformation as a means of bringing surfaces into good face-to-face asperity contact.
Various attempts have been made to provide a noncritical way of bridging what can be a highly variable gap between the two parts of the joint. For instance, surfaces can be raised locally by impacting or indenting, so material is displaced outward and upwards around the rim of the indentation. If this is repeated systematically over a surface it raises an array of asperities to increase the effective diameter--if, for instance, it is done on the outer surface of a shaft. The raised asperities tend to bridge across the gap (to provide the needed rubbing contacts at which galling occurs), and so take up the tolerance gap. This approach has been successfully used in practice, but it lacks flexibility because it depends on deforming the asperities to achieve a fit, and there are practical limits determined by the lack of strength of the deformed asperities towards their tips.
Another solution utilised with some success is to use a metal wedge or shim that when placed between the two parts being joined
REFERENCES:
patent: 5547503 (1996-08-01), Oldiges et al.
Andrews Melvyn
Ball Burnishing Machine Tools Limited
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