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Fusible Alloy Cuts "Prefab" Costs

27th February 1948
Page 34
Page 34, 27th February 1948 — Fusible Alloy Cuts "Prefab" Costs
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WHATEVER may be the pros and cons of the craftsmanbuilt commercial-vehicle body, as opposed to its quantity-produced prefabricated counterpart, it seems certain that the latter mus. play a great and growing part in the equipping of modern transport systems at home and overseas.

Methods of prefabrication and quantity production have not as yet been fully explored ny the majority of bodybuilders. Some concerns have been able to plan in good time and have installed. or are installing plant, but others have either been rushed into semi-improvised mass production or are continuing with hastened individual production while they lay plans for the more modern method.

Of interest to all, however, must be a timemoneyand labour-saving method of setting location points on assembly jigs, because whatever method of prefabrication be employed, assembly jigging can seldom be entirely dispensed with. The proc..ss here described has for some time been widely used by the aircraft industry, and has obvious possibilities for adaptation to other fields.

Bismuth Alloy

The method depends for its operation on a low-melting-point bismuth alloy known as Cerromatrix and produced by Mining and Chemical Products, Ltd.,_ 376, Strand, London W.C.2. It is necessary, before considering the process further, to understand the properties of this alloy Cerromatrix contains tin, lead and antimony, and is lich in bismuth. It has a density of 0.343 lb. per cubic in. and has a good plastic range. Up to 103 degrees C. it is olid and at 120 degrees C. is sufficiently plastic to flow under its own weight At 227 degrees C it becomes completely fluid.

When solidified by cooling, the alloy expands rapidy at first, then more slowly for about 12 hours, when growth becomes complete. After that, there are no measurable dimensional changes. The tensile and compressive strengths respectively are 61toms and 5 tons per sq. in.

With these properties in view, the principles employed will be readily understood. Cerromatrix is used as a self-adjusting picking medium to avoid any need for machined faces or accurate borings at jig location points. Simply put, the jig pick-up point is arranged to be located accurately by means of adjusting screws Or bolts, but is kept clear of its housing or attachment faces.

Then walls pr dams of asbestos or any suitable material are built around the clearances and . Cerromatrix is poured in and allowed to cool. The result is that the location point, which was "suspended in mid air," but in its correct position as determined by jig references, gauges. or Optical equipment.

is now on a permanent and micrometrically accurate foundation, for all future operations.

Clearly, there are various methods, according to the job in hand, by which location points can be accurately set and held in position for casting the spacing material. In many cases the work can proceed in conjunction with the building of the prototype. The prototype, or %Wile of its components, can, in fact, become jig references.

Simple Adjustment

Whatever may be the method of setting, however, the location point is usually adjustable by some simple means, such as jack screws or holding

down bolts. The important point is that clearance must be left in the correct places, so that location points, whilst accurately disposed with reference to the jab. may be " anywhere" in relation to the jig frame,

Thus, taking the most simple instance, an attachment lug might be set by

means of the bolts which hold it to the jig frame It would be spaced, say, f-in. from the frame, and, while held in that position by jig reference, or by being fixed to the prototype, the holding bolts would be drawn just tight.

Location in the plane at right angles to the attachment face would then be automatic, because • the lug would have clearance holes for the holding bolts, of whatever tolerances 'night be required. They might be f-in. holes for Fin, bolts, for example, or even greater clearance might be useful.

That is a simph application, gnd others more complicated are frequently used in the aircraft industry. Typical is the box formation illustrated on this page, which gives universal adjustment and in which all " error " is automatic ally "added " and compensated by Cerromatrix

Complete Rigidity

It must be made clear that the method demands the :.xistence of a free edge at some area of the casting space.. Such provision ensures that any excess growth of the. alloy may take place freely. Rough edges are easily removed_ by means of a warm scraper after the 12 hours' ageinz. There is complete rigidity and no further check on alignment is needed.

Sorrretinies, however, when a large jig has to be moved, there may be a certain amount of distortion or straining. One of the important advantages of the Cerromatrix method is that in such a case it is necessary only to melt out the alloy, make the necessary readjustment on holding bolts of tutustment screws, and then recast.

This simple but ingenious and foolproof technique of. setting up major assembly jigs is employed widely in the British aircraft industry, and it need hardly be said that its virtues are now well established.

The foregoing account is based 'largely on the application of the process to operating conditions at the Weybridg( works of Vickers-Armstrongs, Ltd., where Cerromatrix is to be seen in use on a considerable scale. Through the courtesy of that organization it was possible to examine the method under working conditions.

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