Gas or Arc Welding?
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By P. G. Tucker
Using the Correct Process will Help to Reduce Distortion. Preheating Cylinder and Crankcase Castings Is Important
WELDING is one of those crafts which cannot be mastered without extensive experience. Almost anyone armed with a gas torch and a filler rod can melt blobs of metal on to a joint or fracture, but this is not welding. It is unfortunately true that in many cases the strength or otherwise of a welded joint can be proved only after the component has been put into service, which is one reason why only operators of integrity and those possessing the necessary skill, should be employed on work the failure of which might endanger life.
Many operators who can deal satisfactorily with ferrous metals, find difficulty when faced with a repair job on a light-alloy casting. " Knowing how" goes a long way towards success, but there is no better teacher than experience. Most big shops have facilities for carrying out both gas and arc welding, and I shall explain the application and advantages and disaAantages of the respective processes in specific cases.
Probably the greatest bugbear, either in fabricating by welding or in carrying out repairs and in reclamation work, is that associated with distortion. It is so much waste of time and money if, through excessive distortion, the work-piece cannot be cleaned up to permissible limits.
Then, of course, there is the question of producing a true, clean weld,
by which I mean a joint which is thoroughly sound and not full of blow-holes and slag. Here, one has often to rely on the integrity of the operator, as it is not difficult for him to camouflage a bad weld.
In the case of gas welding, some degree of preheating is essential in practically all cases if distortion is to be avoided. The extent to whieb this is carried out depends on the nature and size of the work-piece. In the case of, say, a valve rocker, the operator would run his torch over the whole rocker for a few moments, whereas with a cylinder head or crankcase the component would be uniformly heated, preferably in an oven over which there is some control of temperature.
A cast-iron cylinder head would be heated until it was a dull red or equivalent to a temperature of 650 700 degrees C. A cast-aluminium crankcase, on the other hand, would not be heated beyond 400 degrees C., and special precautions should be taken to guard against movement of the crankcase walls. This is sometimes done by arranging adjustable tie-bars or bridge-pieces across the open end.
The tie-bars are under compression, so that if the crankcase be overheated there will be a tendetley for the walls to be thrust outwards, apart from the danger of the whole casting
collapsing. In one ot the biggest
relding shops in this country, at the :hiswick depot"of the London Transart Executive, it is the accepted ractice to preheat the. whole of a ylinder head even when the repair is oncerned with only a small area at ne end.
I have seen it stated that valve seats an be built up without the need for general preheat, but the practice the L.T.E. shop is to weld with the ead maintained at a dull-red heat a gas muffle. The head is first berally machined in the area of the alve seat, the ultimate deposit being eep and wide. The valve seat, when nally finished, is far more wear!sisting than the seats as formed in le original cast iron.
Reclaiming a Camshaft An ingenious and efficient method f building up a worn cam is adopted y London Transport. The worn am is first ground undersize to take spring-steel encircling band, which heated and rapidly clenched in osition round the ground cam. This band is next arc-welded at its iges to the old cam, a stainless-steel
ectrode being used. Should the 'reads on the camshaft need buildig up, mild steel is deposited by gas 'elding. The final processes involve ardening and grinding the new cam ice, and reeutting the threads.
Another interesting reclamation th carried out in the same works )ncerns petrol-engine valve rockers. he worn face of the rocker is first round down and then a spring-steel ad piece is bronze-welded in posion. The pad piece is finally ground own as required.
Whether a particular job is suitle for are-welding or gas-welding decided by the nature of the metals )ncerned, the desirability of reducig distortion to a minimum and by anvenience.
Accompanying this article is a Lige in which a number of cornonents is mentioned, together with
the recommended method of welding. Typical uses of arc-welding would be in building up the bearing housing in a wheel hub, the securing of a clutch disc to its hub, or in building up the flange-plate of a differential housing.
Arc-welding is extensively employed on chassis frames in the repair of fractures, the attachment of brackets, or, in some works, in the fabrication of frames from channelsection and plate material. One great advantage of arc-welding is that metal can be deposited immediately the arc is struck, the intense heat being strictly localized.
With gas welding, on the other hand, the torch has to be applied for a period depending upon the size and nature of the component, so that the heat has time to .spread and so introduce the possibility of distortion. This is the reason why preheating is so important in certain cases.
It is economical to arc weld wherever possible but the process is applicable only to steel components. For non-ferrous metals and alloys gas welding should be adopted.
An interesting example of the use of both gas and electric welding on one component is in the fabrication of the sheet-metal "water drip " as used on London Transport buses. Whilst two main sections are gaswelded together, a strip which stands out at right angles on the inner radius of the partly fabricated structure is arc-welded in position. Nine tack welds are first made and then the joint is fully welded. By adopting this method of fabrication distortion is kept to a minimum.
Fusion Welding On the other hand, I saw the offside rear wing of a bus being fusionwelded by gas with so small a degree of distortion that it was readily corrected by harid. In both cases, 20gauge steel was the metal being operated upon. Fusion welding requires far more skill than when a filler rod is being used, but the method invariably produces a much neater joint, and when skilfully executed the need for any final finishing is eliminated'.
Steels having a high carbon content are particularly tricky to deal with and are best left alone so far as repairs or reclamation by welding are concerned. If, however, circumstances are such that there is no alternative, the gas torch should be employed. Apart from ball-races and the like, there are few parts in a commercial vehicle containing a high percentage of carbon. Generally speaking, the welding of road springs should not be attempted, although springs reclaimed by this process have been known to function satisfactorily for a time.