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Aviation History
1932
1932 - 1095.PDF
FLIGHT, OCTOBER 27, 1932 machines, the two last-named are continuously recorded and checked by means of a device known as a weld-recorder, an out standing and unique feature of the apparatus. This recorder is the means whereby an automatic record is obtained of the elec trical conditions under which each weld was made ; it is, there fore, something that brings weld ing on an equality with riveting so far as control is concerned. The recorder is set to condi tions determined experimentally by test welds on the material that is subsequently to be used, a number being made and their strength and ductility ascer tained. When the required re lationship between current and time has been discovered, special current- and time-control swatches in the welder are set, and from this point the process becomes automatic and foolproof. As the electrodes grip the metal and the spring, pressure on the tongs is overcome, contact is automatic ally made, the current passes for the set time, and, if good weld ing conditions obtain, the elec trodes are released for the work to be moved along to the next weld. If, however, for some such reason as faulty electrode contact, bad material, varia tion of line voltage, or the like, too little or too much current passes—conditions that cause bad or overheated welds—a bell rings, and continues to give warning until the machine is switched off, and good conditions re established. It may here be noted that the indicator is set to a tolerance so that while the bell operates when faulty welding conditions occur, it works well within the safety range. In conjunction with this audible control is a permanent graphic record. A continuous strip of paper is fed beneath a stylus, which draws a line for each weld made. The length of this line gives an indication of the time and current conditions relative to each weld, and the strip provides a means whereby conditions can be checked at any desired intervals for purposes of supervision. Thus, the bell acts as an immediate warning to the operator and the shop foreman, while the recorder strip, which is Fig. 4, THE SHOT WELDER : This photograph shows the complete equipment for runs on straight strip or other material that can be fed through a stationary machine. Fig. 3, PNEUMATICALLY CONTROLLED WELDING TOOL : The pressure on the electrodes is automatically determined, and is independent of the skill of the operator. so placed that it cannot be touched by the operator, pro vides the continuous check modern production control demands. The general form of the welder and tools will be gathered from the accompanying illustrations. Fig. 4 shows the complete equipment for runs on straight strip or other material that can be fed through a stationary machine. The box on the right is the actual welder ; it contains the transformers, automatic control devices, warning bell, and weld recorder. The cables leading to the electrodes are taken from the lugs seen to the left of the welder, while above them are the main switch and a small cable leading to the recorder contact. The electrodes themselves are of a special hard, high-con ductivity material that does not distort under heavy pres sure, and are brought together, in the machine shown in Fig. 4, by the action of a pedal provided with a spring- loaded control. In Figs. 2 and 3 special tools for welding more intricate assemblies are shown. The tongs in Fig. 2 are hand operated, but the special tool shown in Fig. 3 has a pneumatic dashpot in order to ensure the absolute control of electrode pressure that is essential. These, tools are connected to the welder in the manner shown in Fig. 1, where the tongs are seen in use on a stainless-steel aileron. It may here be noted that the welder is readily- transportable in order that the leads from welder to tool may be kept as short as possible. Figs. 5 and 6 are typical aircraft members built up by shot welding. Apart from its special merits as an advance in welding technique—which may be briefly summarised as con tinuous control of work and a very high possible speed of operation—shot welding introduces a solution for one of the biggest problems hitherto existing in this field: the welding of the austenitic stainless steels (" Staybrite," " Anka," etc.). As is well known, these materials have presented many difficulties in fabrication, for, though they appear to be readily weldable, it has been established that heating within the range of 500-900 deg. C. causes an internal change that leads to the loss of their corrosion- resisting properties. This is due to the fact that within this temperature range the chromium and carbon, which are normally held in solid solution, tend to precipitate out as chromium carbide, which ranges itself along the grain boundaries to render the material liable to the phenomenon known as intergranular corrosion. It will be evident that the short time required in shot welding is a favourable condition. Shot welding, with its extremelv brief heating period, does not, it is claimed, allow sufficient time for this 1015
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