[Illustration: FIG. 43.—LAP WELD SCARFS.]
The pieces should always be upset where the scarfs are to be formed, to provide excess metal for welding. They should be formed with their end surfaces convex, and at an angle of about 45 degrees, which would not make the joining surfaces too long.
When the fire and all tools are ready, place both scarfs face down in the fire; when they are removed to the anvil, the piece held in the right hand should be turned face up and rest on the anvil, in order that the other may be placed in position on top of it.
The left-hand scarf should be placed carefully, with its point meeting the heel of the other. If placed too high and overlapping, it will increase the surface to be welded and perhaps decrease the dimensions of the material where the points are welded down upon the exterior. If placed too low, in all probability the surplus metal provided by upsetting will not be sufficient to form the weld to a uniform dimension. A little practice with the scarfs before heating is advisable to prevent this difficulty.
The hand hammer should be placed conveniently on the anvil, with the handle projecting sufficiently over the heel so that it can be grasped quickly with the right hand as soon as the two pieces are in position. If this precaution is not taken, the welding heat may disappear before any blows can be struck.
The first blows after the pieces are placed should be directed toward the center of the scarfs; when the center has been thoroughly united, the blows should be directed toward the points to complete the operation, if this can possibly be done in one heating.
It is impossible to give an invariable routine of blows; those given are sufficient for the beginning, the rest must be left to the observation and skill of the operator. Practice and judgment will determine where the blows should be delivered, and when they should cease.
As the welding heat vanishes very rapidly, it requires careful judgment to determine when the pieces cease to unite. All blows delivered after this will reduce the dimensions of the metal; if reheating is necessary, there should be no metal sacrificed by unnecessary hammering. Welds are generally weaker than the metal from which they are made; consequently if the stock is made smaller at the weld, its strength is greatly decreased.