In case it is desired to make a weld in the form of a capital letter T, it
is necessary to heat the part corresponding to the top bar of the T to a
bright red, then bring the lower bar to the pre-heated one and again turn
on the current, when a weld can be quickly made.
_Spot Welding._--This is a method of joining metal sheets together at
any desired point by a welded spot about the size of a rivet. It is done on
a spot welder by fusing the metal at the point desired and at the same
instant applying sufficient pressure to force the particles of molten metal
together. The dies are usually placed one above the other so that the work
may rest on the lower one while the upper one is brought down on top of the
upper sheet to be welded.
One of the dies is usually pointed slightly, the opposing one being left
flat. The pointed die leaves a slight indentation on one side of the metal,
while the other side is left smooth. The dies may be reversed so that the
outside surface of any work may be left smooth. The current is allowed to
flow through the dies by a switch which is closed after pressure is applied
to the work.
There is a limit to the thickness of sheet metal that can be welded by this
process because of the fact that the copper rods can only carry a certain
quantity of current without becoming unduly heated themselves. Another
reason is that it is difficult to make heavy sections of metal touch at the
welding point without excessive pressure.
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