Each piece should be given this shape before proceeding farther.
The scarf may be produced by filing, sawing or chiseling the ends, although
this is not good practice because it is then impossible to give the desired
upset and additional metal for the weld. This added thickness is called for
by the fact that the metal burns away to a certain extent or turns to
scale, which is removed before welding.
When the two ends have been given this shape they should not fit as closely
together as might be expected, but should touch only at the center of the
area to be joined (Figure 51). That is to say, the surface of the beveled
portion should bulge in the middle or should be convex in shape so that the
edges are separated by a little distance when the pieces are laid together
with the bevels toward each other. This is done so that the scale which is
formed on the metal by the heat of the fire can have a chance to escape
from the interior of the weld as the two parts are forced together.
[Illustration: Figure 51.--Proper Shape of Scarfed Ends]
If the scarf were to be formed with one or more of the edges touching each
other at the same time or before the centers did so, the scale would be
imprisoned within the body of the weld and would cause the finished work to
be weak, while possibly giving a satisfactory appearance from the outside.
_Fluxes._--In order to assist in removing the scale and other
impurities and to make the welding surfaces as clean as possible while
being joined, various fluxing materials are used as in other methods of
welding.
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