The amount of this last separation must
be determined by the shape and proportions of the parts in the same way as
would be done for any other class of welding which heats the parts to a
melting point.
Yellow wax, which has been warmed until plastic, is then placed around the
joint to form a collar, the wax completely filling the space between the
ends and being provided with vent holes by imbedding a piece of stout cord,
which is pulled out after the wax cools.
A retaining mould (Figure 55) made from sheet steel or fire brick is then
placed around the parts. This mould is then filled with a mixture of one
part fire clay, one part ground fire brick and one part fire sand. These
materials are well mixed and moistened with enough water so that they will
pack. This mixture is then placed in the mould, filling the space between
the walls and the wax, and is packed hard with a rammer so that the
material forms a wall several inches thick between any point of the mould
and the wax. The mixture must be placed in the mould in small quantities
and packed tight as the filling progresses.
[Illustration: Figure 55.--Thermit Mould Construction]
Three or more openings are provided through this moulding material by the
insertion of wood or pipe forms. One of these openings will lead from the
lowest point of the wax pattern and is used for the introduction of the
preheating flame. Another opening leads from the top of the mould into this
preheating gate, opening into the preheating gate at a point about one inch
from the wax pattern.
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