cs in assembling various
machine details. A cylindrical part which is to be held in position by a
shrinkage fit is first turned a few thousandths of an inch larger than
the hole; the diameter of the latter is then increased by heating, and
after the part is inserted, the heated outer member is cooled, causing
it to grip the pin or shaft with tremendous pressure.

General practice seems to favor a smaller allowance for shrinkage fits
than for forced fits, although in many shops the allowances are
practically the same in each case, and for some classes of work,
shrinkage allowances exceed those for forced fits. In any case, the
shrinkage allowance varies to a great extent with the form and
construction of the part which has to be shrunk into place. The
thickness or amount of metal around the hole is the most important
factor. The way in which the metal is distributed also has an influence
on the results. Shrinkage allowances for locomotive driving wheel tires
adopted by the American Railway Master Mechanics Association are as
follows:

Center diameter, inches 38 44 50 56 62 66
Allowance, inches 0.040 0.047 0.053 0.060 0.066 0.070

Whether parts are to be assembled by forced or shrinkage fits depends
upon conditions. For example, to press a driving wheel tire over its
wheel center, without heating, would ordinarily be a rather awkward and
difficult job. On the other hand, pins, etc., are easily and quickly
forced into place with a hydraulic press and there is the additional
advantage of knowing the exact pressure required in assembling, whereas
there is more or less uncertainty connected with a shrinkage fit, unless
the stresses are calculated. Tests to determine the difference in the
quality of shrinkage and forced fits showed that the resistance of a
shrinkage fit to slippage was, for an axial pull, 3.66 times greater
than that of a forced fit, and in rotation or torsion, 3.2 times
greater. In each comparative test, the dimensions and allowances were
the same.

The most important point to consider when calculating shrinkage fits is
the stress in the hub at the bore, which depends chiefly upon the
shrinkage allowance. If the allowance is excessive, the elastic limit of
the material will be exceeded and permanent set will occur, or, in
extreme cases, the ultimate strength of the metal will be exceeded and
the hub will burst.

CHAPTER IV

THREAD CUTTING IN THE LATHE

When t