re not used
for this particular operation.) When the turret again indexes, the
finished casting is removed and replaced with a rough one. While the
successive operations on a single casting have just been described, it
will be understood that all of the tools operate simultaneously and that
a finished casting arrives at the unloading and loading position each
time the turret indexes. Three hundred of these malleable castings are
machined in nine hours.

=Selecting Type of Turning Machine.=--The variety of machine tools now
in use is very extensive, and as different types can often be employed
for the same kind of work, the selection of the best and most efficient
machine is often a rather difficult problem. To illustrate, there are
many different types and designs of turning machines, such as the
ordinary engine lathe, the hand-operated turret lathe, the
semi-automatic turning machine, and the fully automatic type, which,
after it is "set up" and started, is entirely independent. Hence, when a
certain part must be turned, the question is, what kind of machine
should be used, assuming that it would be possible to employ several
different machines? The answer to this question usually depends
principally upon the number of parts that must be turned.

For example, a certain casting or forging might be turned in a lathe,
which could be finished in some form of automatic or semi-automatic
turning machine much more quickly. It does not necessarily follow,
however, that the automatic is the best machine to use, because the
lathe is designed for general work and the part referred to could
doubtless be turned with the regular lathe equipment, whereas the
automatic machine would require special tools and it would also need to
be carefully adjusted. Therefore, if only a few parts were needed, the
lathe might be the best tool to use, but if a large number were
required, the automatic or semi-automatic machine would doubtless be
preferable, because the saving in time effected by the latter type would
more than offset the extra expense for tool equipment and setting the
machine. It is also necessary, in connection with some work, to consider
the degree of accuracy required, as well as the rate of production, and
it is because of these varying conditions that work of the same general
class is often done in machines of different types, in order to secure
the most efficient results.

CHAPTER VI

VERTICAL BORING MILL PRACTICE