r turpentine is sometimes used when
boring or reaming. If babbitt is bored dry, balls of metal tend to form
on the tool point and score the work. Milk is generally considered the
best lubricant for machining copper. A mixture of lard oil and
turpentine is also used for copper. For aluminum, the following
lubricants can be used: Kerosene, a mixture of kerosene and gasoline,
soap-water, or "aqualine" one part, water 20 parts.

=Lard Oil as a Cutting Lubricant.=--After being used for a considerable
time, lard oil seems to lose some of its good qualities as a cooling
compound. There are several reasons for this: Some manufacturers use the
same oil over and over again on different materials, such as brass,
steel, etc. This is objectionable, for when lard oil has been used on
brass it is practically impossible to get the fine dust separated from
it in a centrifugal separator. When this impure oil is used on steel,
especially where high-speed steels are employed, it does not give
satisfactory results, owing to the fact that when the cutting tool
becomes dull, the small brass particles "freeze" to the cutting tool and
thus produce rough work. The best results are obtained from lard oil by
keeping it thin, and by using it on the same materials--that is, not
transferring the oil from a machine in which brass is being cut to one
where it would be employed on steel. If the oil is always used on the
same class of material, it will not lose any of its good qualities.

Prime lard oil is nearly colorless, having a pale yellow or greenish
tinge. The solidifying point and other characteristics of the oil depend
upon the temperature at which it was expressed, winter-pressed lard oil
containing less solid constituents of the lard than that expressed in
warm weather. The specific gravity should not exceed 0.916; it is
sometimes increased by adulterants, such as cotton-seed and maize oils.

CHAPTER III

TAPER TURNING--SPECIAL OPERATIONS--FITTING

It is often necessary, in connection with lathe work, to turn parts
tapering instead of straight or cylindrical. If the work is mounted
between the centers, one method of turning a taper is to set the
tailstock center out of alignment with the headstock center. When both
of these centers are in line, the movement of the tool is parallel to
the axis of the work and, consequently, a cylindrical surface is
produced; but if the tailstock _h_{1}_ is set out of alignment, as shown
in Fig. 1,