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ig8                           THE  MACHINE-SHOP

dished to suit the section of the wheel, and they only bear against it with
annular seatings, which do not tend to crush the wheel, and, if it should
fracture, the pieces are prevented from flying off. Wheels of parallel thickness
are also gripped with annular contact. Another essential is that the wheels
fit loosely on their arbors and tight only in the flanges, to avoid risk of their
being burst. Fig. 31 illustrates a wheel gripped with washers of leather,
rubber, or cardboard. But the principal feature is that the wheel is mounted
permanently with a screwed flange, to be removed from and replaced bodily
on the tapered end of its arbor, where it is held with a circular nut. Fig. 32
shows a face wheel. The mounting includes an encircling safety ring,
which is set back as the wheel wears. In the Blanchard wheel, used on
the firm's vertical-spindle machines, the principal feature is the provision
of holes in the flange to direct water to the face of the wheel.

The Essentials of Economical Machining
Lubrication.—The efficiency of cutting-tools depends on the lubri-
cation and the cooling of the tool point, and of the surface of the work being
cut. Cast iron and brass are usually excepted. Formerly the chief attention
was directed to the cooling of the tool; now the view-point is changed,
consequent on the increased severity of cutting, with the more rapid
generation of heat. Instead of the drip-can, the cooling liquid is delivered
in a stream, frequently under pressure, and directed with pipe nozzles or
spreaders all over the surfaces being cut.
Cooling Fluids.—With these changed views the practice has undergone
great changes. Special lubricants are now used for certain classes of heavy,
medium, and light work. As of old the best all-round lubricant is lard
oil, but the high cost of it handicaps its general use. The best substi-
tutes contain a mineral oil with a certain quantity of lard, and are termed
" mineral lard oils ". The proportions of lard are varied for different
kinds of work. Soda or potash mixed with a mineral lard oil forms soap.
The soap holds the oil in suspension, and prevents it from floating on the
Distribution and Recovery.—Instead of the drip-can a system of
supply pipes is laid down in modern shops, and each machine is provided
with its own particular equipment for distribution through jets or nozzles,
with means for the collection and return of the liquid. In some cases a
gravity supply is installed. A feed tank is placed in the roof, and the machines
drain to a sump below the floor. More often now the cooling fluid is
delivered by means of a pump. In a few shops different groups of