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Full text of "Modern Mechanical Engineering Vol-I"

230                           THE  MACHINE-SHOP

retain its truth as long as possible. To use a traverse feed only slightly less
than the width of the wheel is more economical than to employ a feed that
bears a small proportion to the width of the wheel. The peripheral speed
of wheels is usually about 5000 ft. per minute, that of the work from 20
to 25 ft. The wheel speed is constant, that of the work is changed when
desirable for making differences between roughing and finishing. Chatter
and vibration are prevented by the employment of a large number of back
steadies.

Surface Grinding.—This has been largely favoured by the employ-
ment of the magnetic chucks. These hold flimsy and awkwardly shaped
pieces, which would give vast trouble if clamped on work tables. Reinforce-
ments in the shape of stops and rings are necessary to prevent side-slip. A
large number of small pieces can be held and operated on thus. Fixtures
are also largely employed. The machines are built in two types, one in
which the work table has linear movements, the other with rotary motions.

Machines for grinding cylinders, for form grinding, and those for tools
and cutters include a large number of designs. The machines for grinding
the cylinders of automobiles and gas-engines have developed with startling
rapidity. The spindles have a planet or eccentric motion, so that while they
are revolved at high speeds they are rotated slowly in a circular pathway,
the diameter of which is increased to impart the feed. The work is carried
on a table that can be adjusted transversely to bring bores in alignment with
the wheel. The work table is fed towards the wheel with changes of travel
for roughing and finishing cuts. Wet grinding is provided for by a pump
and tank and pipe.

Continuous Grinding.—This relates to the treatment of numbers of
small pieces arranged in tandem, or in a circle, to be ground with face wheels.
Much of this work is done on magnetic chucks or in fixtures. The more
awkwardly shaped and the smaller the pieces are, the greater are the econo-
mies of continuous grinding. Often the choice lies between this method
and that of milling done on lineally or circularly moving tables.

CHAPTER   V
The Shops
DIVISION I
Organization.—This must be based on a rigid cost system, from which
the price of work in all its stages can be ascertained, and leakages detected
from day to day. The old method of adding men's time in the aggregate
and lumping contingent expenses and profits on that is no longer followed
in competitive firms.
In order to fix costs at all stages a routine system is essential.   For this