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Full text of "Modern Mechanical Engineering Vol-I"

2O2

THE  MACHINE-SHOP

which explains why machine-tools at the present time take much more
power to drive than their immediate predecessors did. The weight of
material removed in a given time is the real test of the cutting capacity of
a tool. The endurance of a tool is the true measure of its efficiency, since
one that has to be reground at short intervals is not economical. A single-
edged tool should endure for at least an hour, while milling-cutters and
those set up in boxes for turret-lathe work should last for a day or more.
The longer the time occupied in regrinding and in resetting, the stronger
is the reason for maintaining the endurance of the edges.
DIVISION III
Setting and Securing Work.—Broadly there are two methods
employed for holding articles to be tooled. In one the piece is gripped
either directly on the work table of the machine or in a chuck, or on an
arbor, with the help of appliances that are in common use for a multitude of
various jobs. Here in general the
pieces are set and held singly even
though many are identical in shape
and dimensions. In the other
method they are not attached
direetly to the table or other work-
holding element, but to an inter-
cnoss SECTION                        mediate appliance, the fixture, or
Fig. 3S-—Clamping Plates set Diagonally                £O special adaptations of chucks Or
arbors. The first is the older
practice, necessarily retained for all classes of work that are not highly
repetitive. The second is the later method, essential to and inseparable
from mass production, and an interchangeable system.
Work held on Tables.—This chiefly concerns the planer, shaper, and
slotter groups, and the drills, boring machines, and allied forms. The
feature common to all is the level table, provided with grooves of tee-section
to receive the bolt heads for clamping work. The grooves are also used to
hold stops, angle plates, vee blocks, and so on.
The surface of the table provides the accurate datum for ensuring that
the clamped work will occupy its correct relation to the cutting-tool. Hence
the first care is to get the work to bed truly on the table. When practicable
it is well to take a rough cut off one surface of the work in order to secure
contact. If this cannot be done, then a rough surface must be packed care-
fully with wedges where it is out of contact with the table, or the clamping
bolts will pull and spring the work, and it will not be true when the machining
is done and the pressure of the bolts released. Though the effect is more
pronounced in thin pieces, it is present in all except the most massive chunks.
Hence, a safe rule is never to tighten a bolt except in opposition to a machined
surface, or, with a rough surface, near a packing.
Thin, and Substantial Articles.—When dealing with very flimsy