Tc 9-524 fundamentals of machine tools

Table or Base Mounting
When a workpiece is table or base mounted (Figure 4-28),
the strap clamps must be as parallel to the table or base as
possible. All bolts and strap clamps should be as short as
possible for rigidity and to provide for drilling clearance
(Figure 4-29).
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TC 9-524
Parallel bars should be set close together to keep from bending
the work. Washers and nuts should be in excellent condition.
The slots and ways of the table, base, or vise must be free of
all dirt and chips. All work holding devices should be free of
burrs and wiped clean of oil and grease. Work holding
devices should be the right size for the job. Devices that are
too big or too small for the job are dangerous and must be
avoided.
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TC 9-524
GENERAL DRILLING OPERATIONS
THE DRILLING PROCESS
After a workpiece is laid out and properly mounted, the
drilling process can begin. The drilling process, or complete
operation, involves selecting the proper twist drill or cutter for
the job, properly installing the drill into the machine spindle,
setting the speed and feed, starting the hole on center, and
drilling the hole to specifications within the prescribed
tolerance. Tolerance is the allowable deviation from standard
size. The drilling process must have some provisions for
tolerance because of the oversizing that naturally occurs in
drilling.
Drilled holes are always slightly oversized, or
slightly larger than the diameter of the drill’s original
designation. For instance, a l/4-inch twist drill will produce a
hole that may be several thousandths of an inch larger than
l/4-inch.
Oversizing is due to several factors that affect the drilling
process: the actual size of the twist drill, the accuracy of the
drill point, the accuracy of the machine chuck and sleeve, the
accuracy and rigidity of the drilling machine spindle, the
rigidity of the entire drilling machine, and the rigidity of the
workpiece and setup. Field and maintenance shop drilling
operations allow for some tolerance, but oversizing must be
kept to the minimum by the machine operator.