Lathes Page: 45

TM 1-420
LATHES 27-28
cutting action. The distance above center is governed by the diameter
of the work and the end clearance of the tool bit. As a general rule,
the point of contact is raised 3'4 inch above center for each inch of
work diameter.
FIGURE 41.-Tool bit position for external work.
d. Centered work is usually driven by a dog of the bent tail type.
Care must be taken to see that the tail of the dog does not wedge
or bind in the drive plate slot as shown in figure 42 0, thereby pre-
venting the work from seating properly in the live center. Figure
42 Q illustrates a set-up using a dog of correct size. If the dog is
clamped to a finished surface, some means should be provided to
keep the set screw or dog from marring its finish. A piece of soft
metal placed between the work and the dog (fig. 43) will generally
give ample protection.
e. Care must be taken to avoid any condition that will allow the
work to spring or vibrate during the turning operation. When the
set-up is made, the tailstock spindle should be run out only as far
as necessary. Rigidity may also be increased by clamping the tool
bit and its holder as short as possible.
f. In general, for straight turning operations a roughing cut should
first be made to between 0.010 inch and 0.020 inch of the finished
diameter. The finish cuts may then be taken using a finishing tool
and fine feed. The application of a suitable cutting lubricant is
advisable where a high finish is desired.
28. Shoulder turning.-Shoulders in lathe work are usually of
the filleted or square corner types.
a. Filleted shoulders, unless a special radius is designated, are
cut with the round nose of the tool.
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