Tc 9-524 fundamentals of machine tools

7-32

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TC 9-524
When a micrometer and two wires are used. the micrometer
Parting is also used to cut off work after other machining
reading is equal to the measured diameter of the groove plus
operations have been completed (Figure 7-59). Parting tools
two wire diameters.
can be of the forged type. inserted blade type. or ground from
a standard tool blank. In order for the tool to have maximum
To calculate measurement over the wires, use the following
strength, the length of the cutting portion of the blade should
formula:
extend only enough to be slightly longer than half of the
workpiece diameter (able to reach the center of the work).
Measurement = Outside Diameter+ (2 x wires) –
Never attempt to part while the work is mounted between
2 x radius).
centers,
Parting
Parting is the process of cutting off a piece of stock while it
is being held in the lathe. This process uses a specially shaped
tool bit with a cutting edge similar to that of a square-nosed
tool bit. When parting. be sure to use plenty of coolant, such
as a sulfurized cutting oil (machine cast iron dry). Parting
tools normally have a 5° side rake and no back rake angles.
The blades are sharpened by grinding the ends only. Parting
is used to cut off stock. such as tubing. that is impractical to
Work that is to be parted should be held rigidly in
or collet, with the area to be parted as close to the
device as possible. Always make the parting cut at
angle to the centerline of the work. Feed the tool bit
a chuck
holding
a right
into the
revolving work with the cross slide until the tool completely
severs the work. Speeds for parting should be about half that
used for straight turning. Feeds should be light but
continuous. If chatter occurs. decrease the feed and speed. and
check for loose lathe parts or a loose setup. The parting tool
should be positioned at center height unless cutting a piece
that is over 1-inch thick. Thick pieces should have the cutting
tool just slightly above center to account for the stronger
torque involved in parting. The length of the portion to be cut
off can be measured by using the micrometer carriage stop or
by using layout lines scribed on the workpiece. Always have
the carriage locked down to the bed to reduce vibration and
chatter. Never try to catch the cutoff part in the hand; it will
be hot and could burn.