Machinery Repairman

Rough Turning
Figure 6-62.—Rough turning.
Figure 6-62 illustrates a lathe taking a heavy cut.
This is called rough turning. When a great deal of
stock is to be removed, you should take heavy cuts in
order to complete the job in the least possible time.
Be sure to select the proper tool for taking a heavy
chip. The speed of the work and the amount of feed
of the tool should be as great as the tool will stand.
When taking a roughing cut on steel, cast iron, or
any other metal that has a scale on its surface, be sure
to set the tool deeply enough to get under the scale in
the first cut. If you do not, the scale on the metal will
dull the point of the tool.
Rough machine the work to almost the finished
size; then be very careful in taking measurements on
the rough surface.
Often the heat produced during rough turning will
expand the workpiece, and the lubricant will flow out
of the live center hole. This will result in both the
center and the center hole becoming worn. Always
check the center carefully and adjust as needed during
rough turning operations. If you are using a ball
bearing center, feel the area where the bearings are
located and ensure the center is not too warm.
Figure 6-63 shows the position of the tool for
taking a heavy chip on large work. Set the tool so that
if anything causes it to change position during the
6-36
machining operation, the tool will move away from
the work, thus preventing damage to the work. Also,
setting the tool in this position may prevent chatter.
Finish Turning
When you have rough turned the work to within
about 1/32 inch of the finished size, take a finishing
cut. A fine feed, the proper coolant and a keen-edged
tool are necessary to produce a smooth finish.
Measure carefully to be sure you are machining the
work to the proper dimension. Stop the lathe
whenever you take any measurements.
If you must finish the work to extremely close
tolerances, wait until the piece is cool before taking
the finish cut. If the piece has expanded slightly
because of the heat generated by turning and you turn
it to size while it is hot, the piece will be undersize
after it has cooled and contracted.
If you plan to finish the work on a cylindrical
grinder, leave the stock slightly oversize to allow for
the metal the grinder will remove.
Perhaps the most difficult operation for a
beginner in machine work is taking accurate
measurements. So much depends on the accuracy of
the work that you should make every effort to become
proficient in using measuring instruments. You will
develop a certain “feel” through experience. Do not
be discouraged if your first efforts do not produce
perfect results. Practice taking measurements on
pieces of known dimensions. You will acquire the
skill if you are persistent.
Turning to a Shoulder
A time-saving procedure for machining a
shoulder is illustrated in figure 6-64. First, locate and
Figure 6-63.—Position of tool for heavy cut.