Smithy Midas 1220 LTD - Manual (Page 64)

Chapter 13
Cutting off or Parting with a Lathe
You can cut off in a lathe only when holding one end of the work rigidly, as in a chuck.
It is not practical for long workpieces held between centers because the workpiece is not
supported closely with a rest and the free section is long enough to sag and pinch the
blade. Cutting off requires a tight lathe without excess play in the spindle, compound,
carriage, or toolpost. Looseness will almost certainly cause chatter. Cutting off also
requires a narrow cutting edge with ample (5-10°) side clearance, which should feed into
the work slowly to prevent hogging in. Once considered a difficult, costly operation,
cutting off became much simpler with development of narrow tools with special cutoff
blades (Figure 13.1).
The toolpost should hold the cut-off tool as close to the workpiece as possible, with the
top of the blade on dead center and exactly perpendicular to the line of centers. Extend
the blade only far enough to pass through the work-piece, just beyond its center. The tool
should feed to the workpiece on exact center, slowly and evenly with the cross-feed. If
the tool hogs in and the spindle stops rotating, turn off the motor and reverse the
spindle by hand before backing the tool out with the crossfeed.
Figure 13.1 Specially designed tools like this one make cutting off easier.
Always set up the workpiece to cut off as close as possible to the headstock. If you must
make a parting cut on a long shaft or on work between centers, don't complete the cut
in the lathe. Finish the parting with a hacksaw and return it to the lathe for facing. Slow
the spindle speed until you have a good feel for cutting off. Although lubricants and
coolants are not essential on small-diameter workpieces, use them amply on deep cut-off
work.
13-1
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