When it is not necessary that the nut run up tight against a shoulder, the tool can cut its own finishing
groove if you open the half nuts at the same place each time.
To use successfully any of these methods of finishing a cut, you must work quickly with both hands, back
the tool out with one hand while you open the half-nuts with the other. When you return the tool for another
cut, advance the compound slide by the amount of the chip. Never change the setting of the cross-feed
stop after you have started to cut a thread or you will throw the tool out of alignment with previous cuts.
Since the tool is traveling on the 60" angle of the compound, you must divide half the DD figure by the
sine of 60 ", or .866; the resulting figure will be the amount by which the tool is to be advanced on the
compound slide.
Since there is always some springiness or backlash in both the cutting tool and the work, you will have
to take several finishing cuts with the tool at its final setting, without advancing it any deeper, so it can
work out the oversize cutting when possible, keep handy a nut of the required size that has an accurate
thread, and use this to test the new thread.
If the work is driven by a lathe dog while on centers, be sure the slot in the driving plate is marked so the
dog will be replaced in the same slot each time after removal for testing.
Use plenty of cutting oil when cutting steel. Brass and cast iron can be cut dry, but aluminum tends to
stick to the tool and leave a rough surface unless a little oil is used.
THREAD DIAL INDICATOR
Most modern lathes are equipped with a thread dial indicator, which saves time when cutting long screw
threads. The thread-chasing dial is an indicator with a revolving dial. It is either fastened to the carriage
of the lathe or built into it. The dial of the indicator serves as a guide to the operator. It tells him when to
close the half-nut in the apron onto the lead screw so he may take successive cuts in the same groove,
or to space grooves properly for multiple threads.
When this is done, the lead screw and carriage bear the same relative positions as before. In other
words, when the operator engages the half-nut as the proper mark on the dial comes into position, the
threading tool moves into the same groove made on previous cuts.
The face of the dial is numbered to indicate positions at which the half nuts may be engaged. When the
lathe is set up for cutting screw threads, the thread dial indicates the relative position of the lead screw,
spindle, and carriage of the lathe.
This permits disengaging the half nuts from the lead screw at the end of the cut, returning the carriage
quickly to the starting point by hand, and re-engaging the half nuts with the lead screw at a point, which
will assure that the tool follows exactly in the original cut.