Thread cutting in the Small Lathe

By: Gord Thorogood

The lead screw is the part of the lathe that determines the pitch or number of threads per inch that are being cut. Machines that have inch dials and gear boxes will have a lead screw with an even number of threads per inch and will range from two threads per inch to perhaps 16 threads per inch with small bench lathes having 8 TPI or 16 TPI.

If you cut 8 threads per inch with an 8 thread lead screw then the spindle and the lead screw are rotating at the same speed so the half nuts can be closed in any position on the threading dial. You will pick up the thread in the work you are cutting when you close the half nuts. If you wish to cut 16 TPI then the lead screw will be turning at half the speed of the spindle and 12 TPI will be three quarters of the spindle speed or 3 revolutions for every 4 revolutions of the spindle.

If a machine does not have a device to tell you when the lead screw and spindle have come into synchronization then you would have to close the half nuts when starting to cut your thread and not open them again until the thread is complete. This can only be done by stopping the spindle retracting the tool and reversing the rotation until the carriage returns to a point where you can take another cut. This operation is repeated until the thread is finished. Needless to say this can be a very tedious and nerve wracking exercise.

Fortunately thread cutting dials are available that allow the half nuts to be opened and the carriage moved back to the start without losing control over the relationship of the screw and spindle. The dial reads directly from the lead screw using a gear that fits the screw. The gear on the dial will usually have a gear with 4 times the number of teeth as the pitch of the screw. For example an eight pitch screw will mesh with a thirty two tooth gear giving you four positions on the dial where the screw is synchronized with the dial.

The dial will be divided into usually 8 divisions that are numbered one to eight. Each lathe will have a chart that relates to the dial, listing the different pitches to be cut and the positions on the dial where the half nuts can be closed. For example there may be a group of pitches listed and the chart will say you can close the nuts at any number on the dial. You will see another group of pitches listed and the chart will say you can close the nuts only on one, three, five and seven.

You will probably find other pitches where the chart will say that you can only close the nuts at the same number each time. Thread cutting with the dial will become second nature to you with a little practice.

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