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Lathe Operations - Metric Thread Cutting
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Many parts in the machine shop will require the machinist to cut threads. As you have learned earlier in this module the most common form of thread is the V-form thread. Unified National or Standard Unified threads are the norm in the United States, but with the ever-growing influx of foreign manufactured products, especially in machine tools and automobiles, metric threads have become very popular. This unit will show you how to cut a metric thread.

Metric threads come in different forms. The System International (SI) metric thread form is very similar to the American National Standard thread form. The International Standard (ISO) was adopted to standardize metric thread forms.

Metric threads can be identified by the letter M preceding the major thread diameter, and the pitch (Figure 15-1).

Metr_fig1.gif (7985 bytes)
Figure 15-1

The ISO metric thread has a 60-degree included angle. This is the same as the unified thread form. This makes cutting the metric thread form very similar to cutting the unified thread form. Metric thread may be cut on a standard lathe with the proper gear selection. Most of the new lathes come with the proper change gears to cut a variety of metric pitch threads. If the type of lathe that you are using does not have metric thread cutting designations on the quick change gear box, you may have to convert the metric pitch in millimeters to centimeters and then to inches.

To cut a metric thread on a machine which has a Metric/English quick change gearbox the procedure is identical to cutting unified threads, with one exception. At the end of the thread pass, back out the tool, but do not disengage the half nut lever. Because the thread is a non-inch standard or odd pitch metric thread, you can never disengage the half nut until the metric thread is completely cut to depth. To return the tool back to the thread start position, reverse the spindle direction. With the half nut still engaged, the tool will reverse direction. Bring the tool back to the thread start position and continue to make thread cut passes until the thread is cut to the required depth.