Cutting metric threads in a Hercus lathe

Описание: To cut a metric thread on a lathe, you need to set the correct gear ratio for the metric pitch, use a cutting tool with a 60-degree angle, and carefully manage the half-nut and threading dial for each pass. On an imperial lathe with an imperial lead screw, the process is more complex and involves either disengaging the half-nut to reverse the spindle or using a specific gear set to get closer to the desired metric pitch.

On a metric lathe (or with the right conversion)
Set the gears: Use the gear chart or calculator to find the correct gear combination for the desired metric pitch, ensuring the half-nuts and lead screw can accommodate the metric thread.

Set the tool: Set the tool with a 60-degree angle, aligning it with the workpiece.
Set the zero point: Set the carriage to a zero point on the dial.

Cut the thread:
Engage the half-nuts and begin threading.
When the thread is cut, retract the tool.
Disengage the half-nuts and reverse the lathe to return the tool to the starting point.
Re-engage the half-nuts on the same thread line to start the next cut.

Repeat: Continue cutting with progressively deeper cuts until the desired thread depth is reached.

On an imperial lathe with an imperial lead screw

Use a threading dial: You must use the threading dial to ensure accurate re-engagement of the half-nut after each pass.

Disengage and reverse:
Engage the half-nuts for the first pass and cut the thread.

When you reach the end of the thread, disengage the half-nuts and immediately shut off the lathe.

Back the tool out of the thread.
Reverse the spindle and watch the threading dial.

Engage the half-nuts again when the dial returns to the original line, and repeat the cutting pass.

Change gears: Some lathes can be converted to metric with a gear change. This can involve adding a metric lead screw or using a gear train with a specific ratio to cut metric threads on an imperial lathe
This video demonstrates how to cut metric threads on a lathe, including setting the tool and the carriage: