An engine lathe is a 500-arm octopus.

I was reminded of this by a newb machinist who asked on Google on where the dial is on a LeBlond for threading on a manual lathe.

Here is what the thread dial looks like:

If you feel like you are wrestling with the 500-arm octopus or staring at the million switches and controls in an airplane cockpit, we have a visual guide to remind you of what to trigger and where when threading on a manual lathe.

WHERE IS THAT PESKY THREAD DIAL?

When threading on a manual lathe, you can EITHER always engage the half-nut on the lead screw and return the carriage by reversing the spindle rotation through the motor drive …

… OR, you can use a thread dial or indicator which meshes with the leadscrew and shows when the half-nut can be engaged, so that the tool will cut along the same thread.

Here is a picture of a thread dial on a LeBlond RKL manual lathe:

The thread dial consists of a worm wheel meshing with the lead screw. It is connected by a short shaft to the indicating dial. The dial is calibrated with four numbered lines and four others midway between them.

These lines are used when cutting threads on a manual lathe. When set to a position, the line becomes your starting point to pick up the lead of the thread and, during additional passes, to achieve the proper depth of thread.

HIT YOUR MARK WHEN THREADING ON A MANUAL LATHE

On the thread dial, push the button and it moves inward.

You want to engage the half-nut in the same spot every time.

Before engaging the half-nut, run your machine and make sure the lathe is turning slowly.

Check out the thread dial and make sure it is turning slowly. If it isn’t turning, you’ve got a bigger headache.

When engaging the half-nut, you want to make sure your numbers on the thread dial or reaching the top repeatedly. Look for the mark at the top where they align during rotation.

EVEN & ODD PITCH THREADS

Engage and disengage the half nut. Try to find the sweet spot repeatedly.

For the exact position to land on for even, odd and other threads, refer to this chart:

CUTTING METRIC THREADS ON A MANUAL LATHE

When cutting metric threads, always engage the half-nut. Return the carriage by reversing the spindle rotation through the motor drive.

For more information on cutting metric threads, check out this video.

Hopefully, this visual guide will help you identify the threading dial, one of the many octopi appendages to engage when threading on a manual lathe! If something is unclear, hit us up in the comments below.

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