metric thread on inch type lathe

A couple of years ago I looked for a solution to this metric problem. My 13x40 grizzly will cut metric threads with the 120-127 transposing
gear. We knew that but my problem is that I often cut very close to a shoulder. The usual story is that you cannot disengage the half-nuts at the end because if you do it will not reengage at the correct spot for the next cut. After a lot of research someone smarter than I had a solution. It will align once in 127 turns. I took an aluminum plug and lightly pressed it into the left end of the spindle with a 10-24 or similar bolt tapped about one half inch off center. It travels similar to the throw of a small crankshaft. I took an old counter. It's mechanical with a lever on the end. I fastened it to a screw on the top corner of the front plate of the front cover of the headstock. I connected the lever to the cam/bolt with a light wire and spring. Most threads are cut with less than 127 turns. Set the counter to zero. Move the carriage to the right and stop it with the number one mark on the thread dial lined up. Then move the tailstock up against it as a positive place to stop against next time (tightened down). Engage the feed lever with the motor off. Start the motor and make the first thread pass. Disengage at the end. The thread dial will usually be short of 127 turns. Keep the spindle running until it gets near 120. Stop the motor and turn the spindle by hand until 127 comes up. Reset to the tailstock and mark number one should line up again. Reset the counter and feed in the compound as needed. Engage the feed lever. Start motor. Make the next cut. It is a bit of a pain but it gets a metric thread cut properly to a shoulder. At least there is a solution. If you have more questions write snipped-for-privacy@aol.com
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There is another method that will permit opening the half-nuts also. Start the first cut with the thread dial at 1. As soon as the half-nuts are opened, stop the spindle motor and watch the thread dial. Back up the spindle as needed to re-align the thread dial at 1 without letting it go completely around as the carriage is returned to a suitable starting point. Close the half nuts and proceed with the next cut. The exact same timing will be maintained as long as the thread dial is at the same point and has not turned completely around.
This is basically the same as backing up with the half-nuts still engaged but it permits the spindle and carriage to be temporarily unlocked and then returned to the same relative position.
Don Young
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