Possible to turn left-hand thread with a right-hand lathe ?

YOU CANNOT CUT LH THREADS WITHOUT AN INTERMEDIATE (IDLE) GEAR TO REVERSE THE DIRECTION OF THE LEAD SCREW.
Simply reversing the spindle direction won't do it. Cutting from the back side won't do it.

This is commonly done with a reversing tumbler but it is possible to install the intermediate gear in the gear train. This may involve modification of the lathe. The idler gear can be any tooth count and is free rotating on an arbor.
 
RJSakowski said:
YOU CANNOT CUT LH THREADS WITHOUT AN INTERMEDIATE (IDLE) GEAR TO REVERSE THE DIRECTION OF THE LEAD SCREW.
Simply reversing the spindle direction won't do it. Cutting from the back side won't do it.

This is commonly done with a reversing tumbler but it is possible to install the intermediate gear in the gear train. This may involve modification of the lathe. The idler gear can be any tooth count and is free rotating on an arbor.
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Absolutely correct.
 
Use an idler gear (easier, all mechanical) or do an electronic lead screw (electrical and mechanical) for left hand threading. Been working on my own ELS for a while. Have most of the electronics and programming done. (Rolled my own.) Have to design and fab the mechanicals for the lathe next.
 
I have, and still occasionally use, a Grizzly version(G1550) of the 9X19, 9X20, whatever they are this month. A reversing tumbler is required for cutting left hand threads. It usually, per many web sites, mounted at the spindle. The principle is to add one more gear into the threading train, which then reverses the lead screw.

The chuck does not reverse, it turns the same CCW as for turning. Only the lead screw reverses in relationship to the spindle. I built a slim profile because there is also a planetary gear on the outboard end of my spindle. Ask and I'll send a copy. But adding one gear in the train simply reverses the output direction. That's all that's required. Perhaps extending the banjo is all you need.

.
 
Bill is correct, the chuck turns in the same direction so it will not unscrew when cutting LH threads- only the carriage reverses direction
I didn't realize the Compact 8 doesn't have a tumbler reverse- learned something today
Doesn't the Compact 8 have a bolt-on chuck anyhow? afaik the original machine has a three-bolt hub (DIN style)
 
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C-Bag said:
You are correct. My HF 9x20 is a knockoff of the Compact 8. There are many websites dedicated to modding the 9x20 and one of them is modding to reverse(called a tumbler) the lead screw. You must be careful because the chuck can unscrew. Here‘s one of the many sites dedicated to modding/improving the 9x20.

9×20 Class Lathes – Mini-lathe.com

www.mini-lathe.com
Click to expand...
For left hand threads you would reverse the leadscrew but the spindle still turns counterclockwise so the chuck would not unscrew.
 
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