Metric threads on pm1236

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FTlatheworks

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I tried cutting metric threads on my pm1236 and it didn’t work out. I cut standard threads just to make sure that would work and it did, but the threads weren’t pretty. I’m cutting aluminum. One question I have is where is the chart for where to start on the lead screw. Is there a different process for metric then standard besides the lead screw speed settings?
 
You can start anywhere on the leadscrew BUT you cannot release the half nuts once started. There are people who debate this but as a new user best succeed in making good threads first. Aluminum can tear instead of cut, use Kerosene/WD40 or commercially available cutting lube for the metal being cut. I suggest watching videos to help things along.
Pierre
 
Don't know how the PM 1236 is set up to cut metric threads. On my craftsman 12x36 the procedure is similar to imperial threading but you don't release the half nuts. Instead you stop the lathe at the end of the thread. Back out the cross slide, Reverse the lathe with the half nuts still engaged to bring the carriage back to your starting point. Stop the lathe. Move the cross slide in to zero. Advance the compound a few thousands and start the lathe in forward to make another pass. This process gets repeated until you have finished cutting the thread.
 
Did you change the lower gear to be driven by the 120t gear instead of the 127t that is used for English threads in the drive train?
 

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FTlatheworks said:
No I didn’t change those out. I disengaged the half nut too.
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Many of us have made those mistakes at one time or another, make those two changes and you should get good threads.
 
How do you stop at the right spot if you can’t disengage the half nut. I got good threads, but I had to use the foot brake or stop the motor with the tool in the work. Either way there are risks to these methods of stopping.
 
Mount the tool upside down, run in reverse away from the shoulder. Or place it behind the work the usual way up.
 
Most threads have an undercut at the end of the thread where the tool tip should be when you stop the spindle. Since you have a brake, that is a good way to stop in the right spot. I have a VFD on mine with a fast deceleration time and it stops in less than 1 rotation when I shut off the spindle. If you don’t have an undercut, you need to stop in the same spot each time, or when you get deeper into the thread and run a bit past the last cut, you can break the tool tip by cutting too much.

The undercut should be slightly deeper than the minor diameter for an external thread, and about 1.5x the pitch. That should be enough room to stop the spindle without crashing into a shoulder unless you are going really fast. For your first threads, you might want to make it wider until you have more experience.

For metric threads, stop the spindle with the tool in the undercut, retract the tool, turn spindle in reverse to your starting position, move the tool back to the correct depth for the next pass and then run forward to the undercut. Repeat until you are at the correct thread depth.
 
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