Hi Bog,
Had me thinking about this during my drive in to work.
If I had to cut a double thread, in the chuck, without releasing the half nut, I would turn the compound over to parallel with the lathe bed, zero the dial on the compound and make sure all the lash was taken up in the compound feedscrew by turning it to advance toward the lathe chuck. Now lock the compound slide gibs so it won't move. I would chuck up a piece of stock, prep it for threading, and then make my first thread. Back the tool out using the cross slide. I would be threading by the plunge method or cutting on both sides of the tool . I wouldn't want to do this with a coarse thread, but with the threads found on the average ME project, this should not be a problem
Now release the lock on the compound and advance the compound a distance equal to the pitch of the thread toward the headstock ( not the lead of the thread mind you, the pitch of the thread which is for a two start equal to one half the lead).
Now back up the carriage, advance the cross slide and cut the second thread by the plunge method. You should be 180 degrees out of phase from the first thread.
Example:
A 1/4-20 pitch 2 start screw will have a lead of .100
You would cut the first thread, move the compound 1/20th of an inch or .050" or .100/2 ( lead / 2) and then cut the second thread.
It would be useful to have a 3 phase motor on the lathe to make stopping the spindle easy. However, with a spindle crank, as has been detailed ad neausium in ME and MEWS, you can hand turn the spindle to cut the thread just as easy and those with single phase machines will have less anxiety coming up to a shoulder or the chuck.
If the thread has a oddball pitch with a fractional thousandth or so, you may want to use a dial indicator to track the movement of the compound instead of the compound feed screw in case the screw has some wear or is sloppy.
It is important to move the compound the EXACT amount. This is important!
Cut the screw long and make half of it into a tap to make the nut. Or make two if double length is too woobly.
This should work a treat with fine threads ( 20 or less per inch).
Hope that is clear, If I can explain something better let me know.
Best wishes,
Dave
Had me thinking about this during my drive in to work.
If I had to cut a double thread, in the chuck, without releasing the half nut, I would turn the compound over to parallel with the lathe bed, zero the dial on the compound and make sure all the lash was taken up in the compound feedscrew by turning it to advance toward the lathe chuck. Now lock the compound slide gibs so it won't move. I would chuck up a piece of stock, prep it for threading, and then make my first thread. Back the tool out using the cross slide. I would be threading by the plunge method or cutting on both sides of the tool . I wouldn't want to do this with a coarse thread, but with the threads found on the average ME project, this should not be a problem
Now release the lock on the compound and advance the compound a distance equal to the pitch of the thread toward the headstock ( not the lead of the thread mind you, the pitch of the thread which is for a two start equal to one half the lead).
Now back up the carriage, advance the cross slide and cut the second thread by the plunge method. You should be 180 degrees out of phase from the first thread.
Example:
A 1/4-20 pitch 2 start screw will have a lead of .100
You would cut the first thread, move the compound 1/20th of an inch or .050" or .100/2 ( lead / 2) and then cut the second thread.
It would be useful to have a 3 phase motor on the lathe to make stopping the spindle easy. However, with a spindle crank, as has been detailed ad neausium in ME and MEWS, you can hand turn the spindle to cut the thread just as easy and those with single phase machines will have less anxiety coming up to a shoulder or the chuck.
If the thread has a oddball pitch with a fractional thousandth or so, you may want to use a dial indicator to track the movement of the compound instead of the compound feed screw in case the screw has some wear or is sloppy.
It is important to move the compound the EXACT amount. This is important!
Cut the screw long and make half of it into a tap to make the nut. Or make two if double length is too woobly.
This should work a treat with fine threads ( 20 or less per inch).
Hope that is clear, If I can explain something better let me know.
Best wishes,
Dave