I have recently had the need to machine a 25mm metric thread up to a shoulder with a minimal undercut.
As I have to use conversion gears for this the half nuts must remain closed so stopping the lathe precisely was going to be a problem, I decided to run the lathe in reverse and start at the undercut using a rear tool post.
Problem No 2, I have a screw on chuck which could unscrew at any point with this setup.
I have seen people resort to drilling through the back plate into the mandrel register and relying on a cap screw to prevent this, this is bodge at best and cannot be repeated with another chuck with any accuracy.
I decided to machine a .125” deep recess into the chuck back plate and make a clamping disc which is threaded, peened over and lock nutted to a length of 8mm all thread through the mandrel so clamping the back plate firmly against the register.
This does involve removing the chuck from the back plate to fit it but it is a small inconvenience for the sake of peace of mind and can be used with all four of my chucks.
Like you, I prefer running the lathe in reverse (and the lead screw direction reversed) when single point threading to a shoulder minimal undercut. I do not have the rear tool post so I turn the threading tool upside down in the QCTP tool holder. I had to buy a left-hand cutting tool for holding the thread cutting insert in order to cut as close as possible to the shoulder near the chuck. I use the A R Warner T15 HSS tool inserts most of the time because I seem to accidentally chip the carbide inserts too often on parts with limited access for threading. By the way, Joe Pieczynski has an excellent YouTube video on threading with the lathe in reverse for those have not seen this technique.
Thank you for the tip in locking the thread-on chuck in place.
I have to use a rear tool post on my lathe (South Bend) as the front of the carriage has a tendency to lift off the bed under load with an upside down tool. It was obviously designed so the normal cutting forces keep it down.
I have watched some of Joe Pieczynski's video's, he certainly knows a thing or two.
It won't stop the problem with screw cutting to a shoulder but this is a tip to remove drive to the chuck quickly and reduce rotating inertia, literally at the touch of a button.
Air conditioning pumps from a car have a nice electro/magnetic clutch. Apply 12v and the clutch is engaged, remove it and it is free. A/C repair shops usually have dud pumps. They are easy to gut and place in the lathe drive chain. Torque capacity seems quite high but I have no figures nor have I measured that, I have just used them. I haven't done it yet but you could fit a second one as a chuck brake. Connect them such that when one is on the other is off. Almost instant stop, but screw on chucks will need your retainer or watch out for flying chucks.
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