I am moving on to inside thread cutting.

[jbmauser]

I know I need a relief cut to run the cutter into at the back of the cut and so I need a way to know when to lift the handle and disengage the half nuts. It can't be a hard stop because if I goof up I could snap the gears that drive the shaft. I can't figure any way to automatically stop the lathe or to lift the handle automatically. I have to do it myself. I thought about rigging some kind of light with contacts that let me know when the cutter has passed into the relief area but I must be overthinking this. I see lots of carriage stops on Youtube and they are not for screw cutting. I have an old SB9 with change gears.

 

[mikey]

I'll contribute some ideas and the rest of the guys can correct me.

• Ideally, your thread relief should be 0.015" deeper than the minor diameter and, if possible, the lip of the thread relief should have a 45 degree angle leading into the thread relief. Micro 100 (and others) solid carbide thread relief tools have this 45 degree configuration ground into the tool; very handy to have.

• I would have suggested you consider threading from the inside out but if I'm not mistaken, the SB9 has a threaded spindle so that's out.
• You can always thread normally at the lowest possible speed and put a mark on the threading tool with a Sharpie that tells you when to disengage the half-nuts. If you stay focused and disengage the half-nuts right on time, this works just fine because the saddle will stop immediately when you disengage the half-nuts. I do this all the time, even though my leadscrew has a clutch.
• You can also make a crank that is attached to a mandrel that fits inside the back of the spindle and use it to thread under hand power. With light cuts, this is an easy, near fool proof way to thread without danger of crashing. See this: https://www.hobby-machinist.com/threads/spindle-crank-for-south-bend-10l-lathe.66531/

Hope this gives you some ideas. Keep your speeds down and practice - it will come.

 

[rock_breaker]

Mikey does it the neat way, I put an "O" ring on my boring bar at the end of the work rhat marks the spot to open the half nuts.
Have a good day
Ray

 

[Bamban]

Here is advice from a rookie.

I would definitely get the thread relief cutter suggested by Mikey. Set up the thread relief cutter and threading tool so they have the same stick out from the tool holders to put the threading bit where you want in the relief cut.

To be on the safe side you can make some practice passes on the outside on a practice material to make sure with the same distance traveled from the face of material the two cutters dovetail together. Use a dial indicator to monitor the saddle movement. Once confident that they do, go ahead and start on your workpiece.

Cut the relief, before retracting relief cutter set the dial indicator on the left side of the saddle to zero

Install your threading tool holder, set up the lathe for threading. Make a few dummy passes, not cutting, but just go in and disengage the half nuts to hit the zero mark on your dial indicator. Once confident you can hit your mark and with a few practice runs you could within 0.002, go ahead cut your internal thread.

Again, this is coming from rookie, others may find faults in it.

 

[Asm109]

Mount a dial indicator on the bed of the lathe. Open the half nuts when the dial comes around to the depth you need. I usually set the indicator so it sweeps one rev and I stop at 0.

 

[dulltool17]

I have a little micro switch on a mag base that I place on the ways. When the switch closes, it activates an alarm. Runs on 24 VDC. I also put tape on the boring bar as a visual indicator.

 

[Cadillac STS]

Would it be possible to use a left hand thread cutter, run the lathe backward and cut the back of the cylinder? So you could see it easier looking right at it cutting instead of it hidden by the work?

 

[mikey]

Sure, I don't see why not. Got a LH threading tool?

If you mark your tool or use an O-ring like Ray said, it is not really an issue to cut internal threads.

 

[Brento]

Other ways ive seen is actually turning the tool upside down and then you can cut on the back side with no issues. You can then see what you are doing.

 

[12bolts]

When all is said and done, its not that difficult for a one off job to cut the power and just hand feed up to the end point

Cheers Phil