Broken threads on the QCTP of my compound slide

The basic premise here is (was) a fairly common problem on the earlier versions of most of the smaller lathes. It is in no wise limited to the Jet It sounds like you crossed up metric and imperial threads trying to mount a newer type tool post. If I understand the damage correctly, the simplest solution is to replace the center post with the next larger size and drill/thread the top or compound slide to match. If the compound is disabled, perhaps the top slide is still usable by adding spacers.

I have a Grizzly G1550, an almost exact duplicate of the Jet. When I mounted the AXA tool holder, I had to make a special post for the tool holder to fit properly. That has improved over the years, the proper post being available now.

BTW, threading on such machines involves changing a couple of gears sometimes. Imperial threads are available when the 127 tooth gear is in place. Threading is otherwise a piece of cake, just make very light passes.

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better-lathe-than-never said:
Yes - I have about 12'' steel rod I can test the die on as well as a steel plate that I can use to test the tap on. Realistically, I'm not going to learn single threading and then do that accurately on a new stud - it's too much too soon.
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A chamfer at the start help and the rod should be a little undersized.

There are standard fit class's for threads that give you an idea of the tolerances, someone might have a chart :)

Stu
 
Well, my first attempt tonight went like this:

First, I took a flat 2''x2''x1/2'' steel plate which was already threaded to 9/16'' 18tpi just like my base; I re-drilled it and re-tapped it to the new dimension: 5/8'' 18tpi - that worked well, the new threads looked solid. This was a practice run for re-drilling and re-tapping the compound slide's base to a larger diameter and it worked well using my drill press to keep the work and the tap perpendicular.

I then tried to make a new post out of some mystery steel I had on hand. I turned one end down to .616x'', gave it a generous chamfer, mounted the drill chuck in the tail stock and pressed against the die handle all while spinning the lathe chuck by hand and maintaining pressure with the ram against the face of the die. I had trouble keeping the die handle going straight. Even with the ram pushing against the face it wandered around and produced somewhat 'wavy' threads which I wasn't happy with. The new post fit in the threaded hole OK, but it wobbled off center axis as I threaded it down in place - all in all the quality of the new post was not good. This try failed. I have to decide where to take it from here: use another type of material, and either buy a better die (chipped one of the teeth on my die) or wait until I'm able to learn single threading and do it all on the lathe.

The one good thing is that my old post is still holding up well through all this.
 
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better-lathe-than-never said:
Well, my first attempt tonight went like this:

First, I took a flat 2''x2''x1/2'' steel plate which was already threaded to 9/16'' 18tpi just like my base; I re-drilled it and re-tapped it to the new dimension: 5/8'' 18tpi - that worked well, the new threads looked solid. This was a practice run for re-drilling and re-tapping the compound slide's base to a larger diameter and it worked well using my drill press to keep the work and the tap perpendicular.

I then tried to make a new post out of some mystery steel I had on hand. I turned one end down to .616x'', gave it a generous chamfer, mounted the drill chuck in the tail stock and pressed against the die handle all while spinning the lathe chuck and maintaining pressure with the ram against the face of the die. I had trouble keeping the die handle going straight. Even with the ram pushing against the face it wandered around and produced somewhat 'wavy' threads which I wasn't happy with. The new post fit in the threaded hole OK, but it wobbled off center axis as it threaded it down in place - all in all the quality of the new post was not good. This try failed. I have to decide where to take it from here: use another type of material, and either buy a better die (chipped one of the teeth on my die) or wait until I'm able to learn single threading and do it all on the lathe.

The one good thing is that my old post is still holding up well through all this.
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not unusual. 3/8 and above start getting hard to thread with a die. That's why I single point larger threads. I will chase it with a die if necessary.
The die will usually refine a larger thread quickly once single threaded to 75%-80% of thread depth.
 
Looks like that lathe has a gearbox. I bet you could single point on it pretty quick. If you're new, skip all the stuff with the compound, set the tool at 90 degrees to the work, and take light cuts using the cross slide. If you're worried about timing, do it like a metric/imperial crossover. Take a pass, stop the lathe, back the tool away from the work, run in reverse to get back to the start and do it again. Do not disengage the half-nuts. If you can do that to get even half depth, the die will be a lot easier to run down the thread.

If you're really stuck, post some dimensions and I'll make you one.
 
ttabbal said:
If you're really stuck, post some dimensions and I'll make you one.
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It will take me a long time to start threading on this machine.... yes, the machine is capable of doing it - but I don't know anything about the threading chart on the face of the gearbox, or what gear arrangement to use or how to control the speed with the belts. Plus the half nut on the timing dial is jammed (won't go down). I do have some thread cutting bits that dad left me but that's about all the assets in this category. Appreciate the info and support, but don't wait around - it may be a long while.
 
better-lathe-than-never said:
Plus the half nut on the timing dial is jammed (won't go down).
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Time to take it apart and see why. Operation of half nuts is pretty simple, you will be able to fix most anything if you have a mill & a lathe. Might need to buy a part or two. Being able to thread on a lathe opens up a lot more of it's capabilities. Just another challenge that will broaden your abilities.
 
better-lathe-than-never said:
It will take me a long time to start threading on this machine.... yes, the machine is capable of doing it - but I don't know anything about the threading chart on the face of the gearbox, or what gear arrangement to use or how to control the speed with the belts. Plus the half nut on the timing dial is jammed (won't go down). I do have some thread cutting bits that dad left me but that's about all the assets in this category. Appreciate the info and support, but don't wait around - it may be a long while.
Click to expand...
the half nuts might not engage because of a lockout ... when you have the fine feed mechanism for turning set, it locks out the half nuts usually.
 
woodchucker said:
the half nuts might not engage because of a lockout ... when you have the fine feed mechanism for turning set, it locks out the half nuts usually.
Click to expand...
Don't over analyze it - it stopped working when I accidentally rammed the tool into the spinning chuck... I'm firmly in the destructive stage of the learning curve... ;) Need to take it apart and examine the damage...
 
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