Running a threaded spindle lathe in reverse to cut heavy threads in a blind hole.

[pestilence]

I want to make an ER32 collet chuck for my Logan 922, but I'm pretty scared of threading 2 1/4 - 8 TPI into a blind hole when I cut the spindle threads into it.

I'd really like to run the lathe in reverse so I can cut the threads going out instead if in, but I've read horror stories of chucks unscrewing themselves. If I go easy on it and make sure my chuck is tight, do I need to worry about it?

If I DO need to worry about it, could I get around the problem by making a thin jam nut to put behind the chuck? I figure if I screw the jam nut in behind the chuck and then lock it and the chuck together really well, I might get by for one job.

Anyone tried this before?

This will be literally the second time I've tried cutting threads on a lathe. Third if I end up making a jam nut. The first try went very well, but it was an outside thread with good clearance and much slower feed.

 

[Ray C]

Dont do it.

I don't see how the spindle has enouth thread length to accommodate a jam nut.

A 30 lb block of metal coming off and rolling around unconstrained (even slowly) is never a good thing and could get you hurt.

 

[Codered741]

I agree with Ray. Don't do it!

Just cut a good relief in at the bottom of the hole, put a mark on the ways so you aren't trying to watch the tool as you cut. Then just thread as normal.

It is intimidating to not watch the tool as it cuts, but watching the machine, not the tool, is easy when you get used to it.

You could also make a threading stop, which will automatically disengage the half nuts at the same spot every time. This has been done by other forum members, and involves making an attachment for the half nut lever, which will hit the stop, usually mounted in the chip pan.

 

[pestilence]

Well the jam nut would only be about .25 or maybe less, I'm thinking, and I'd only install it for the threading operation. Once that's done I'd be installing the part directly on the spindle to finish it.

Another alternative would be to thread the inner surface of my existing chuck's base plate left handed and making a draw bar to run through the spindle.

 

[Tony Wells]

Some people with threaded spindles drill and tap a hole in the mounting plate and partly into the spindle for a cone point set screw. I'm not crazy about the idea, but it would keep the chuck from unscrewing.

I've threaded countless internal threads blind, and as codered says, it's easy once you get used to it. Maybe a practice part or two with a generous relief would help you get comfortable with it. I use a 1 or 2 inch dial indicator with the "last" zero as my stop. That way I get the rhythm of the stopping point.......and count down to it. 5-4-3-2-1 STOP!!. I can usually hit the same point within a couple of thousandths that way, even at a pretty good clip RPM-wise. Try it, you may find it works for you. And if you do use a relief, you don't have to worry about pulling the threading tool out of the cut at the same time. I do it without when I have to, because you're not always allowed a relief. But since this is your project, you get to make the call.

 

[rgray]

Hard to imagine that you're chuck would come loose. I always thread away from the shoulder. Even if it means running the tool bit upside down.

 

[Ray C]

If you spin in reverse with a threaded chuck/spindle... oh yes, it will come off and only fools luck will keep it on.

If it's spinning fast and if a jaw extends past the outer surface, it will take a wild bounce in a random direction as soon as the jaw hits the ways. Could easily cost you your life! Heads pop like melons when hit with heavy metal objects.

Ray

Hard to imagine that you're chuck would come loose. I always thread away from the shoulder. Even if it means running the tool bit upside down.

 

[Codered741]

Making a 1/4" jam nut is not going to reliably keep the chuck from unscrewing itself. I have seen plenty of nuts jammed together, unscrew themselves under load. It might help, but I wouldn't chance it. Not to mention that it is possible that this will wreck your accuracy, being that the chuck aligns with the shoulder on the spindle, not necessarily the threads themselves.

Personally, I wouldn't risk it, if the chuck were to come off mid thread, say good-bye to the boring bar, part, and maybe toolpost, cross slide, compound, and chuck! And have fun filing the chunks out of the ways! And this is all assuming that you don't get in the way! Not worth the risk in my mind.

If you are set on doing this in reverse, see if there is a way to put a set screw into the chuck adapter plate, with a brass plug to keep from marring the threads. But I still wouldn't do it.

Personally, I feel that if you are not comfortable machining a part with your tools and capabilities, find someone who is. A friend or professional who has the right tools, will get it done right the first time, and have no chance of damaging your part, or YOU!!

There are also backplates that are pre-threaded for most chucks. They require machining, but the threads are already cut. You thread them on and turn the register and face that the chuck mount to, so they run completely true to the lathe. Like this one: http://www.shars.com/products/view/5351/63_2148_Threaded_Back_Plates_


-Cody

 

[Tony Wells]

I think in reality, the unscrewing of a chuck is a little overblown. If it is properly tightened, it would take substantially more torque to unscrew than you are putting on it while threading. Unless of course, you crash into something. But then, if you are running inside-out, that doesn't seem likely.

I agree with red though. You either need to practice until you are comfortable with it, or buy a nut, or get someone to make it for you - the least tasteful option for any machinist.

 

[stevecmo]

I have to agree with the others.....don't do it. If you don't have one, make a DI holder that clamps on the front way of the lathe. This will tell you when to disengage the half nuts. When you are in back gear it is slow enough that you shouldn't have a problem. Like others said, you need to have a good relief groove. Then set your DI at the front of the groove. If you haven't had too much coffee you should be fine. )

Steve