Need a way to turn a 30 tpi screw.

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Jerry_H
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Re: Need a way to turn a 30 tpi screw.

Post by Jerry_H »

Harold we, on this board, are so very lucky to have you for our teacher.

Jerry
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Harold_V
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Re: Need a way to turn a 30 tpi screw.

Post by Harold_V »

Thanks, but please don't discount the excellent advice offered by others, in particular, Glenn. His work demands the best possible shop practice and he's very well tuned to working properly. Even more importantly, he understands machining. It's a hard trade to master otherwise.

H
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John Hasler
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Re: Need a way to turn a 30 tpi screw.

Post by John Hasler »

Thank you, Harold. Looks like my lathe dog is going to be less of a toolbox pet and more of a working dog.
earlgo
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Re: Need a way to turn a 30 tpi screw.

Post by earlgo »

John & Glenn: 3 reasons why I chose the way I did.
1) The ATLAS chucks are well worn and I don't have the jaws ground straight to the last gnat's eyebrow.
2) When turning between centers, one can take the part out to check and verify and still get it back in register.
3) This is the way my Dad taught me.

If I used one of my worn out chucks, the threads may not be axial with the part, and if Jack finishes the heads, etc. on his lathe, his chucks may not grip the stock the same way and he will end up with maybe not-so-straight parts. If I turn all between centers then the threads, and OD of the stock will all be axial. One of the last ops will be a light skin cut on the central stock diameter.

I have to use the equipment I have and have learned what it will and will not do. Did I ever mention that this ATLAS is well worn?
---earlgo
Before you do anything, you must do something else first. - Washington's principle.
JackF
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Re: Need a way to turn a 30 tpi screw.

Post by JackF »

Though this part is being turned for me and not by me I am learning lots from this "thread". :wink: :)



Jack.
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GlennW
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Re: Need a way to turn a 30 tpi screw.

Post by GlennW »

Thank you, earlgo.

Excellent reasons, and added good info to the thread.
Glenn

Operating machines is perfectly safe......until you forget how dangerous it really is!
pete
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Re: Need a way to turn a 30 tpi screw.

Post by pete »

My thinking is a lot like Earls, except what little I've learned is swayed by a lot of the books and magazines from the UK. For shaft type work and if accuracy is important I'll always chose to do it between centers if possible. In fact I prefer that over chucking it. That method seems a lot more commonly used in the UK or Europe than over here for some reason. A good dead or live tailstock center and one just turned at the headstock end is always going to be more accurate than any chuck I can afford. I can dial parts in ok with a 4 jaw, but trying to do it to the level between centers can do is going to take a very long time. I just don't use a cut off tool when turning a part between centers. :-) And no safety complaints from me Earl, I likely would have done exactly the same given the same equipment. Chain saws don't have guarding over every tooth, and if you want to keep all your body parts you keep anything far enough away to not lose em.
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GlennW
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Re: Need a way to turn a 30 tpi screw.

Post by GlennW »

You just make the part in one setup if using a chuck.

Rough the part, then finish and thread it. Everything will be coaxial on a part such as being discussed here.

Grab the stock, turn the threaded end and the body down to within .020 of finished diameter and then cut your relief groove. (since this part has one) Then go back and turn the features to the finished diameter and thread it and part it off. Flip it around and face the parted end to the proper length.

Chuck runout means nothing that way. The axis of the part will be the axis of the lathe spindle. Nothing will be eccentric or "bent".

How would you ever make parts on Screw machines, CNC Turning Centers, or Swiss machines if turning between centers was the only wayr to produce good parts?

There's way too much "internet Hypochondria" regarding three jaw chucks, runout, and inaccuracy of parts.
Glenn

Operating machines is perfectly safe......until you forget how dangerous it really is!
earlgo
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Re: Need a way to turn a 30 tpi screw.

Post by earlgo »

My buddy bought a Myford Super 7 lathe and it is the most accurate hobby lathe I have seen. He fitted it out with a small collet chuck and the run-out is hard to read with a 5tenths DTI. Unfortunately it does not cut 30 TPI or I might have camped out in his shop to do this as you all have suggested.

I hold with my choice to turn this between centers so JackF can have a concentric axial double part blank.

Pete: Cutoff with the part between centers is certainly not a good idea as you suggested. :shock: Unless you are friends with the local ER nurse.

--earlgo
Before you do anything, you must do something else first. - Washington's principle.
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Harold_V
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Re: Need a way to turn a 30 tpi screw.

Post by Harold_V »

GlennW wrote: Wed Mar 21, 2018 6:27 am There's way too much "internet Hypochondria" regarding three jaw chucks, runout, and inaccuracy of parts.
True. I've encountered numerous individuals who don't understand the relationship between the jaws of a chuck and the centerline of the spindle. For what ever reason, they assume that material gripped in less than a true position will translate to parts that do not run true.
The only time that would be true is if the part being machined was removed and replaced, or there was a second operation to machine the opposite end. That, in fact, is why I promote the use of soft jaws, as they eliminate the problem, assuming they are used properly.

There's also WAY too much attention devoted to backlash, which is a fact on machine tools. Unless it's excessive, it just doesn't matter. If a guy can't work with backlash, he really needs to back up and gain an understanding.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
pete
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Re: Need a way to turn a 30 tpi screw.

Post by pete »

I'd 100% agree about the fixation many on the forums have about obtaining very low runout numbers on there 3 jaw. If the parts never taken out of the chuck until it's fully finished and the O.D. is completely machined it's going to be as concentric as the lathes headstock bearings allow it to be. 3 jaw scroll chucks are a time saver and convienience and parts were machined concentric long before they were ever invented. That said I did spend a hell of a lot for a couple of high accuracy Emco 3 jaws before I knew that. :-( That backlash seems to be another where it's importance is overblown. If it can move it has to have at least some clearance to allow that. Even the worlds best ground and lapped ballscrews will have some measurable amount of backlash. So they add backlash compensation to the programs for a reason.

Earl are you 100% certain that Myford can't cut 30 tpi? I've maybe got a defective memory but I've got a whole lot of U.K. written books and magazines and I'd swear I've read about 30 tpi being cut on a Super 7 at least a few times. Myford made just about every tooth count for there change gears you could think of so I'd almost bet it's possible. I don't have a link handy, but I've run across at least a couple of websites that have programs that allow you to input the pitch of the leadscrew and the tooth count of the gear on the spindle and it will give you the gear combinations required to cut pretty much whatever thread pitch you'd like. Not having the correct gearing combination might be a problem tho.
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liveaboard
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Re: Need a way to turn a 30 tpi screw.

Post by liveaboard »

I generally use the chuck; my work is usually not too demanding anyway.
recently I tried center to center mounting for the first time, since I needed to check and remount a few times, and take small adjustment cuts to a bearing holder; normally adjusted by shims, which turned out to be priced at a very ridiculous rate, I adjusted the bearing preload by machining the holders instead.
I was quite pleased by the result, it was so quick I was left wondering why anyone would bother with shims.
shortening bearing holder.jpg
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