Leading (Screw) Thoughts, My 618

[flylo]

That's a 621 I believe, the last 6" they made, also called a 6" square head . I had a new one still shrink wrapped. If you contact Atlas they'll e-mail the manuals for it, just didn't want you to order a short lead screw.

[Richard P Wilson]

Thank you for the techniques and advice given here, good stuff, don't recall reading about this before. I will probably continue to use the lathe as is. How about cutting off the ends, flip the center, and hold it together with a collar at each end? Modifying the lead screw I like, if the rest of the machine was in better condition that would work.

Thanks, JFLingg

Could do. Cut off the ends, drill up ecah cut face, say 1/2" dia, 1" deep, locset a 1/2" dowel, pin the joints, put back in machine. Thats been done before in these circumstances. Personally, I don't think your leadscrew is that bad, I've worked with worse than that.

[JFLingg]

Thank you for the techniques and advice given here, good stuff, don't recall reading about this before. I will probably continue to use the lathe as is. How about cutting off the ends, flip the center, and hold it together with a collar at each end? Modifying the lead screw I like, if the rest of the machine was in better condition that would work.

Thanks, JFLingg

[Baz]

If you want an accurate long thread just run in reverse and cut away from the chuck using the unworn side of the thread.
Alternative. While you still can use the above to cut a new leadscrew but since you have a short bed it will only be threaded say over 6 inches having turned the rest to clearance core diameter using some ingenuity and a fixed steady. It won't matter that it is short because you only do short threads.
Another possibility is to move it all to the left, cutting off that plain bit and extending the RHS with unthreaded section.

[wdtom44]

[QUOTE=Mcgyver;1171910]This is a lathe's leadscrew not a feedscrew on a mill - how does backlash matter? I suppose it would be something to be aware of if there was a feed handle on the screw (a useful accessory). But when feeding or threading, backlash is irrelevant.

Yes I agree it is irrelevant in the lead screw. Since the it came up I was thinking generally. The lathe does have a cross feed and compound too.
L

[Mcgyver]

As for backlash, unless you have NONE, and who has that on home shop manual machines, you have to work around it. I have used machine tools with a LOT of backlash and been able to do work to .001 or very close to that. You just have to work around the backlash.

This is a lathe's leadscrew not a feedscrew on a mill - how does backlash matter? I suppose it would be something to be aware of if there was a feed handle on the screw (a useful accessory). But when feeding or threading, backlash is irrelevant.

Looking at the above photo's it seems an incredible amount of wear for late model home shop (low duty cycle) machine. I don't get it. As for does it matter? Only if you are cutting long threads where consistent pitch is a requirement - i.e. a feedscrew. Not an everyday occurrence, some will never do a job like that. If it bothers you, you can get acme thread stock and machine a replacement without too much trouble, but I don't think its necessary or would derive any benefit for most work

[wdtom44]

I think that unless you are going to need long threads that are accurate you will be OK. Even in a long thread a nut will probably run OK as it only engages a short section of thread at any one time. As for backlash, unless you have NONE, and who has that on home shop manual machines, you have to work around it. I have used machine tools with a LOT of backlash and been able to do work to .001 or very close to that. You just have to work around the backlash. Know the number on the dial you need, go past it, then back to the desired setting, called working across backlash I think. As to hand feeding, you will get a more consistent finish with power feed and a lot less concentration and effort on your part, especially on a longer cut.

[MattiJ]

All true, but most threads are short and cut near the chuck, thats why the wear is located there. If the OP is cutting short threads near the chuck, then all will be fine. If he suddenly decides to cut a long feedscrew, and wants accurate pitches the whole length, then problems may arise. I've seen (and used successfully) much worse leadscrews than that. It sometimes helps, when the leadscrew is worn and screwcutting is in progress, to keep a little pressure on the carriage handwheel, holding the carriage back, so the half nuts are always in contact with one flank, not floating about between flanks.

Wear near the chuck can be also problematic if you don't pay attention to it. On the chuck end the lead screw goes from badly worn to virtually new within 1" with rather abrupt change whereas the wear on the middle part towards the chuck develops gradually. Tool post or top slide set in different way than previously could get you in that ugly 1" transition area that has never been used before.

On bigger lathes you can usually swap the lead screw around with little machining. Tail stock end is usually virtually unused.

[Richard P Wilson]

The only place where there might be a problem is right where the wear area ends.... If your threading crosses that area, there can be a "knuckle" in the thread pitch. I do not know that I have ever seen that be a problem, but the chance at least exists.

There is a shift between the thread flanks in the unworn area and the worn area... so there will be a shift when the halfnuts pass over that area. How much depends on how much wear there is. May be nothing to worry about.

All true, but most threads are short and cut near the chuck, thats why the wear is located there. If the OP is cutting short threads near the chuck, then all will be fine. If he suddenly decides to cut a long feedscrew, and wants accurate pitches the whole length, then problems may arise. I've seen (and used successfully) much worse leadscrews than that. It sometimes helps, when the leadscrew is worn and screwcutting is in progress, to keep a little pressure on the carriage handwheel, holding the carriage back, so the half nuts are always in contact with one flank, not floating about between flanks.

[J Tiers]

The only place where there might be a problem is right where the wear area ends.... If your threading crosses that area, there can be a "knuckle" in the thread pitch. I do not know that I have ever seen that be a problem, but the chance at least exists.

There is a shift between the thread flanks in the unworn area and the worn area... so there will be a shift when the halfnuts pass over that area. How much depends on how much wear there is. May be nothing to worry about.

[danlb]

Like Richard said; worn but usable. As long as 1) the spacing of the teeth are reasonably uniform and 2) the flank to flank distance matches the original pitch and 3) the teeth are not worn to the point where they have sharp crests, you will be able to cut usable threads with it.

You will have more backlash, but that is a common problem with any acme leadscrew.

Dan

[Richard P Wilson]

Worn, yes, but useable in my book. i'd be more concerned about bellmouthing of the chuck jaws than I would about the wear on the leadscrew.

[JFLingg]

Unused end
Used area

Wornout? Usable? I cut 40 TPI with it, 14 TPI coarsest so far. I've not cut threads more than and inch or so in length. Occasionally use .0024" feed, finest that can be set. Usually use the carriage hand wheel for feed. (Could set compound parallel to axis and use that for feed.)

Bought the lathe used, took it all apart and cleaned it, not too dirty. Found the 4 jaw was bell-mouthed, I use it in that condition, just need to be judicious how things are set in it. The finish is good if I use properly ground HSS cutters. It does not like any stick-out, but I'll do some anyway. Or clamp one end in the chuck, other end, live center in the tailstock. Not possible to use stock smaller than 1/4 inch diameter.

Oddity, the lathe came with two steady rests, no face plate, and the 4 jaw chuck. I bought it from a third party, no previous owner info to be had. I really did not check it out, I thought it didn't look abused. I've learned a lot with it, especially how to deal with feed screw backlash on the cross feed and compound. I can hit a thousandth using techniques learned from HSM magazine and this site.

I've made Elmer's first wobbler engine, bushings, dust plugs for my tractor hydraulic couplings, tools and accessories, swarf, and other things. It is so much better than no lathe.

JFLingg Again, thanks for this incredibly useful site.