View Full Version : Change the lead for an acme thread?

Bob Farr
12-13-2008, 03:10 PM
The cross feed on my Atlas/Craftsman 12" lathe has an intolerable amount of backlash. I'm making a new lead screw. The stock one is 1/2", 10n acme.

My first thought was to oversize the diameter of the new leadscrew thread a bit to accomodate for wear in the nut. Ok idea? The other option is to make a new 10n nut, then cut the new 10n leadscrew for a nice fit. Better idea?

If I should make the nut and the leadscrew, is there any advantage to choosing a finer lead than 10n? Yes, I know the last option involves making a new handwheel mic dial, but I don't mind that if there's something to be gained from a finer lead.

Disclaimer: I'm new and trying to learn. If I've said something that discloses ignorance, don't mistake it for stupidity. Thanks in advance for any advice.


12-13-2008, 04:16 PM
On that machine there is nothing to be gained by going to a finer lead. You would be better off to make a new larger dial that has room for 1/2 thou marks instead after replacing the lead screw and nut.

12-13-2008, 05:47 PM
If you change the pitch of the lead screw, all of the threading data will be useless!

12-13-2008, 06:12 PM
I have a recollection that Evan made follower nuts from plastic on his mill and have been wondering what kind of success it would be on a cross slide nut. That is an area of annoyance for me as well - I don't have any kind of cross slide lock but could make one easily, but slop is just undesirable on the cross slide. The nut I have in place is a split nut but that just wears the screw faster.

I have room under my cross slide for a longer than standard nut so plastic may be a slop-free option. It's a small lathe so abusing the nut should not be a problem.

Bob Farr
12-13-2008, 11:05 PM
Thanks for the info guys. A 1/2-thou dial would be nice. Assuming the machine (or I) could are capable of holding that tolerance, my time might be better spent working on that instead.

I had not thought of using another material for the nut. I have some small blocks of 660 bronze, and I also have some black Delrin. Using Delrin for a nut would allow a close fit with the screw and might reduce friction compared with the bronze. Any thoughts on using Delrin?

The combo of this machine and my rookie skills will probably not permit me to split a thou on a repeatable basis, but I've enjoyed fixing it up and I appreciate the input. I already have the screw blank made, so chosing the lead/pitch and whether to make the nut are all that is left.




12-13-2008, 11:14 PM
I presume that you have access to another lathe to make the new screw. Otherwise, there is a guy in Kansas who advertises on e-bay. He will sell you a new screw and nut for about $50.00. His number is (913)636 6107. There is no learning process, but it IS immediate. I just stripped the horizontal feed nut on my Atlas shaper and am facing the same problem-1/2-10 LH. I will try to make my own using 12L14 for the screw and leaded brass for the nut because that is what I have. The original lasted 50 years, so I guess the design and materials were "adequate."

12-13-2008, 11:34 PM
I am using Delrin for the Y axis leadscrew on my CNC mill. The X axis has bronze nuts in an anti backlash setup. I just had to readjust the bronze nuts and not the acetal nut. It still only has about .001" backlash, if any. Keep in mind that a CNC machine is capable of cranking those handles much faster and more often than a manual operator. Acetal plastic leadscrew nuts are a stock item now and are widely used on commercial machines. Commercial equipment has been using plastic for gears and other power transmission parts for decades.(1) I wouldn't hesitate to give it a try. The absolute worst that can happen is that you will have to make a nut from metal instead.

(1) The basic cost to produce a given category of machine such as a lathe, bandsaw, automobile, photocopier etc can be closely estimated by simply weighing the machine. The less it weighs the cheaper it is to make regardless of size or what exact materials are used. Plastic is light although not necessarily inferior.

Bob Farr
12-13-2008, 11:44 PM
I have an Astoba multimill that will handle the work but maybe ordering would be better.



12-14-2008, 07:42 AM
I have an Astoba multimill that will handle the work but maybe ordering would be better.


Now that's a machine you don't see every day. Tony, and some of us would like to see some more photos of it.


John Stevenson
12-14-2008, 07:46 AM
If you change the pitch of the lead screw, all of the threading data will be useless!

Cross slide screw, not leadscrew.


12-14-2008, 08:45 AM
It really doesn't matter if the thread and or nut is worn or not if you have a DRO on the cross-slide or if you take the final few cuts using a good dial indicator on the tool-post. In all cases, its not that the amount that you turn the lead-screw that is important but rather the amount that you move the tool in or out as the case may be that is important.

Use the DRO or Dial Indicator method to make your new lead-screw - you may be pleasantly surprised.

I'd be more concerned about the gib settings on the cross-slide and the top-slide than I would about the the lead-screw and its nut. A lot of perceived "back-lash" is in fact "end play" at the "dial end" of the lead-screw and "rocking" of a nut due to loose holding-down screws.

"Back-lash" it not important if you are only moving in one direction, as is the general case in non-CNC-ed lathes or mills. "Taking-up" the back-lash does all that is required in most cases.

Bob Farr
12-14-2008, 11:12 AM
Oldtiffie, thanks for the reminder about the many sources of wear and slop in the system. I'm trying to reduce as many as possible. When I made the blank I changed the diameter of the shaft running through the carriage to get a better fit. I also shortened the threaded end so that the handwheel bore would be seated on a solid shaft instead of the threads. The nut stub is a tight fit in the bore and there is no elongation of the screw hole, so I know the nut isn't moving. So I'm doing what I can to get the best fit and feel from the machine, given what it is:


The current screw shaft is terribly worn in the center section, as expected, and the threads are nearly knife-edge there:


Compared with the ends of the shaft:


The tool-post screw is in similarly poor condition. If I tighten down the gibs I can't move the cross slide smoothly, but if I adjust the gib for smooth movement with no "stiction" then the cross slide will move laterally on its own within the wear limit of the screw/nut and chatter on fine cuts. The reduced friction offered by using Delrin as a nut (why not as the gib, too?) is an interesting option.

My inexperience may be contributing as much to the problem as the screw, but I figure it will be easier to improve my technique if the machine wear is at a minumum. Besides, making a new screw is a good experience builder for me too, even if it doesn't make a big difference in machine performance.

Jim, thatnks for your interest in the Astoba. It is a very interesting machine and I am fortunate to have it. I sent Tony an email offering any pictures he would like. Let me know if you want some and I'll send them to you too.


12-14-2008, 12:00 PM
Thanks Bob.

You really DO have some wear there!! And I can see why you want to improve it.

So, providing that your screw-threading lead-screw and half-nut combination is in reasonable shape, there is no good reason why you can't make a very serviceable and satisfactory cross-slide lead-screw and nut on it.

Best of luck. I am sure that it will work out OK and that there will be a much better lathe and a very happy owner of it in your shop.

12-14-2008, 08:29 PM
On my old artisan I just bought a piece of left hand acme threaded rod from mcmaster. also bought a matching brass nut. The rod is good to .003" per foot. I just tigged it in place of the old screw. Works good.

12-14-2008, 08:52 PM
I have an Astoba multimill that will handle the work but maybe ordering would be better.

Wow, that's a neat little machine! It looks brand-new -- did you restore it?

12-14-2008, 09:12 PM
Cross slide screw, not leadscrew.

I guess thet slipped by me,I saw lead in the thread title!

Bob Farr
12-16-2008, 09:01 AM
Woo hoo that feels good! After an evening spent sweating the details of grinding a proper single point tool, hand-cranking the threading process, spring cuts and test fitting the nut, I now have a much better fitting cross-slide screw and nut. I don't have it assembled yet to quantify the backlash improvement, but it feels perfect and this new/old closeup should give you some idea:


I took some liberties when sizing the blank to gain some more usable threads, and to improve the fit in other places too:


I am very happy with the outcome and I'm ready to tackle one for the toolpost now too. This was a huge confidence booster and should make working with the Atlas much more enjoyable.

Lazlo, the Astoba came to me in very good shape. All it really needed was a thorough cleaning and bearing lube. I repainted the motor casing in wrinkle finish black to match the rest of the machine, but that's it. You can see that is is rather small, but quite capable as both a lathe and a mill. This screw shaft is 12.5" overall length, which will not turn between centers without slipping a bit of the shaft into the headstock:


Thanks to everyone for the advice and encouragement!


12-16-2008, 09:41 AM
Way to go Bob. Looks like you did a great job and I'm sure you'll have a big smile on your face when you crank the handle for the first time and don't feel 3/4 of a turn of backlash. I agree that they are no fun to run when they have that much backlash in them. We have two Monarch 10EE's at work that are in need of a new leadscrew and nut. They have every bit of 3/4 of a turn of backlash in the cross slide if not more. They do get used 7 days a week though. Thanks for sharing.

12-16-2008, 10:17 AM
Nice job. Now you need to make a proper dial for it too. This is something I need to do for my SB9 as well.

I sure like that Astoba. That's the size a Unimat should be.

12-16-2008, 01:37 PM
Nice job. Now you need to make a proper dial for it too. This is something I need to do for my SB9 as well.

I sure like that Astoba. That's the size a Unimat should be.


If your lathe has the older 1" dials you can easily replace them with 1-1/2" that are much nicer to read. You can also just squeeze 200 graduation marks around it for direct reading if that's your style.


12-17-2008, 04:59 PM
I've given the topic some thought from time to time and I came up with an idea for making the nut. I haven't tried it so I can't vouch, but it seems to me that a bronze nut could be cast right around the new shaft. Yes, I am sure that shrinkage could be a problem but a fella could try an experiment with a plain piece of threaded rod or whatever before commiting the real screw. Solves the problem of the L H Acme tap that costs rather dear. I will eventually try it myself. But who knows when.


12-17-2008, 05:21 PM
Bob, great work. Your old lead screw looks like my SB9 crossfeed screw. It's on my to-do list.

It looks like you cut the threads on your Astoba machine.
How does it cut threads? Does it have a change gear setup of some sort?
Very interesting machine!


Errol Groff
12-17-2008, 05:26 PM
Here is how I approached a similar repair on a SB 13" lathe.


Errol Groff

12-17-2008, 06:09 PM
A left hand 10 pitch acme tap is easily made and will cut a new nut just fine. The old lead screw is still good at the ends and will serve as stock or a piece can be turned in the lathe to make a tap.

An acme tap needs a very long lead in because it is removing a lot of material at once. Here is one I made, all hand ground, from a piece of lead screw stock. The tap was hardened by heating to red and quenching in water since the material is 1040 steel.


A close up of the nut shows that it does a reasonable job of tapping bronze. Because the tap is the same size as the leadscrew clearance may be obtained by freezing the nut stock before tapping and tapping it while cold.


The tap itself should be ground with three flutes to make it centralizing. The way to grind it is this:


Spin Doctor
12-17-2008, 06:49 PM
I realize this is not germaine to the OT but when I was doing repair work on K&T 4 axis machining centers (K&T MM800s) the 4th axis had a variable lead worm on the table drive that you could adjust axially to keep the 4th axis backlash in specs. If you looked at the lead of the worm the tooth thickness on the "fat end was close 50% greater than the thickness on the "thin" end. This idea might be applicable to a home brewed dividing head or gear hobber where you want to reduce backlash to an absolute minimum.

Bob Farr
12-17-2008, 09:39 PM
*** It looks like you cut the threads on your Astoba machine. How does it cut threads? *** Steve

Steve, the Astoba uses change gears to cut threads and is hand-cranked when doing this. It can cut left-hand and right-hand, metric or standard threads.

Because the headstock changes to a lathe or mill position it does not have the traditional lathe leadscrew for thread cutting. Instead, a plate holding the change gears also uses a sort of "driveshaft" that turns the toolpost screw in sync with the spindle and workpiece:



There is also a device that drives the toolpost via the motor for turning while in the lathe configuration, but threading is done with the hand crank, very slowly! It really is a very versitile machine. It is exactly like the Hommel machine shown on Tony's site, and is probably an earlier, later or simply re-branded version of the same model. I don't know when it was made.

Evan, thanks for posting the info on how to make a left-hand acme tap. I kept my job simple for the cross slide screw by using the old nut and cutting the screw to a close fit to accomodate for the wear. But I may make a new tool post screw, which is considerably shorter than the cross slide screw. That would allow me to make it a few inches longer, then cut the extra off to use as the tap blank for a new nut. The thread form would then be from the same source, which should make for a nice fit using your "chilled-nut" technique! Did you grind the flutes before or after hardening?

Thanks to everyone who suggested making a larger dial for the Atlas/Craftsman. I may do that after I get a chance to work with the machine a bit more and improve my skills, but assuming that I can split thou's right now (even with a bigger dial having better resolution) is probably expecting too much, too soon.



12-17-2008, 09:59 PM
I grind before hardening and then touch it up after hardening to refine the edges. I use 1040 a lot for tooling that doesn't have to cut steel because it hardens so easily. A simple propane torch is sufficient to heat a small tool to red heat and then it may be quenched directly in water as may all carbon steel (as opposed to alloy steels). After hardening it may be drawn back to straw or a touch of blue by waving it through the torch again.

A note about using a propane torch to heat metal: The hottest part of the flame is the center light blue cone but that isn't where the majority of the heat is. A propane flame has the majority of the heat in the outer flame sheath. When heating something hold it at the very end of the flame instead of close in to the center cone. The difference is substantial. It isn't necessary to take the entire tool to red heat. The teeth will heat first and that is all you need. By hardening just the teeth the tool is much tougher and less likely to break.