Lead Screws

[Paul ScholeyParticipant @paulscholey17560]

[Paul ScholeyParticipant @paulscholey17560]

I have an Adcock and Shipley 1ES with a Bridgeport head, I am trying to reduce backlash as much as possible, it has been reground but I still have wear in the middle of the lead screws, my question is does any firm do such a thing as recondition lead screws in this country ?

[John HaineParticipant @johnhaine32865]

If it's in the middle then the screws are probably worn rather than the nuts. Hard to put metal back on! Best option is to replace them, with new nuts as well. There are a number of suppliers for ACME threaded screws – try these:

**LINK**

[Dave HalfordParticipant @davehalford22513]

You will have to joint the new thread to the old 'front end'

HPC also do lead screw

[Paul ScholeyParticipant @paulscholey17560]

Thanks for the replies , I was wondering about the lead screws advertised , if I could make them fit, like you say it's the ends. I did see a firm in America that grinds them, I presume they take a small amount of the ends where they are not worn to make it all the same ? I will have a better look at the ends of the screws to see what's involved. Thanks for the links

[Martin ConnellyParticipant @martinconnelly55370]

DRO may be a cheaper option.

Martin C

[Paul ScholeyParticipant @paulscholey17560]

I am going to fit DRO but I would still like to stop as much backlash as possible

[BazyleParticipant @bazyle]

What you need is a lathe that is long enough. Is the leadscrew hardened enough to need grinding or just turning? Don't be intimidated, it is just a screwcutting job.

You might try characterising the screw for error, in both directions though if it has power feed most of the wear will be on one flank of the thread.

Have a look at how much room wrt diameter there is at each end of the screwed section. If you are adapting a bough screw you would want to turn down each end and fit into a socket on new end sections.

[Paul ScholeyParticipant @paulscholey17560]

I did think about putting it in my lathe but I think it will be to long , I will check , but wouldn't that just duplicate the wear on my worn lathe leadscrew ?

[Paul ScholeyParticipant @paulscholey17560]

Sorry it's not the leadscrew on the lathe is it it's the thread cutting thread

[BazyleParticipant @bazyle]

If a leadscrew is worn it just means that some of the flank is thinner (in the middle section), but it is still (probably in this case) 4tpi. The middle is probably evenly worn but as you transition from the unworn end to the middle there will be an error and as the thickness of the flanks changes you cant snug up the nut to take out the backlash. You just have to thin the flanks of the thread at the end down to the same as the middle so the nut can be adjusted the same in the middle as the ends,

Since you are matching the ends to the middle which is your reference you only have to do half at a time so take off the tailstock and hold the outer end in a fixed steady. Don't worry about it not being precise. If you have 20 thou (half a mm) backlash you have to be a pretty rotten machinist not to get that down to better than 5 thou. If you are looking for <1thou for a CNC conversion you should be looking at a ball screw conversion anyway.

Have I made that clear enough. I can do a step by step if necessary.

[Paul ScholeyParticipant @paulscholey17560]

Thank you for that Bazyle, you have made it perfectly clear,

[Clive FosterParticipant @clivefoster55965]

Further to what Bazyle says its important that you do all the thinning on the same side flanks of the threads. Not too tricky if you can do the whole thing in one set up but if working in sections you do need to keep your brain engaged.

A classic "No way will I cock up something that simple!" trap.

Worst thing when re-furbishing a worn screw is that the master, thinnest, thread which all the rest have to be matched to is in the middle. If doing in sections you need a relatively long, nicely fitting bush, to support the hanging end. An ordinary fixed steady isn't enough. I'd be thinking in terms of a solid bush a couple of inches or so long made a close running fit on the screw and held in a solid support bolted to the lathe bed. Although a special support sounds more work than simply setting up in a steady it isn't silly hard to make dead right if you have a between centres boring bar. Done well it gives you the option of working on a short length at a time to minimise deflection.

On a job like this where you want right, really right, a bit more trouble at set-up time is worthwhile effort. Hafta say I've often spent time setting up with general purpose kit to "good enough to do the job if I'm careful" standard when saying "stuff it, I'll make something proper" right at the outset wouldn't have taken much longer and allowed me to "just do it" without crossed fingers and toes.

I've always felt that it should be possible to make a thingy with an internal thread and adjustable cutter that could simply be spun along the thread trimming the unworn parts back to match the worn ones as it goes. Never got my head round how to ensure that all the trimming goes equally on the same side when running both ways from the middle though.

Clive.

[egaParticipant @ega]

Posted by Clive Foster on 20/02/2019 11:04:23:
… An ordinary fixed steady isn't enough. I'd be thinking in terms of a solid bush a couple of inches or so long made a close running fit on the screw and held in a solid support bolted to the lathe bed …

Something like this, perhaps?

[Paul ScholeyParticipant @paulscholey17560]

Thanks for all that information, I certainly will be taking my time, I have no intention of rushing into anything.

[Paul ScholeyParticipant @paulscholey17560]

The long one of the two lead screws overhangs the end of the lathe bed by about 6" , Could that end be held by a steady or make something up ? but how would I get that end on centre ? The other thing I imagined I would do was wind the cutting tool in roughly in the centre of the lead screw just clear of touching then let it travel to the other end taking a skim of the non worn part, if I make up a steady near the chuck end for extra support it would be in the way of the traveling cutting tool, How would I know how much to take off ? if you can understand what I am getting at ?

[Nick Clarke 3Participant @nickclarke3]

Posted by Paul Scholey on 20/02/2019 16:00:09:

The long one of the two lead screws overhangs the end of the lathe bed by about 6" , Could that end be held by a steady or make something up ? but how would I get that end on centre ?

Could you make a bushing to fit in the tailstock, possibly needing to remove the tailstock spindle first? It depends upon your lathe of course. Once the bush was fitted you could bore it to size with a boring tool held in the lathe chuck.

[Paul ScholeyParticipant @paulscholey17560]

That's an idea ! thank you I will look into that, I have a Harrison 140

[Clive FosterParticipant @clivefoster55965]

Paul

If you make up something like the excellent device shown by ega and arrange it to be dead on centre at the headstock end it will still be on centre wherever it is placed on the bed. (Assuming the lathe is in good order and properly aligned of course.) So you can work on whatever length be convenient.

There are a multitude of ways to make such a device. I'd probably arrange for the support bush carrier to have a modicum of adjustment accepting the slight reduction in rigidity as compared to a monolithic approach. The important thing is that the screw be well supported on centre-line and exactly parallel to the bed. Standard steady fingers are generally too narrow for this sort of job and the faces are rarely exactly parallel to the bed. Close but not dead nuts.

If you do make such a steady device put some thought into designing for versatility so it can be used for other jobs where the standard steady isn't ideal.

Clive

Edited By Clive Foster on 20/02/2019 17:18:23

[Paul ScholeyParticipant @paulscholey17560]

Having thought about what you've said about it going through the tailstock or making a support up, could it go through the headstock spindle and do it that way somehow ? maybe cutting from left to right ?

[Paul ScholeyParticipant @paulscholey17560]

Second thoughts would that be a left hand thread ?

[Chris TriceParticipant @christrice43267]

Posted by ega on 20/02/2019 11:53:01:

Something like this, perhaps?

[BazyleParticipant @bazyle]

Further to what Bazyle says its important that you do all the thinning on the same side flanks of the threads. Not too tricky if you can do the whole thing in one set up but if working in sections you do need to keep your brain engaged.

Nope. It is very important that the centre line of the thread stays the same otherwise the pitch will no longer be true. The wear on a lathe leadscrew tends to be 90% on the side dragging the saddle towards the headstock but on a mill it will be both sides.

You don't even need a special cutting tool because you can use a parting tool thin enough to fit the thread. Set the topslide almost parallel to the cross-slide over by the Acme angle (14.5 degrees). At the worn middle having taken up the slack in the gear train move the tool in its holder to be alongside the thread flank. Wind it out, move to the end of the leadscrew and move it in. You will see it is overlapping the thread by several thou, about half the wear. You can now advance in a few thou and take a cut, step by step inwards. Don't start the cut in the worn bit with no actual cut being taken as it will just rub along the side pushing the tool sideways. You might end up with little witness marks at each cut owing to tool flexing which is the drawback of not grinding a pukka tool.
Then swing the topslide over and do the other side.

[Clive FosterParticipant @clivefoster55965]

Paul

Whatever you do you will need to support the screw where it passes over the bed. You need a clean accurate straight cut on the thread so its important to ensure that there is no way the job can climb over the cutter or waggle out of the way.

Were I to do the job I'd make up a support along the lines of the device shown by ega to hold the far, tailstock, end and make up an extra deep L block for my travelling steady to ensure that the cut is properly controlled. My Smart & Brown 1024 uses an L shape bronze block rather than the usual fingers for its travelling steady so its easy to make an extra deep one that won't run into trouble as the coarse leadscrew thread runs over (inevitably narrow) fingers.

I'd use a collet rather than a chuck to hold it and fit a bush in the back end of the collet tube to ensure no wiggles. I modified my collet tube to take interchangeable bushes of that ilk to do the job normally entrusted to a spider. Worked out well for me in damping out wiggles when working on relatively slender rods. Means I have to make a bush for each size rather than simply adjust a spider. But I reckon a simple running fit on a inch or so of nylon bush gives pretty equivalent support to a nipped up spider and I don't have to loosen the spider to push the bar in further for the next piece.

Clive.

[Pete RimmerParticipant @peterimmer30576]

Before attempting to effect any re-cutting I suggest that you map the condition of the screw in both directions to record the wear on each side. Set up a tenth-reading dial indicator and use it to record the actual travel at each turn or half-turn of the screw. Do this in both directions because if the flanks are worn in both directions and you only machine one flank you'll still have a loose spot where the other flank is.

A few things to note:

If your lathe leadscrew is worn you will create a screw that has the same wear characteristics as the lathe screw, so you need to use a lathe with a very good screw to make an accurate screw for your mill.

If you map the wear on the screw then machine both sides of the thread you'll have a screw with an asymmetric threadform and you'll need to produce a matching nut or some kind of backlash compensating system for the nut you have.

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