Lead Screws

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 ?

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**

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

HPC also do lead screw

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

DRO may be a cheaper option.

Martin C

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

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.

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 ?

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

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.

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

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.

Something like this, perhaps?

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

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 ?

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.

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

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

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 ?

Second thoughts would that be a left hand thread ?