Lead screws

Discussion on all milling machines vertical & horizontal, including but not limited to Bridgeports, Hardinge, South Bend, Clausing, Van Norman, including imports.

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myles.lawrence
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Joined: Fri May 08, 2020 10:31 am

Lead screws

Post by myles.lawrence »

I have two Benchmaster mills, one horiz and one verticle. I want to replace the lead screws and nuts but don't know what to get or where. I replaced the nut on my South Bend lathe and that dropped the back lash from 32 thou to 9 thou so want to do the same to the mills too. Does anyone know what size (tpi) these are and where I might buy one? Both mills have over 30 thou backlash so I'd like to fix them.
Thanks in advance for your help.
whateg0
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Re: Lead screws

Post by whateg0 »

Do you not have access to the screws to measure them?

Dave
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Harold_V
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Re: Lead screws

Post by Harold_V »

I'm curious why you think they need to be replaced. Backlash with machine tools is a fact of life, and is generally dealt with by understanding. Thirty thou is not excessive, so unless the machine in question has an extremely fine screw (not usually the case), it most likely isn't a problem. Please note that I could be wrong.

My thoughts on the matter:

How much backlash a machine has isn't nearly as important as one may be lead to believe. So long as it is consistent (which indicates that the screw is not worn unevenly), learning how to operate the machine correctly is far more important than the amount of backlash.

Unless the screw/nut assembly is yielding error (uneven wear), you'd be far better served to learn to machine with backlash in mind. That is very much a part of learning how to operate machine tools (not CNC).

H
Wise people talk because they have something to say. Fools talk because they have to say something.
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NP317
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Re: Lead screws

Post by NP317 »

And inspect the nuts.
They might be 2-piece nuts that allow adjustment of the backlash, like the Bridgeport mills I have maintained.
RussN
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tornitore45
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Re: Lead screws

Post by tornitore45 »

Some machinates have a provision to distort the nut and reduce the backslash. I would explore that, if it is an option, before doing anything.
Mauro Gaetano
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pete
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Re: Lead screws

Post by pete »

While technically using the term lead screw is correct in general conversation it's a hindrance when searching for replacements. So instead try running a Google search using Feed Screw Manufacturer's or even Motion Control added to that and you'll find what your looking for with a lot more hits. And I'd certainly agree that .030" of back lash isn't excessive on an older machine and that number really shouldn't a huge problem to work with. Even a brand new Bridgeport will come with roughly .004". Any manual machine is going to have some backlash, so having to compensate for that or doing the same with your .030" is really no different. As others have mentioned I'd investigate what you already have and see if those feed nuts are adjustable or can be modified to be. If your still determined to replace what you have then fwiw the reputable manufacturer's web sites all have detailed drawings for what are usually over size nuts that the end user machines to fit there application. If that for any reason can't be made to work then even ACME taps are available to make your own. https://www.roton.com/products/taps-for ... formation/ Both the correct bronze nut material and those taps are going to be expensive.

ALL feed screws and nuts are going to have varying amounts of lead and lag errors within the length of the screws. And replacing all 4 of those XY feed screws and nuts will again not be a cheap. Plus there's going to be a good amount of fairly precise machining required for you to do before you'll have 100% correct drop in replacements. Yes the better feed screw manufacturer's also offer custom machining, but if you have to ask the price you better be sitting down. And as those manufacturer's web sites show the costs rapidly rise as the accuracy specifications get tighter in allowable deviation per foot or meter. Since I don't know very much about the Benchmaster mills you have, then measuring your feed screws for diameter and reading the machines dial markings for how much one complete turn moves each axis you'll then know your feed screw pitch and there diameter. Pull the table and Y axis on one of your mills and post some pictures here of what you have and then we might come up with some better suggestions.
RMinMN
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Re: Lead screws

Post by RMinMN »

I'm not an experienced machinist so take this with a grain of salt.

CNC machines need nearly zero backlash. To get this they replace the acme leadscrew with rolled ballscrews. These come it different diameters, lengths, and quality. Making one fit your machines may not be possible but it is something to look into. Once caveat: Acme screws tend to hold the tables in position if not being actively held. Ballscrews do not so you would need to lock any table direction not being used.
Richard_W
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Re: Lead screws

Post by Richard_W »

RMinMN wrote: Mon May 11, 2020 5:32 am Ballscrews do not so you would need to lock any table direction not being used.
I have not had this problem. Unless you are doing some heavy hogging. Generally there is enough drag on the table slides for it to not be a problem. Actually I would love to have ball screws on both X and Y axis on Bridgeport type mills. Makes it nice to climb mill a finish cut.

Richard W.
whateg0
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Re: Lead screws

Post by whateg0 »

My buddy's CNC mill uses ball screws on all axes. If the z does not have a brake, gravity is enough to cause it to fall. Even on my old mill, if I didn't lock the table or hold the handwheel, it would move into the cut on heavy cuts. None of my machines have ball screws, but if the table will move with 10tpi acme, why wouldn't it move with a coarser ball screw that had less friction?
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