# Measuring a (maybe worn) ACME



## graham-xrf (Dec 30, 2019)

A very mundane question, I know, but how does one discover where the wear is in an ACME thread rod and nut?

No question, the pair are a bit in trouble! The rod is the 1/2" x 10TPI  left-hand ACME used on a tailstock, and the "nut" is the internal thread cut into the quill. One flops about within the other in really sloppy manner. Functionally, not such a big deal, because a quill gets clamped up anyway, but other damage has forced a re-make. The plan is to now use a phosphor-bronze insert for the quill. The question is about the external thread on the rod..

The minor diameter of the ACME external thread should still be at it's original size (one hopes), because I think it is not supposed to ever rub on the minor diameter of the nut. Similar for the major diameters. I am thinking any wear has to be on the 29° side faces.

Just looking at two ACME rods, one in better condition, I can see one has thread "tops" wider than the other. I get the impression the "valleys" in the worn external thread are wider than the tops. Threading the rod into it's quill, and measuring the axial "play" is a bit awkward to do, but I get it at about 0.021", using a digital caliper.

I just don't know how to tell how much of that belongs to the rod, and how much to the nut. I was hoping to only replace the nut.
Is there a wisdom about estimating ACME thread wear?


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## macardoso (Dec 30, 2019)

I would expect wear in the screw to manifest itself as backlash that is greater than the ends of travel where the machine is rarely used. Quick and easy way is to jiggle the handwheel to see how much backlash is present at maybe every inch of travel.

Ideally the nut is softer than the screw and will wear first. This would be detected as increased backlash across all ranges of motion. This effect can be combined with screw wear.

Some nuts are adjustable for backlash. If so, tighten them up. Some are not and can be replaced rather inexpensively.

Worn screws can be replaced with precision screw stock. Not crazy expensive but certainly more than the nut.


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## macardoso (Dec 30, 2019)

For the size of a quill nut and screw, I would just replace them both with precision stock from the same MFG. Cost shouldn't be unreasonable and will give you a very long life of that component.


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## macardoso (Dec 30, 2019)

Thomson Linear has a matching pair of precision ground ACME screw and bronze threaded nut for under $75 on a 0.75" screw. Don't know what size you need.


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## machPete99 (Dec 30, 2019)

If you have thread wires you can check the pitch diameter of the screw at a number of places and compare.
I am not sure how that translates to backlash but there  is probably a relationship if you do enough trig...


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## graham-xrf (Dec 30, 2019)

macardoso said:


> For the size of a quill nut and screw, I would just replace them both with precision stock from the same MFG. Cost shouldn't be unreasonable and will give you a very long life of that component.


 I would go for that I think, though it would need a little fiddle to join a ready-made threaded rod onto the salvaged back end which has the thrust disk and taper pin hole for the handle. Maybe drill it, press on a somewhat interference fit, or use some Loctite.

The size is 1/2" x 10 ACME . It is for a South Bend 9A - so not large.


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## macardoso (Dec 30, 2019)

Looked up a pic...




I'd cut the old screw off, tap the original part maybe 1/4-20. Then cut your new screw to the length needed and drill and tap one end 1/4-20. Join them with a stud and red loctite. Super easy and a good practice part for your lathe. Be sure to dial in the runout using a 4 jaw or your new screw might wobble about the shaft.


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## benmychree (Dec 31, 2019)

Why not just MAKE the new parts?


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## pontiac428 (Dec 31, 2019)

One could machine an undersized internal thread into a delrin bushing, then heat the part to partially melt-form a precision fit nut out of the delrin to your existing threaded rod.  Then machine the delrin to replace your bronze bushing, and voila!  Backlash free.


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## graham-xrf (Dec 31, 2019)

benmychree said:


> Why not just MAKE the new parts?


I am getting there, though I admit I am to some extent hedging my bets. Little things like getting in enough material to allow for "collateral damage". I have invested in one of these -->


So my little run of only using one Sandvik triangle insert, and a couple of ready-made carbide tools for everything has to come to an end. I have to get down to fixing up the side-to-side vibration wobble on my bench grinder that comes from wheel tilt relative to the axle. After that, true it up, and try again with the tool grinding. Most things I attempt usually have a little upstream sequence of jobs like this to be done first!

The "easy" option of getting a piece of LH 1/2" x 10 from a UK outlet is not as straightforward as you might think. There is some, but expensive! A better option would be to go for T12 or T14 x 3mm metric, but even that in LH is minority stock. From eBay, the 1/2" is  $11.72 + $27.34 for shipping. That is for a chunk 36" long, which can be thought of as either "a lifetime supply", or "just about enough to cover the said collateral damage"!

--> LINK

Another method is to use a  HSS ACME  die cutter. The  1/2 x 10 (left-hand) is £ 18.50 ($24.53). Maybe not such a bad option as a backstop. Anyway, I am already scanning around for a suitable piece of steel. It does not have to take load except a little when drilling, so any carbon steel rod that machines reasonably well should be OK.

So first we have a go at making it!


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## Chuck K (Dec 31, 2019)

Cutting acme threads is no more difficult than cutting 60 degree threads. Once you get past the chatter you'll make it work. If it were me, I would cut the screw to fit the worn nut.


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## whitmore (Jan 1, 2020)

graham-xrf said:


> A very mundane question, I know, but how does one discover where the wear is in an ACME thread rod and nut?
> 
> No question, the pair are a bit in trouble! The rod is the 1/2" x 10TPI  left-hand ACME used on a tailstock.


There's quick-and-dirty solutions, like this one 
<https://www.practicalmachinist.com/vb/south-bend-lathes/quickie-repair-worn-crossfeed-nuts-273712/>
but maybe there's a second cause: the thrust bearing in the tailstock could be the loose element,
it'd have the same play effect as if the threads were worn.


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## graham-xrf (Jan 1, 2020)

Chuck K said:


> Cutting acme threads is no more difficult than cutting 60 degree threads. Once you get past the chatter you'll make it work. If it were me, I would cut the screw to fit the worn nut.


Thanks Chuck. Now that is an option I had not considered - and why not?  This time, given that the whole quill was wrecked by earlier misadventure before I got the lathe, and has to be re-made anyway, a new nut is to be part of that. I am using one reasonably condition South Bend 9C lathe to fix up parts on the less fortunate SB-9A.

I got some PB104 phosphor bronze, and I am happy to attempt making a new nut insert for it.
So can your suggestion be varied to turn it around, and cut the nut insert with slightly thicker threads to sweetly fit the screw instead?


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## graham-xrf (Jan 1, 2020)

whitmore said:


> There's quick-and-dirty solutions, like this one
> <https://www.practicalmachinist.com/vb/south-bend-lathes/quickie-repair-worn-crossfeed-nuts-273712/>
> but maybe there's a second cause: the thrust bearing in the tailstock could be the loose element,
> it'd have the same play effect as if the threads were worn.


Wow! This one is quicker and dirtier than most! Perhaps not-so-dirty, given how "clean" things need to be to make it work. I will keep it in mind. This time, I am going to at least try for the "make it from scratch" solution as a starter.

Making a zero-backlash rework like that one has something in common with the heat-and-form-to-shape trick using a Delrin nylon nut, but using tin on the screw instead.

You are right about the other cause of backlash. Taking up the sloppiness at he thrust ring is as simple as using the right spacer. For this case, I was waggling the screw in the nut with all disassembled. The poor fit is real! Back end thrust disc sloppiness would only add to it!


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## Chuck K (Jan 1, 2020)

So can your suggestion be varied to turn it around, and cut the nut insert with slightly thicker threads to sweetly fit the screw instead?
[/QUOTE]
I would much rather make the screw than the nut. It's difficult to cut small diameter internal acme threads.


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