Acme Thread Issues

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gwarner

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I got a Nichols mill a short while back and one of the issues I had with it was when they were in production they used hydraulic cylinders to move the bed. The one I got, a PO had converted the bed to hand screw. While the set up he did was tight it did not give the full length of travel to the bed and was rigged to what I would consider a weak spot.
As an experiment I bought some all thread and some standard nuts and routed it the way the factory would have. It worked great. It was smooth and reasonably tight (about .004 lash) and gave me full travel on the bed. I spent less than $8 for every thing.
Happy with the results I ordered some acme rod and some nuts. Once they came in I set it up the same way I did the all thread and frankly the Acme is crap. It is stiff, grabby and has over .019 backlash. I have tweaked it every way I can think of with out improving it. I up milled a piece last night and actually got some minor chatter on the bed for the first time ever. Tonight I switched back to the all thread and am happy again.
What are your thoughts on why it is so much worse with the Acme. I think these results are worse than you would see from worn out parts. I did buy it from a good source and while I did not spring for the precision stuff I spent a little over $50 for the rod and nuts. I would think that it should be at the very least as good as the off the shelf all thread. It makes no sense to me.
 
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This is a timely thread. I have the same set up on my Williams Mill with the Kearney and Trecker Midget mill head. I used fine all thread rod and it takes a lot of turning to move the table each way. I had been thinking about an Acme rod, but was afraid it would be too hard to turn
Thanks
Martin
 
I have made all new acme screws and nuts for my mill and they work great. You HAVE to buy PRECISION acme screws and nuts. ( I make my own nuts) I get my screws from McMaster Carr and they are reasonable. The nuts , however , are horribly expensive, so that is why I make my own.
Now , the first thing most say is " I don't know if I can cut acme thread". It is no different than single pointing any other thread. Just a different tool and angle. I replaced two axis on my mill, the screws and nuts for less than $50 and now have only a few thousandths backlash.


EDIT: I try for a slightly snug class 3 fit when I make the nuts and they are kind of stiff at first, but after the axis is run back and forth a dozen or so times , the tiny high spots wear off and you end up with a few thousandths backlash( YOU WILL NEVER HAVE ZERO LASH). Oil it up really well and you are good to go.
 
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Low cost acme thread threaded rod is meant for hanging pipes and stuff. They won't work for anything close to precision. As a reference, my cheap asian mini mill had .005" of backlash in the X and Y. The stuff you ended up with is really bottom drawer. It will cost more bucks, but its money well spent in the long run.
 
Thanks for the responses guys. I want to clarify that I am not in the pursuit of zero backlash. I just want a tight enough machine that chatter is not an issue. Any time I need precision I set up digital calipers as a cheap DRO.
The lesson I take from this is that Acme thread, while stronger, is not any more precise or smooth than low grade all thread. So buy the precision stuff.
My whole reason for going with the Acme was the 10 TPI was easier to gauge travel when turning the handle and fit a dial I had. Additionally like Martin, I had hoped to save on some turns of the handle.
I am going to leave it as is for now until,it starts to wear then I will get the precision stuff. Since I am just a hobbyist that could take years.
 
Your 2G or 2C class fit for Acme threads is too sloppy for precision screws for lathes and mills. You need to go with class 3C threads if you can afford it. As Mark said buy the Acme threaded rod and make your own nuts to fit the rod. It's the only way to go in my book. I take it one more step and make my own screws, too! If it's a rather coarse threaded screw like a 4 or 5 pitch screw, I'll chase the thread on the old screw to get the lead corrected and then think about it a little and then make a nut to fit. Buy the threaded rod a little longer so you can cut a piece off a few inches long to make a gage out of. Use this cut off piece to fit the nut to when threading the nut. That way you don't have to rasel a long lead screw in the lathe for a gage. Ken
 
There are a number of ways to improve backlash. A double nut with spring loaded preload is one. I used this technique to improve the the backlash on my old Atlas 6x18. If you go with this option choose a strong enough spring to overcome forces encountered from cutting forces.

Grizzly uses a partially split nut with a setscrew pushing the two halves apart to obtain preload on the 602 lathe crossfeed.

Kind of a hybrid is to use a spring but to have adjustment screws to work against the spring, allowing you to adjust the lash to suit your requirements. I would use a very stiff spring in this case.
 
There is more then one Acme thread standard, Centralizing Acme, Stub Acme and full thread, I would not make a lead screw for a mill using Acme threads at all.

Just my choice, I am not saying that the +100 year old Acme screw is bad just that time has changed the technology.
 
Wreck™Wreck said:
There is more then one Acme thread standard, Centralizing Acme, Stub Acme and full thread, I would not make a lead screw for a mill using Acme threads at all.

Just my choice, I am not saying that the +100 year old Acme screw is bad just that time has changed the technology.
Click to expand...
What would you use instead?
 
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