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View Full Version : Cheap, low-backlash, low-friction leadscrew nuts?



DICKEYBIRD
09-13-2010, 07:05 PM
I know, impossible but what are some options a feller can make in the home shop or purchase on the cheap. The X & Y axes have 1/4-20 60 deg. threaded screws and the Z is a 3/8-20. Not even Acme threads.:rolleyes: The Z has at least a quarter turn backlash. The X/Y axes are better but if I'm going to take it apart, I should try to make it better.

As mentioned before, I'm trying to make this thing as good as possible without spending much...at all.:) I assume Sherline used better than hardware store variety all-thread for the screws as the threads appear to be pretty smooth. Is there a plastic material that can be tapped and fitted tight to the screw that would hold up? I will be cutting aluminum, plastics and brass with the mill at relatively slow feed-rates

I apologize in advance for asking yet another question about this CNC Micromill (converted Sherline) I'm working with but you guys are the best when it comes to this kind of stuff.

Evan
09-13-2010, 07:25 PM
You haven't been paying attention to some threads in the past DB. Acetal or Delrin is the material of choice for that application. If you can find a chunk at a reasonable price the only thing better is one of the PTFE filled varieties such as Torlon or Delrin AF or Ertalon and so forth. Plastic is the way to go as it is self wiping and doesn't wear the screw or itself. The Y axis nut on my mill is the original that I made from Acetal and it now has perhaps .0005 backlash. I made the shell for it in such a way that I can compress the nut and it removes any trace of backlash. I have made that adjustment only once.

DICKEYBIRD
09-13-2010, 07:39 PM
You haven't been paying attention to some threads in the past DB.Yep, I guess I'm guilty as charged.:o I tend to have a one-track mind at times and was probably focused on something else when that came up. I'll be doing some searches now, thanks.

oldtiffie
09-13-2010, 09:26 PM
I seem to recall some comment or advice here about casting a block about/on the lead-screw and then separating them and machining the rest of the block to suit the application.

You'd need to do the casting on the least-worn part of the lead-screw.

I seem to recall that the casting material was a multi-part/pot mix. I can't recall the material name.

alanganes
09-13-2010, 09:35 PM
I seem to recall some comment or advice here about casting a block about/on the lead-screw and then separating them and machining the rest of the block to suit the application.

You'd need to do the casting on the least-worn part of the lead-screw.

I seem to recall that the casting material was a multi-part/pot mix. I can't recall the material name.

I'd guess you are thinking of "Moglice".

oldtiffie
09-13-2010, 09:42 PM
Thanks alanganes.

That seems to be it.

It shouldn't be too hard to incorporate a "back-lash adjuster" as on the nuts for many milling machines.

darryl
09-13-2010, 11:37 PM
I've made a few from plastic, in that size. I haven't used acetal or delrin, but HDPE worked for me. The best rendition I had of a leadscrew nut was making it in two parts. Each part was grooved a bit on its edge, then the two edges put together side by side, leaving a sort of square hole between them. I used a piece of aluminum channel to contain the two pieces side by side, then ran a tap through that hole, which left me with two half-nuts which I then screwed into place in the piece of channel. There was no play in that setup, and I had no need to press the two half-nuts together to eliminate any play.

It's been years, so I don't recall the details- I think I made the leadscrew nut an inch long. It would have been longer except for being limited by the length of the tap.

Two more things about it- the leadscrew was a piece of stainless threaded rod. It seemed to be smoother right from the start than the garden variety rod, but I also polished it up using a cordless drill and some green scrubbies. Maybe I should have used purple, but in any event I figured that if I got it smoother it would wear better in the plastic nut. It certainly was smooth in operation.

The plastics that Evan mentioned would most likely be better, but the same method could be used. If a play adjustment was required, a couple of set screws on each side of the channel could press against the side of each half-nut.

The machine it's on is long gone now, but I'll see if I can draw a picture to show how I made it.

darryl
09-14-2010, 12:15 AM
http://www.glacern.net/free_photo_upload/lead%20screw%20nut%20assembly.jpg

There it is, pretty much as it was made. I don't recall if I drilled through the notched hole before running the tap through, but that might be a good idea. Make the notches only deep enough to guide a drill bit. You actually don't need to angle the notches- if you made them 1/16 inch deep in each part, the resulting square hole will guide the drill bit properly. If you don't make the hole to end up square, you could have trouble drilling the hole to be true.

Another idea- if you place a couple thicknesses of paper between the plastic blocks before you assemble them into the channel, you'll end up with a slight gap between the pieces once the hole is tapped and a leadscrew is inserted. The gap will allow you to use the setscrews to adjust side play against the lead screw, and consequently the play and the level of friction when the lead screw is turned.

lazlo
09-14-2010, 08:34 AM
I'd guess you are thinking of "Moglice".

I made near-zero backlash nuts for my mill/drill with Moglice 1000, which is Teflon loaded epoxy. It works great, but it's a lot of work, and the Moglice is very expensive.

You have to thoroughly clean an unworn section of the leadscrew, and coat it with mold release. If you don't use enough mold release, the nut epoxies to the leadscrew, and you truly do have zero backlash :)

Then, after it's cast, you have to use a wrench or pliers to break it loose, and it will be incredibly tight, due to the pitch variance in the leadscrew. Devitt (the distributor) recommends using Bon Ami scrubbing powder as a non-embedding lapping compound. You have to repeatedly run the nut back and forth, adding lapping compound, until the nut runs smooth.

As you can probably tell, I wouldn't do it again.

Bill Pace
09-14-2010, 09:22 AM
When I got that little Barker mill that went thru Katrina, it had the levered axes and I kept pondering on converting it to conventional crank axes. So, one day in a fit of "wonder if I could..." I had some 1/2" acme all thread and some plastic of unknown type (I'm pretty sure it was Delrin) off ebay and off I went. I first cobbled a tap out of a piece of the all thread - no treatment, just to see what would happen in a piece of the plastic -- it was so effective that I ended up converting all three of the axes in just a few hours,

One peculiar thing that I found was the reaction that darryl is talking about in his description/drawing. Drilling through the plastic was a hit & miss thing to get it where the tap would go through and get a threaded hole the all thread would go through without too much friction! Splitting the piece sounds like a good way. I finally settled on a 1/2" drill and then running the tap back and forth thru the hole till it would seem free enough to try the all thread. If I would've slowed down long enough to make the tap better - more attention to the cutting edge and maybe case hardening it may have made a difference..

I was so impressed with the results that I wouldn't hesitate to try plastic on most anything!

lazlo
09-14-2010, 02:20 PM
For another datapoint that's probably going to cause a lot of angst :rolleyes:, the Excello 602 turret mill (which I own) was available for a brief period in the late 80's with Delrin table nuts (bronze nuts were standard).
The problem was that grit would trap in the nuts, and very quickly wear-down the leadscrews.

Perhaps that only impacted manufacturing applications, but Excello pulled the Delrin nuts and never offered them again.

That might explain why Delrin nuts are not used on industrial machine tools (although they are often used on high-speed positioners). I've got several anti-backlash (spring loaded) teflon-coated acme screw/delrin nut assemblies off Ebay, and often wondered about using them on my lathe compound.

Evan
09-14-2010, 02:27 PM
The problem was that grit would trap in the nuts, and very quickly wear-down the leadscrews.


Grit? What grit?

lazlo
09-14-2010, 02:31 PM
Grit? What grit?

If you look under the table at the leadscrew of any mill, including mine (which is usually surgically clean) it's covered with nasty black gunk: a mixture of swarf, carbon, dust, rust, burnt cutting oil...

Evan
09-14-2010, 02:40 PM
My plastic Y axis nut hasn't even completely worn off the black oxide finish on the lead screw yet. It has hundreds of hours of running time on it. I purposely used bronze nuts and linear bearings on the X axis with plastic and PTFE on the Y axis so I could compare the long term wear and reliability. The Y axis is nearly pristine even though it is much more exposed to all the "grit" while the X axis shows significant wear both to the nuts and the lead screw as well as the linear bearings and rails. I will take some photos this evening when I have time. I will have to replace the bronze nuts, screw and the linear rails and shells fairly soon on the X axis. It will be with plastic nuts and bearings this time.

Bill Pace
09-14-2010, 02:50 PM
The problem was that grit would trap in the nuts

That is a very valid point, and, on reflection should have occurred to me - course that also happens in the conventional screw nut, probably contributing to wear more than use. Although, I have a feeling it would be a long time happening in a home shop - mine at least. Even though, as you point out, my shop constantly has an almost black "dust" all over everything, probably from all the variety of activities I do there - grinding, sanding, paint, etc.

Evan
09-14-2010, 03:01 PM
My mill is actually exposed to quite a lot of grit. Because the rails and plastic bearings are cheap and easy to replace I also use it as a surface grinder from time to time. Even so the wear is all on the metal to metal surfaces with virtually none on the plastic to metal surfaces.

Because of the wear potential I designed a system of easy adjustment for the linear bearings and the nuts to compensate for such wear without sacrificing accuracy. Part of that system is the ability to rotate the round rod ways to place a new surface on top which is the weight bearing area. It only takes a few minutes and no readjustment is required. Even replacing the rails doesn't require readjustment and also only takes a few minutes each.

BillDaCatt
09-14-2010, 03:10 PM
In the event that you can afford to spend a little money - McMaster-Carr.com carries 5/8" ball screws for $10.22 per foot and the nuts are only $26.69 each. Not cheap but darned near free when compared to how expensive the other sizes they offer are. Shipping is always quite reasonable too.

The part numbers are:
Ball Nut 5/8" @ 13/64" per turn: 5966K16
Ball Screw 5/8" @ 13/64" per turn: 5966K26

DICKEYBIRD
09-14-2010, 10:03 PM
Cheers Bill, thanks for the info.:)

I hope to build a machine someday that will need 5/8" ballscrews but the whole mill I'm working with right now probably weighs less than a few feet of 5/8" ballscrew and a a few nuts.;) It's a little Sherline hobby mill.

BillDaCatt
09-15-2010, 12:17 AM
Thought it might be a tight squeeze, but I figured it was still worth sharing. The cross-slide in my 9x20, although small, has lots of room inside and I thought yours might too.

Evan
09-15-2010, 12:25 AM
I just developed a really easy and nearly foolproof technique to make perfect leadscrew nuts from Delrin. Right or left hand, any thread form. As soon as I massage the pictures I will post it in a new thread.

PrecisionSpindle
09-15-2010, 04:33 AM
imperial sized lead screws and bronze nuts are available off the shelf at reasonable costing. The screws are hardened. Course thread screws are precision machined; fine thread screws are thread ground. Bronze nuts are finished machined to fit the screw selected for you. Nuts are available in flange style and pillow style. For a listing, contact -

Precision Spindle & Accessories Inc.

www.precisionspindleinc.com tel. 519 671 3911
haviland@precisionspindleinc.com fax. 519 652 5994

Evan
09-15-2010, 05:39 AM
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