Anti-backlash screw help

[rrbeaupre]

Hi - I'm new to the board and new to CNC. I am converting a Rong Fu 45 table and column to a cnc machine. I have designed and built an anti-backlash screw consisting of two 1" threaded delrin blocks sandwiched between an aluminum piece that attaches to the table. One of the delrin blocks is bolted directly to the aluminum piece. The other delrin block rides on four 1/4" machine screws under spring pressure. In my experimentation using a digital caliper, I can't seem to get better than about 9-10 thous. backlash. And that is when the spring-loaded screws are really pretty darn tight. Is that normal? Will it be acceptable with software correction? I'm open to any suggestions experienced users might have.

I have attached a couple of photos showing my design.

Thanks!

Bob

[spoiledbrat]

I am not sure exactly how you set up to measure the backlash, so please forgive me for being "Captain Obvious". Perhaps the "backlash" is really in the screw supports. Do you have double-row, angular contact bearings installed on one end of the screw, or does it ride in radial bearings?

Another possibility is that when you apply force to the mechanism, the sprung-nut may be rotating a slight amount, manifesting itself as backlash. I doubt this is the case, since it is not a ballnut (capable of back-driving). Just a thought...

Good luck with this problem,

Rob

[Stepper Monkey]

I would bet the sprung nut really is actually at least part of the problem. The sprung nuts bores appear to have raw thread passing through them, so it isn't going to be a tight fit. It isn't bushed or running on smooth shafting, so I'd bet there would almost have to be enough slop in there for it to be rotated slightly - thus getting rocked back and forth a bit by the leadscrew whenever it changes direction and drags the nut along for the ride.
Could be a number of other things as well, but that and bearings are a good place to start looking. 9-10 thou is a hell of a lot, you should easily be aiming at under 2.

[ZipSnipe]

I'm trying to figure out how your measuring the backlash with a caliper? Maybe on your design you could eliminate two screws and replace with alignment pins and let the springs ride on them.

[HackMax]

Shouldn't the springs be on the inside of the block pressing out against the threads. I think you are working WITH the threading instead of against it.

[Stepper Monkey]

The point to exerting pressure here simply ensures each of the two nuts remain firmly seated upon their opposing screw faces.
Either way should work equally well for this, but most nuts do indeed work through expansion and not compression.

I have a feeling this is more due simply to space and design requirements than any difference in performance, but if anyone knows differently I would be very interested to hear!

[irving2008]

I'd go with spoiledbrat's view that what you are seeing here is end-float not backlash.

You cannot measure backlash accurately until you have counteracted any end-float in the system, or at least have measured it and can take it into account. For clarification 'backlash' is the amount you have to rotate the screw to get a change of direction of the table, converted back to linear form. For example, if you have a 10tpi screw and you can turn it 1/4 turn before the table reverses direction (as measured using a DTI) you have .025" backlash ASSUMING the screw doesn't move axially (along the screw). If you can push the screw and table assembly back and forth relative to the screw's mounting then that's end-float and needs to be counteracted by appropriate angular contact bearings or other approaches. End-float should be < 2thou before attempting to counteract backlash in the nut else you're on a hiding to nothing.

[rrbeaupre]

Thanks for all the info! First of all, you can see in the pic how I was measuring backlash using a digital caliper. With the assembly in place, I moved the screw in one direction, extended the caliper, and then turned in the opposite direction, watching both the caliper display and the graduated collar on the screw itself.

The screw has bearings only on the handle end and appear to be just two thrust type bearings, the inboard one bearing against a thick washer which, in turn, press against the screw itself.

Can someone explain what type of bearing would be ideal for this operation, etc.?

i agree that I should start first by making sure that any backlash problem stemming from the screw bearing should be eliminated.
Thanks!

Bob

[Torsten]

Originally Posted by HackMax Shouldn't the springs be on the inside of the block pressing out against the threads. I think you are working WITH the threading instead of against it.

I agree with this statement, the design pictured dose not take up any Backlash it just uses the Springforce against the second Nut.
Moving the Springs to the inside of the Blocks would improve the design, then you will be able to adjust the screws to take up the Backlash.
Still Nylon Blocks are not Ideal, they are a bit rubberish and when under load will increase friction and sticktion, so you may still end up with some Backlash.
But it would be far less then the 9-10 thou you are getting now.

[spoiledbrat]

I disagree with Torston. Your design does in fact work to eliminate backlash. I firmly believe that a spring need to be in the design when using non-precision screws, or any screw in an environment that a speck of dust can find its way to the screw. I use a similar setup, with an aluminum block tapped to 15/16-16, and a bronze block tapped the same. My ballnuts thread into them. There are SS guide pins that run through the bronze block for alignment. I set the "preload" (to me, that is the tension/compression between the nuts) to about 50 lbs. As long as my machining load is less that 50 lbs, I effectively have no backlash. Without the springs, an imperfection in the ball screw, or dirt in the ball screw could cause binding, or require a lot of torque to rotate the screw.

It truly does not matter wither you compress the nuts together, or push them apart. Hell, when the nuts start to wear, then use a mechanism to push them apart. Just like new!

Rob

[HackMax]

Spoiledbrat and stepper monkey are correct. I just keep thinking in the way a screw and nut work that comes naturally to me - turn the nut on the screw then the nut moves and the screw remains in one place. The other way it's just the opposite - you turn the screw then the nut stays and the screw moves up and down.



Rick

[Stepper Monkey]

All that matters with an a/b nut is that one nut is firmly bearing on one screw face and the opposing nut is firmly bearing on the opposite face. The relative position of the two is irrelevant as long as they are on opposing faces.
I think the layout is fine either way, compression or tension, you just need to keep the nuts from moving axially relative to one another (i.e. bushes or guide pins), and to remove any end float. I would suspect there is a little of both going on here, but the basic arrangement itself is sound.