Bit of backlash

[blighty]

Hi all,

I have just fitted the ballscrews and steppers ( x and Y ) to my mill and i'm getting some backlash.

the mill is a RF45, i have class5 screws and nuts, 2 angular contact's at the fixed end's that are preloaded. they have no play and don't move any where. the nuts are fixed in there mountings by 6 screws and they don't move ether. both steppers are 960oz, with olden couplings? to the screws.

when testing for backlash..... X .02mm/.03mm, Y .04mm

any idea's where this backlash could be coming from?

thx in advanced.

[HackMax]

Man I hope I can get that tight when I get mine in. Are your ballnuts preloaded? I think I read somewhere to expect backlash even with ballscrews unless they're double-nutted. Sounds like you are running single nuts.

[blighty]

hi MackMax,

yep, i have preloaded single ballnuts "zero backlash". well i hope so, they cost enough

[HackMax]

Hmmm, I was thinking that in order for the nut to be preloaded it was a double nut. My bad. I know the double nuts are expensive and I surely can't afford those. If yours are supposed to be zero backlash I wouldn't have a clue to find that slack you have according to the info you provided.

Rick

[blighty]

i may of got the terminology wrong on the preloaded ballnut bit. another way of saying it would be........"it's got big ball's in it"

[alexccmeister]

I am hoping to retrofit my x2 with double ballnut. Not the one piece but two pieces opposing each other. You guys think it will work?

Alex

[TOTALLYRC]

Originally Posted by blighty Hi all, I have just fitted the ballscrews and steppers ( x and Y ) to my mill and i'm getting some backlash. the mill is a RF45, i have class5 screws and nuts, 2 angular contact's at the fixed end's that are preloaded. they have no play and don't move any where. the nuts are fixed in there mountings by 6 screws and they don't move ether. both steppers are 960oz, with olden couplings? to the screws. when testing for backlash..... X .02mm/.03mm, Y .04mm any idea's where this backlash could be coming from? thx in advanced.

I know this will sound silly, but something is not as tight as it seems. The screw people will blame the bearing people who will blame the screw people who will blame you as the builder. Look for Vince's bport cnc conversion log and there is a great portion where he fights exactly the same thing.

Originally Posted by blighty i may of got the terminology wrong on the preloaded ballnut bit. another way of saying it would be........"it's got big ball's in it"

.
I believe the proper term is it has been fitted with oversize balls to get to zero backlash. When we talk oversize, we are talking .0001" oversize balls.They keep going up by that amount untill there is zero backlash.

Originally Posted by alexccmeister I am hoping to retrofit my x2 with double ballnut. Not the one piece but two pieces opposing each other. You guys think it will work? Alex

2 nuts working in opposition is a great way to do it. If you allow for some adjusability or spring load them, you will be able to compensate for any wear in the future.
It can be more expensive, but if you have the room and the money, I like it better than the oversize ball route. Just my opinions of course.

[alexccmeister]

Originally Posted by TOTALLYRC 2 nuts working in opposition is a great way to do it. If you allow for some adjusability or spring load them, you will be able to compensate for any wear in the future. It can be more expensive, but if you have the room and the money, I like it better than the oversize ball route. Just my opinions of course.

not too expensive really. I got quite a few ballnuts I bought over the course of my cnc hobby life and all I need to do is drill and tap the other end of the cncfusion kit where the piece bolt onto the table. Put the other ballnut on in teh opposite direction, that's it. I am getting some taps off ebay, what a bargain. 3 15/16" tap for just USD30.

Alex

[Fixittt]

are you sure your backlash is not coming from the motor itself? I know my small 269 oz steppers have a bit of backlash in them when they change directions. it takes about 3 steps to fully change direction within the motor itself.

[blighty]

TOTALLYRC,

i'll have a look at Vince's log. thx

i had a little play with the X fixed end and found the A/C bearing is sitting flush with the bearing holder. so i faced off a tho or 2 so when i put the holder back on it would make dam sure the 2 bearings are being pushed together. that should make the outer of the bearings nice and tight. only thing is, i dropped my DTI and snapped the end off so a cant do any thing till monday.

Fixittt.

i will have i look at the motor when get a new DTI

[scudzuki]

The way to measure backlash that is inherent in the nut/screw/table interface is to put the tip of an indicator held in the spindle on a vertical face (side of a vise, angle plate, whatever) then grab the table and shake it back and forth. You'll find out in a hurry how much slop is in the axis. I just totally rebuilt a retofit RF45 that utilizes Thomson preloaded ballscrews and servos. I bought it secondhand from a guy that didn't know the difference. When I disassembled the X and Y axes, I discovered that the builder had not locked the nuts to prevent them from rotating in their mounting block. Good components do not make up for poor implementation and attention to detail. The X and Y axes went from .008" and .006" backlash respectively to .0015". Most likely this is being introduced by the bearing and not the screw/nut interface. Regardless, I can live with it now.
Joe

[philbur]

Where/how did you measure this?

Phil

Originally Posted by Fixittt are you sure your backlash is not coming from the motor itself? I know my small 269 oz steppers have a bit of backlash in them when they change directions. it takes about 3 steps to fully change direction within the motor itself.