Z axis backlash.

[Crevice Reamer]

Those set screws are holding the lock nut in place. If it were possible to tighten that nut until the next set of holes lined up, then you'd be good--But I doubt if that is possible. You could at least make sure the screws are tight though.
CR.

[Al_The_Man]

Originally Posted by wyc But, there must be another source of error as there is over 1/2 turn when turning the coupler where it is still in free space. So, if I wiggle it back and forth I can see some in the coupler, but, the rest is above where the leadscrew is connected to the load. Don't know if that's the ballhead or not.

One other source is the thrust bearing at the end of the ballscrew, this can also show up as actual vertical movement of the screw when under a load and the nut stay stationary, if it is loose.
Al.

[wyc]

I reassembled the unit and checked the Z backlash. It has gone down from 86 thous to 80 thous. So, the coupler has improved the backlash 6 thous.

Woohoo! ( I guess)

I'll just have to keep an eye on it and modify my quick Z move from 0.1" to 0.2" since tools coming back up barely clear the work surface and it makes me worry.

When the machine was new the Z backlash was down around 8 thous.

I don't think the backlash at the moment has any impact on the work or performance of the machine. I'm mostly cutting aluminum at the moment.

I also just opened a set of T handle allen wrenches and I didn't apply too much force putting back in the bottomside coupler allen when it went "Snap". Now I have a chunk of allen key stuck in the allen screw. I hope I don't ever have to work on that allen screw anymore.

Got them off ebay. They must be junk.

Is there a parts diagram of the machine or is there an upgrade to any parts on the Z axis?

Wayne

[neilw20]

You probably needed a 4.0 mm allen key. If the key is not a good tight fit get the correct key. The Syil machines use METRIC HARDWARE.
No doubt you got a 5/32" which is 3.968mm. Only a little bit under size. Buy some good quality METRIC KEYS, and don't use the 'BALL END TYPE' for getting things really tight. They only contact in ONE SPOT, and can damage the hex socket.
But once you've damaged the screw that makes it difficult.
If it is the earlier coupler with multiple straight slots then the screws tightens against flats on the stepper shaft and the ballscrew end which also should have flats. IMHO dimples would have been better than flats.
With the POWER ON so that the stepper is held from moving, jog Z using page-up/down keys until the screw is accessible.
Now gripping the ballscrew firmly but without causing it any damage turn it back and forth. You can grip with pliers and some aluminum right NEAR THE BOTTOM as the head will never go down more than 380 mm from the top without hitting the bed.
There should be NO BACKLASH.
Make sure the screws line up with the center of the flats and then tighten.
I had the same problem. You should be able to get it better than 0.001" if loose gibs are not creating subtle errors with head tilt.
After getting the coupling tightened adequately, you can check the backlash in the nut.
TURN OFF the power, temporarily LOCK the GIB with the locking lever, then attempt to turn the coupling and hence the screw. Any movement there will be backlash in the nut AND/OR the thrust bearings.
Don't forget to unlock the gib.

[wyc]

I have revisted the Z axis problem on my machine and found several things.

First off the ballscrew nut assembly appears to be fine. I've taken apart the ballscrew thread detaching at at the coupler.

What I discovered is I believe a brass nut on the bottomside the bottom bearing. The end near the coupler.

This nut had completely come loose. I spun it hand tight and reassembled the unit. Now I have backlash down to 0.005" or so. That's much better, but, I believe the nut may be the preload on the ballscrew.

I have a few questions. If this is the preload screw then what should the nut be preloaded to in foot pounds.

If the nut has nothing to do with this then maybe there is somewhere else I should be looking? If so, where?

Any thoughts or ideas would be most welcome.

[iGG1e]

If that nut was only finger tight then there would've been very little preload on the two angular contact bearings that should be behind the washer the nut sits on. There should be shims between the bearings, so tightening it to an ever increasing level shouldn't really do much. Without the proper equipment to set and measure the preload, I just tried to get a balance of backlash vs smoothness when I had to change the factory ones on my X axis as they were really notchy. Foil shims and lots of fiddling.

For some reason the factory bearings (memory fails me exactly) are a high angle and these are expensive when you compare them to the standard low angle ones you see at most bearing suppliers. Luckily I was able to get an exact high quality equivalent pair for favours owed, otherwise I would've tried the much cheaper lower angle ones.

I can't remember my Z backlash measurement, but I will be doing it again very soon as I've got the thing apart...yet again. I don't think it was any better than what you've measured though.

[smallblock]

Sure looks like a bad place for the stepper mount. Any lubricant on the ball screw will eventually migrate down to the exposed bearing on the motor.

[wyc]

I'll admit I'm not mechanically inclined. I'm all thumbs and lucky to still have all 10 digits!

That said I've been pondering this and it takes me a long time to see things clearly. Now that I've pondered over this overnight I have a couple of observations.

Even if I get the nut tightened to pre-load the ball screw nut and it takes out the backlash there is nothing to prevent the nut from working loose again in the future. Of course if this happens in the future I will hopefully remember where to tighten it up again.

But, what you can't see is the upper bearing mount has two pins in it. Great. It won't be moving. But, why? there is no pressure or load on the upper bearing mount.

Now the bottom bearing mount seems super critical if the nut needs to maintain precise pressure. If the bottom mount moves so does the nut and thus the pre-load.

How is the bottom bearing mount held in place? With two screws and the bearing mount holes are slotted allowing it to adjust. That means the bottom bearing mount can move over time since it has no pins to hold in place.

Yes, any grease will flow down the ball screw lubing the bottom bearing which is good, but, also onto the nut that shouldn't be moving. Bummer.

I wonder if this is different on the X4 machine.

Off to the hardware store to get an open ended 15mm wrench to tighten the nut up. I'll post an update if I can get rid of all the backlash.

Wayne

[Old Megawatts]

The ball nut that you have pictured is actually two nuts in oposition to each other. They are screwed towards each other on the screw and are spaced apart by a two piece spacer. These spacers are held in place by the two black retainers that are just showing above the mount. The only way to change the preload on the two nuts is to change the thickness of the spacers between them. I haven't tried this but that is the way they work. I speak from experience as I succeded in spilling all of the balls out of my Y axis and had the pleasure of re-assembling them. Hope this helps you understand how they work.

OM

[wyc]

Originally Posted by Old Megawatts The ball nut that you have pictured is actually two nuts in oposition to each other. They are screwed towards each other on the screw and are spaced apart by a two piece spacer. These spacers are held in place by the two black retainers that are just showing above the mount. The only way to change the preload on the two nuts is to change the thickness of the spacers between them. I haven't tried this but that is the way they work. I speak from experience as I succeded in spilling all of the balls out of my Y axis and had the pleasure of re-assembling them. Hope this helps you understand how they work. OM

I may have my technical terms all wrong which I have no doubt that I'm speaking jibberish. That's good to know and I'll keep that in mind. I hope I never have to go that route and have to take the ball screw nut assembly apart.

Even still the ball screw rod can move back and forth if the retaining nut is not set. I've got it assembled now and have the nut with a few pounds of torque and the backlash is now 0.003". Took a break to make some lunch. Back to the garage to see if I can get out the last few thous.

Wayne