Hi, got an OmioCNC machine, not used it very much yet, but started to play with it today and cut some bits of plastic. Part way through it started making a nasty sound... Video here:
I presume one (or more) of the balls in the ballscrew has broken? The noise appears to come from the ballscrew area, it's like a crack crack noise. It's *not* coming from the end bearings for example. When I first got the machine I jogged it around and would get the odd crack like this, but very intermittent, I had previously assumed it was the loose ball cycling around... I guess not though?
Any tips on dismantling and how to repair? I have written to Omio to ask for help, but perhaps if I need a complete new ballscrew there are other options I should look at (price/quality wise?).
I guess this is a good opportunity to replace the end bearings, previously there was a good 0.05mm backlash if I pulled the machine forward/back along the Y (holding the ballscrew still). I'm guessing this would be play in the end bearing rather than play in the ballscrew? Tips on sources of appropriate angular contact bearings for this machine?
Thanks for any help!
Sending you a PM Ed as I may be able to help.
Also don't run the machine as it could make this worse/harder to fix, the issue is with the nut internals I'm pretty confident of that.
Last edited by Lee Roberts; 05-08-2016 at 03:27 PM..Me
Sounds like a trapped ball between the internal track'sJohn S -
Agreed, here is a snippet of my PM for anyone with similar nut issues:
Unfortunately it is quite common for these nuts to leave the factory (china) contaminated internally, I've seen and repaired everything from bits of dried out grass/straw, missing balls, misaligned returns and more commonly bits of metal swaf/chips.
Really your machine builder should of checked for this before assembly, anyway it's common on the forum now to advise anyone buying a screw and nut combo to strip it down and check the nut before use to be sure everything is good. Doing this isn't really hard to do as soon as you take the nut off the screw the balls will fallout and bounce all over the workshop, unless your paying attention!
Where people struggle is the repacking and loading of the balls back into the nut, providing you take your time, apply a little patience, a little finesse and can hold a small set of tweezers steady you should be able to do it. It dose sound like quite a corse sound on the video, it can't be ruled out that a ball bearing hasn't collapsed but to be fair I've never seen this to date, though this could just be my experience.
It sounds to me like a ball has gone off course and ended up in an area of the nut their not supposed to be in, this happens when one of the plastic returns becomes damaged or is inserted incorrectly, crap in the nut can assist in this and escalate the problem.
I would suggest stripping the screw down off the machine and seeing what you can find in the nut, as I say the worse you can come off is finding it tricky to reassemble the nut. If that happens and you can't do it, give me a shout and I can do it for you, I carry spare balls in the right sizes, for the different size nuts and a few spare parts.
A 16mm nut should have a total of 51 balls, 17 per track/return and the ball size is normally 1/8th (3.17mm) in size.
The offer of help is there to anyone who needs it, just give me a shout :)
Last edited by Lee Roberts; 05-08-2016 at 05:51 PM..Me
A ballnut once assembled should never drop a ball unless a ball return pops out.
If this happens, balls will be all over the floor.
The cheap ballnuts will have plastic ball returns and these are more prone to this but will normally only happen if you put them under excessive load.
Lee is right, the Chinese ballnuts can come full of shit, (Literally) but the amount of balls is dependant on a number of things like lead, ballsize and BCD, so different ballscrew manufactures use different ballsizes and BCD, so without knowing the manufacture of the ballscrew you are pissing in the wind to get a matched ballnut.
Luckily most of the chinese ballnut manufactures buy all of TBI out of spec screws and pack the nuts to suit, so even if you were to get a new nut to fit the screw, there is a good chance it wont fit well anyway and could be too loose or too tight.
the out of spec screws will normally be not making C7 or in most cases have a slightness under or oversized BCD.
Most of the 1605 ballnuts have 3.175mm balls, but in all cases they will either be slightly under or oversized to suite the screw.
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OK, thanks for all the advice! Have now dismantled the machine and
- Good news, ballscrew seems to be ok
- Not so bad news, looks like the problem was one of the bearings on the ballscrew ends. Specifically the "fixed end" with dual bearings has a crunchy bearing which looks extremely worn (wobbles significantly with finger pressure)
Some photos of the busted bearing here. You can see that one side of the bearing is heavily worn:
Now this machine is not setup ideally.
On the motor side is
- Single bearing 26x10x8mm (OD x ID x width), labelled 6000Z
On the opposite (to motor) side is the dual bearing, each is:
- 22x10x6mm, labelled 6900Z
The bearing housings are literally just holes cut in the aluminium chassis plates (reasonably accurately drilled, bearings are a tight fit). On the motor side the bearing fits from the outer side. On the dual bearing end there is a hole each side of the frame, so the machine frame is between each bearing (I guess you could describe the mount as an I shape with the bearings each side of the I). There is a large nut applying clamping force to the outside end of the ballscrew (dual bearing end) and I think no washers.
So working backwards.
- I think I need to replace the dual bearings with angular contact type?
- Google finds me these: http://www.wychbearings.co.uk/71900-...C5-Enduro.html
- Any comments about price, alternatives, suppliers?
- I guess I need to understand and check the mounting arrangement for these? My limited understanding would be to arrange the bearings back to back, such that they are being clamped together by the two ends of the ballscrew? However, I would need to check that the machine frame isn't touching the inner race (which looks like it was in the photo above, causing a bunch of wear due to the inner race being ground between the outer nut and inner frame mount)?
- What kind of torque levels should be applied to the AC bearings? Keep increasing until there is no measured backlash?
- I'm also planning to change the larger motor side bearing (since it has evidence of damage on the oil seal). I believe this should be a deep groove bearing? Any thoughts on suppliers? I plan to add a wavy spring washer to tension it, but still allow for some machine movement? Perhaps another angular contact bearing would be better though? (machine is aluminium framed)
I also observe that the linear bearing carriages are quite stiff to move. I'm unclear on how smooth these linear carriages should be though...? I will post something separately once I have the bearings replaced, however, any thoughts in the meantime appreciated. Note I have no idea if the carriages (hiwin) are already greased. There is nothing left after the wipers from the track move past. There is a bunch of chatter from the balls. It does have grease nipples already fitted (unlike the ballscrew...), any thoughts on what to lube with (uncovered linear rails, so dust is a problem, prefer to use dry/non sticky lubes where feasible)
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