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marbles
17-09-2021, 10:00 PM
Hi there,

I had a moment today to look at the Z axis stepper to ballscrew connection on my DIY gantry cnc. The two shafts have been out of alignment for some time. The plastic bushing was really worn, see pick.

I took my time this afternoon to line up the shafts properly finally and put on a new flex coupling. Its defo running a lot quieter and smoother.

As I run the z axis pretty fast iím still getting a slight vibration however. The z axis is 500mm with a stepper perched on the top :), see pic. I suspect one or both shafts having considerable run off and with the current coupler i'll not be able to eliminate that.
Q: Would I be better to get a couple of 1:1 pulleys and a belt and support the stepper upside-down off the back? I suspect the belt will do a fair bit to take out the micro vibrations.

Any thoughts or links welcome :)


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BeagleBrainz
18-09-2021, 12:44 AM
First off, is the undriven end of the ball screw constrained, from your photos I can not tell.

There does seem to be a large portion of the Z axis is unsupported, is there any flex/for & aft movement in the axis where the stepper motor is mounted.Could a redesign be possible to reduce the unsupported portion ?
The plate supporting the stepper seems a little on the thin side, this maybe something to look at. A little more mass and better (as good as you can get it ) alignment would be one thing I would look at.
The cheap couplers as shown in the photo are not the best choice, a Lovejoy\oldham coupler is a far superior solution, I have been using them on my mill for 5 years with no issue( tho I think they may have been in the vicinity of $80 AUD per piece ).
Be aware that some of the bearing blocks do not contain angular contact bearings but just regular roller bearings (and cheap ones at that) that are not suited to the application.
Is your ballscrew\ball nut aligned ? I mounted mine in such a way that there is some movement prior to snugging the mounts, as to allow the ballnut to find its position on the ball screw. This was due to the design of how the original lead screw & nuts were mounted. I did have an issue with the Z that required some fettling.

There could be probably a dozen more things that may be causing your vibrations.

Please note that I am playing devil's advocate.

marbles
18-09-2021, 09:07 AM
The underiven end is supported with a bearing block. The z axis backing plate on which the stepper is mounted has vertical ribs to add ridigity.

The lovejoy coupling looks similar to the jaw couplings i'm using but with more spider teeth. The Oldham looks like a superior solution. I'll investigate.

I'm going to re-align the two shafts again and see if I can improve on yesterdays attempt. If no improvement i'll look to other areas and investigate the pulley's & belt option, which I think I have 'in stock' in the workshop.

Clive S
18-09-2021, 11:56 AM
The underiven end is supported with a bearing block. The z axis backing plate on which the stepper is mounted has vertical ribs to add ridigity.

The lovejoy coupling looks similar to the jaw couplings i'm using but with more spider teeth. The Oldham looks like a superior solution. I'll investigate.

I'm going to re-align the two shafts again and see if I can improve on yesterdays attempt. If no improvement i'll look to other areas and investigate the pulley's & belt option, which I think I have 'in stock' in the workshop.

I personally would go with the pulley and htd belt as that can help stop resonance.

I also noted that you don't have all the bolts in the bearing blocks and as BeagleBrainz has pointed out the type of bearings in the block need to be checked.

JAZZCNC
18-09-2021, 12:45 PM
This is not all due to poor alignment.?
Because your rear plate and front plate are so long and so thin you are always going to struggle with screw alignment because has your front plate extends and becomes a very long lever the 10mm plate will flex and because the ball nut mounting is attached the screw alignment will also shift. Combine this with a tall and thin rear plate, even though you say it's supported, you will get some flex. On top of this, you have other weak links like the stepper mounting being thin and weedy so together they play a part in the total misalignment you are getting.

Yes changing to a better coupling will buy you extra time but eventually the same will happen unless you remove the cause of the problem which to me is mostly down to the weak nature of the Z-axis causing misalignment while in use which you will never be able to adjust out as it's random with the flex.

To me, you either live with it and replace it with another cheap coupler, buy a spare, and realize you'll need to replace them often.!.... Or . . Upgrade the Z-axis with a stiffer setup.

Changing to a pulley and belt setup will remove the alignment issue but it could introduce other issues because now you will be putting lateral forces on the ball screw and Bk bearings and like was mentioned lower down if your BK bearings are the cheap Non AC kind they won't handle those forces for very long and you'll introduce other problems like binding and stalled motors etc.

If it was me I'd beef up the Z-axis before wasting any money on expensive couplers.

dazp1976
18-09-2021, 04:31 PM
+1...

You have a machine that's running so why not make some custom parts and beef it up!?.
Make some bearing carriers that are 1 inch thick to house 2 AC bearings back to back to replace those BK things.
I did that on my mill using 7201-2rs (1605 ballscrews). I'll never go back to niether standard or thrust types ever again!.

marbles
18-09-2021, 04:46 PM
+1...

You have a machine that's running so why not make some custom parts and beef it up!?.
Make some bearing carriers that are 1 inch thick to house 2 AC bearings back to back to replace those BK things.
I did that on my mill using 7201-2rs (1605 ballscrews). I'll never go back to niether standard or thrust types ever again!.

That seems to be the general consensus from comments. I didnt know the ballscrew bearings/units were a weak area but I do now. I could just get better quality bearings and swap out the generic ones. Take your point about custom bearing housings espec to support a pulley/belt system as the z axis driver.

JAZZCNC
18-09-2021, 08:07 PM
. I didnt know the ballscrew bearings/units were a weak area but I do now. I could just get better quality bearings and swap out the generic ones.

There is nothing wrong with the BK-type housing, it's the correct component for the job, it's just that the cheap ones come with crappy bearings. So just replace the bearings with angular contact bearings.

This will be much simpler than making plates and fitting bearings into them because if you don't bore the opposing bearing pockets perfectly concentric to each other then you'll introduce more issues.!

marbles
18-09-2021, 11:48 PM
There is nothing wrong with the BK-type housing, it's the correct component for the job, it's just that the cheap ones come with crappy bearings. So just replace the bearings with angular contact bearings.

This will be much simpler than making plates and fitting bearings into them because if you don't bore the opposing bearing pockets perfectly concentric to each other then you'll introduce more issues.!

Cheers Dean, will get on it and order up a set. I've got 1605 ballscrews so prob 7201-2rs as dazp1976 says

dazp1976
22-09-2021, 12:03 AM
Cheers Dean, will get on it and order up a set. I've got 1605 ballscrews so prob 7201-2rs as dazp1976 says

The 7201 have a 32mm od. (32xx12x10mm).
I think the BK blocks take a 28mm od bearing. 28x12x9mm if memory serves correct.

I chose to custom mine to fit the 7201 just for a little more strength.

BeagleBrainz
22-09-2021, 09:59 AM
Here's section of THK's literature with a good reference of Bearing block models and bearing part numbers.

https://tech.thk.com/en/products/pdf/en_a15_299.pdf?#6