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Z axis Problem?

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Z axis Problem?

Post by CKAFamily on Tue Mar 01, 2016 10:42 pm

Hi,
I have done my first print today. No problem uploading the files onto the SD card as long as the file name is prefixed with 3D or 3d.

I do have a Z axis problem.

I noticed a lot of stiction and judder from the front Z axis linear bearings but smooth operation at rear Z axis linear bearings. It got worse when I operated the machine for the first time.
The front guide rails and linear bearings lock up and the Z axis lead screw slips. From a careful look at the construction, there is a lot of build plate flex from front to back. There is a large moment arm from the lead screw to the front rails and linear bearings. This moment arm in combination with the flex causes the front rails to stick, judder then jam and the whole Z axis locks up. Does anyone else experience this? I tried to realign the Z axis rails. I set the Z axis to the very top, loosened the rail mount at the chassis and retightened. I did the same at the Z axis bottom position. I thought this solved the problem of locking up but when I next went to print the Z axis still locked up.
Anyone else experienced this?
Anyone have a suggestion?

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Re: Z axis Problem?

Post by Mikael on Thu Mar 10, 2016 5:32 pm

Ill look in to this when I get mine.
If I find a problem then ill post a fix.

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Z axis problem

Post by sid on Tue Mar 15, 2016 7:13 am

Hello,

I have the same problem with the Z axis. The friction on the linear guide on the front result the bed is not flat ... to have a flat bed I had to screww a lot in the back and really small on the front but that really bad.
I think we have to renforce this construction, maybe add a helicoydal screw on the front also with a belt coming from the back.

But this is a real problem from this machine
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Re: Z axis Problem?

Post by CKAFamily on Fri Mar 18, 2016 12:06 am

Hi,
Yes that is a problem. Not sure how to fix that issue. I have added two linear bearings to the front rails (see image) and that reduced the judder noticeably but not entirely. I loosened each front rail mounts to ensure correct alignment. I also detached the lead screw nut to slide the plate up and down to check for any sticking points.
Good luck, but I agree, an extra lead screw at the front would solve the flex issues.
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Re: Z axis Problem?

Post by Mikael on Fri Mar 18, 2016 3:43 am

Hi,
I got my printer yesterday and I think I know what you are talking about.
My front support is flexible but was initially very bent on the right side. Seems like they forced the packages under the bed and that offset the left bearing.
I applied a bit of force on it and it snapped back, now its fully horizontal.
If I run in to any issues I have an idea on how to add a support to that part to make it less flexible.

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Re: Z axis Problem?

Post by Mikael on Sat Mar 19, 2016 6:46 pm

Hi guys,

The issue is friction.
The bearing sticks and jams, dont think its because of the alignment.

I turned the screw by hand and help it past the sticky part with a bit of force and applied wd40.
After that it seems to work when I screwed it up and down by hand, at least temporary.
Testing it now.

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Re: Z axis Problem?

Post by CKAFamily on Sat Mar 19, 2016 7:53 pm

Hi, i wish I could say the same. Despite lube, I found at times one of the front linear bearing would jam.  The whole build plate has very little rigidity. Flat plate designs are poor for rigidity, especially in this case where the lifting force is so far from the front rails. Doubling up the front linear bearings has eliminated the lockup problem and almost removed the judder. I will experiment with the rigidity issue in the future.

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Re: Z axis Problem?

Post by Mikael on Sat Mar 19, 2016 8:07 pm

[quote="CKAFamily"]Hi, i wish I could say the same. Despite lube, I found at times one of the front linear bearing would jam.  The whole build plate has very little rigidity. Flat plate designs are poor for rigidity, especially in this case where the lifting force is so far from the front rails. Doubling up the front linear bearings has eliminated the lockup problem and almost removed the judder. I will experiment with the rigidity issue in the future.[/quote]
Where did you buy the bearing you are using?
If this doesnt work then I need to do the same mod.

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Re: Z axis Problem?

Post by CKAFamily on Sat Mar 19, 2016 8:41 pm

Hi,
Aliexpress, 
LMF8UU  for 4pcs/lot 8mm flange linear ball bearing for 8mm linear shaft cnc
Store:- CNC Automation
Only 10 days to deliver which was surprisingly fast for an Aliexpress store. Hope yours is the same speed.

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Re: Z axis Problem?

Post by gregster on Sun Mar 20, 2016 4:53 am

Thanks for the info. Looks like a good improvement!
My build plate is not running smooth if going up/down fast. But small steps (during build process) are going fine.

But will definitely keep your solution in mind for the future.

Cheers,
Gregster

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Re: Z axis Problem?

Post by Mikael on Tue Mar 29, 2016 6:05 pm

Doing the bearing mod now.
Is harder then it looks.
Needs to be almost fully disassembled to get access to the front bearings.

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Re: Z axis Problem?

Post by CKAFamily on Tue Mar 29, 2016 9:10 pm

Yeh it is fiddly. You will need longer M3 bolts for the two bearings back to back. You probably noticed that anyway. Good luck!!

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Re: Z axis Problem?

Post by Mikael on Wed Mar 30, 2016 2:33 am

I was so lucky.
The tightening screw on one of the bearings didnt want to come out and the material they are made of is soft. So either I leave it like it is or fix it.

Seems like I need to basically disassemble everything.
Spent an hour on it last night without anything major happening. Everything needs to be removed in a specific order, trying to do anything in any other order will result in swearing and loss of sanity.

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Re: Z axis Problem?

Post by Mikael on Sun Apr 03, 2016 10:47 am

Ok, this was a pain.

To do it 100% correct I was forced to both modify all parts of the assembly and also replace many parts with either new ones or better ones.

First to get access to the front Z-axis bearings you need to remove both the Y and X axis.
The order you need to remove them does not matter much, but its a must have.

All the bearings near the front for the Y and Z axis are attached via bearing housings.
The bearing housings are screwed on the the chassis with 2 screws.
Each screw has 2 shims, one nut and a bushing.
The shim that is placed in between the bushing and the bearing is far too small. So I replaced mine with larger ones.
The Y axis is connected to the chassis in a different way on the rear side. It rests inside a plywood piece.
For me, this plywood piece didnt really reach far enough to catch the Y rod when I was assembling things again, so again I needed to modify.
The modification was to grind down the corner that points towards the belt with about 3mm then when attaching it to the chassis adding 2 shims on the back for each screw. This solved the problem and the Y axis could reach.
I am going to design a new piece for replacing the plywood one.

When it comes to the Z axis removal there are 2 bearings for each rail in the front.
Each bearing has, again, a screw, some shims, a nut and a bushing.
Again the shim in between the bushing and the bearing housing was too small and needed replacing.

All bearings are screwed in with small screws to each rod. The small screws are allen screws.

Once you have been able to remove both the Y and Z axis you can start adding the new bearings.

The only problems I really ran in to that were hard is the alignment afterwards.
The Y axis didnt line up afterwards because the bearings were not close enough to each other.
I added 2 extra shims in between the bearing housing and the bushing on both sides and that solved the issue for me.

The only thing you need to make sure of when doing this mod is to test all bearings BEFORE you screw things back together.
There are major alignment issues with this machine and if you "hope" that things will line up then you will be in for a bad surprise afterwards.
What I did was to travel the Y and Z by hand many times and just look at the bearing performance. See if they look sticky on any point or if the rod is flexing.

I hope this helps anyone trying to fix this issue.

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Re: Z axis Problem?

Post by Snootch on Fri Apr 08, 2016 4:21 am

Similar issues with the z-axis sticking. unfortunately only noticed after one end of one of the y-axis sliders vibrated itself completely loose forcing the print head into the bed, which prompted a complete round of bolt tightening & loctiting.

The front z axis shafts were out of alignment - to the extent that the shaft would not turn freely on its bearings (incidentally does anyone understand why the vertical shafts are fixed in bearings at the top and bottom instead of blocks like the ones on the rear are - comes in handy for testing the friction on the shaft while adjusting, but doesn't seem necessary otherwise?) i removed all the bolts and nylon spacers holding the bearing housings and replaced with some compressible spacers, by adjusting the bearings top and bottom i was able to return the shafts to vertical such that it now runs smoothly in both directions on the z-axis.

http://imgur.com/cCWRGHA

[url=http://imgur.com/cCWRGHA]http://imgur.com/cCWRGHA[/url]

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Re: Z axis Problem?

Post by Mikael on Mon Apr 11, 2016 3:52 am

I think you might run in to problems with stability when using the CNC with compressible spacers.

The method I applied works fine regarding the Z axis without risking any stability issues.

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Re: Z axis Problem?

Post by sid on Thu Apr 14, 2016 5:57 am

Mikeal, did you have some pictures to explain the modification you have done on axis Z and Y? Thanks

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Re: Z axis Problem?

Post by Mikael on Thu Apr 14, 2016 6:01 am

Sid, ill get you some later on today.
I had another bearing failure on the top right bearing. Its just not built to handle the load of the stepper motor belt tension.

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Re: Z axis Problem?

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