SO3: Z belt skipping [SOLVED]

I have yet to put rooter on my Shapeoko, I am still at the step of drawing and calibrating.

I probably have drawn the round Shapeoko calibration model found there:

a dozen times and at the 11th print, my Z belt is skipping. I am 100% pure positive its not the stepper motor, I am pretty sure its either the new springs (yes I have the new one) overcome the idler tooth gripping force, or I was lucky and I got one of the longer Z belt that were sent a while ago. I tried to see marking on the belt but I cannot see any.

My stepper is at the very top alignment position and the adjustment screw is at the bottom.

I do have some 6mm bearings here, I could try to replace the Z bottom one with larger ones, but the documentation does not mention if they are 606 or 626 bearings.

While drawing, My bottom wheels are close to the and of the rails, and the springs are almost at their maximum extended length. It might be that, my carriage is too low, and the springs are too strong for so such light 3D printed carriage see

But I still think I got the wrong belt…

Help !!!

We have a bit on the wiki:

The belt was originally 524mm (not 624), then some were out-of-spec, so it was changed to 520mm (not 620)

yeah, the method I used to tension the Z axis belt, is what is listed.

As mentioned, my motor sits atop the slot and the adjustable spacer is all the way down the slot.

So, I must have the wrong belt, I can’t see another reason.

Contact and they’ll work it out w/ you.

Well, I might be out of luck.

I took the belt out and its 520 not 620 like you said. But, I do run a beta upgrade for the Z plate, which might be it. Maybe the adjustment slots are not at the right place, or the belt is printed wrong. Who knows.

I have measured the bearing on the bottom idler (6mm screw), and they are 12mm in diameter. I’ll get 626zz flanged bearing which are at 19mm in diameter, so that would enlarge a bit the distance to be covered by the belt.

But since they are going to come from china (I won’t pay 10$ per bearing locally), I am 4 weeks out to figure out if it will be enough.

Sight :frowning:


Did you recently receive your machine or has it been awhile? 520 IS the correct belt model…though, as has been mentioned, there were some of the original size SO3’s that were sent out with belts that were out of spec from the vendor. I’m curious as to whether your machine was from that “era” or more recent. It’s not unusual (at least in my own experience) to have a kit arrive and not be able to assemble it for 6 or 12 months because of “life,” so I thought I’d ask.

I’m hoping you can get it worked out without having to mod your assembly with oversized bearings.


Life you said :smiley:

Yeah, I can work on it basically only during the week-ends. Its SO3: #00947

I started my “build” thread with my upgrades in November, so its been a while

The more I look at it, the more I think going from 12 to 19mm diameter for the idler could work. I am not missing much. It is definitively not playing the same “music” when I pinch my X and Y axis compared to my Z axis. Much more play on the Z axis.

We will see in a few weeks (I hope).


Hi Jeff,
No need to wait 4 weeks, email support and we will take care of it.

Ed was nice enough to send a beta plate for the Z carriage (not flat, U shape to reinforce it) so, I feel it is ok if I have to work a bit to make it work. I might be a 520mm belt that is wrong, but I cannot tell.

I already had to mod the carriage a tiny bit my using 3mm shimms on the wheels instead of the washers supplied because the U part of the plate was too long and was hitting on the back plate.

I guess its part of the fun, but, yeah, if I cannot get it work play nice, I’ll ping support.

However Apollo, I’ll have you tell me if you are within weeks of releasing your official improved Z plate, so I don’t have to fiddle with the beta one, only to see the official release out in the wild before I am done :stuck_out_tongue:


After much trial and error during a sleepless night, I finally managed to realign everything and get the carriage to play nice.

I indeed had a 520mm belt, and because of my setup, the belt was not 100% truly vertical as I had to shim the wheels with 3mm spaces instead of simple washers.

No need of 626 bearings to increase the travel distance of the belt path. Only 2x M6 washers were needed to line up the belt correctly.


One thing I found was that I hadn’t tightened the Z Motor/Idler tight enough and the belt would skip on a +Z move. I found that using carpenter’s shims (wooden shims used to level various home constructions) fit very nicely between the Z and X Motors and, loosening the Z Motor mounts slightly and jamming a pair of shims between the two motors allowed me to tighten the belt so it was ‘twang’ tight and no more skipping!

PS: This should be a tip in the construction guide…