Y axis wobble and belt teeth against bearings

The biggest issue with prints I'm producing at this point is what looks like wobbling or inconsistent speeds in the Y direction. If I print a cube, sides parallel to the Y axis end up with periodic bands.

This is a Sell's mendel. I realized that at both ends of the Y belt it is redirected by two bearings, and on one the teeth go right against the surface of the bearing, and it really looks like that affects the speed of the travel. Has anyone else dealt with this issue? Did I run my belt wrong? Am I on the wrong track and its likely to be something else?

Also, because I know its the most common cause of this, I'm not using old style, file down both sides of your motor shaft and wedge on the pulley and hope its even type pulleys. I'm using nophead style set screw pulleys and they look perfectly straight.

Thanks for any insight

See this thread.

You may be able to eliminate the problem by adjusting belt tension.

If not, one approach to solving the problem is to use a toothed idler pulley. It does appear to improve the situation significantly.

I ended up replacing the 5 mm belt with a 1/4 inch wide MXL (0.08 inch, 2.032 mm) belt and used a commercially made pulley. An extra washer was added to each idler bearing assembly to allow space for the wider belt. I fact I did this on both the X and Y axis. This completely eliminated the effect as far as I can tell.

Interesting. I knew my Y belt was pretty loose, but I wasn't seeing backlash so I didn't think it was a problem. I'll try playing with its tension, and if that doesn't fix it I'll try one of those idler pulleys.

Your pulley and belt might be mismatched. This was the case on my Prusa. It was barely noticeable, but whenever there is a direction reversal, the motor would begin moving and the belt would lag behind for a fraction. My belts wer XL types, and my pulleys were not.

Its the same belt and pulley combo I'm using for my X axis, and I'm not seeing any start/stop lag on small features. The only issue that shows up is banding whenever there's a long stretch in the Y axis (or diagonal so it has a Y component). I've also made calibration prints, objects are the right size, so missed steps aren't likely. It really does look like hiccups in travel speed.

But you say the bands are periodic...

Tighten the belt (or loosen it), and see how it affects the results. The only periodic thing there would be gear against belt.

Make sure you don't have LASH turned on in SF.

Could it be from vibration of the frame in the x-direction? If the motherboard panel is not mounted on the frame as designed, then the Mendel frame is weak in the x-direction and it would tend to vibrate when printing at fast feed rates (>30 mm/s).

The other thing that's periodic, and what I'd suggested earlier, and what madscifi pointed out, is the belt teeth rubbing against the idler bearing. I don't have lash turned on. I do have the motherboard panel mounted, but I'll check for vibration.