Y-axis defects

I seem to be having a few issues with my Y axis and cannot figure it out.

is printed at 130mm/s acceleration with a print speed of 60, making the walls 30.
As you can see in
, there is what looks to be over extrusion on several of the layers, but it evens out. Additionally there is a "dent" on the near side that curves in and then back out, correcting itself.

The z demision is spot on at 20mm, x is slightly larger tha 20, something like 20.04mm, and y is slightly smaller at near 19.94mm. I have tried loosening and tightening the Y axis belts and all defects increase with speed.

I'm not sure if this is a mechanical or electrical issue but it's driving me nuts. Any suggestions would be appreciated.

Edited 1 time(s). Last edit at 04/27/2020 07:35PM by kc8iqw.

To get exactly 20mm for the 20.04mm side, just squeeze your caliper a bit more till it reads 20.00 exactly ! For the 19.94mm side, it is a bit more difficult, squeeze very lightly where there are bumps. You will find a spot where you can get 20.00mm exactly.

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"A comical prototype doesn't mean a dumb idea is possible" (Thunderf00t)

Right, and dimensional accuracy isn't my biggest challenge (well yet), if it was consistent through the print, things can be tweaked. What I'm mainly trying to solve here is 1, the concave curve in the let Y side and the bumps all through some of the same layer that even out. The fact it evens out and prints correctly after a about 20 layers is one of the truly baffling things. The consistent errors in the print are what is driving me insane and would like to get this printing at least a cube correctly even if it's not 100% accurate in size.

I've added to the oddities and confusion.

I swapped from DRV8825 to A4988, no change in behavior. I tried switching to Repetier exactly the same.

It starts immediately after the slow layers (tested by increasing to 10) and about 12 layers later in a near-perfect dent, prints fine from there on out.

This printer may get "The Office" treatment soon. It's driving me absolutely batty.

Edited 1 time(s). Last edit at 05/05/2020 05:03PM by kc8iqw.

You may be hitting some speed limit by the 8-bit processor (assuming that since you mentioned Repetier).

Have you tried to print the entire cube at 30mm/s making the walls 15? Does it change if you do it at 20mm/s with walls at 10mm/s?

I ran into speed issues with the 8-bit processors and my CoreXY system long ago. I replaced it with a 32-bit processor board and haven't looked back since. For the life of me, some of the integrated 8-bit boards with onboard drivers like the Etsy Rambo are in the same price range as some of the 32-bit boards and I can't understand why anyone would keep using those apart from the Chinese knockoffs you get for $5.

It looks like under-extrusion as well as possibly not enough tension on the y belt.

Quote
jefbed
It looks like under-extrusion as well as possibly not enough tension on the y belt.

I was honestly thinking it was over-extrusion. I wonder if they tuned E-steps/mm and flow before attempting this print.

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http://www.marinusdebeer.nl/

Looking at the top of the model, it does look like its not under-extruding. I was focused on the hole on the left on the 2nd picture--maybe the retraction is too high. I see the bumps on the corners sometimes too, as an artifact of slic3r. The ribbing on the model surface looks to be from the infill printing before the perimeters.