20mm test cube is never square on Y axis only.

Prints more like a slight parallelogram. All other sides of the cube are square. Just the side facing the Y axis idler, and the side facing the Y axis motor.

Post a picture it'll help. Sounds two of the axes are not square with one another though it could be lots of other things.

Hopefully this shows what I'm talking about. The side in question was facing the front of the bed, with the other side facing the rear of the bed. Its lean is the same way, so the object appears to be a parallelogram, and not square. This is becoming a major issue. I've previously torn apart the entire printer and squared up everything. The printer is screwed down to a base, with clips holding it down, level, and square.

Are all your belts tight and drive pulleys secure, do your axes slide freely?

It must be something to with the y axis, is the belt angled underneath (assuming it's an i3 variant printer). If its on an angle it might give slightly poor positional accuracy but you wouldn't expect it to be different along the same line.

What printer, electronics and firmware is this happening on?

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

The belt IS slightly angled.Meaning...from the base of the Y sled, running to the front bearing, it's nearly flat. However at the back portion of the Y axis base, it slopes DOWN to get to the Y motor, loops around the pulley, and is straight to the front.

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dc42
What printer, electronics and firmware is this happening on?

Folgertech Prusa I3 (plexi original not 2020) Mega2560 + ramps 1.4 shield running Marlin.

I'm only floating it as a possibility as it might manifest on a large part like the one you showed in your photo. If you can extend the belt clip under the bed so that the belts are parallel and the belt clip sits in line with the belts you will get more precise movement of your y axis.

Yeah I'm going to print an adjustable Y axis motor mount and try this again.

It looks like your x and y axes are not orthogonal. You will have to move one end of one or the other to correct the problem. I designed a test cube just for this purpose and wrote a spreadsheet that calculates the necessary correction. You can download them here: [www.youmagine.com]

Essentially, the object is a 100 mm cube. You measure the lengths of the sides and the diagonals and enter that data into the spreadsheet and it tells you how much you have to move the axis to correct the error.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

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the_digital_dentist
It looks like your x and y axes are not orthogonal. You will have to move one end of one or the other to correct the problem. I designed a test cube just for this purpose and wrote a spreadsheet that calculates the necessary correction. You can download them here: [www.youmagine.com]

Essentially, the object is a 100 mm cube. You measure the lengths of the sides and the diagonals and enter that data into the spreadsheet and it tells you how much you have to move the axis to correct the error.

Wow...that is...amazing. WAY over my head though I'm afraid. I'm not a math guy. I will try and print this however, and document all my findings. I had a look at your spreadsheet, and I must admit, it's confusing to me. The physical length is from the x/y thread rod where it joins into the printed plastic connectors? So the entire length of the actual thread rod "frame"? Is that right?

And that is what I'm actually going to be adjusting? The frame itself?

The math is just Pythagorean theorem and a little trigonometry- nothing beyond high school level.

The physical length of the axis is the length of that axis between the points that attach it to the machine's frame. For example, the x axis length of an i3 type design is the distance between the Z screws. In an i3 design it is a little difficult to adjust the x axis, so I suggest you adjust the y axis.

Most likely you're going to shift one end of the Y axis a couple mm. When you do so, loosen the attachments at both ends so that you don't produce a twisting or bending force by shifting one end. Shift whichever end you choose and then tighten both ends down again.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

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the_digital_dentist
The math is just Pythagorean theorem and a little trigonometry- nothing beyond high school level.

The physical length of the axis is the length of that axis between the points that attach it to the machine's frame. For example, the x axis length of an i3 type design is the distance between the Z screws. In an i3 design it is a little difficult to adjust the x axis, so I suggest you adjust the y axis.

Most likely you're going to shift one end of the Y axis a couple mm. When you do so, loosen the attachments at both ends so that you don't produce a twisting or bending force by shifting one end. Shift whichever end you choose and then tighten both ends down again.

Awesome. Thanks for all your advice. I'll let you know how it all works out!