Mendlemax NOT printing Circles.

Hi there everyone. I have just completed my Mendlemax its actually most of the components from the Ultibots Ultimax kit. Now i have tried everything to get it to print near to perfect circles and it just wont. I have made sure all the belts are tight and of equal tightness there is no backlash that i can tell and the steps per mm are as bang on. In fact i have ended up going back to the original steps per mm that i calculated using the gt2 belt calculator. But its still printing oval circles and now im at a loss. My theory is that perhaps the belts i got are not quite right i know that the x axis has a slight twist in it that wont pull out perhaps that is it i dont know. As the printer is finished and prints amazingly well apart from the circles. Im open to any ideas. Bed is level to within .01 of a mm. Although there is a small dip in the glass of about .1 on one side just off center im getting new glass which should fix that. i keep check and re-checking measurements but im lost now. Here are a couple of photos. [drive.google.com]

Ok so after some more messing about i had discovered that the Y axis was not traveling at 90 degrees to the X axis. So it better than it was before not that it squared up a bit better but i may have some other alignment issues that i cant work out right now. from what i can tell its square to within about .02 of a mm but circles still not round. I may be correcting for other things that aren't within tolerance like the belts. I may be chasing my tail on this one.

If your pulleys are tight on your stepper shafts and your steppers are not skipping steps, it sounds to me like you need to adjust steps per mm on either the X or Y axes. Measure your oval in X dimension and in Y dimension and then scale the steps per mm of one or the other to make it produce a circle. i.e. if the STL is 100mm outside diameter and your part prints measuring 100 in the X dimension and 102 in the Y dimension, scale your Y steps per mm down by a factor of 100/102. If you suspect differences in belt pitch, you can check that by counting teeth on the belt against a ruler.

When calibrating a machine I prefer to fine tune final steps per mm of all axes and the extruder by trial and error. I use a vernier caliper to measure the travel and scale the steps per mm accordingly.

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My Prusa Mendel i2 inspired Repstrap with welded steel frame: [youtu.be]
And my Smartrap derived Briefcase 3D printer: [youtu.be]

Yes I measured the steps per mm and calibrated each axis precisely but when it prints say a 60x60x15 mm cube with a .4 mm wall it's still not printing the dimension exact so I am still a bit lost I think it's an alignment thing but it's getting hard to measure things to check for perfect alignment.

OK so i measured a length of belt that i had left over from the build. Over a 300mm length without any stretch there is about a 1mm difference as i assume that GT2 belt is supposed to be 2mm tooth to tooth i didn't think the belt was supposed to stretch as its a timing belt etc. I'm using no name brand but im wondering if i need to go to a gates belt or equivalent as that's not good enough for me.

If the belt error is consistent, you would be able to compensate for it. It sounds like it is not consistent i.e. the pitch varies on a length of belt. This could explain you getting ovals even though your X and Y axes appear to calibrate accurately. This would be the case if the circle you are printing is not being printed in the region where you calibrated your axes - and your belt pitches are different between the two regions.

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My Prusa Mendel i2 inspired Repstrap with welded steel frame: [youtu.be]
And my Smartrap derived Briefcase 3D printer: [youtu.be]

Yes the pitch change is not linear I have ordered new belts hopefully this will fix the issue. I measured my friends belts on his printer and they are exactly 2mm pitch and mine clearly Aren't. I will post back once I have replaced them.

Timing belts shouldn't stretch or vary in pitch. Make sure the belts you get are fibreglass-reinforced.

The ones I have on it now are fiberglass reinforced but there is a clear defect in them. I have ordered Gates belts for it.

So i got the new belts and i must say what a difference it now prints very well. So if there are issues with printing circles check the belt pitch.