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Non round circles, undersized prints

Posted by ncq 
Re: Non round circles, undersized prints
June 12, 2014 10:34AM
If friction were a problem, I'd expect inconsistencies between layers. The prints look fine aside from the dimensions.

I still think it's something to do with belts or pulleys. You mentioned that you checked the pulleys. Do you have flats for the set screws on the motor shafts? Are you sure they're tight? Printed pulleys (esp. PLA) can warp around the set screw with time and motor heat, and cause backlash as the set screw loses contact with the shaft. This causes backlash, and will cause circles to come out lemon-shaped. Your example of a Prusa i2 vertex shows this a bit in the threaded rod hole.

It's possible that your attempts to compensate with steps/mm adjustments is causing the scaling issue, and the real problem is backlash. Backlash is hard to see on parts with edges not orthogonal to the axes (like the cal cube & L-piece).

If possible I recommend getting quality pulleys and more appropriate belts. A GT2 belt and pulley set from tridprinting.com will run you about $20 shipped.
Re: Non round circles, undersized prints
June 12, 2014 03:10PM
@MrDoctorDIV - I will try what you advised

@pokey9000 - I have flats for the set screws on the motor shafts and they are tight. I have ABS pulleys. Things you mentioned make a sense, but I think it really could not be a backslash.

I will be on a competition for next week, so I will do experiments next Saturday. I will let you know.

For now, thank you guys!
Re: Non round circles, undersized prints
July 01, 2014 05:47AM
Hi, I am sorry for late reply, but I didn't have a time for printing.

I have done something you have advised - removed friction between belts and washers (I added a second bearing so now belts couldn't slip off). I also set steps per mm to the same value for X and Y axis.

Unfortunately problems didn't disappear. I added a pen on my printer for easier analyse - see the attached file. I think that X dimension is OK, but there is a strange behaviour on the Y axis - as you can see in the picture.

I still don't understand what's happening there.
open | download - tuzka2.gcode (9.8 KB)
open | download - IMG_20140701_0001.jpg (356.9 KB)
Re: Non round circles, undersized prints
July 01, 2014 08:23AM
I'm at a loss. I honestly don't know what else to try or where to even start finding answers.

Realizer- One who realizes dreams by making them a reality either by possibility or by completion. Also creating or renewing hopes of dreams.
"keep in mind, even the best printer can not print with the best filament if the user is the problem." -Ohmarinus
Re: Non round circles, undersized prints
July 06, 2014 11:47PM
Judging by your pen plot, I'd say that the Y axis has quite a bit of backlash. This could be friction related, especially since you are using microstepping, so the available torque is not constant. Try lowering your acceleration values for Y to see if that makes any difference.

Mike Anton
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