Round circle calibration

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Round circle calibration

Postby nismobg » Sun Dec 09, 2012 11:31 pm

Ok here is advanced question about calibration , my Z is +/-0.01mm my X,Y are +/-0.01 on square objects . Now if i print for example a 5cm cube with 2cm hole the cube x,y,z dimension are in the 0.01mm tolerance but the hole radius is off by roughly .5mm. I tried different hole sizes and cube sizes but the holes are always off by about .5 mm. Any ideas ? I'm using Slic3r with Rep Host and SolidWorks for cad . Can it be Slic3r problem ?
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Re: Round circle calibration

Postby cvoinescu » Sun Dec 09, 2012 11:42 pm

Two possibilities I can think of: the nozzle squishes the filament as it deposits it; the half of the filament on the inside of the curve gets distributed over a shorter distance, so it spreads out more. That's why the holes end up smaller than expected. The effect is greater on smaller holes and tighter radii. The Stretch operation in Skeinforge deals with exactly this effect (although I can't tell you from experience how effective it is). The nozzle can also drag the freshly extruded filament slightly closer to the center of the circle as it turns, making this effect even more pronounced.

The other possibility is that the X and Y aren't exactly perpendicular to each other. Dimensions along the axes will be correct, but one diagonal will be expanded slightly and one contracted slightly. If you happen to measure your circle along the "short" diagonal, it may appear too small.

Edit: Both of these assume the hole is round. I thought of one more thing that would affect any shape of hole. You got the outer surfaces calibrated perfectly for a given size of the cube. Could you have arrived at that by having a perimeter slightly too wide but compensating by scaling everything down just a little (e.g. slightly fewer steps-per-mm than required)? The tell-tale symptom is if a square 30mm on a side prints out 30.0mm, but a square 60mm on a side prints out, say, 59.7mm. Then a 20mm hole would print about 19.5mm across.
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Re: Round circle calibration

Postby nismobg » Mon Dec 10, 2012 12:20 am

Thanks for the reply I am a little confused by the 3rd solution however if the x-y are not 90 degree that it would obviously show on any cube prints but the all have perfect corners . The layer height I printed those is .2mm and the nozzle is .35 with .5mm infill.
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Re: Round circle calibration

Postby nismobg » Mon Dec 10, 2012 1:39 am

Ok it has to be slic3r , just printer 3 different circles , OD-19/12/9mm and the ID had to be 15/8/5mm but its 14.5/11.5/8.5 so the outer diameter is dead on and the inner is -.5mm , it just have to be the software.
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Re: Round circle calibration

Postby nismobg » Mon Dec 10, 2012 2:43 am

I have found the problem for anyone who faces it http://reprap.org/wiki/ArcCompensation.
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Re: Round circle calibration

Postby cvoinescu » Mon Dec 10, 2012 11:58 am

Told you so! (Well, I also told you two other things, but at least the first one was the correct guess.) :)
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Re: Round circle calibration

Postby orcinus » Mon Dec 10, 2012 2:25 pm

Try using Slic3r 0.7.2 for dimensionally critical stuff.
It also handles hole diameters much better.
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Re: Round circle calibration

Postby nismobg » Mon Dec 10, 2012 9:28 pm

Well I have fixed the issue and have dead on hole diameter and cube dimension by lowering the extr. mult. to .7 after fine calibrating the extruder. That's with slic3r 0.9.7, I have tried 0.7.2b and it produces the same results and 0.9.7 after the calibration and the .7 . Hope that helps a lot of people having the same issues. BTW the feed rate for QU-BD if you don't use the machined channel on the raptor gear but the other part is 106.707 in Marlin.
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