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Joined: Mon May 09, 2016 1:55 pm

Problem with dimensions

Sat May 14, 2016 10:52 am

Hey everyone, hope someone got the knowledge to help me, because i'm getting pretty lost now
I've been trying to calibrate my printer for a while, but i still no luck.

I've made a square 20x20x10 (X Y Z) it contains a square hole/pocket in the middle, and the walls is designed to be 5mm thick.
Here is a drawing on how it's supposed to be.
Here is my problem:
The outer wall is supposed to be 20mm, but measures 19,75
The inner wall is measured to be 10mm measures 9,75
and the walls between is exactly 5.00mm, as it should be.
(the height is also precisely 10.00mm high.)

The problem here is exactly 0,25 differences from the model to the printed part.
I even tried a bigger model which was 60x60x20 in size, and the problem still persists, the difference is also 0,25mm here

I have calibrated my nozzle, the extruded width is set to 0,42 and is calibrated and measured to be 0,42 so this is all fine.
I've tried with different size models, and its not by a percentage, but a constant value of 0,25mm
I've thought it might be the PLA shrinking, but why is the middle wall and the height precise then?
I've tried printing with different temperatures, and i am now printing at 168c to minimize shrinking, but i still end up with the same result as if was printing at 210c
I've even tried to look for backlash, but everything is tight and smooth.

So if anyone got some ideas, or any clues - i would really appreciate the help
(i have uploaded the gcode and the fff profile aswell)

Thanks in advance
K8400 Vertex.fff
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Posts: 161
Joined: Fri Apr 29, 2016 5:43 am

Re: Problem with dimensions

Sat May 14, 2016 12:19 pm

I read through your gcode file. I will take a specific example for you from the file.

You set your extrusion with to 0.42mm, so you're telling the software that's how wide each extrusion is.

If you look at the movement coordinates, you can see that the XY values go from 90.210mm to 109.790mm. That is a difference of exactly 19.58mm. But of course you also have to account for the extrusion width, so when you add 0.42mm to that distance to account for the extrusion on each side, you have exactly 20.00mm. So the coordinates in the file are 100% correct.

So I would say you can rule out anything that the software is doing. However there are plenty of other factors like plastic shrinkage, filament quality and extrusion multipliers, firmware steps/mm values, mechanical tolerances and backlash, etc.

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Re: Problem with dimensions

Sun May 15, 2016 6:01 pm

plastic shrinkage as andrew pointed out is probably what your seeing. the shring for each plastic type is different. for dimensionally accurate parts most abs for instance will need to be scaled up in x and y by 0.8-1.0%. petg is somewhere around 0.4%. no idea where pla is. this also varies from one mfg to another. remember its only in the x and y

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