Print too small, steps/mm wrong or something else?

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MTVDNA
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Print too small, steps/mm wrong or something else?

Post by MTVDNA » August 18th, 2016, 5:54 pm

Hey guys,

Today I designed and printed a lid for the washing pole hole, the old one had pretty much turned to dust.

Original:
Image
My design
Image

I measured the old one using calipers and modeled it according to those dimensions. In fusion 360 everything looked fine, and the dimensions were the same in cura (I checked). When the print was done, I noticed it didn't fit as snugly as the old one. When I measured them next to eachother, I noticed the one I printed was about 1mm smaller (the diameter of the bottom part).

I had printed parts before to a specific size, e.g. stuff to fit bearings in, and that always turned out fine. So I made a calibration object to check if the steps/mm were set correctly. I measured a bunch of distances in both the x, y and z directions, but I'm not quite sure how to interpret them. For example:

Code: Select all

axis          modeled (mm)          measured (mm)
X              100.0                   99.75
                20.0                   19.80
Y              100.0                   99.80
                20.0                   19.70
The print seems to be a bit smaller than intended, but the error doesn't seem to increase for bigger distances. This suggests that it's not related to the steps/mm setting, but rather some systematic error. I did measure 5, 10 and 15mm as well, but the results were similar. For the smaller distances the error was a bit smaller, unfortunately I can only measure up to 1/20th of a mm with my (analog) calipers.

On the Z-axis the print was consistently ~0.1mm too tall, which I don't think is much of an issue.

So although the size is not exactly right, the error is not very big (at least I think, do you guys get similar results?) This does not explain why my print turned out too small. Both prints were made with the same filament (MakerPoint Ultramarine PLA), printed at the same temperature, and approximately the same print settings.

Any ideas on what I could've done wrong, and how I can improve my prints?

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Amedee
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Re: Print too small, steps/mm wrong or something else?

Post by Amedee » August 19th, 2016, 2:49 am

0.2mm error is not that bad for 3D printing given the mechanical parts on one side and the material on the other (plastic shrinking)...

From your table it seems that the error is more or less constant regardless of the size of the model, so I would exclude bad settings (like step/mm). I would look at the printer itself: ensure there is no play. In particular the belts tension, but also the linear bearings.
Phil.

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MTVDNA
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Re: Print too small, steps/mm wrong or something else?

Post by MTVDNA » August 20th, 2016, 1:05 pm

I agree, .2 mm accuracy isn't bad at all. I'll check my next prints to see how they turn out. I've looked at the belts before and they didn't feel loose, but to be honest I don't really know how tight they should be. I'll give them a closer look.

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