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Inner deminsion error

Posted by 999alfred 
Inner deminsion error
October 24, 2017 02:43PM
I have found something that I am trying to figure out. Using a 0.3mm nozzle. I have my printer calibrated so that printing the 20mm cube I am only +- 0.02mm. I think that is pretty good.
But when I am printing the dimensions on an inside opening, like a bearing hole, is smaller then the given dimension. I printed a test piece, a square frame with the outside being 30mm and the inside being 20mm, The outside lengths are fine. But the inside opening is 19.59mm. That is -0.41mm off.
Is this normal? Do you have to allow for this in your original drawing? Which then throws off dimensions if the drawing is used for CNC or any other non 3d printing steps.

Re: Inner deminsion error
October 25, 2017 03:56AM
Yes this is an artifact of the printing technology, as the filament is deposited being stretched, those forces tend to bring it toward the inside of "holes".

See the excellent blog post from nophead on that subject for more details : [hydraraptor.blogspot.fr]

You have multiple workarounds :

- Adapt your 3d models by trials and measurements.
- For somewhat small circular holes, you can use the "polyholes" method described in the blog post above. I use that drawing as a reference for this : [reprap.org]
- Or you can post process your prints : circular holes can be drilled, other types filed ,etc.
- There exist some software settings/adjustments in the different slicers which could help mitigate the problem. Cannot advice you on that topic as I have no personal experience about it, but I believe it tends to create problems in other parts of the prints.

Edit : by the way, as you are using english, unless you specifically want replys from local people you should post that general question in a more appropriate section of the forums to get a wider audience. For instance in "printing" section : [forums.reprap.org]

Edited 1 time(s). Last edit at 10/25/2017 04:00AM by DeuxVis.

Most of my technical comments should be correct, but is THIS one ?
Anyway, as a rule of thumb, always double check what people write.
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