Fit variation, probably quantization
 
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rickwise
(@rickwise)
Active Member
Fit variation, probably quantization

I'm doing a series of test prints to try to fit to a Lego 2x2 block.  The goal isn't important, though.

I did a print with 4 samples with 4 different inside dimensions, 12.5, 12.7, 12.9, and 13.1, and found that it's between 12.7 and 12.9.

So I printed 4 more samples with 4 dimensions, 12.75, 12.8, 12.85, and 12.9.  I was surprised to find that the 12.9 fit was different between the two groups of samples.

Finally, I printed a set of 4 12.8 dimension parts, and they had different fits.  The obvious conclusion is that it depends on the X, Y position.  I assume this is caused by the quantization that happens in slicing.

I did another test, and found that it seems to be associated with the relative position compared to the X, Y of the start of the print, not the absolute X, Y of the printer.

So (finally) my question.

If I want to print a plate of 3 objects that all have the same internal fit, do I need to set their positions to be a multiple of the extrusion fit (I'm using the default 0.45mm)?  Is there some other way to make the objects consistent?

Posted : 08/09/2022 5:52 pm
Diem
 Diem
(@diem)
Illustrious Member

The new slicer has features designed to address this sort of issue.  It would be worth trying it before making any specific changes.

Cheerio,

Posted : 08/09/2022 6:36 pm
rickwise
(@rickwise)
Active Member
Topic starter answered:
RE: Fit variation, probably quantization

 

Posted by: @diem

The new slicer has features designed to address this sort of issue.  It would be worth trying it before making any specific changes.

Cheerio,

I'll that, thanks!

Posted : 08/09/2022 6:40 pm
rickwise
(@rickwise)
Active Member
Topic starter answered:
RE: Fit variation, probably quantization

I upgraded to 2.5.0, it still behaves the same.  When I print a series of objects with inside width of 12.95 in the STL, what gets printed varies from 12.83 to 12.88 mm.

I can correct if it's a constant factor, but I don't know how to correct when the printed dimension varies by where it ends up on the print surface.

Posted : 08/09/2022 8:01 pm
Diem
 Diem
(@diem)
Illustrious Member

So the error is approximately a quarter of the default nozzle diameter and varies by one eighth.

If you are 'measuring' the stl by the design diameter be aware that almost all design software creates a cylinder by approximating a polygon with its vertices on the designed circumference and slicer will take this as a centreline position for the extrusion.

You are not going to get much closer.  Do these parts have to be snap-fitted repeatedly?  They are likely to wear in use...

I would be inclined to calibrate out about half of the absolute error and wear the parts to fit by repeatedly fitting and removing them from a typical lego block.

Cheerio,

 

Posted : 08/09/2022 11:50 pm
rickwise
(@rickwise)
Active Member
Topic starter answered:
RE: Fit variation, probably quantization

 

Posted by: @diem

So the error is approximately a quarter of the default nozzle diameter and varies by one eighth.

If you are 'measuring' the stl by the design diameter be aware that almost all design software creates a cylinder by approximating a polygon with its vertices on the designed circumference and slicer will take this as a centreline position for the extrusion.

You are not going to get much closer.  Do these parts have to be snap-fitted repeatedly?  They are likely to wear in use...

I would be inclined to calibrate out about half of the absolute error and wear the parts to fit by repeatedly fitting and removing them from a typical lego block.

Cheerio,

 

 

Thanks, Diem!  That will be my plan B.  I'm doing some experimenting to see if I can add a couple of minimal objects to force consistent location on the print plate, then ensure the objects are at multiples of the nozzle diameter.

Regards;

Rick

Posted : 09/09/2022 2:21 pm
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