Problem with hole sizes

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Problem with hole sizes

Postby tmccafferty » Mon Apr 08, 2013 1:36 am

I am having problems on my hole sizes coming up small. It doesn't seem to be a linear thing based on the size of the hole. A 6mm and a 16mm come up about .4mm small. Also, I am accurate on outside dimensions of parts. I am using a .35mm nozzle. It's almost like I am laying an extra perimeter inside the hole, but that must just be a coincidence. It's not a problem compensating for this on parts I model, but when I down load someone elses .stl file, I'm in trouble.

Any ideas.
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Re: Problem with hole sizes

Postby tmccafferty » Mon Apr 08, 2013 2:08 am

Hmmm...

I just did another test. I printed a "washer" with 24mm OD and 16mm ID and 3mm thick.
Printing slow (30), PLA, temp at 190C, .35mm nozzle, fill .25

Test 1: 2 perimeters
ID = 15.65
OD = 24.03

Test 2: 3 perimeters
ID = 15.35
OD = 24.03

Test 2: 4 perimeters
ID = 15.30
OD = 24.03

Any ideas?
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Re: Problem with hole sizes

Postby fma » Mon Apr 08, 2013 6:18 am

This is because PLA/ABS retracts when cooling down, and holes tend to shrink. Slic3R does not compensate that.
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Re: Problem with hole sizes

Postby BenJackson » Mon Apr 08, 2013 6:28 am

Another factor is that STL doesn't have circles, it only has flat triangles (and thus in a plane, polygons). If you inscribe any polygon in a circle (as parametric CAD programs do at STL export, and polygonal modellers like Sketchup do when you draw your circle) you will see that it produces a smaller opening than the circle itself. The fewer sides (and parameters for this vary between CAD programs) the worse it is.

For whatever reason my experience is that holes are almost always about 0.5mm smaller than drawn once you print. Bottom layers are often worse because they're "smushed" but I fix that with a tapered hand reamer (best 3D print cleanup tool ever).
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Re: Problem with hole sizes

Postby Digitalmagic » Mon Apr 08, 2013 7:24 am

Take in account that a 0.35 nozzle produces a 0.41mm spaghetti.
Entering real nozzle diam in your slicer is like telling your slicer your spaghetti is 0.35mm.

You could try a higher value in nozzle diam, between nozzle geometry and measured spaghetti diam.
Human has 20 nail ... extruders.
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Re: Problem with hole sizes

Postby flurin » Mon Apr 08, 2013 12:32 pm

I had the same problem at the beginning. I reduced the Extrusion multiplier (slic3r) as much as possible, now the holes are nearly perfect.
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Re: Problem with hole sizes

Postby Digitalmagic » Mon Apr 08, 2013 12:41 pm

Always the same goal: Indicating the real filament diameter (by averaging, using a caliper), and the real spaghetti diameter.
In order to compute the real flow, and extrude the right quantity.

Now, for the small hole/perimeter, the spaghetti width is rather managed by nozzle dia field than by extrusion factor (flow control).
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Re: Problem with hole sizes

Postby SystemsGuy » Mon Apr 08, 2013 3:27 pm

? Can you elaborate on that?

Digitalmagic wrote:Now, for the small hole/perimeter, the spaghetti width is rather managed by nozzle dia field than by extrusion factor (flow control).
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Re: Problem with hole sizes

Postby tmccafferty » Mon Apr 08, 2013 3:45 pm

I did the washer test again...
I changed the nozzle size parameter of my .35mm nozzle to .4 ----- 16mm hole still ~15.5mm
I changed the extrusion factor from 1 to .9 ------- 16mm hole still ~15.5mm but the top surface finish degraded.

It seems like if it was totally due to shrinkage, it would vary based on hole size (i.e. a percentage). However, all holes are off about .5mm

Again, it's not a big deal if I model the part as it's easy to compensate. However, when I print something like flurin's Z support bracket, the parts don't fit together and there is little I can do with the files.
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Re: Problem with hole sizes

Postby flurin » Mon Apr 08, 2013 4:52 pm

If you have the same parameters (PLA, 1.75mm, 0.35mm ...), you can try with my gcode-files.
Attachments
Z-SupportBracker.zip
(786.67 KiB) Downloaded 661 times
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