External dimensions OK, internal too small.

Hi,

I am new to 3D printing, my DIY CoreXY machine is funcional since last weekend. I did make a fairly decent 3DBenchy already so I am happy, but there is an issue I need help with. I printed some rings yesterday, 20mm external diameter, 12mm internal diameter, 10mm high. The products are nice and round, external diameters are fairly good, varying between 19.9 and 20.0mm. The height is OK, too. But the internal diameter is 11.5 to 11.6mm. That is way too far of for me. Now I don't want to correct the drawing, I hope I can change it in settings. I have a RAMPS 1.4 set-up running Marlin, slice with Cura 2.3.1 32bit, as I don't have a 64bit computer. I hope I can teach Cura to make the holes bigger. I have a very constant wall thickness of 0.5mm, but I haven't found a spot in Cura to tell it this.

Any help is appreciated,
Best regards,

Hugo

That is normal behavior for FDM printing. You fix it by making the holes bigger in the CAD model. Typically, 0.2-0.5 mm larger is required to ensure the desired print dimension. It is partially a function of your extruder's calibration (over extrusion tends to make the holes smaller). You will have to experiment to determine the best tweak to the hole sizes. When printing small holes for screws, etc., you can always run a drill through to get an exact size after printing.

The size reduction occurs for a couple reasons. The STL file is a polygonal approximation of the round hole and tends to result in holes being smaller that the CAD drawing. Also when extruding plastic inside a hole, the viscosity of the plastic coming out of the nozzle pulls at the just laid down plastic a mm or two behind the nozzle, which tends to pull it inward toward the center of the hole.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

I read in an article that the size reduction happens from printing internal perimeters first. So as the filament is being laid down in sequence its being pushed towards the inside of the holes there by reducing your hole size. Or is this the same phenomenon your talking about Digital Dentist?

Probably some of that, too.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

I always thought, it was the nozzle squishing the plastic down, that caused the inner diameter dilemma?
It squishes in the direction where no other perimenters block the filament.

Hmmmm, OK, thanks for the input. So there is no setting in Cura to cure this. I'll make a table of hole-sizes required in the CAD file for the desired holes and work with that. I could with smaller sizes run a drill through, but on this 12mm hole drilling out 0.5mm is drilling out half the inner layer, I don't like that.

Cheers,

Hugo

You can use modifiers in slic3r to beef up the plastic around the holes and allow drilling without weakening the print. See: slic3r.org/blog/modifier-meshes

Edited 1 time(s). Last edit at 09/29/2017 08:34AM by the_digital_dentist.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]