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Overhang threads

Posted: Tue Aug 22, 2017 5:02 pm
by Douglas74
Hello. I'm trying to print this Q-tip dispenser for my wife, and I'm getting this overhang stringy issues.

Any thoughts on how to combat it?


Re: Overhang threads

Posted: Tue Aug 22, 2017 5:12 pm
by ednisley
Douglas74 wrote:how to combat it
First rule of consumer-grade 3D printing: you cannot print over thin air.

Cut the box in half where the overhang meets the sides, flip the top part over, print both with no overhangs, then glue them together.

Or cut the top front panel out, print it flat on the platform, then glue it in place.

In either case, putting alignment features inside the corners will simplify the glue job and improve adhesion, particularly if you're using epoxy with PLA.

Easier than futzing around with interior support structures, fer shure!

Re: Overhang threads

Posted: Tue Aug 22, 2017 5:14 pm
by Douglas74
Thanks for the quick response.

It should be printing at a slope. I thought it would be fine. I'm not great yet at playing around in software to edit and slice and so on.


Re: Overhang threads

Posted: Tue Aug 22, 2017 10:25 pm
by Gwhite
As was mentioned, printing over air is tricky. It's called "bridging", and it can be done, but only for short distances, and even then, only if everything is right.

If you are using Simplify3D to do your slicing (highly recommended, but costs ~ $150), there are settings for bridging that you can play with. From the looks of it, one problem you may be having is one I ran into. It prints the perimeter OK, but when it goes to do the fill, it's zig-zagging, and the ends aren't adhering to the perimeter. The zig-zags "unravel" downward & make a birds nest. In Simplify 3D, there is a setting so you can tell it to do fills in bridging regions in a specific linear direction. That way the fill strands can be aligned with the perimeters, and you have a much better chance that they won't make a mess. You may still get some uneven fill, but it won't trash your print nearly as badly.

Re: Overhang threads

Posted: Wed Aug 23, 2017 4:37 pm
by ednisley
Douglas74 wrote:It should be printing at a slope.
That works only for angles up to maybe 60 degrees from vertical; those edges look like they might be over 80 degrees.

The outermost perimeter thread in each layer shouldn't overhang the next-lower layer by more than (about) half the thread width, which works out to 50-ish degrees with the typical 0.4 mm width and 0.25 mm thickness.

At 80 degrees, the overhang is more than the thread width: it's extruding into thin air.

Your mileage will vary, but overhangs get ratty starting around 60 degrees.