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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.

Re: Overhang threads

Posted: Mon Nov 30, 2020 4:32 pm
by Gwhite
This an old thread, but I've run into a print job where I'd like to push an overhang to it's limits. I want a small shallow angled section, with the best possible print quality. It's basically a ramp something needs to slide on, so avoiding supports & roughness is important. A couple questions for the experts:

1) Can going to a thinner layer help? I've always printed a 0.40 mm wide extrusion with a 0.35 mm nozzle & 0.2 mm layer height. I've played it safe & never tried to go to more than 45 degs before I use supports. If I print 0.4 mm wide at a 0.1mm layer height it seems to me that I should be able to maintain a 50% overlap out to ~ 60 degrees or so off vertical. That would help a lot. As an aside, are there other parameters I will need to tweak if I cut my layers in half?

2) The area where I need the overhang could be supported on both sides. In effect, the ramp could also be semi-bridging. I've never had much luck at getting smooth bridging, and I'm not sure it's even possible to avoid the usual "ropey" bottom surface you get even when it doesn't sag. The width of the ramp only needs to be ~ 3 mm wide. I'm guessing (hoping) that a semi-bridged ramp at ~ 75 or 80 degrees will work OK.

All comments & suggestions welcome. Thanks!

Re: Overhang threads

Posted: Mon Nov 30, 2020 6:59 pm
by ednisley
Gwhite wrote:
Mon Nov 30, 2020 4:32 pm
width of the ramp only needs to be ~ 3 mm
Because the threads run perpendicular to the slope of the ramp, you're not ever going to get a sliding-smooth surface, particularly if the matching object is also 3D printed with a matching thread orientation.

The central problem is building a slope by squeezing molten goo from a nozzle, with part of the opening in free air. The goo will preferentially squeeze from that side and, because it has nothing below, tend to form a nice round (half-ish) cylinder.

If you must have a smooth ramp, coat it with epoxy (or whatever) to fill the notches, sand to a flat surface, then recoat with epoxy / wax / whatever.

Futzing with layer heights may keep you occupied, but doesn't address the cause of the problem.

Re: Overhang threads

Posted: Mon Nov 30, 2020 7:59 pm
by Gwhite
Thanks! It doesn't have to be perfect, but for example, when I've done bridging in the past, I often get alternating extrusions. I increase the extrusion width over normal, but instead of sticking together straight sideways, they scoot up or down a little, and then the next pass goes the opposite direction. The result is something that has an unevenness at twice the spacing of a regular printed section.

I also discovered that as soon as it isn't perfectly horizontal, S3D doesn't figure it's bridging, and all the bridging options don't apply.

I think I figured out a dodge. The sliding piece is smooth and round. If I cut a vertical walled Vee, the bottom edge perimeter is nice and smooth. Instead of sliding on the rough surface of a flat ramp, it will slide up along the smooth edges of the Vee. It will take some fiddling to get the angle of the Vee to produce the desired "ramp" angle, and it will tend to wear, but if I go to nylon, I think it will be OK.