Bridge with circular hole
I have a design which needs a few pockets to receive square M3 nuts. These have to lie flat during the print, with a hole for the M3 screw above them. (The orientation is determined by the larger structure I want to print, an electronics enclosure.) That means that a bridging layer needs to be printed over the square pocket, but with a circular hole in the middle of the bridging area.
With the default settings, the first thing PrusaSlicer wants to print is a perimeter around the circular aperture -- in mid-air above the pocket; that doesn't work too well... I have found the experimental setting "Quality > Extra perimeters on overhangs", and that's a step in the right direction, but does not quite get me where I need to be:
Referring to the figure below, it first wants to print all the outer blue rings. But then it skips the second-to-last ring and, again, tries to print the inner perimeter in mid-air (innermost blue ring as shown). Only afterwards will the gap be closed by another ring.
Is there a setting which makes the slicer fill the bridging layer from the outside in, all the way to the hole perimeter? I am aware that the results will still not be perfect with 100% overhang. (May try to improve on this with a dome structure in the design itself, but have limited height to work with). But it would certainly be an improvement over printing in mid-air...
Best Answer by Scott:
You can design your holes in a way each layer can be a bridge:
Unsupported Holes
Recommended Value: Extrude 1 = layer height, Extrude 2 = layer height x 2
Without special design considerations, holes through unsupported bridges will not print successfully. This technique can be used to print such holes without adding supports to your model.
1. When designing your part, extrude two rectangles tangent to your hole
2. The first extrude should be as thick as the layer height you intend to print with
3. The second extrude should be twice the layer height you intend to print with
This technique can be a little tricky to visualize until you have tried it a few times.
RE: Bridge with circular hole
You can design your holes in a way each layer can be a bridge:
Unsupported Holes
Recommended Value: Extrude 1 = layer height, Extrude 2 = layer height x 2
Without special design considerations, holes through unsupported bridges will not print successfully. This technique can be used to print such holes without adding supports to your model.
1. When designing your part, extrude two rectangles tangent to your hole
2. The first extrude should be as thick as the layer height you intend to print with
3. The second extrude should be twice the layer height you intend to print with
This technique can be a little tricky to visualize until you have tried it a few times.
RE:
If you didn't design it, you can try adding support...
Or, you can add a modifier (right click "add a shape), make it really small (1 layer height) and place it to make the hole plain just for 1 layer.
Doing this it will print the hole as a bridge, and then print the hole normally.
After the print finish, you just have to break that layer with a screw or anything.
Video example for my previous message:
RE: Bridge with circular hole
Thank you, Scott! The two layers of straight bridges are probably the way to go to obtain stable bridging without adding excessive height.
It's my own design, so I can implement this. I have just shied away from letting the layer structure "bleed through" into the design typically -- trying to maintain a level of abstraction and assuming that a given design might end up being sliced in different ways, layer heights etc. But as with any design (be it mechanics, electronics, or software), "proper" abstraction only gets you so far, and there comes a time when you need to tweak the details one level down...
I will try this, thanks!
RE: Bridge with circular hole
Personally I often just make a sacrificial layer (make 1 plain layer just before the hole) I have to break afterward. It's easy, doesn't demand any time during designing, and works very well.
RE: Bridge with circular hole
A while ago I wrote an OpenSCAD module to make a bolt that implements this 'bridging helper' approach. I find it useful to just use this module for any bolt hole (subtract the bolt from the solid), regardless of it's orientation, because the bridging helpers do no harm in the inverted orientation. I made a similar one for hexagonal nut recesses too.
Here's my code for the bolt:
module bolt(head_height=2.9,screw_length=10,head_diam=5.5,screw_diam=3.3){
cylinder(d=head_diam,h=head_height);
cylinder(d=screw_diam,h=screw_length);
intersection(){
cylinder(d=head_diam,h=head_height+layer_ht);
translate([-head_diam/2,-screw_diam/2,0]) cube([head_diam,screw_diam,head_height+layer_ht]);
}
translate([-screw_diam/2,-screw_diam/2,0]) cube([screw_diam,screw_diam,head_height+2*layer_ht]);
}
And here's an image of the result with those defaults:
I made the layer height a (global) parameter, but I seem to remember that the 0.2mm model works ok with a 0.3mm slicer setting, but the other way around was less successful. Or have I got that backwards.......? Anyway, I mostly print everything at 0.2mm so that's the setting I typically use.