3d printing design: countersunk screws without supports

We design lots of kits with screws. One of the great parts of our service is that we provide the exact screws you need, so you're not shopping for tons you don't. Since that's one of the big benefits, we design to include that instead of doing lots of snap fit pieces. We think the screws make the kits feel more solid and easily repairable. 

Since we're doing lots of stuff with screws, we have to design screw holes in our stuff! Lets talk a bit about how we design the half that the screw passes through. I'm talking mainly about the part that a screw must pass completely through, not the part that we are screwing into. 

Basic hole

as the heading implies, this one is basic. its just a hole. You can make a hole slightly larger than your screw shank. This totally works, but then you have the downside of the screw head poking out proud of the surface. This is especially egregious when you're using socket head cap screws like we do. The heads are pretty tall and can catch on things easily. 

Counter sunk

Counter sinking the head is a very nice feature that we nearly always add. It is pretty easy to do, you create your hole for the shank, then you create a larger pocket that the head of the screw can fit in. This spot is where the tips come into play. Countersunk holes often mean that you're going to have overhangs. Not only are there going to be overhangs, they're circular which means that your printer can't bridge from one side to another, making the overhangs even messier. 

There are 2 ways we've used to make our screw holes be countersunk without needing supports and resulting in a very nice clean hole. 

-the square trick

This trick requires that you reshape the inside of your countersink. By overlaying 2 rectangles at different heights, you provide your slicer with nice bridgeable sections. You'll find that your machine can bridge this without issue and you'll get nice clean screw holes without any drooping or poops. 

Start with a square hole to make things easier and add overlapping rectangles to what would be the overhang. These small spaces can be bridged so easily that you don't need supports. To make it even easier on your printer, you could do square holes for the countersink too, but I left mine round for this example. 

this one is hard to visualize, so I'm adding a few angles here. Above, is looking up into the hole that would otherwise need supports. Below, is looking at it from the side, with section view turned on, so you can see it like it is cut in half. 

Personally, I don't like this method that much. Its a bit more work to create and I think it is ugly. Who is going to see it or care? I don't know. Maybe it is illogical, maybe I'm just lazy, but I don't like it as much as the next solution. 

-An aggressive chamfer

Adding a chamfer that is so aggressive that there is no circular overhang left is the next best way. 

I start with a simple pocket and hole that meet my requirements. maybe I make the pocket a little deeper than I need, so that the next steps don't make it to shallow. 

I start adding chamfer at the hole itself and aim to take up half the circular overhang with it. After that, I add a chamfer to the outside corner of the overhang, consuming the rest of the overhang. You could probably just do this with a singular chamfer. Now that I think about it, I'm not sure why I do it in 2 parts. 

what you're left with is a big fat chamfer that your printer can do without supports very easily. The upsides are that this is super easy and it also guides your screw into place. The downside is that this takes up a lot of depth in the material. If your part is fairly thin, this method may not work at all. 

one last tip, holes on a vertical surface. 

if you need to have a nice round hole on a vertical surface (the side of your object), you may be frustrated to find that any circle needs support or it gets a little poop right at the top. There's a super easy fix for that. Just cut a notch at the top, where the angle is the most shallow. It doesn't have to be fancy. I chamfer mine but that's not even necessary, your printer can bridge that pretty easily when it isn't trying to make a smooth circular curve. 

Look at the picture above. On the left is a regular hole, on the right is a hole with a notch. 

now look at both printed on this test piece. You can see that both are functional but the one with the notch is cleaner. The one on the left isn't horrible, but it has an obvious flat spot on the top of the circle. If you have tight tolerances for parts fitting together, that notch will save you!