I’m designing a 3D print that will have something attached with self-tapping screws but after doing a bit of research it seems that the best way to determine what size hole you need is through trial and error.
My design required me to use 9.5mm long #1 or #2 size screws which have a metric diameter of 1.85mm and 2.18mm respectively. To that end I decided to created a test piece with holes of varying sizes to see what would work best for me.
I created a test piece with x20 screw holes ranging from 2.15mm to 1.2mm in diameter in 0.05mm increments. I was intending on using an extrusion based PLA print with 100% infill and 100 micron layer thickness for my actual design so I created my test piece using those same settings. To account for variations during printing I made three test pieces.
I thought that using step increments of 0.05mm would prove too fine a resolution for the printer but this doesn’t appear to be the case. You can see changes in diameter across all the holes.
Starting with the larger #2 screw I managed to screw them completely into the 2.15mm and 2.1mm holes. After this is screws did not fully seat. This happened in the 2.05mm, 2.00mm and 1.95mm holes, with increasing effect, after which I stopped.
Looking at the photo you can see on the 1.95mm hole a significant burr has formed from the material pushed out of the hole when driving the screw into the print. This plastic deformation, which gets worse as the holes become smaller, prevents the screw from fully seating.
I then started inserting the smaller #1 screws into the test piece starting with the 1.8mm hole and ran into the same problem from 1.70mm.
With both sets of screws you could definitely notice the increasing torque required to seat them as the holes got smaller. With the smaller #1 screw there the were certain times when I thought I would cam the head of the screw as it was proving that difficult to drive them down.
I did play around with seeing if the #1 screw would fit in the large holes and found that even in a 2.00mm hole the 1.85mm diameter #1 screw was secure, although I did feel that if I drove the screw too hard it would tear out the threads. I figured that even though the hole was meant to be 2.00mm in diameter the way the heated filament squeezes out and forms ridges as each layer is applied means that the screw was able to get some purchase. You can hear these ridges when you run your nail across the layers of a 3D print.
I got the same results with the other two remaining test pieces which gave me confidence in the the process.
After all of this I decided to settle on using the #1 screws in a 1.8mm sized hole and felt confident that I would get strong enough fastening for my needs.
Although not a formal way of determining the size of a hole required for a self-tapping fastener it did successfully achieve the same objective and I would recommend this process if you are looking to attach items with self-tapping screws to a 3D print.