There are four common ways to add female threads to a 3D-printed part:
Print them: This is only practical for larger threads and, depending on the print geometry, the support material may need to be removed with a tap if it’s not water soluble.
Tap the printed hole: This is only practical for small thread pitches unless you’re printing with a 100% fill which significantly increases cost, print time and weight. The issue is that most slicers only allow you to specify a global value for the following parameters: number of floor/roof layers, number of wall layers, and fill density percentage. Thee default settings for my printer are four layers for floors and ceilings, two layers for the walls (they can have different values) and 37% fill density and once you’re past the wall/floor/ceiling layers, you’re into the honeycomb. There might a slicer that enables you to increase the amount of structure around specific internal features (i.e., a hole), but I’m not aware of it.
Embed a nut: Depending on the geometry, printing may need to paused to insert the nut.
Install a threaded heat-set insert: Depending on the geometry, printing may need to paused to install the insert.
I haven’t found a situation on the car where printing the threads was feasible, so I will compare the other methods vs. heat-set inserts.
Threaded Heat-Set Insert vs. Tapped Hole
Pros:
Up to 3x stronger
Unlimited assembly/disassembly
Eliminates creep
Stronger resistance to torque
Cons:
Higher cost
Requires more space
More weight
Higher potential to damage part during installation
Several years ago had found an excellent article that published the results of strength testing performed on heat-set threaded inserts vs. tapped holes, but the link is now defunct. That article was the source of the above strength and torque claims.
Threaded Heat-Set Insert vs. Embedded Nut
Pros:
Requires less space
Lighter
Cons:
Less strong
Less resistant to torque
Potential to damage part during installation
So far none of my parts have been structural, so the lower strength and torque resistance aren’t an issue.
InstallatiNG THREADED HEAT-SET INSERTS
The inserts are available from a variety of places and McMaster has a good selection here. A tapered hole is required to accommodate the insert (the McMaster inserts have 8° taper). The first couple of times I sketched a tapered profile, created a reference-geometry axis and performed a revolved cut of the profile about the axis. I subsequently realized that I could do an extruded cut and specify a Draft — duh, that’s a lot easier, especially since a single extruded cut operation can be applied to an unlimited number of holes.
The inserts are installed by heating them with a soldering iron while gently applying pressure. Although a standard soldering iron tip can be used in some circumstances, I recommend using an installation tip which offers the following advantages:
The shoulder on the installation tip sits flat on the top of the insert which makes it much easier to install the insert straight (i.e., orthogonal to the part).
It indexes the insert which prevents the soldering iron from slipping off of the insert and melting the plastic on the soldering iron and marring the part’s surface.
It heats the insert more evenly which speeds up the process. Specifically, a standard soldering iron tip will only transfer heat via a small contact area at the top of the insert and it will take time for the tip to get hot enough to melt the plastic.
Soldering iron tips; standard (front), #4 (middle) and #10 rear (heat discolors the brass)
The installation tips cost ~$18 at McMaster, but I’m sure that they can be obtained for less elsewhere. Keep in mind that ruined parts can sum to that amount pretty quickly and you don’t need to use the optimal tip because a smaller than ideal version will work better than a standard tip. For example, a #10 installation tip would work fine for a 1/4” insert.
Like the holes, the inserts are tapered so you want to ensure that you orient them properly. Yeah, that’s obvious, but the taper is visually indeterminable and I’ve ruined a couple of parts by not paying attention. The knurling it typically located at the top of the taper, but if you’re not sure you can just mic both ends.
The installation/heating process seems to make it difficult to wind a screw into the insert post installation and some plastic may have been pushed forward into the hole and or into the insert. I found that running a tap through the insert from the soldering iron side solved those issues.
EXAMPLE PARTS
Tube separator for two -8 transaxle oil lines and a -4 air line for the shift servo; two 10-24 threaded heat-set inserts were used on the bottom side
End cap for the side-impact bars. It is tight enough to remain in place without a screw, but I added a heat-set insert to ensure that it wouldn’t move if bumped.