How to Add Threads to a Part
You want a screw to hold your part together — but should the part have a threaded hole, or a plain hole the screw passes through? Getting this right is the difference between a part that assembles cleanly and one where the screw spins uselessly. Here is how threads work, how to read a callout like M3 or 1/4-20, and how to add the right hole to your model fast.
Tapped hole vs clearance hole
Almost every screw joint is one of two things, and you need to know which before you add a hole:
Clearance hole (the screw passes through)
Slightly larger than the screw so it slides through freely and bites into something on the far side — another threaded part or a nut. No threads in this part.
Tapped hole (the screw threads in)
Cut to a smaller diameter so the screw's threads grip the walls. This is the “threaded hole.” In metal you tap it after drilling; in plastic a thread-forming screw can cut its own.
The simplest rule
If the head of the screw should pull two parts together, the near part gets a clearance hole and the far part gets the threads (or a nut). You almost never want threads in both.
Reading a thread callout
A thread callout is just a size plus a pitch. Two systems cover nearly everything you will meet:
- Metric — M3, M4, M5: the number is the screw's outside diameter in millimetres. An M3 is a 3 mm screw.
- Imperial — 1/4-20: the first number is the diameter (1/4 inch), the second is threads per inch (20).
For a clearance hole you drill a little over the screw diameter (about 3.4 mm for an M3). For a tapped hole you drill the smaller tap-drill size (about 2.5 mm for an M3) so there is material for the threads to form in. Our bolt hole size chart lists the clearance and tap-drill sizes for M3, M4, and M5 so you do not have to memorize them.
Threading a 3D-printed part
Printed threads are possible but coarse. For anything that will be assembled more than a few times, two approaches are far more reliable than modeling fine thread geometry:
- Thread-forming screws into a plain hole. Size a small pilot hole and let a self-tapping or thread-forming screw cut its own threads in the plastic.
- Heat-set inserts. Model a hole sized for a brass insert, press it in with a soldering iron, and you get clean metal machine threads in a printed part — the strongest option.
Either way you are really just adding a correctly-sized hole, not modeling a helix. And remember printed holes shrink slightly — the same tolerance rules apply.
Add a threaded hole in seconds
You do not have to look up tap-drill tables by hand. Describe the hole to PartWork.ai — “an M3 tapped hole” or “a 1/4-20 clearance hole” — and get it placed on your model, sized correctly, ready to print or machine. See creating parts for how holes and features are added, and exporting when you are ready for a file.
Start with 2 free AI generations — no card required
Open the studio, describe the threaded hole you need, and export the part. More credits: 100 for $4.99 (~5¢ each).