What hole diameter does an M3 heat-set insert need?
An M3 heat-set insert with a 4.6 mm outer diameter needs a 4.0 mm hole. Ruthex and CNC Kitchen both publish 4.0 mm for their standard and short M3 brass inserts. A blind hole should be at least 1 mm deeper than the insert. The 5.0 mm Voron M3 insert needs a 4.4 mm hole.
The number
| Manufacturer | Insert part | Outer diameter | Recommended hole diameter | Recommended hole depth |
|---|---|---|---|---|
| Ruthex | RX-M3x5.7 (5.7 mm) | 4.6 mm | 4.0 mm | L + 1 mm min (blind) |
| Ruthex | M3 Short (4.0 mm) | 4.6 mm | 4.0 mm | L + 1 mm min (blind) |
| Ruthex | RX-M3x5x4 Voron (4.0 mm) | 5.0 mm | 4.4 mm | L + 1 mm min (blind) |
| CNC Kitchen | M3 x 5.7 Standard | 4.6 mm | 4.0 mm | L + 1 mm min (blind) |
| CNC Kitchen | M3 x 3 Short | 4.6 mm | 4.0 mm | L + 1 mm min (blind) |
| CNC Kitchen | M3 x 5 x 4 Voron | 5.0 mm | 4.4 mm | L + 1 mm min (blind) |
Figures from the Ruthex and CNC Kitchen size tables, 15 September 2026.
Why
The hole is smaller than the brass insert so the knurls bite. A soldering iron or insert press melts the plastic, and the knurls drive it into the grooves. That displaced plastic is what resists pull-out.
The hole also needs room below the insert, since melting pushes a slug of plastic ahead of the tip. Both makers draw a blind hole at least 1 mm deeper than the insert. CNC Kitchen adds: straight, not tapered, no chamfer.
Printed holes come out undersized. In CNC Kitchen’s tests a 4.0 mm hole had to be modeled at 4.1 mm. Shrinkage varies with material, printer and orientation, so print a test part. A burr on the back of the boss means the hole was too small.
How thick should the boss wall be around an insert?
Keep at least 1.6 mm of plastic around a 4.6 mm M3 insert. Ruthex and CNC Kitchen both give that minimum wall in their size tables for standard and short M3 inserts. For the 5.0 mm Voron M3 insert, Ruthex lists 1.3 mm and CNC Kitchen 1.6 mm.
Why
The knurl drives plastic outward. That puts the wall around the hole in hoop stress, the tension a barrel hoop carries. A thin wall bulges while soft, then splits along a layer line as it cools or as a screw is torqued. The datasheet minimum is the wall that carries that load.
How to check it on your own print
- Print a test boss at the datasheet hole and wall, in the real part’s material and orientation.
- Install with a soldering iron 10 to 20 °C above printing temperature: about 225 °C for PLA (polylactic acid), 245 °C for PETG (glycol-modified PET) and 265 °C for ABS (acrylonitrile butadiene styrene), per CNC Kitchen.
- Inspect for bulging, whitening or a crack along a layer line, and a burr on the back.
- Thread in a screw and pull hard by hand. A sound boss holds and the insert stays flush.
Sources
- ruthex M3 thread insert RX-M3x5.7 product page and size table (accessed September 15, 2026)
- CNC Kitchen Heat Set Insert M3 Standard product page and metric dimensions table (accessed September 15, 2026)
- Tips & Tricks for Heat-Set Inserts used in 3D printing, CNC Kitchen (accessed September 15, 2026)
- Threaded Inserts for 3D Prints - Cheap VS Expensive, CNC Kitchen (accessed September 15, 2026)