3D Printing Calculators

3D design & tolerances

Heat-Set Insert Hole Sizing

A soldering iron and a brass insert only work if the printed hole is slightly under the knurl. Pick the thread, enter insert length, and get hole diameter, depth and a boss that will not split.

Inputs

Units
mm

The brass body, not the thread length in a mating screw.

Results

Pilot hole diameter
4.20mm
Hole depth
6.50mm

Insert length plus 0.8 mm clearance.

Minimum boss OD
8.0mm
Insert outer diameter
4.60mm

Recommended gear

Calculation method

Each ISO metric size maps to a typical tapered heat-set insert: outer knurl diameter, recommended printed hole (a few tenths under the knurl) and a minimum boss so the wall does not split. Depth is the insert length plus 0.8 mm so melted plastic has somewhere to go and the iron does not punch through a thin floor. Lengths are millimetres; the unit toggle shows inches.

Exact formula

Hole Ø and boss OD come from a taper-insert table (M3 hole 4.2 mm, OD 4.6 mm, boss 8.0 mm). Hole depth = insert length + 0.8 mm. Limits: Generic taper-insert table, not a named brand. Check the packet; oversized holes spin, undersized holes bulge PLA. Iron temperature is not modelled.

Installation direction

Put the taper (smaller OD) into the hole first if the insert has one. Flush the large face with the part so a screw head sits flat.

Frequently asked questions

Soldering iron temperature?

Start around 180–200 °C in PLA, hotter in PETG/ABS. If plastic chars or the boss slumps, you are too hot or too slow.

Can I tap the plastic instead?

You can, but heat-set brass survives reuse. This tool is only for inserts.

Why is the hole smaller than the insert?

The knurl needs plastic to reflow into. A line-to-line hole spins under torque.

M3 in a 0.4 mm nozzle wall?

The 8 mm boss wants several perimeters. Check the wall-thickness multiplier.

More in this topic

All tools in this topic