Heat-set inserts, self-tapping screws or captive nuts: hole sizes and bosses

A printed part needs a thread wherever you want to open and close it: a battery lid, a board held on standoffs, a bracket on a wall. There are three common ways, and the right one depends mostly on how often the screw comes out. This lesson compares them, takes hole sizes from manufacturer tables, and works out a boss for an M3 insert.

Numbers assume FDM with a 0.4 mm nozzle. Holes always print a little small; see Print to the right size for hole compensation.

The three options side by side

	Heat-set insert	Screw into plastic	Captive nut
What it is	Brass sleeve melted into a hole	Screw cuts its own thread in the plastic	Standard nut held in a hex pocket
Best for	Lids opened often	Parts assembled once or twice	High clamping force, cheap hardware
Extra tool	Soldering iron, ideally an insert tip	None	None
Wears out?	Brass thread lasts	Thread loosens each time it is re-cut	Steel thread lasts
Design cost	Boss with enough wall	Boss with enough wall	Pocket, often needs a bridge above it

Heat-set inserts: hole sizes from the table

The hole for an insert is set by the insert maker, not by the thread size. Here is one maker’s table (CNC Kitchen inserts, as listed by its retailer). Other makers use different sizes, so always check the table for the insert you bought.

Insert	Length	Hole diameter	Minimum wall
M2	3.0 mm	3.2 mm	1.3 mm
M3 standard	5.7 mm	4.0 mm	1.6 mm
M3 short	3.0 mm	4.0 mm	1.6 mm
M4	8.1 mm	5.6 mm	2.1 mm
M5	9.5 mm	6.4 mm	2.6 mm

Design rules from CNC Kitchen:

• Blind holes about 1 mm deeper than the insert, so displaced plastic has somewhere to go. Through holes need not be longer than the insert.
• Straight, not tapered, and normally no chamfer at the edge.
• The hole should be just big enough for the installation tip to slide in. Printed holes come out small, so print a test plate with a few diameters and pick the one that works.

Wall thickness is the part people skip. Too thin and the boss splits or bulges when the brass goes in.

Worked example: an M3 boss for a PETG lid

The lid is opened often, so we use an M3 standard insert (5.7 mm long). The part is printed in PETG at 240 °C.

1. Hole: 4.0 mm from the table. Print a test; if the tip will not enter, try 4.1 mm.
2. Depth: 5.7 + 1 = 6.7 mm, round up to 7 mm.
3. Wall: at least 1.6 mm. With a 0.4 mm nozzle, perimeters are about 0.45 mm each, so 1.6 mm is 3.6 lines. Round up to 4 perimeters, about 1.8 mm, so the wall is solid plastic with no gap.
4. Boss outside diameter: 4.0 + 2 × 1.8 = 7.6 mm, call it 8 mm.
5. Iron: 10–20 °C above the print temperature, so 250–260 °C.

In the slicer, set enough perimeters that the boss is all wall, no infill. Infill around an insert gives the brass nothing to grip.

Installing an insert

CNC Kitchen’s procedure:

1. Set the iron 10–20 °C above the print temperature. Their examples: about 225 °C for PLA, 245 °C for PETG, 265 °C for ABS.
2. Put the insert on the tip (or on the hole) and let it sink in with light pressure. Do not force it; the heat does the work.
3. Stop at about 90% of the depth, pull the iron away, and press the last part flush with a flat tool so it sits square.
4. Hold it for a few seconds. A freshly melted insert tends to creep back out.

Too hot is worse than slightly cool: overheated plastic degrades and the insert sinks too deep or tilts.

Screws straight into plastic

Screw makers for plastic parts publish boss rules for moulded parts. Screwerk’s table for its plastic screws gives, as multiples of the screw diameter d:

Plastic	Hole	Boss outside	Thread engagement
ABS	0.86 × d	2.0 × d	2.0 × d
PET	0.81 × d	1.85 × d	1.7 × d
PC	0.89 × d	2.5 × d	2.2 × d

Their general rule is a hole of about 0.8 × d. For a 3 mm screw for plastics: hole about 2.4–2.6 mm, boss about 6 mm, at least 6 mm of thread. This is moulded-plastic data; a printed hole comes out smaller, so print a test boss before committing. Designing a board case uses a 2.5–2.8 mm hole for an M3 machine screw driven into plastic, a similar starting point.

Use this for parts you assemble once. Each removal shaves the thread a little.

Captive nuts

A hex pocket holds a standard nut so it cannot turn. ISO 4032 puts an M3 nut at 5.5 mm across the flats at most. The OpenFlexure microscope project, after testing, uses a 5.7 mm trap: 0.2 mm of clearance on the flats while the corners still stop the nut from turning. Make the pocket about 0.2–0.3 mm deeper than your nut is thick (measure it; M3 nuts are around 2.4 mm).

Two layouts:

• Slot from the side. The nut slides in from the edge. Easy to print, and the nut cannot fall out once the screw is in.
• Pocket from below. The screw hole above the pocket starts in mid-air, so the printer has to bridge. Use a sacrificial layer or crossed bridges, covered in Design for no supports.

Common mistakes

• Using the thread size as the hole size. An M3 insert needs about 4 mm, not 3 mm.
• Thin bosses or infill around the insert. The boss cracks or the insert spins.
• Iron far too hot, pushing hard. The insert goes in crooked and too deep.
• Blind hole exactly as long as the insert. Displaced plastic has nowhere to go and the insert stands proud.
• Screwing straight into plastic on a lid opened weekly. It strips within weeks.

Safety

The iron and the brass are at 200–270 °C. Use tweezers or a tool to hold the insert, never fingers, and set the iron back in its stand between inserts. Melting plastic gives off fumes; work with ventilation, especially for ABS/ASA.

Sources

• Tips & tricks for heat-set inserts used in 3D printing — CNC Kitchen
• CNC Kitchen threaded inserts, size table — 3DJake
• Screws for plastics, boss design table — Screwerk
• Basing M3 nut trap sizes off ISO 4032 — OpenFlexure Microscope (GitLab)