Why is the hole smaller than the insert?

Heat-set inserts grip by melting the surrounding plastic so it flows into their knurls. The boss hole is deliberately a little smaller than the insert's knurl diameter, so as you press the hot insert in, displaced plastic locks around it for strong pull-out resistance.

How deep should the boss hole be?

Make the hole roughly the insert length plus about half a millimetre, so the insert seats flush or just below the surface and the screw can fully engage without bottoming out on the print.

How much wall do I need around the insert?

Leave enough plastic so the boss doesn't split when the insert is pressed in or when the screw is torqued. A boss outer diameter of roughly two to three times the hole diameter is a safe starting point, more for thin or brittle parts.

What temperature should the soldering iron be?

Set it to the plastic's melt range: around 200 °C for PLA, 230 °C for PETG and 240 °C for ABS. Too cold and the insert won't sink; too hot and you'll char the plastic or push it out of the boss. Press slowly and keep the insert square.

Why does Nylon need a larger hole?

Nylon is tough and flows less freely when heated, so a slightly larger hole gives the insert room to seat without cracking the boss. Increase the hole a touch and apply steady pressure while it's hot.

Heat-Set Insert Hole Size Calculator

Heat-set inserts give 3D-printed parts strong, reusable metal threads — far better than screwing directly into plastic. The key to a solid insert is the boss hole: too big and it spins, too small and the boss cracks. This tool gives the right hole for your insert and material.

How it works

A heat-set insert grips by melting the plastic around it. When you press a hot insert into a slightly undersized hole, the displaced molten plastic flows into the insert’s external knurls and freezes there, locking it in place.

The rule is simple: the boss hole is a little smaller than the insert’s knurl (major) outer diameter. How much smaller depends on the plastic:

PLA flows freely when hot, so it takes the smallest hole.
PETG and ABS flow a little less, so they get marginally more clearance.
Nylon is tough and resists flow, so it needs the most room to avoid cracking the boss.

The tool also gives the hole depth (insert length plus a small margin), the recommended boss wall so the surrounding plastic doesn’t split, and a small top chamfer to help the insert start square.

Tips and notes

Press inserts in with a soldering iron tip matched to the insert thread; let the part cool fully before loading the screw.
Keep the insert square to the hole — a tilted insert sits proud and won’t seat the screw properly.
For load-bearing or pull-out-critical parts, increase boss wall thickness and use inserts with more aggressive knurling.
If an insert spins after cooling, the hole was too large — reprint with the recommended diameter rather than relying on glue.