Updated 19 October 2026

Heat-set insert hole calculator for FDM parts

A heat-set insert is a threaded brass bush pressed into a printed hole with a hot tip. The plastic around it melts, flows into the knurls and sets, which gives a printed part a thread that survives repeated assembly. The insert maker's datasheet gives the hole the insert needs. A printed hole does not come out at the size it was modelled, so the model has to be corrected by what a test print loses.

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Size the insert hole

How it is calculated

c (diameter) = modelled test hole - measured test hole diameter = datasheet diameter + c (diameter) c (depth) = modelled test depth - measured test depth depth = datasheet depth + c (depth)

• The datasheet diameter is the hole the insert needs in the finished part. The correction adds back whatever the test print lost, so the printed hole comes out at that size.
• Leave both test depth fields empty and the depth correction is zero: the depth to model is the datasheet depth.
• No insert sizes are built in. They differ between makers and between insert types, so take them from the datasheet of the insert you will use.

Printing for heat-set inserts

• Print the test hole in the same material, layer height and orientation as the real part, at the datasheet diameter or close to it: the correction changes with the diameter.
• Give the wall around the insert enough perimeters for the molten plastic to grip. A wall that is mostly infill lets the insert pull out or spin.
• A blind hole needs room below the insert for the plastic it displaces. Where the datasheet gives a minimum depth, model at least that.
• Set the insert along the axis of the hole, with the tip at the temperature the insert maker gives for the material, and let it cool before loading the thread.
• For a fit to a pin rather than an insert, use the FDM hole compensation calculator.

3Descu publishes no tolerance table, because none has been measured on its own parts. Where a fit is critical, mark it on the order. The design rules explain why.

From the numbers to a part

A model with its insert holes corrected is ready to price. Upload the model and the instant quote returns a price and a delivery date in under a minute, with no account. 3Descu is an ISO 9001:2015 certified factory with 200+ printers, FDM and SLA only. A part that does not match the agreed specification is reprinted or refunded.

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