Heat-set inserts in 3D prints: hole sizes and how to install them

Short answer

A standard M3 heat-set insert goes into a 4.0 mm hole with at least 1.6 mm of plastic around it. Make a blind hole about 1 mm deeper than the insert. Heat the insert with a soldering iron set 10–20 °C above the print temperature of the plastic, melt it in to about 90 % of its length and press the rest flush with a flat tool.

Printed holes come out a little smaller than designed, so print a test hole before the real part.

What a heat-set insert is

A heat-set insert is a small brass sleeve with a metal thread inside and knurls on the outside. You heat it and push it into a hole in the print. The plastic around it melts, flows into the knurls and sets again as it cools. The result is a metal thread in a plastic part.

Why bother? A screw cut straight into plastic wears the hole out after a few turns. A metal thread can be opened and closed many times. That matters for lids, battery covers, printer parts and anything you take apart for cleaning or repair. Inserts also let you use normal machine screws instead of self-tapping screws.

Inserts are made for thermoplastics: PLA, PETG, ABS, ASA, nylon and similar. Resin prints don't melt, so there the inserts are glued in instead. CNC Kitchen suggests thick CA glue or two-part epoxy for that.

ruthex and CNC Kitchen sell inserts made for 3D printing. Their M3 insert is 5.7 mm long, and there are shorter M3 versions for thin parts. Both publish a dimension table with the hole size for every insert, and the M3 numbers in this guide come from those tables.

Hole size and depth

The maker's table gives three numbers that matter: the hole diameter, the minimum wall around the hole, and the insert length. These are the values for the standard ruthex and CNC Kitchen inserts:

• M2 (3–4 mm long, depending on the maker): 3.2 mm hole, at least 1.3 mm wall.
• M3 (5.7 mm long): 4.0 mm hole, at least 1.6 mm wall.
• M4 (8.1 mm long): 5.6 mm hole, at least 2.1 mm wall.
• M5 (9.5 mm long): 6.4 mm hole, at least 2.6 mm wall.
• Short versions use the same hole as the standard insert of their thread size. The "Voron" M3 insert (5 mm across, 4 mm long) needs a larger 4.4 mm hole.

Depth, walls and why printed holes differ

Depth: for a blind hole, CNC Kitchen advises making it about 1 mm deeper than the insert, so a 5.7 mm M3 insert wants a hole of about 6.7 mm. The extra room takes the plastic the insert pushes ahead of it. A through hole can be as long as the insert.

Shape: keep the hole straight. A tapered hole or a large chamfer is not needed.

Wall: the minimum wall is the plastic between the hole and the outside of the part. Thinner walls bulge or crack when the insert goes in. More walls in the slicer (3 or 4 instead of 2) give the insert solid plastic to grip instead of infill.

Why printed holes differ: a printer lays down plastic in round lines, and the inside of a small hole tends to come out smaller than the drawing. How much depends on the printer, the material and the slicer. If the hole is too small, a burr of plastic forms under the insert and the screw gets hard to turn at the end. If it is too large, the insert doesn't hold. Our guide on fit and tolerances explains where the difference comes from.

Find your real hole size with the tolerance test

The tolerance test generator prints a plate with a row of holes that grow in small steps. Each hole is the peg size plus twice the clearance printed next to it. Use it to find out which designed hole size gives a real 4.0 mm hole on your printer:

• Set Peg size to 4 mm and Shape to Round.
• Set Smallest clearance to 0 and Clearance step to 0.05. The holes are then 4.0, 4.1, 4.2 mm and so on.
• Set Plate thickness to 6 mm or more, so an M3 insert fits fully inside the plate.
• Print it in the material and orientation of the real part, with your normal settings.
• Melt a spare insert into two or three of the holes. Pick the smallest hole where the insert goes in straight and leaves no burr below it.
• Design the real part with that hole size. To measure the holes before you start, see measuring for a good fit.

Installing them step by step

• Pick the insert. For most small projects M3 is the common size, and a ruthex M3 packPartner link holds 100 of the standard 5.7 mm inserts. If you don't know yet which sizes your projects need, an assortment box with M2 to M5Partner link covers the four common sizes.
• Set the soldering iron 10–20 °C above the print temperature of the plastic. CNC Kitchen gives about 225 °C for PLA, 245 °C for PETG and 265 °C for ABS. Irons without a temperature setting often run too hot and can burn the plastic.
• Use a tip that fits into the insert. A pointed tip works, but a tip shaped for inserts holds the insert straight and heats it evenly.
• Set the insert on the hole, small end down. The narrow lead-in at the bottom centers it in the hole.
• Put the hot tip into the insert and let it sink in with light pressure. Don't force it. Let the heat do the work.
• Stop at about 90 % of the depth and lift the iron away.
• Press the last part flush with a flat, cold tool and hold it for a few seconds while the plastic sets. Inserts tend to creep back out if you let go too early.
• A CNC Kitchen insertion aid for M3 and M4Partner link is a flat aluminium tool made for this step: its base rests on the part so the insert ends up square to the surface.
• Let the part cool, then test with a screw. It should turn in by hand to the end of the thread.

Common mistakes

• Hole too small: the insert pushes a ring of plastic ahead of it, which ends up in the thread. Make the hole a bit larger.
• Hole too shallow: the plastic that the insert displaces has nowhere to go, and the insert stops short of flush.
• Iron too hot: the plastic burns or turns runny and the insert sinks too deep. Turn the temperature down.
• Melting the full length with the iron: this often leaves the insert crooked or sunk below the surface. Stop at about 90 % and press the rest.
• Letting go too soon: the insert comes back up while the plastic is still soft.
• Walls too thin: the part bulges or splits around the insert. Keep to the minimum wall from the table and add perimeters.
• Wrong insert for the hole: "Voron" style M3 inserts are wider than standard M3 inserts and need a 4.4 mm hole, not 4.0 mm.

Frequently asked questions

Make it

Tolerance Test

Find the right clearance for your printer

3D Printing

Sources

1. CNC Kitchen: tips and tricks for threaded inserts in 3D printing
2. CNC Kitchen Threaded Inserts Set, dimension table (3DJake product page)
3. ruthex M2/M3/M4/M5 Threaded Insert Assortment Box, dimension table (3DJake product page)
4. ruthex Threaded Insert M3, M3x5.7 (3DJake product page)
5. CNC Kitchen Insertion Aid for Threaded Inserts (3DJake product page)