Heat-Set Inserts and Magnets in 3D Prints: Hole Sizes and Cost

When to use one

Wherever a screw will come out again: a battery door, a lid that opens for refills, a mount that gets moved. In CNC Kitchen’s M3 tests, an insert held until the screw head sheared, at 3 to 4 Nm; a screw driven straight into the plastic stripped at about 1 Nm.

The hole

Straight-sided, a little smaller than the knurl so the brass melts its way in, and about a millimetre deeper than the insert so the displaced plastic has room. For our M3 that is Ø4.0 mm, 6.7 mm deep, for a 5.7 mm insert.

The wall

Leave plastic all round: 2.0 mm for M3, 2.5 for M4, 3.0 for M5. A thinner wall bulges or splits as the insert goes in.

Short, long, flanged

Short inserts (3.0 mm for M3) suit lids and thin walls; long ones (8.0 mm) take more pull. Flanged inserts cannot sink crooked, press-fit ones go in cold, and blind ones seal the far side.

Imperial sizes are in the chart too, from #4-40 to 1/4"-20, the thread on a camera tripod.

Plastic-thread screws

BT screws are cut for plastic and need only a pilot hole: Ø2.4 mm for a BT3, with at least 3 mm of boss around it. They are the cheapest way to close something.

Strong enough, once

Straight out, a screw in plastic holds nearly as well as an insert: 142 kg against 181 in the same test. It is the turning that strips the thread, so they suit parts closed once or a few times.

They can ship loose

A buyer can drive a screw with a screwdriver, so a box closed with plastic screws can ship with the screws in a bag and no fitting charge. Inserts and magnets cannot.

The pocket

Press-fit pockets are 0.1 mm wider and 0.2 mm deeper than the magnet: Ø6.1 × 2.2 mm for a 6 × 2 disc. A glue pocket is 0.3 mm wider and holds a drop of adhesive.

Pull

From about 0.13 kg for a 3 × 1 mm disc to about 8.5 kg for a 25 × 3. One larger magnet can often do the work of several small ones, and it is one piece to fit.

Heat

N52 magnets are rated to 80°C. That is fine indoors, but a part on a summer dashboard runs close to it.

Poles

Two magnets that must attract need opposite faces out. Mark one face of every magnet before fitting, or a lid repels its box.

Keep them in

The CPSC’s magnet rule covers products made for entertainment, jewellery, mental stimulation or stress relief with magnets that can come loose. Fix magnets so they cannot come out, and keep loose magnets out of anything for children.

For the heat a car puts on a part, see car accessories, and for magnets in a drawer system, Gridfinity bins.

Count the pieces

Fitting is charged per piece, so two screws where two will do, one magnet where one holds, and plastic screws where a lid closes only once.

Model the hole as given

Draw each hole at the catalogue’s size. Every insert, magnet and screw we stock lists the hole or pocket it needs, and the reference box in the panel uses exactly those.

Pick hardware when you create the product

Add the parts by quantity, tick “Assemble this product”, and add written steps or photos if the order matters. Unticked, the pieces ship loose in the box.

For how a part is checked before it ships, see what we check before a part ships, and for designing the rest of the product, designing 3D prints that sell. For pins, bearings, sheets and bolts, measure them and draw the hole from the same catalogue.