A Tiny OpenSCAD Test Coupon Can Save Your Heat-Set Inserts

This post was created with AI assistance. Please review the code and technical details before publication.

Heat-set inserts are an excellent way to add reusable metal threads to 3D-printed enclosures, sensor brackets, and robot parts.

The difficult part is often not the insert. It is the hole.

A hole that measures 4.2 mm in CAD may print smaller or less circular because of extrusion width, shrinkage, filament behavior, layer settings, and printer calibration. Using one “recommended” diameter for every printer can produce loose inserts, cracked bosses, or a brass insert that slowly leans sideways during installation.

Instead of testing the final enclosure, print a small calibration coupon first.

A Simple OpenSCAD Coupon

The following model creates four holes with slightly different diameters:

hole_diameters = [4.0, 4.1, 4.2, 4.3];
spacing = 12;
coupon_height = 6;

difference() {
cube([
len(hole_diameters) * spacing,
12,
coupon_height
]);

for (i = [0 : len(hole_diameters) - 1]) {
    translate([
        i * spacing + 6,
        6,
        -1
    ])
    cylinder(
        h = coupon_height + 2,
        d = hole_diameters[i],
        $fn = 48
    );
}

}

Change the diameter values to match the insert you are testing.

Print It Under Real Conditions

The coupon should use the same:

Filament
Layer height
Wall count
Print orientation
Nozzle
Temperature settings

After printing, install one insert in each hole and compare:

How much pressure is required?
Does the plastic flow around the knurl?
Does the insert remain straight?
Does the boss crack or bulge?
Does the insert rotate when the screw is tightened?

The best hole is not necessarily the tightest one. Excessive interference can damage the surrounding plastic, while insufficient interference may allow the insert to rotate or pull out.

Hole diameter is only one part of the design. Boss diameter, wall thickness, installation depth, plastic type, load direction, and print orientation also affect the final joint. This design guide for threaded inserts in plastic provides a broader checklist for evaluating those variables.

A ten-minute calibration print is much cheaper than discovering the wrong hole size after a six-hour enclosure print.

Small coupon, fewer surprises.