Heat-set inserts can make repeated assembly more practical, but “M3” names the thread, not the outside geometry or the required hole. Choose the insert first, then design its seat from that manufacturer's drawing.

Start with the exact insert

Record the part number, outside geometry, length, intended plastic and installation method. Heat-set products may specify straight or tapered holes. SPIROL's heat/ultrasonic insert guidance ties performance to suitable hole size, surrounding wall and plastic filling the retention features.

Do not apply one table of “typical M3 holes” to every brass insert. An outside-diameter measurement alone does not replace the manufacturer's recommended hole geometry.

Design and print a small test piece

Use the specified hole, boss and depth as your baseline. Print a coupon in the same filament, orientation and wall structure as the intended part. Measure the resulting hole rather than assuming the CAD dimension survived printing unchanged.

Label the coupon with the insert and filament identities. If you test dimensional variants, keep each change explicit so the successful result can be repeated.

Control the installation

Use equipment and a tip intended for the chosen installation method. Follow the insert supplier's temperature, force and dwell guidance, together with the polymer and equipment limits. There is no universal PLA, PETG or ABS soldering-iron setting in this guide.

Hold the work securely and guide the insert squarely. Let the specified process soften the plastic; do not compensate for a wrong hole by forcing the insert. Allow the joint to cool before loading or checking the thread.

Keep the tip clean using its approved method. Do not spray an unspecified PTFE lubricant or other compound onto a hot installation tip. A lubricant's compatibility and temperature rating cannot be inferred from its name.

Inspect the joint, then test the application

Check alignment, seating, cracks, bulging and thread access. Use the intended screw and assembly method. If retention matters, define a repeatable torque or pull-out test suited to the application; tugging with pliers does not establish a load rating.

Keep the accepted dimensions and process with the part drawing. A successful cosmetic installation is the start of verification, not proof that a printed boss can carry any requested clamping load.