Printing Replacement Parts

A replacement part is harder than a part you design from scratch, because the shape is fixed by something else: the shaft it slides on, the holes it bolts to, the slot it clips into. The steps are the same as A First Functional Project, with more care at the measuring and material stages.

1. Decide whether to print it

Some parts should not be printed at all. Do not print a replacement for anything whose failure could injure someone or cause a fire: brakes, child seats, climbing or lifting gear, pressure parts, anything in contact with mains wiring or a heater, or parts on the printer's own heating system. Contact the original maker for those. Also check whether the original maker sells the spare; for a part that carries load in a machine, the original is usually the safer choice.

Good candidates are low-stakes: knobs, caps, battery covers, cable clips, drawer slides, feet, brackets and organizers.

2. Measure the original

• Measure every surface that touches something else: shaft diameters, hole positions, slot widths, clip thicknesses. Calipers make this far easier.
• Measure the mating part as well as the broken one. A worn or broken original may no longer show the true size, but the shaft or slot it fits usually does.
• Photograph the part from several sides next to a ruler, and sketch it with the dimensions written on.
• For a part in pieces, tape or hold the pieces together to measure overall length.

Non-critical surfaces, such as the outside of a knob, can be approximate. Spend your care on the fits.

3. Model it with clearance

Prusa warns that parts modeled with zero tolerance usually will not fit together and that the right tolerance depends on the model's size, orientation, the interlocking shape, calibration, settings and material shrinkage. Prusa states its own printers are accurate to at least 0.2 mm Maker spec, and suggests at least 0.3 mm of clearance for moving parts such as hinges as a starting point Starting point. Your printer and material may need more or less, which is why the fit test below matters. Fits and Tolerances and Dimensional Accuracy cover measuring your own printer's error.

If the original part was injection molded, it may have thin walls and sharp internal corners that suit molding but not printing. You are free to thicken walls and add fillets on internal corners where they do not affect the fit. Keep walls to whole perimeters: Prusa notes one perimeter from a 0.4 mm nozzle is about 0.45 mm wide Rule of thumb.

4. Choose the material for the conditions

The material has to survive where the part lives. Prusa's general material guides give these working limits. They are rules of thumb, and individual brands and blends differ, so check the technical data sheet for the filament you buy:

Material	Prusa's general material guides Rule of thumb	Good for
PLA	Softens and deforms above about 60 °C; brittle on impact; degrades under UV	Indoor, room-temperature, low-wear parts
PETG	Suited to most exterior use below about 80 °C; tough and somewhat flexible	Clamps, holders, mechanical parts indoors and out
ASA	Temperature resistance up to about 93 °C; high UV resistance; needs ventilation and works best enclosed	Outdoor and technical parts

A part in a parked car, near an engine, beside a heater or in the sun should not be PLA. A clip that has to flex should not be brittle PLA either; PETG's toughness suits it better. Prusa notes ASA releases potentially dangerous fumes while printing, so read FDM Emissions and Ventilation first.

5. Orient for strength

Prusa explains that FDM parts are weaker between layers than along them, so orient the part so the load does not pull the layers apart. A snap clip should be printed so its flexing arm lies flat in the layers, not standing up. A shaft or pin that will be bent should lie down, not stand up. If one orientation cannot suit every feature, consider splitting the part and joining it (see Gluing and Joining). Strength and Orientation goes deeper.

For strength, add perimeters before infill. Prusa says a model's strength is mostly set by the number of perimeters, not the infill.

6. Fit test, then the real part

1. Print just the fitting section: the bore of a knob, the tab of a clip, the hole pattern of a bracket as a thin plate.
2. Try it on the real mating part. Note whether it is too tight, too loose or right.
3. Adjust the clearance in the model and reprint the test until the fit is right.
4. Print the full part with the same material, orientation and settings as the final test.
5. Install it and check it under normal use before relying on it.

If the part will need threads or inserts, read Threads and Snap Fits and Heat-Set Inserts before modeling.

7. Keep the file

Save the model with the fit clearance and material in its name or a note. Parts that broke once tend to break again, and a saved file turns the next repair into a reprint.