How to measure a broken part so the copy fits
What to measure, with what, and which photos to send so a printed spare fits first time. With the measuring mistakes that ruin most parts.
Most printed spares that do not fit did not fail in printing: they failed in measuring. Half an hour well spent with a caliper saves a week and a second print.
What to measure with
A digital caliper costs little and is the difference between a part that goes in and one that does not. A ruler is fine for the overall outline, but not for anything that has to fit: by eye, half a millimetre is invisible and is enough to stop a shaft entering. If a ruler is all you have, measure the big dimensions with it and tell us clearly which figures are estimates.
What to measure, in order of importance
- 1Whatever mates with something else. Shaft and hole diameters, centre-to-centre distance between screws, slot width, tab thickness. These are the dimensions that decide whether the part works.
- 2Centre-to-centre distances, always centre to centre and never edge to edge. Adding up edges accumulates error and is the classic reason four screws never line up at once.
- 3The overall envelope: length, width, height. They cross-check everything else.
- 4Wall thicknesses. They define the strength and are the first thing lost if you copy only the outline.
- 5Angles and tapers, if any. A wrongly assumed angle shows up far more on assembly than you would expect.
The four most repeated mistakes
- Measuring a worn hole. A hole that has been rubbing for years is bigger than it was. Copy that figure and the screw rattles. If you know which screw it takes, the right figure is the screw’s nominal size.
- Measuring a heat-deformed part. A part that has warped next to a motor no longer holds its original dimensions.
- Forgetting the orientation. A part that looks symmetrical in the photo but only goes in one way round. Mark on the photos which way is up and which way it faces.
- Not saying what it is for. Knowing the part holds a pane of glass, or rotates, or vibrates, changes the material, the thickness and the clearances. It is more useful than half a dozen dimensions.
The photos that help
- On a plain, light background in soft light. A black part on a dark table is invisible.
- With a ruler or caliper in the same plane as the part, to give scale.
- All six faces, straight on and not foreshortened. A diagonal shot distorts every proportion.
- One photo of the part fitted in place, or of the gap it goes into. It is the one that most often answers what the dimensions cannot.
- A hand sketch with the dimensions written on it. It does not need to be pretty: it needs to say which figure belongs to what.
Can I bring the part to the workshop instead of measuring it?
It is by far the best option, and it is why having the workshop in central Barcelona is useful. With the part in hand it gets measured with calipers, the orientation is obvious and wear is spotted. If you cannot come, photos with scale plus a sketch are the reasonable substitute.
Should I add clearance when copying a part?
Do not add it yourself: tell us what mates with what and give us the real dimensions. The right clearance depends on the type of fit — press, sliding, rotating — and on the technology it is printed with. If you add clearance and we add clearance too, the part ends up loose.
What if the part is so broken that pieces are missing?
It can be reconstructed if what remains gives enough information, and often the cavity the part sits in says more than the part does. Send photos of the housing and of whatever it contacts: the missing geometry can usually be inferred from what surrounds it.