Most custom jobs that go wrong go wrong at the measuring stage, and almost always for the same reason: one dimension was assumed rather than measured. Fifteen minutes of care here saves a reprint.

What to measure

Overall length, overall width, overall height, the diameter of any hole, and the distance between the centres of any two holes. That last one is the one people skip and it is the one that matters most for anything that bolts to something else.

Use the right tool, or improvise well

Digital calipers are twenty dollars and worth owning if you do this more than once. Without them, a steel ruler is fine for anything over about 20 mm. For holes, the reliable trick is to find a drill bit or a round object that fits snugly and measure that instead.

Photograph it properly

Lay the object on a plain, contrasting surface with a ruler beside it – not underneath, beside, with the markings visible. Take one shot from directly above and one from the side, both square on rather than at an angle. Perspective distorts, and we cannot measure from a photo taken at 30 degrees.

Tell us what it does

This matters as much as the dimensions. A bracket that holds a light fitting and a bracket that holds a shelf may look identical and need completely different wall thicknesses. Tell us what it attaches to, what it carries, and whether anything about it moves.

Note what failed

If you are replacing something broken, tell us where it broke. A part that snapped at the same point twice has a design weakness worth fixing rather than reproducing. This is genuinely one of the advantages of printing a replacement – we can make the new one stronger than the original.

Tolerances

If a part needs to slide into something, it needs clearance – typically a few tenths of a millimetre. Tell us which surfaces need to fit and which are free, and we will build the allowance in. Guessing at this is the second most common cause of a part that is technically correct and practically useless.

Got it all? Send it over. Our guide to commissioning a custom part covers what happens next.