Build a box with a lid that fits

Build a box with a lid that fits — video walkthrough

A shelled tray and a plug lid — and the clearance that makes the difference.

This is the project that teaches the thing most first prints get wrong. On screen, a 56 mm plug in a 56 mm hole looks perfect. Printed, it will not go in — an FDM printer lays plastic slightly wide of where the model says, so two nominally identical parts interfere. The fix is Clearance: a negative value shrinks a solid on every face, and −0.25 mm turns a plug that jams into one that slides. The measurements chain together, which is worth seeing once. The box is 60 mm outside with a 2 mm wall, so its cavity is 60 − 2 × 2 = **56**. That is where the plug's 56 comes from, and why the clearance is applied to the plug rather than the box. One detail that looks like a mistake and is not: the plug is **7 mm** tall but only needs to sit under a lid plate seated at Z 6, so it overlaps by a millimetre. Clearance shrinks the *top* face too, so a plug that merely touched the plate would be pulled 0.25 mm clear of it and the Union would join nothing. The finished lid measures 9.75 mm tall, and that number is the proof the clearance took. If your printer runs tight or loose, this whole model re-fits by changing one value. Open Help → Build a box with a lid that fits in the editor to build it hands-on, step by step.

  1. Add a Box 60 × 60 × 4 at Position Z 6 — the flat top of the lid.
  2. Add a Box 56 × 56 × 7 for the plug: 56 mm is exactly the size of the hole it must drop into.
  3. Apply Clearance of about −0.25 to the plug, so it comes out a quarter-millimetre smaller on every face.
  4. Select both and Union them into one lid.
  5. Add a Box 60 × 60 × 40 beside it, Hollow it to a 2 mm wall, and open the Top.
  6. Run the print check, then export both parts.