Design and print

A box has a wall on every side, so its inside is the outside minus two walls. To fit something inside, start from the thing: its size, plus a gap each side, plus a wall each side, gives the outside.

1. Inside width
2. Try: walls and inside › Thicker walls
3. Try: walls and inside › Inside 60
4. Worked example: a case for an Uno › From the part outwards
5. Worked example: a case for an Uno › Find the mistake
6. Vary: change one thing › The case
7. Vary: change one thing › Thicker walls
8. Practise: sizes › Inside
9. Practise: sizes › Outside
10. The smallest case

Drawn the same size, a printed peg won't go in its hole: printers make holes smaller and pegs bigger. Plan a clearance: about 0.2–0.4 mm for a sliding fit, less for a tight press fit. Every printer differs, so calibrate with a test card.

1. Same size
2. Try: make it slide
3. Worked example: design the hole › Work it out
4. Worked example: design the hole › Find the mistake
5. Vary: change one thing › Press fit
6. Vary: change one thing › A 6 mm peg
7. Practise: fits › Will it go in?
8. Practise: fits › Press fit
9. A lid peg

A printer builds in layers: an overhang more than about 45° from vertical needs supports, and a part is weakest across its layer lines. Choose the way up that needs no supports and runs the layers along the load.

1. Strongest way
2. Try: turn it round › Supports
3. Try: turn it round › Strongest
4. Worked example: two questions for every part › Overhang, then load
5. Worked example: two questions for every part › Find the mistake
6. Practise: which way?
7. Choose and justify › A phone stand
8. Choose and justify › My part

The slicer's settings trade time and plastic for finish and strength. Halving the layer height doubles the time. Infill past about 20–30% costs a lot for little strength; extra walls add more strength for less.

1. Thinner layers
2. Try: the settings › Layers
3. Try: the settings › Infill
4. Worked example: what buys strength › Walls or infill?
5. Worked example: what buys strength › Find the mistake
6. Practise: trade-offs
7. Strong and quick