Build a screw-top jar

A container and a lid whose threads mate because they were generated together.

Threads are the thing people assume a browser CAD tool cannot do, and they are also genuinely hard to get right by hand: a lid's thread must be the exact negative of the jar's, plus a clearance, or it either seizes or spins loose. The threaded jar generator cuts both from the same numbers, so they mate by construction rather than by luck. **Go coarser than feels right.** Raising the pitch to 4 mm means fewer turns to open — but the real reason is printing. A coarse thread has big, blunt teeth; a fine one has teeth roughly the size of a layer line, which is exactly the scale at which an FDM printer stops being able to tell them apart. **The wall does double duty.** At 2.5 mm it survives being dropped, and it also gives the thread something solid to be cut into. Too thin and the teeth have nothing behind them. **Print both parts standing up, open end down.** Every thread is then a horizontal ring instead of an overhang, so no supports are needed anywhere. Threads printed on their side almost never mate. If the lid binds, do not reprint blind — re-open the generator and add clearance. The part stayed parametric, so the fix is a number rather than a remodel. Open Help → Build a screw-top jar in the editor to build it hands-on, step by step.

1. Open the Generate panel and choose Threaded jar; set Diameter 50 and Height 60, with the lid left on.
2. Raise the Thread pitch to about 4 mm — coarser threads print far better than fine ones.
3. Set the Wall to about 2.5 mm, since the thread is cut into it.
4. Run the print check, then export and print both parts standing up, open end down.