Lids that fit: clearance, lid types and test prints
A printed lid fits when there is a small, deliberate gap between it and the box, usually 0.15–0.3 mm on each side. Too little and it jams; too much and it falls off. Because every printer prints slightly differently, the right gap is found with one small test.
Why lids need a gap
A printer's lines spread a little as they are laid down, so outside edges come out slightly too big and holes slightly too small. Two parts drawn to exactly the same size will not go together. The extra room you add for this is called clearance.
Pick the lid for the job
| Lid style | How it closes | Good for |
|---|---|---|
| Flat (lift-off) | Sits on top | Display boxes, dust covers |
| Friction-fit | Pushes in and holds by grip | Pill boxes, small parts tubs |
| Telescoping | A deep skirt slides over a collar | Tall round containers, gift boxes |
| Screw-on | A thread turns it shut | Jars, anything carried in a bag |
| Twist-lock | Push down and turn a quarter | Quick open and close with one hand |
| Swing lid | Hinges up on a bin | Bins you open often |
Friction-fit lids need the tightest gap, because grip is the whole point. Threads and twist-locks need more, because their parts slide over each other at an angle.
Snug, Standard or Loose
In SO3D every lid that fits into or onto the box has a Fit setting (called Thread fit for screw-on lids). Each step adds a little more clearance:
| Lid style | Snug | Standard | Loose |
|---|---|---|---|
| Friction-fit | 0.08 mm | 0.15 mm | 0.25 mm |
| Telescoping, Twist-lock | 0.15 mm | 0.25 mm | 0.4 mm |
| Screw-on | 0.18 mm | 0.3 mm | 0.45 mm |
Start with Standard. A well-tuned printer, such as a recent Bambu Lab or Prusa, often suits Snug. An older printer, or PETG, which squishes wider than PLA, often needs Loose.
Test before the big print
- Make a small version of your box, only a few centimetres tall, with the same lid style and fit.
- Print it with the same filament and settings you will use for the real one.
- Try the lid. Too tight: go one step looser. Rattles or falls off: one step snugger.
Also check your slicer's elephant's foot setting: a squashed first layer makes the bottom of a lid wider and is a common reason a lid will not start on its thread.
Worked example: a pill box that clicks shut
You want a small round box with a friction-fit lid, printed in PLA on a well-tuned printer.
- Choose Friction-fit and Standard: about 0.15 mm of play all round.
- Print a test version only 15 mm tall. It takes minutes, not an hour.
- The lid pushes on but needs real force: choose Loose (0.25 mm). It falls off when you shake the box: choose Snug (0.08 mm).
- Print the full-height box with the fit that worked. Note it down, because the same fit will work for your next lid on that printer and filament.
Common questions
Can I just sand a tight lid? A little, yes: fine sandpaper on the rim fixes a lid that is only just too tight. If it needs more than that, reprint one step looser; sanding a thread ruins it.
My screw lid starts, then jams after half a turn. The thread is too tight, or the first layers are squashed. Try Loose, and check your slicer's elephant's foot setting.
Does a looser fit make the box bigger? No. The outside of the box stays the size you set; Fit only changes the small gap between box and lid.
Why did a lid that fitted in PLA jam in PETG? PETG is laid down slightly wider. Go one step looser when you switch to PETG.
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