# Fits and clearances

How much gap to leave between parts that sit, slide, turn or link together, for each finish.

**Use this when:** two printed parts touch, fit together or move against each other.

**Don't use this for:** a single part's size accuracy. That is tolerance, in Design rules.

All clearances are per side, surface to surface. A hole diameter is the shaft diameter plus 2 times the clearance (`CLR-01`).

![Diagram: a shaft inside a hole with the gap c marked on each side.](/docs/diagrams/clearance.svg)

*Clearance c is the gap on one side. The hole is the shaft diameter plus 2c. An open sliding fit needs 0.5 mm per side matte, 1 mm glossy (`CLR-04`).*

## Open fits

An open fit is one where only one pair of surfaces must clear, or the outer part can flex: a lid on a ledge, a part sliding along one rail, a shaft in a bore. Glossy values are larger because coating adds material and fuses small gaps.

| Rule | Name | Limit (mm, per side for gaps) |
| --- | --- | --- |
| `CLR-01` | Clearance convention | All clearances are surface-to-surface gaps per side. Hole Ø = shaft Ø + 2 × clearance. |
| `CLR-02` | Loose static fit | Minimum gap per side for parts that sit together without moving. MATTE: 0.3; GLOSSY: 0.8; CLEAR: 0.8. |
| `CLR-03` | Removable lid or cover | Minimum gap per side for lids that are taken on and off by hand (open, non-enclosing fit). MATTE: 0.3–0.5; GLOSSY: 0.8–1.0; CLEAR: 0.8–1.0. |
| `CLR-04` | Open sliding fit | Minimum gap per side for a part sliding along one rail or face (not enclosed on all sides). MATTE: 0.5; GLOSSY: 1; CLEAR: 1. Bands: AUTO_PASS ≥ value; REVIEW MATTE 0.3–<0.5; GLOSSY 0.8–<1.0; ROUTE MATTE <0.3; GLOSSY <0.8. |
| `CLR-05` | Rotating shaft or hinge (round bore) | Minimum gap per side for a round shaft or hinge pin in a round bore. MATTE: 0.5; GLOSSY: 1; CLEAR: 1. |
| `CLR-06` | Interlocking chain links | Minimum gap per side between links. MATTE: 0.6; GLOSSY: 1.0–1.5 (prefer separate finishing); CLEAR: separate finishing only. |
| `CLR-07` | Print-in-place assembly | Assemblies printed already joined. MATTE: 0.6, REVIEW; GLOSSY: not recommended (ROUTE); CLEAR: not allowed (ROUTE). |
| `CLR-08` | Enclosed moving mechanism | A moving part fully enclosed so it cannot be washed, inspected or finished. MATTE: REVIEW; GLOSSY: ROUTE; CLEAR: ROUTE. |
| `CLR-09` | Precision alignment | Gaps below 0.3 mm per side for locating features. MATTE: REVIEW; GLOSSY: ROUTE; CLEAR: ROUTE. |
| `CLR-10` | Press and snap fits | Interference and snap fits are never auto-approved; resin- and geometry-specific testing is required. MATTE: ROUTE; GLOSSY: ROUTE; CLEAR: ROUTE. |

## Closed fits

![Diagram: on the left a part resting on a rail, an open fit; on the right a box inside a sleeve, a closed fit.](/docs/diagrams/open-vs-closed.svg)

*An open fit has one pair of surfaces to clear. A closed fit surrounds the part, so every face must clear at once: 1 mm per side matte, 2 mm glossy (`CLS-02`), against 0.5 and 1 mm for an open sliding fit (`CLR-04`). Wide flat walls bow and can need more.*

A fit is closed when the mating surfaces form a complete loop around the inner part in cross-section: a box in a sleeve, a drawer in a case, a plug in a socket, a lid inside a rim (`CLS-01`). Every error on every face must clear at the same time, and the ring around the part cannot flex. A closed flat-walled fit needs at least 2 times the open sliding clearance: 1 mm matte, 2 mm glossy, 2 mm clear, per side (`CLS-02`).

| Rule | Name | Limit (mm, per side for gaps) |
| --- | --- | --- |
| `CLS-01` | Detect closed perimeters | A fit is CLOSED when the mating surfaces form a complete loop around the inserted part in cross-section (box in sleeve, drawer in case, plug in socket, lid inside a rim). CLOSED fits use CLS rules instead of CLR-04. |
| `CLS-02` | Closed fits need at least 2× the open-fit clearance | Minimum gap per side for a closed flat-walled fit is at least 2 × the open sliding clearance; wide flat walls can need more. MATTE: 1; GLOSSY: 2; CLEAR: 2. Bands: AUTO_PASS ≥ required; REVIEW ≥ open-fit value and < required; ROUTE < open-fit value. |
| `CLS-05` | Long engagement | If the sliding engagement length exceeds 2 × the opening width, REVIEW for squareness and straightness. |
| `CLS-06` | Round bores | Round closed fits (shaft in hole, cap on round neck) use CLR-05: a round hoop has no flat span to bow. Polygonal and rectangular closed fits use CLS-02. |
| `CLS-07` | Design fixes for closed fits | Prefer contact on narrow rails or bumps rather than full faces; add 0.5–1.0 mm lead-in chamfers; add ribs to sleeve walls to cut bow; mask contact faces from coating. |

Round bores are the exception. A shaft in a round hole has no flat span to bow, so it uses the open rotating fit (`CLS-06`, `CLR-05`).

## Use instead

- **Wrong move:** a box sliding in a sleeve with the open sliding gap. **Why:** the sleeve is a closed fit: walls bow toward each other and coating builds up on every face, so it jams. **Right move:** use the closed-fit gap (`CLS-02`), slide on narrow rails, and add 0.5 to 1.0 mm lead-in chamfers (`CLS-07`).
- **Wrong move:** a glossy chain or print-in-place hinge. **Why:** coating wicks into the gaps and fuses the links (`GLS-02`, `GLS-03`). **Right move:** print it matte, or finish each link separately and assemble afterwards (`CLR-06`, `CLR-07`).
- **Wrong move:** a press or snap fit. **Why:** they are never approved automatically; they need resin-specific testing (`CLR-10`). **Right move:** a screw with a heat-set insert, or a magnet closure.

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Last checked: 2026-11-09. Womp version: Womp web app, November 2026.