# Captive panel screw (envelope)

A captive panel screw envelope for assemblies: a shank and a round knob, M3 to M5, any length.

## Geometry

The shank (diameter d, the thread's nominal diameter, no thread) runs along +Z from z = 0, `length` long; the knob
(diameter 2d, height d) is centred on the shank's top end, overlapping it by d / 2. Shank and knob are two bodies, as
in the models. Overall height: length + d / 2.

| Thread | Shank Ø | Knob Ø × height |
|---|---|---|
| M3 | 3 | 6 × 3 |
| M4 | 4 | 8 × 4 |
| M5 | 5 | 10 × 5 |

## Parameters

| Parameter | Meaning |
|---|---|
| `thread` | `M3`, `M4`, `M5` |
| `length` | shank length, mm |
| `material` | `unspecified` (no material assigned), `steel`, `stainless-304` |

## Sources

These generic screws name no maker (PennEngineering's PF11/PF12 come in other lengths) and no standard covers them.
**The geometry was measured from the published reference models** below (the cylinders of each model's two solids),
and every preset is checked against its own model: bounding box within 0.05 mm and volume within 2 %.

- **step.parts**, `captive_panel_screw_m<d>_l<length>` for M3, M4, M5 × 10 to 30 mm (https://www.step.parts,
  repository https://github.com/earthtojake/step.parts, commit c6113328, `catalog/step/`): step.parts' own models,
  original project material under the repository's **MIT** licence (© 2026 earthtojake). They name no material, so
  the presets have `material = "unspecified"`.

Simplifications (the models'): plain cylinders: no thread, retainer, spring, ferrule or knurl.

Preset list: from the step.parts catalogue (MIT), see `library/NOTICE.md`.