# Timing pulley (GT2, GT3, MXL)

Flanged timing pulley for 2GT (GT2), 3GT (GT3) and MXL belts, as an envelope for assemblies.

## Geometry

Axis along Z, bottom flange's outer face on the XY plane:

- the toothed body at the **outside diameter O.D.** over the width **W**, with its grooves cut in;
- a flange of diameter **F**, 1 mm thick, at each end of W;
- the bore; with `hub`, a hub of diameter **D** up to the overall length **L**.

**MXL** grooves follow the published rack: depth 0.64, bottom 0.67, flanks 20° (28° under 24 teeth). **2GT and 3GT:** the source draws a rounded groove and dimensions its depth (0.75 and 1.14) only, adding that it varies with the number of teeth, so the groove's width is a proportion for looks, not for design.

The pitch diameter **P.D.** sits outside the O.D. and is in `part.py`, for centre distances. `detail = "simple"` is a plain cylinder at O.D., for big assemblies. Set screws aren't modelled.

## Parameters

| Parameter | Meaning |
|---|---|
| `profile` | `2GT`, `3GT` or `MXL` |
| `teeth` | Number of teeth: 2GT 14 to 60, 3GT 16 to 60, MXL 20 to 72 |
| `belt_width` | Nominal belt width: 2GT 6, 9; 3GT 6, 9, 15; MXL 4.8, 6.4, 9.5, 12.7 |
| `bore` | Bore, mm |
| `hub` | Hub above the top flange, on sizes that offer one |
| `detail` | `full` (tooth grooves) or `simple` (plain cylinder at O.D.) |

## Source

MISUMI, *FA Mechanical Components* catalogue (2014), Number of Teeth / Dimension and Belt Nominal Width / Dimension
tables and drawings:

- 2GT: High Torque Timing Pulleys pp. 1381-1382, <https://uk.misumi-ec.com/pdf/fa/2014/P1_1381-1382_F19_EN_001.pdf>
- 3GT: pp. 1383-1384, <https://uk.misumi-ec.com/pdf/fa/2014/P1_1383-1384_F19_EN_001.pdf>
- MXL: Timing Pulleys pp. 1389-1390, <https://uk.misumi-ec.com/pdf/fa/2014/P1_1389-1390_F19_EN_001.pdf>

Every row is in `part.py` (`PULLEYS`, `WIDTHS`); flange thickness t1.0 is on the drawings.

The source lists which bores it machines per tooth count. This part accepts any bore that leaves at least 1 mm of wall
under E, so common bores (such as 16 teeth, 5 mm) can be drawn; check a real pulley's bore with its seller. MXL pulleys
under 20 teeth have another shape in the source (shape K) and aren't offered. Names of belt profiles identify the belt;
this part isn't made by or affiliated with any maker.

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