layerling 1.58.1

• Ring gear and rack: The gear has two more types. The Ring gear has its teeth pointing in - the outer wheel of a planetary set - and is set by teeth, module and backlash like an involute gear, plus the rim round the teeth; its outside is module × (teeth + 2.5) plus twice the rim, with round teeth module × (teeth + 1.7). A pinion of the same module runs in it at a centre distance of module × (ring teeth − pinion teeth) / 2, which the properties panel gives for the selected pair; ring teeth = sun teeth + 2 × planet teeth makes a planetary set. The Rack is the gear of endless radius, its involute flanks straight lines at the pressure angle: a bar as long as teeth × π × module, so racks of one module line up end to end, its depth free; a gear of the same module rolls on it with its centre teeth × module / 2 in front of the pitch line, a module behind the tooth tips. Both take round teeth too, both are exact bodies for the edge tool and STEP, and switching the type keeps the module. The MCP bridge takes gearType: "internal" with gearRim, and "rack". Promised to @prmod3d and @luk-saw in #201.