What blank diameter do I turn for a module 2, 30 tooth gear?

The outside diameter: 2 × (30 + 2) = 64 mm (2.52 inch).

How do I convert diametral pitch to module?

Module = 25.4 ÷ diametral pitch. A 10 DP gear is about module 2.54.

What is the smallest number of teeth?

With a 20° pressure angle, fewer than about 17 teeth start to undercut unless the profile is shifted.

Spur gear calculator: module, PCD, OD and centre distance

Module m (mm)Inch gears: module = 25.4 ÷ diametral pitch.Number of teethPressure angleTeeth on the mating gear (optional)

Pitch circle diameter (PCD): 60 mm (2.36 inch)
Outside diameter (blank size): 64 mm (2.52 inch)
Root diameter: 55 mm (2.17 inch)
Base circle diameter: 56.382 mm (2.22 inch)
Whole depth of tooth: 4.5 mm (0.177 inch)
Circular pitch: 6.283 mm (0.247 inch)
Tooth thickness (on PCD): 3.142 mm (0.124 inch)
Diametral pitch: 12.7

Standard full-depth spur gears without profile shift. Fewer than 17 teeth at 20° may undercut.

How it is worked out

PCD = module × teeth; outside diameter = module × (teeth + 2); root diameter = module × (teeth − 2.5).
Whole depth = 2.25 × module; circular pitch = π × module.
Centre distance = module × (teeth of gear + teeth of pinion) ÷ 2.

Questions

What blank diameter do I turn for a module 2, 30 tooth gear?: The outside diameter: 2 × (30 + 2) = 64 mm (2.52 inch).
How do I convert diametral pitch to module?: Module = 25.4 ÷ diametral pitch. A 10 DP gear is about module 2.54.
What is the smallest number of teeth?: With a 20° pressure angle, fewer than about 17 teeth start to undercut unless the profile is shifted.

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Results use standard formulas and typical values. Use them for estimating and planning; check critical figures against the drawing, the material certificate or the maker's data.