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Spur Gears, 20 Teeth: Cut One Space at a Time on a Dividing Head
Martin

Created by

Martin

14. October 2026NO
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Spur Gears, 20 Teeth: Cut One Space at a Time on a Dividing Head

Without a hobbing machine, a spur gear is cut one tooth space at a time: a formed involute cutter on a milling machine, the blank turned by a dividing head between cuts. On a 40:1 head, a 20-tooth gear is two full turns of the crank per tooth. Eight involute cutters cover every gear from 12 teeth to a rack. The picture: cutting a spur gear with the spiral (dividing) head, from "A Practical Treatise on Milling and Milling Machines".
Advanced
A day for a pair

Instructions

1

Turn the blanks

Saw two slices of round bar and turn each to the outside diameter the cutter's data gives for your module and 20 teeth. Bore the centre to fit the mandrel and face both sides.

Materials for this step:

Mild Steel Round BarMild Steel Round Bar1 piece

Tools needed:

HacksawHacksaw
Metal LatheMetal Lathe
Turning Tool Set HSSTurning Tool Set HSS
Cutting OilCutting Oil
Vernier CaliperVernier Caliper
2

Set up the head

Press a blank onto the mandrel and mount it between the dividing head and its tailstock on the milling table. Fit the involute cutter for 20 teeth of that module and centre it over the blank.

Tools needed:

Milling MachineMilling Machine
Dividing HeadDividing Head
Mandrel RodMandrel Rod
Involute Gear CutterInvolute Gear Cutter
3

Cut, index, cut

Set the depth from the cutter's data, cut the first space with oil, back off, give the crank two full turns, and cut the next. Repeat until all 20 spaces are cut and you are back at the first.

Tools needed:

Milling MachineMilling Machine
Dividing HeadDividing Head
Involute Gear CutterInvolute Gear Cutter
Cutting OilCutting Oil
Safety GlassesSafety Glasses
4

Second gear and deburr

Cut the second blank the same way. Deburr the tooth edges and check that the pair mesh smoothly at their centre distance (the sum of the two pitch radii).

Tools needed:

Deburring ToolDeburring Tool
Vernier CaliperVernier Caliper
5

Indexing 20 teeth

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Materials

1

Tools Required

11

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