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### Design of helical gears

Any helical gear will be defined by a few key parameters.

A The number of teeth

B The size of the teeth. This will determined the cutter to be used. This will also define the tooth shape, i.e., involute and the pressure angle.

C The helix angle.

The lead is the distance the milling table has to move whilst the dividing head rotates through 360º. But, of course, the input to the dividing head is divided by 40. So in the time it takes for the milling table to move 39.38mm this input must rotate 40 times

the pitch of the lead screw is 5.00mm

so the number of turns of the lead screw to move 387.6 is:

turns = 387.6 / 5

turns = 77.53

so 77.53 turns of the leadscrew must make 40 turns of the auxiliary input of the dividing head.

so 77.53 / 40 is the ratio required

ratio = 77.53 / 40 = 1.938

The gears train required can be computed using the “Hobnail” program. For the set of change gears that was available, the gears were 26, 28, 48 and 23.