Spiral Helical Gear cutting structure

Given,
Pai=12'
OD=45
M=1.5

Then,
PD=OD-2M
=45-2×1.5
=41

Job lead= (pai×PD)÷(Tan12)
=605.98

Machine lead=203

Gear Ratio=(Job lead÷Machine lead)
=605.98÷203
=2.98