Lead Angle (λ):

The angle at which the teeth are inclined to the normal to the axis of rotation. As illustrate in given figure. Lead angle is the complement of the helix angle,

That means ψ+ λ= 90^o

 As angle between axes of shafts is 90^o

       ψ₁ + ψ₂ = 90^o

 Or, (90^o - λ) + ψ₂ = 90o

 or, λ= ψ₂

That means lead angle of worm = helix angle of work gear

∴  p_n of worm = p_n of wheel

 or, p_x  cos λ= p₂  cos ψ₂

But

 λ= ψ₂

∴  p_x = p₂

It refers axial pitch of worm is equivalent to the circular pitch of worm gear.