Uhm.. i made a rack in Solidworks and checked the tooth depth.. So the distance between tooths is 3.14 * module. ..
But it seems at a Module 1, the depth of the tooth is 2.25.. i cant reverse or derive this number (and i need to understand why its 2.25).
Uh huh, the pitch of a rack ( or a worm...) is "unwinding" the teeth, so going from diameter of the pitch circle to the circumference means multiplying by Pi, for metric module pitches, mod times Pi, but for "English" DP divide Pi by the DP, simples
so for your 1.25 mod hob, you need the "teeth" to repeat at Pi Times 1.25 mm.
The depth of cut, 2.25 x mod, is because an involute gear needs clearance at the bottom of.the tooth space, in an ideal world the cut would be 2 x mod, but that leaves no room for grease, grit, badly-formed teeth...
On larger gears, the clearance can be less, as there's more room to play with, so for a 5 mod it could be reduced to 0.125, giving q cut of 2.125 times the mod, or less.
So... Your hob needs teeth pitched at Pi x Mod, with an included angle of twice the pressure angle, and a depth of 2.25 x Mod. This won't be a "topping" cutter, as far as I can tell, so the gear blank will need to be the correct diameter ( teeth+2 x Mod)...
Hope that makes.sense!
Dave H. (the other one)
