Gear hobbing with a tap questions

[torker]

A couple of questions...I assume that the gear turns as the tap pulls it into it. The examples I've seen have all been aluminum. Would this work with steel gears as well? I'm thinking there is a lot more pressure involved and was wondering if a more elaborate bearing setup would be needed for the gear to turn on. Also..the setups I've seen seem rather light...when doing this with steel gears would you need a much stiffer setup overall? I'd like to try this. Thanks.
Russ

[G.A. Ewen]

Russ,
I assume that you are talking about hobbing with a tap. I only tried it once on brass and it worked very well. I think that if I were going to try it on steel I would gash the teeth partly out first and then finish it with the tap. A regular tap is not the best for hobbing because the setup can jump out of time when the flutes come around. A spiral tap always has a tooth engaged.

[Dr. Rob]

A regular tap usually gives a 30* pressure angle on the gears (60* thread form) which is kind of steep for regular spur gears and may require closer attention to proper center distance for the gears to run properly when in mesh.

A small gear blank offers a smaller radius and thereby requires more force on its perifery to overcome rotational resistance. IE the tap has to pull harder, so make sure the setup rolls very easily. Any resistance will make the tap slide 1/nth of a degree x number of teeth, which makes the net result of a smudged mess of teeth on the blank.

So yeah- Strong, steady and smooth setup.

When starting out on the first cut, make the depth of cut coincide with the PCD of the desired gear. This ensures proper tooth spacing on the blank. The alternative being the 32 1/2 tooth syndrome.

Oh yeah- don't forget the taps own helix angle in your equations. You probably know that though.