Mill or Hobb for Worm Gear

[epanzella]

I've been watching countless utube videos on making a worm gear. NOBODY ever talks about the diameter of the blank. The effective diameter is constantly changing as the cut progresses into the circumference of the wheel. I need four tpi so do I need a .250" multiple at the PD, or the root of the hole, or what? Also, some people use a hobb and some people use a gear cutter or fly cutter on a mill. Now as I understand it a hobb is identical to the worm in pitch and tooth form. This says to me that the form it cuts is a constantly turning helix around the centerline of the hobb. To use a gear cutter on a mill the the blank is presented to the cutter at the helix angle and fed thru by the table advancing. It seems to me this would be a straight cut at the helix angle but would not have an advancing helix like the hobb. One of these (or me) must be wrong. Help!

 

[benmychree]

The hob could make a theoretically correct worm wheel, generally if the work is to be hobbed blank needs to be gashed with a gear cutter before hobbing to establish the tooth spacing, unless it is done on a gear hobber where the blank and hob are gear driven at the ratio.
Find a copy of Brown & Sharpe's book "Practical Treatise on Gearing