If I can help you with your scraping feel free to PM me. My part 3 on the Bridgeport should be posted soon. There will be some good things I've learned on the metrology of scraping in that post.
Thanks for commenting. I was debating how to explain the generation process. Not all of this was hobbing. In fact, the Fellows Shapers do not use hobs, but the manner in which the involute is generated from a straight sided cutter profile is almost identical. Which is why I find them fascinating. Hobs use a rotating helical cutter with straight sided cutter teeth like the tooth of a rack gear. The hobbers shown at 5:30 and again at 6:20 in the video show the angle of the hob shaft relative to the work shaft. This is to cancel out the helix angle of the hob in order to cut a straight tooth relative to the axis of the work. If it looks like a corn cob it’s probably a hob. The hob tooth geometry to cut helical and angled gearing gets complex. In contrast to hobbing, the Fellows cutters move in a reciprocating straight line, like a shaper, to cut the teeth on a gear blank. But both work and cutter rotate slowly in sync to form the involute. This makes cutting helical gears very simple by using a helix internal to the machine (shown at 4:00) to rotate the cutter stroke. The same cutting tool and involute generation remain the same for both helical and straight gear profiles. Both methods have the advantage of generating an involute which is matched to the pitch diameter of the gear being cut. If you buy a set of single tooth gear cutters for a mill you’ll see they have 8 different tooth profiles for a given D.P. depending on the tooth count. Unfortunately, the profile is only correct for one pitch diameter, while anything greater or less will be an approximation. These can produce noisy and less efficient gears, but for most purposes work just fine. The home shop alternative to a truly hobbed gear is to pre-cut the pitch of the desired gear teeth and let a hob rotate the free wheeling blank as the hob finishes the tooth profile. That’s on my bucket list after I get my Bridgeport back on-line. Thanks for watching.
Being a vermont native who recently took an interest in hand scraping, I never knew what was in my backyard!
If I can help you with your scraping feel free to PM me. My part 3 on the Bridgeport should be posted soon. There will be some good things I've learned on the metrology of scraping in that post.
Thanks for taking video while you were there. Really interesting stuff!
It's callled gear "hobbing".
Thanks for commenting. I was debating how to explain the generation process. Not all of this was hobbing. In fact, the Fellows Shapers do not use hobs, but the manner in which the involute is generated from a straight sided cutter profile is almost identical. Which is why I find them fascinating.
Hobs use a rotating helical cutter with straight sided cutter teeth like the tooth of a rack gear. The hobbers shown at 5:30 and again at 6:20 in the video show the angle of the hob shaft relative to the work shaft. This is to cancel out the helix angle of the hob in order to cut a straight tooth relative to the axis of the work. If it looks like a corn cob it’s probably a hob. The hob tooth geometry to cut helical and angled gearing gets complex.
In contrast to hobbing, the Fellows cutters move in a reciprocating straight line, like a shaper, to cut the teeth on a gear blank. But both work and cutter rotate slowly in sync to form the involute. This makes cutting helical gears very simple by using a helix internal to the machine (shown at 4:00) to rotate the cutter stroke. The same cutting tool and involute generation remain the same for both helical and straight gear profiles.
Both methods have the advantage of generating an involute which is matched to the pitch diameter of the gear being cut. If you buy a set of single tooth gear cutters for a mill you’ll see they have 8 different tooth profiles for a given D.P. depending on the tooth count. Unfortunately, the profile is only correct for one pitch diameter, while anything greater or less will be an approximation. These can produce noisy and less efficient gears, but for most purposes work just fine.
The home shop alternative to a truly hobbed gear is to pre-cut the pitch of the desired gear teeth and let a hob rotate the free wheeling blank as the hob finishes the tooth profile. That’s on my bucket list after I get my Bridgeport back on-line. Thanks for watching.