Using Hobs without a Hobbing Machine

I believe that hobs also assist in turning the gear blank to generate the correct forms but could a hob be used set so that there is no turning moment and would it generate the correct forms if the blank was rotated a tooth at a time as used with gear cutters?

Brian

A hob with helical teeth would just cut away all your gear blank if it were not rotated in time with the hob. But you can make straight tooth hobs with just a Rack form that can be used by indexing the blank and they will form close to the correct curved profile on the teeth.

This might interest anyone who likes studying gear cutting principles.

It is my Alexander Master Toolmaker milling machine set up as a sort of Sunderland gear shaper.

The indexing head has been geared to the table traverse so it rotates the gear blank as the table moves, the Sunderland cutter is going up and down on the slotting head with a clapper box similar to a shaping machine.

A few teeth are generated at one go, then the table is returned to its start point ready for the next pass, but crucially the gear blank is manually rotated so as to overlap the previous teeth with the next new ones.

I cannot claim this idea as mine, it was my late father who designed it, he cut a full set of Myford changewheels on it going up in one tooth increments so there is quite a large stack of them.

Phil

Isn't that the hat from the first Harry Potter film?

Yeah, it looks a bit wonky though because it's about five photo's stitched together.

Phil

Wow! To both the machine set up and what must surely the the world's most comprehensive set of Myford change gears.

Dad did not really need all those change wheels, his Myford S7 (now mine) has a screw cutting gearbox as well.

He just did it because he could, he was a very clever fella.

He also designed a spiral bevel gear cutting attachment for the Myford, but that's another story.

Phil

Phil P,

beautiful machine. Nice work. Kudos.

I agree! A VERY nice machine you have there! DO you have all the head attachments - Horizontal mill, vertical, etc?

Very nice..

Joe

Makes me wonder who had the most patience; your Dad; or your Mom. Nonetheless a beautiful piece of kit.

Yes

The machine is fully tooled up with all the goodies, the only one I dont have is a spiral milling attachment.

Phil

Just to put the subject back on track.

If you make a hob with circular grooves which conform to the rack tooth dimensions, that is a hob with NO helix angle.

You can take one pass through the blank mounted on a dividing head and with the hob centred, then index one tooth and take another pass until you have gone all the way round.

You will get a good approximation of an involute tooth form, the difference being that it will be made up of a lot of tiny flats as opposed to being a generated curve, for a lot of slow rpm jobs they will perform adequately.

Phil

Phil, surely you don't index one tooth but a fraction of a tooth and displace the blank slightly? Otherwise you will just get straight sided teeth. I seem to recall an article by Neil Wyatt (?) describing this in MEW.

John, the cutter will cut several teeth at one setting so you get a sort of faceted shape to the tooth,here is one I prepared earlier

Edited By JasonB on 02/01/2019 09:57:06

That depends on the number of gear teeth surely? At some number it only cuts one; then 2, 3 and so on as the number of teeth increases. For lower numbers of teeth the tooth form tends to a rack, which is the exact opposite of normal involute where higher numbers tend to rack form. I'd like to see the numbers.

Incorrect; it's basically the same technique as the "Sunderland" planer shown by Phil. As illustrated by Jason you end up with the correct tooth form, but approximated by a series of straight lines. If you index by a fraction of a tooth width and offset the hob you simply increase the number of straight lines, and they're shorter, so you end up with a better approximation.

When free hobbing a worm wheel the last operation, once at full depth, was to move the worm wheel along the axis of the hob by a fraction of the worm pitch:

If I recall I did four increments; at each increment cutting noise could be heard, until the worm wheel had made one rotation.

Andrew

In the example shown in the three sketches it would not be hard to go around again at "half tooth" spacing with the cutter moved by half the pitch which should double the number of facets.

Probably not needed on Brian's traction engine gears that may well have had cast teeth to start with.

You would need to use less table traverse per stroke to keep the facet-count down and reduce chip load on smaller tooth counts but the shape would always be correct IMO.

The straight tooth hob sounds ideal, thanks Phil and Jason.

Brian

Yes, I can confirm the parallel hob approach works well:

A major advantage is that it makes  cutting gears with non-standard centre distances very easy, although if overdone the tooth shapes start to suffer.

Edited By Neil Wyatt on 02/01/2019 16:30:04

Edited By Neil Wyatt on 02/01/2019 16:32:33