Gear Hobbing on a Universal Mill Without an Index Head

[Dr. Hillbilly]

As I slowly learn about vintage machining practice I realize that I have been inquiring about an index head for hobbing, when I do not need an index head at all, and I am once again sabotaging my searches with incorrect terms. (I need a Rosetta Stone to translate from CNC to Vint.!)

If I hob a worm gear there is no table motion at all, so rotation of the gear must be driven by the spindle (a gear set from the outboard end of the arbor?).
Helical and spur gears will require table motion for hobbing, but the the traverse rate is very slow. For example if I am hobbing a 20 tooth gear with a single tooth hob my spindle speed must be 20 times the table speed. So if the hob is turning 100 rpm the table must turn 5 rpm. If an index head with a 40:1 reduction ratio is used then the input shaft must be turning 5X40= 200 rpm. Gearing the slow rotation of table up to 200 rpm seems problematic.

So were simple tables without index plates or reduction worms ever used?
Was outboard arbor drive of the table every pursued?
How common was it to use the hob to drive the head, so there is no head drive from the table or the spindle?

 

[johnoder]

when I do not need an index head at all,

No, just a bunch of stuff that has to be made. You think a Dividing head is difficult to find? Try finding "stuff" mostly existing in folk's noggins

A link to an authoritative text from 1916

A treatise on milling and milling machines .. : Cincinnati Milling Machine Company : Free Download, Borrow, and Streaming : Internet Archive

Page 317 shows FREE hobbing of a worm wheel. Blank is accurately "gashed" before hand (on page 316). The finishing set up allows the worm wheel to FREE WHEEL - where it is DRIVEN by the real hob. No table movement at all, not applicable to other than worm wheels. Knee is raised to effect the cutting. This process was not applied to high accuracy worm wheels.

 

[Dr. Hillbilly]

No, just a bunch of stuff that has to be made. You think a Dividing head is difficult to find? Try finding "stuff" mostly existing in folk's noggins

A link to an authoritative text from 1916

A treatise on milling and milling machines .. : Cincinnati Milling Machine Company : Free Download, Borrow, and Streaming : Internet Archive

Page 317 shows FREE hobbing of a worm wheel. Blank is accurately "gashed" before hand (on page 316). The finishing set up allows the worm wheel to FREE WHEEL - where it is DRIVEN by the real hob. No table movement at all, not applicable to other than worm wheels. Knee is raised to effect the cutting. This process was not applied to high accuracy worm wheels.

Yes, the Cincinnati treatise was the first reference I found to self-feeding of the gear by the hob.

Obviously table feed will be required for spur and helical gears, but it seems that the hob would still self-feed?

 

[Dr. Hillbilly]

speaking of table feed, after repeated lube and reassembly the seized actuating rods for feed are working, but only 16 of the 18 feed combinations are happy.
There are three selections in back, and they all work. The six in front work on speeds one and two in back. However speeds 5 and 6 in front do not like the third speed in back. So feeds 17 and 18 run for a few seconds and then the front selector lever pops out of position violently. I do not want to test it again for fear of breaking something.
Any ideas?

 

[johansen]

Yes, the Cincinnati treatise was the first reference I found to self-feeding of the gear by the hob.
View attachment 278164
Obviously table feed will be required for spur and helical gears, but it seems that the hob would still self-feed?

You could gash a crude spur gear (you could even try to cut the teeth out on a bandsaw, as close as you can to manually scribed lines) and then use the hob to drive it but you would end up with a wobble. there is nothing to guarantee it would turn into a mostly straight spur gear or a helical gear with a slight twist.

Think about the torque applied to the gashed crude spur gear by the hob.. and then ask the question: is the torque is going to stay the same across the full width of the gear, or did your gashed teeth have a bit of a taper to them? There is no guarantee you're going to get a spur gear, no guarantee a second gear cut is going to have the same helix angle.

also the hob is only going to average out the gear tooth profile across 2 or 3 teeth. even if you have a 100+ teeth the amount of contact that the hob has with the 4 or 5th tooth is so small that it can't provide much correctional force to correct the profile of a tooth that is really out of position.

its worth an experiment though, on a hob you don't really care about.


this type of gear tooth generation would be appropriate for a gear set where all the teeth are made the same way.. then you make them all work together by running them in a sandbox until they mesh correctly.

there is a video on youtube of making manual transmissions from the 1920's where they take the gears and "run them in" with an abrasive slurry.
edit: found it.
YouTube

R. G. LeTourneau discovered he could flame cut gears and run them in a sandbox until they mesh correctly, but he was dealing with very large gears for earthmoving equipment.

 

[EmGo]

Helical and spur gears will require table motion for hobbing, but the the traverse rate is very slow. For example if I am hobbing a 20 tooth gear with a single tooth hob

This is not hobbing. There are no "single tooth" hobs in the sense you are using it. This is just cutting spaces with a form cutter.

 

[Dr. Hillbilly]

This is not hobbing. There are no "single tooth" hobs in the sense you are using it. This is just cutting spaces with a form cutter.

When the old documents refer to "single tooth" I believe they are referring to "single pitch" in current terminology. Of course using a multiple pitch (or multiple tooth) tool changes the ratio of rotation of tool to work.

 

[EmGo]

When the old documents refer to "single tooth" I believe they are referring to "single pitch" in current terminology.

No. The words you are using are confusing me. They are wrong. Those are single start, double start, triple start hobs and always have been. There is no such thing as a "single tooth" or "single pitch" hob.

Hobs MUST rotate the work in a timed relationship with the cutter.

You can't hob anything without a physical connection between the cutter and the workpiece. The freewheeling method described which people have used for worms is

1) not very good
2) requires pre-cutting of the teeth.

You cannot do this with a spur or helical gear. Will not work. Period.