Group Project: Dividing Head - The Build

[rwm]

Here is my solution to the tilt index.

This is not very precise, but it is really there just to keep me from screwing up and selecting say 60 deg when I really want 45.

And I'm gonna call this FINISHED!

 

[rwm]

LOL
Looks great.

I have survived the worst of my hospital stay, hope to come home next week.

Good to hear that.
Thank you! And thanks to everybody for all the help and commentary along the way.

 

[Flyinfool]

Now what are you going to make with it.

Or as pretty as it is it might just have to on the mantle in the living room......
Tell the boss I said it is OK to put it there.

 

[extropic]

Congratulations to both of you.

 

[rwm]

She loves the time machine. She says it has already traveled about 8 weeks forward in time.

 

[extropic]

It's not exactly to the plan specs but I think I have a functional 40 tooth worm gear:

View attachment 454358
Stop reading here is you don't want the long version!
I ran into problems and learned a few things. My plan for making this was to make a hob by cutting slots into the acme lead screw (kind of like a tap with flutes.) I was hoping the hob would cause the gear blank to passively rotate as it cut the teeth.
I started off by removing material from the center and cutting slots in the 3.6" diameter blank:
View attachment 454361
I used a 3/4" end mill to get close to the minor diameter of a 1-4 acme screw.
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I then mounted the blank on ball bearings attached to this table I made. This positions the blank at the correct height on my lathe:
View attachment 454364

Like so:
View attachment 454365

This first attempt WAS A TOTAL FAIL!
The hob simply would not follow the grooves I cut and was "cross threading" with the teeth. After much thought and gnashing of teeth (my teeth) I came up with this theory. The blank diameter of 3.6" may be ideal in theory but the depth of tooth and coverage of the screw that geometry creates is just too ambitious for my home made hob. Here's why- the pitch of the lead screw is 4 TPI or .25". When you start to cut the OD of the 3.6" blank you cannot physically have a .25 pitch and a 3.6" gear. The circumference is too large and the hob wants to make a 43 tooth gear or so. In fact I calculate the pitch for a 3.6" gear at .2826" For small gears with large teeth this becomes a problem. If the tooth depth is a much smaller percentage of the gear diameter then the change in pitch circumference from the OD to the pitch diameter is not so much of an issue.
After ruining the first blank I decided to machine off the bad teeth and start with a smaller OD blank. I machined it down to 3.4" diameter and recut shallow grooves with a 1/8" end mill. This actually worked as planned! The hob cut the center of the blank and turned the blank as it cut.

View attachment 454369

Try this link?: https://photos.app.goo.gl/97AEtNLx19TU22XJ7

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So the tooth depth is not as much as spec but it is adequate to smoothly drive the gear. The tooth coverage does not extend all the way to the edges of the gear, but again, it is adequate. Tooth spacing appears to be excellent (which is critical.)
The hob is now not cutting well. I may try to modify the teeth to make it more functional. I may also make a specific acme tooth shaped cutter and try cutting each tooth individually, then follow up with the hob. It would be great if I could get the teeth about 50 thou deeper. Or I could quit here and call it a win!

View attachment 454375

I just watched a new Youtube video from OX Tool (Tom Lipton) where he walks through making a worm wheel. He used an involute (spur gear) cutter (not Acme) and produced an approximation of the ideal tooth form, but he explains his rationale. The video is a good two years late for this project, but I thought it might be food for thought for any future pilgrim.

 

[rwm]

Interesting that he used a large diameter cutter and made straight teeth. I used a cutter with the same diameter as the worm so the valleys were curved to match the worm. In retrospect, this was probably not necessary. This is not a load carrying gear. A DP cutter could simplify the process as you see. This would require minor design changes as the OD of the driven gear will be smaller than shown in the plans. It still might be useful to "run in" the cut gear against and Acme hob if possible.
He didn't really get into which cutter he chose and why? I assume the cutter indicated for the number of teeth on the driven gear?