Can free hob acme/trapezoidal threads on a lathe?

[strantor]

Plenty of videos out there on free hobbing but they all use common fastener thread taps. I only found one where someone used an ACME tap and it seems his results weren't great.

I'm not sure if the result was due to him or if the concept just isn't feasible. It seems he didn't try for a better result that seemed possible.

 

[Jake M]

When you do it that way, it's make shift, so you expect some fiddling and some mixed results right from the start.

I see multiple geometrical descrepencies between the cutting edges of the tap (and their lack of clearances) and the variable diameter/radius of the part. The deeper parts of the thread are on a smaller diameter than the outer most parts. That's part of the magic that makes hobbing work to make anything fit right, but it also is getting kinda deep, with a big sidewall on the thread, and I think that is interfering with his cutting. On a "standard" thread, these issues would be a much smaller percentage, and probably a lot easier to bury as the cut reaches full depth.

I've only done this with half inch 13 threads, and my results were.... well.... It looked good from across the room, but if you studied it up close you could see that it wasn't cutting right, and given the cutting geometry of the tap's individual cutting tips, I think it just couldn't decide which part of the engaged cutting area(s) to follow. Pretty much exactly what I see here in the video, except like I say, the scaling is quite different.

I suspect the process could work, but I think to do it you'd need a dedicated hob to make a worm wheel like that. The pattern is just too deep to do it that way.

Or on the other hand, I could be smoking crack again. I'm curious what other's say. I'd kinda like to be able to make a worm and wheel on demand.....

 

[RaisedByWolves]

Plenty of videos out there on free hobbing but they all use common fastener thread taps. I only found one where someone used an ACME tap and it seems his results weren't great.

I'm not sure if the result was due to him or if the concept just isn't feasible. It seems he didn't try for a better result that seemed possible.

That video was soooo cringeworthy........

As to the issues he had, I think his TPI was too low for the diameter amongst other things.

I'm just glad I didn't see a finger fly off.

 

[strantor]

That video was soooo cringeworthy........

As to the issues he had, I think his TPI was too low for the diameter amongst other things.

I'm just glad I didn't see a finger fly off.

Yeah obviously if I try this I will be smart about it and wear gloves. And long sleeves and a gold neck chain.
(Sarcasm)

I just ordered an acme tap (needed one anyway) so I'll be giving this a shot even if I'm not confident it will work.

I will try it with a larger diameter than he did, and a few other things I think would help, like turning the OD slightly concave before attempting to hob.

 

[RaisedByWolves]

Yeah obviously if I try this I will be smart about it and wear gloves. And long sleeves and a gold neck chain.
(Sarcasm)

I just ordered an acme tap (needed one anyway) so I'll be giving this a shot even if I'm not confident it will work.

I will try it with a larger diameter than he did, and a few other things I think would help, like turning the OD slightly concave before attempting to hob.

LOL, I almost fell off my chair!

 

[Ecosta777]

I was able to do this with success for a bandsaw gearbox I needed to fix. I cannot find the photos at the moment except for the finished product, but I will look later. I used a similar setup as the video above, had to make a special "tool post arbor" for the gear to mount on the compound.

For the hob I actually just made two of the worm gears I needed, cut teeth in one and hardened it to make my hob. I still have that lying around and will post a picture of that, and the in process photos if I can find them.

I also roughed out the teeth in the gear with a small endmill, with my mill head angled and the gear on the rotary table, which helped a lot with the hobbing.

 

[Ecosta777]

Found a photo of the hobbing. Looks like I have not actually started cutting yet, and you can see how the teeth were roughed out. Also some better shots of the hob. Just made some basic teeth in it, nothing crazy. It actually worked quite well and the gear / worm have been in use for a few years now.
I did have the original gear to copy, it was severely worn but I think I must have been able to get the correct OD, as I did not have any of the timing / clocking issues as the above video did.

 

[strantor]

Wow @Ecosta777 that is a really good looking hob and final result with the worm and gear. What you did is pretty much the same process I will be doing. Is your lathe able to hit 2.5TPI with its own gears or did you have to do something special? Mine only goes up to 4TPI.

 

[Ecosta777]

Wow @Ecosta777 that is a really good looking hob and final result with the worm and gear. What you did is pretty much the same process I will be doing. Is your lathe able to hit 2.5TPI with its own gears or did you have to do something special? Mine only goes up to 4TPI.

I had to use a friend's lathe that went down to 2.5 TPI. When I had the South Bend lathe it would only go down to about 6 TPI I think