[HELP] Can someone help me to re-gear my lathe?

[Dan Boy]

So I found this website and even though I don't understand all of the numbers the relative RPMs work out correct.

I think the limiting factor will be the shaft size on the driver gear. I'll have to remove the gear to see but if I simulate the 22T driver gear with an 18T gear and substitute the 66T driven gear with an 88T gear I only get a slow down of RPM to..... I can't do the math

At the speed I was running at the driven gear was 29.4 RPM and with the changes as above I get an RPM of 17.94 so thats not even half of my current slowest speed. :/

https://geargenerator.com/#200,200,25,87...,0,1,3,804

   

[Dan Boy]

So I took the current gear train off today.  It's super dirty.  I'm gonna clean and Ill come back.

Found an old lathe tool stuck in it, I wonder how long it's been there.  Certainly not one of mine!

   

   

[Pete O]

It's all a little clearer now with your gear train apart; the gear that you had indicated as the 'driver' in your first photo is actually part of a compound gear train- presumably the gear at the very top-right of the first photo is the actual drive gear? This gives you more options for speed reduction as you can alter the relationships between the drive gear and the larger compound gear, then between the two gears of the compound gear set, then between the small compound gear and the final driven gear.

[Dan Boy]

Oh wow, I didn't mean to mislead anyone, it was just how I saw it.  Sorry for any confusion.

Here is a pic of the rest of the gears.  I hope I have made it clear in the picture.  

So Pete, you think I should change the lower gear in this latest picture?

   

[Dan Boy]

Maybe I should shoot a video if thats easier so you can see the whole lot in context.

[Pete O]

I'm far from an expert Dan, but I guess you have a number of options for speed reduction across that gear train; smaller gear on the lathe spindle (chuck shaft) (the fwd/rev tumble pair would need to still be able swing into mesh); larger gear on the big side of the compound gear; smaller gear on the small side of the compound gear; larger gear at the final driven shaft. I think I'd start by seeing if it is possible to achieve the desired reduction by rearranging the gears you already have; if not (probably not) then I'd look at the most economical place in the chain to achieve it, i.e. where the least number of new gears are needed. It looks like it might be possible to put a bigger 'big' gear on the compound set without needing to change any of the idlers, as the banjo can adjust (swing) to accommodate that? Looks like the way the banjo is made, a bigger gear at the final driven end will necessitate a new idler and unfortunately the banjo has been manufactured without a way to accommodate this.

One of the electronics wizzes on here could probably devise a way to drive the leadscrew electronically at an infinitely variable relationship to the spindle. That would be cool. Possibly end up cheaper than gears!

[rleete]

Varmint Al did something like that many years ago. I always intended to try it, but never got around to it.

http://www.varmintal.net/alath.htm#Ultra_Fine

[Dan Boy]

Well, that is a very interesting point regarding motorising the lead screw. If I did that I could just keep the banjo gear chain to one side.

I wonder how much HP on a motor I would need for the lead screw. I think not much.

[Dan Boy]

Just been out to the workshop.  To turn the lead screw takes very little effort even though it is going through the gear box.

Now I have this 24v motor that I am intending to use for a powered bead roller I'm building but maybe I should use it on the lathe.

The plate on the motor is not easy to read but looks to be an output speed of 6.8 RPM.  I guess I don't know the duty rating of this motor or how much load will be on it turning the lead screw while I am cutting material. But maybe I should mock it up and see how it performs.

   

   

[Pete O]

Looks like you have yourself project. A much smarter person than me would be able to synch the motor to an encoder on the spindle for thread cutting. I'm off on holiday so I'll be interested to see what you've done when I get back.