[How do I?] Need to Install Metric Transponder Gears on a Smith Lehmann Oil Country Lathe

Blair

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Wondering if anyone could be of some assistance

I have a gearing problem, this lathe is English threads only, and I need to install metric transponder gears
this is how the gears are setup
It has a 24T Stud gear, meshed with a 40T Idler, to another 40T Idler, Meshed with a 48/18 compound gear set, which is Meshed with a 72/24 compound gear set, which is the Screw/Input Gear to the gearbox for the Feeds/Threads
The 72/24 compound gear slides on the gearbox input shaft, which is splined, from the A position to B position to select the Feeds/Threads selection

Now this is what I am thinking, correct me if I'm wrong

These two compound gears are not accessible to change so essentially they are sort of, a part of the gearbox
The second 40T idler gear meshes with the 48/18T compound gear
So my question is can I use the second 40T Idler as my Screw Gear Directly or would I have to place a gear beside it to make it a compound gear, but replaceable to act as my Screw Gear making all the gears after it basically part of the gearbox
I am trying to be able to put a metric 127/100 transponder gear set in between the stud gear and the 40T Idlers so I can put the proper Stud and Screw Gears to cut metric threads. The stud gear is not changeable at this time, but i can set it out so i can change it, and make a plate to hold the necessary gears in the area where the two copper pipes are, I'll move the pipes
I just need to know the correct gearing and where to put the gears

My plan is to set the Gearbox to cut 8TPI which would basically convert my lead screw to 8TPI like on the old Southbend lathes with no QC on it, then use the southbend metric chart and gears to cut metric threads. A little awkward but should work. Just got to get a setup where my screw gear is out where i can change it

I should be able to do this but I cant get my head wrapped around this gearing setup

Thanks for any and all help you can give me
Blair
 

hman

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Here's an article that may be of help. Note that to get exact metric threads, you'll need a 127 tooth gear somewhere. Your gears appear to have a pretty large pitch, so a 127 tooth gear might not fit. The article suggests several alternate sets of transposing gears that give pretty close approximations.
 

Blair

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I’ll check it out thanks
You’re right it’s 8dp
But for the conversion I can use smaller DP my problem is understanding where and how to put the gears that would go on the screw normally because I cannot access it, so I will have to make an arrangement between the stud gear and the 48T gear so I can access it and put what ever gear I need to
Thanks I’ll go check it out at noon, at work now
 

benmychree

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What pitch leadscrew do you have on the Lehman lathe?
 

Blair

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It’s 2tpi
The stud gear is 24T the Screw gear is either 24T or it’s a 72T depends on wether it’s in A or B position
So basically I need a setup between the stud gear and the 48T gear that relocates the screw gear before the 48T gear so the 48T compound gear and the real 24/72 compound screw gear thinks or acts like it’s part of the QC gearbox and the 40T idler that meshes with the 48T compound gear would now be my Screw Gear so I can change it to what ever I need to to get my metric pitch
If I put the QC in 8 tpi position and leave it there then essentially my leadscrew would be 8 tpi like on the old Southbend lathes
 

Illinoyance

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Metric transposing gears normally involve a 127 tooth gear. It doesn't look like you will have space for it.
 

benmychree

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It’s 2tpi
The stud gear is 24T the Screw gear is either 24T or it’s a 72T depends on wether it’s in A or B position
So basically I need a setup between the stud gear and the 48T gear that relocates the screw gear before the 48T gear so the 48T compound gear and the real 24/72 compound screw gear thinks or acts like it’s part of the QC gearbox and the 40T idler that meshes with the 48T compound gear would now be my Screw Gear so I can change it to what ever I need to to get my metric pitch
If I put the QC in 8 tpi position and leave it there then essentially my leadscrew would be 8 tpi like on the old Southbend lathes
My lathe has a 4 TPI screw, you could also use my chart with a suitable multiplying the pitch by two.
 

Blair

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That’s not the problem
I can make smaller gears to fit don’t need gears that big to cut threads
Take lighter cuts
I need to know if the 40T idler gear can be used for screw gear in place of the actual QC input shaft cause I cannot change those gears
 
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