View Full Version : Gear Train for 27 TPI

03-28-2009, 10:47 AM
I have 3 gears in a train. The central one is 120 teeth. the top and bottom are 60 teeth. The change gears are 40,44,52,63,66,69,78. The lathe has a quick change gear box and 8 tpi leadscrew.
Any ideas on how to get 27 TPI with a combination of these?

03-28-2009, 11:07 AM
We need some more info:

I'm assuming this is on a change gear lathe that doesn't have a quick change gear, but I may be wrong.

To figure this out, we need to know the thread pitch on the lead screw used and the number of teeth on the gear at the back end of the lathe spindle. A photo might help.

Somehow you need to include three reductions of 3 to 1 and a multiplier of the lead screw pitch. With what youve got listed it looks possible, but I would need the rest of the info on the lathe.

03-28-2009, 11:24 AM
This lathe has a quick change gear box and a 8 tpi lead screw. It is a Victor 14x40

03-28-2009, 12:03 PM
Hmmmm... I guess I just answered on the other site:


03-28-2009, 12:25 PM
Hmmmm... I guess I just answered on the other site:


How many gears in your train and what number of teeth are they?

03-28-2009, 12:41 PM
Gears? No idea. In my case it didn't matter.... all I had to do was adjust a single gear for the exact change ratio needed.

Using my example (selecting 24 tpi) it was 24/27 = 0.8888888.

03-28-2009, 01:59 PM
Looks like 18/27=.666666666 x 60=39.999996 might work. I will try it and see,. Thanks.

Paul Alciatore
03-28-2009, 06:10 PM
OK, assuming you have a 18 TPI setting and by top and bottom gear you mean the stud gear which turns at the spindle rate and the gear box gear which is on the input shaft of the QC gear box AND the 120 tooth middle gear is just an idler.

Change the top gear to 52 tooth and the gear box gear to 78. Set the QC box to 18 TPI and cut a test thread. It should be dead on.

18 = 2 X 3 X 3
27 = 3 X 3 X 3

The ratio of the two threads is 18/27 or 2/3 (0.66666).

You presently have a 1:1 gear ratio from the spindle to the QC box with 60 teeth both top and bottom. By using 52:78 you get a ratio of 2/3 or 0.66666.

You will be cutting a thread with a smaller pitch (more TPI) so you need to step down. Therefore the smaller tooth count gear goes at the input side of the chain or at the top.

03-28-2009, 06:37 PM
Thanks Paul,this shows that there is more than one way to skin a cat!
I used the same 18 TPI but came up with a 40 tooth on top, 120 in the middle and 60 on the bottom..
I will definitely record your method for future use in case any of the gears get lost I will have another option!
And thank you for the excellent explanation of the prime factors.
This is what I was looking for, I hope I can remember it next time I need it!