South Bend 9" metric lathe question

[josef]

I have an old circa 1950 South Bend lathe that has a metric gear box for cutting threads. I've used the lathe for many things but am now trying to learn threading. When I set things up according to the chart on the gear box, I should get a 1mm thread pitch. Instead I get 2mm. I don't know if the previous unknown owner made any changes to the gearing between the main shaft and the gear box. It appears that one turn of the main shaft results in one turn at the input of the gear box. As far as I can tell, one turn at the input of the gear box gives 3mm movement and checking the thread pitch with calipers gives the same 3mm thread pitch for the lead screw.
Can anyone help me figure out what is going on? I have the South Bend "how to run a lathe" book and a parts list but the parts list does not give gear teeth numbers.

 

[woodchucker]

I am not familiar with the Metric gear box. Does it reference different stud gears.
On my Imperial it calls for both 40 and 20 on my gear box chart. 20 for most, and 40 for the coarsest threads. Is it possible you have it set up with the coarsest stud gear?

 

[benmychree]

A gear on the shaft running into the feed box that is twice the number that is currently on it, or a gear on the stud that is half the number of teeth should solve the problem.

 

[Jim F]

According to 55th edition of HTRAL, you need a 50 tooth and 20 tooth gear for the stud position.

 

[josef]

Well, I think I figured out the problem. I think that there should be a 50T gear on the lead screw shaft. There is a 20T gear there now. This makes the ratio from the main shaft to the lead screw 1:1. On the metric gear box it calls out a 20T stud gear for small pitches and a 50T for larger pitches. Working through the math, as the machine sits now with a 20T stud gear and 20T lead screw gear it acts as if you are using a 50T stud gear.
There is also a 45T gear stuck on extra. Not sure what that was for. Anyhow, I think I'm okay now. Thanks for all the suggestions.