SB-9C Thread Cutting Gears Larger Than 80T...What Is Their Purpose?

[Metal-Masher]

When I search feeBay for the two elusive thread cutting gears that I need (32 and 36), I often see gears for sale with more than 80 teeth, despite the fact that the chart on the lathe only shows gear sizes up to 80.

What is the purpose of these larger gears? Is it only to create a slower x-axis feed rate?

Examples:

112T
South Bend 9/1K lathe 112t change gear 9/16" bore | eBay

116T
South bend lathe 9"1K Turning gear 116t .563id .125 Key .38 Face 18dp... | eBay

 

[garyphansen]

You answered your own question, it is to provide a slower feed rate.

 

[Metal-Masher]

You answered your own question, it is to provide a slower feed rate.

Thanks gary. I was actually hoping there was more to it. Kinda like "these larger gears belong to the secret society of the Brotherhood of Mysterious Thread Sizes..."

 

[Dobermann]

The "turning" gears were, I think, provided for lathes that did not have longitudinal and cross feed capabilities--Model C in present day parlance. Those lathes without the aprons with the drive could only be moved by the half-nuts and if you wanted to make a fairly smooth turning job you would mount up the turning gear. Those lathes still did not have powered cross feed capability. However, I have two lathes like that, a 1936 Workshop lathe and a 1936 9 Jr and the turning gear can come in quite handy and can provide a nice finish. Of course, continual use of that means of feeding the carriage travel would wear the half-nuts more than you might like.