beginner question about name plates

I recently bought and restored a '45 South Bend 9a and I have started learning the basics of making parts and doing some threading. My first project was a thread dial so I can now use my quick change gear box to do what it is supposed to do!

I have to say the learning is one of my favourite parts of this machine and I have gains tons of knowledge and inspiration from lots of people on this forum. What a great resource!

I have a question about the quick change gear chart plate on my 9a. I understand how it work but I am unsure about what the small numbers are for under the TPI numbers. For example, I am currently making a collet closer bar and need to cut 26tpi on the inside of the tube to hold the 3c collets. On my 9a plate the number 0.0131 is under the 26 (with a 20T stud gear) on other plates I've seen the number 0.0129 under the 26 (with the 40T stud gear).

I've included a picture that shows both. I got the image from Jim that restores them. Nice work by the way Jim!

Can someone shed some light on what that 0.0131 means?

thanks!!

cheers,

monty

It's the feed per revolution in thousandths.

Michael

Thanks Michael.

Be aware, that the feed rate movement is different from the thread rate movement, because the gearings are different.

Best wishes --- Allen

Just remember that the apron has a three position selector, which you use to choose either CROSS FEED, LONGITUDINAL FEED, or THREADING (which is really neutral). For the power feeds, you use the star knob (or lever) to engage the carriage feeding movement. Do NOT use the half-nuts for power feeding (only threading).