Several shop made tools I've done

[ronzo]

The threads are the same, just two different diameters. They are both 20 TPI.. One left hand and one right hand. The larger one moves thru the block, the smaller one out of the larger one. That is why one is left and one right. If they were both the same it would not work. You are correct on the movement direction.

"Billy G"

Another variation of this is a differential screw which I have used several times when I was designing mfg. equipment. it is used for a very fine adjustment. Lets say you used a 32 pitch thread as the primary screw, then a 40 pitch inside the first one. When the primary screw is turned one revolution it would advance .032 while the secondary screw would retract .025 the net difference being .007 per revolution.
Ron
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[DMAM]

I can most certainly attest to Mr. Clines teaching methodology, and the fact that had I not went through his class I would never have got to where I am today. With the knowledge I gained from him I have machined everything from parts for Hendrick motorsports to prototyping parts for our armed forces....... And I still use my little hammer daily that I made on the lathe in his class.