Arbalest,
In response to your question:
Thanks for that Jerry. Forgot to add it's a clever idea!
Can you tell me though, if I had say a .5mm pitch thread on one end and a 1mm on the other would I get .75mm movement per revolution? I have to ask these things as I'm nowhere near as clever as some of you guys!
Note that Jerry is using 40 tpi on one end and 56 tpi on the other. In his case, when one complete revolution is made, one end will move 1/40" in one direction while the other moves 1/56" in the other - since both threads are right handed. That gives him the difference in movement. If one thread was right handed while the other was left handed he would get the
sum of 1/40" and 1/56".
In the example you cite, one end will move 1 mm while the other moves .5 mm in the opposite direction. What it will do is allow a .5 mm extension or contraction per turn. To get the same movement with a right hand thread on one end and a left hand thread on the other, you would make both ends with a .25 mm threading.
Simply stated, if both threads are right or both are left you will get the
difference in pitch movement while if they are opposing you will get the
sum. In practice, you want to pick thread pitches that are close to each other that don't share a small divisor. The idea is to get a very small movement without having to cut an extremely small pitch thread. For instance, you could cut one thread at 1 mm and the other at .8 mm and get a movement per turn of .2 mm. Or you could use .35 mm and .40 mm to get .05 mm per turn.
Alan