Questions on making a self-centering jig

[sleepy23]

hey guys,
i am looking at a design that needs a self centering jig for aligning parts. i have not had that much experience in capturing parts with anything other than grippers. Essientally, i want a hand crank that closes the jaws towards the center. i cannot purchase a self centering vise due to special needs. i think I know how they work, but just need some verification from someone that has been around them. here is how i think it should go:

1 shaft: one end has an area to attach a handle followed by right hand threads followed by an idle area followed by left hand threads
so it would look something like this:

p----rrrrrrrr-------lllllllll---

where 'p' is the handle, '-' is idle shaft, 'r' is RH thread, and 'l' is LH thread

is that correct? if not, could someone please give me some insight as I am under the gun to get a design together.
Thanks

[Zumba]

Yes that will work. You probably want to swap the "r" and the "l" though.

http://www.nookind.com/acme/AcmeTwinlead.cfm

[sleepy23]

thanks, i greatly appreciate the answer and the link

[dfurlano]

OMG!!!! Do you know how long I have been looking for these???

[sleepy23]

it was a great link. i already downloaded a model and buit my design around it.