Watchmaker's lathes often have a specialised chuck that look like a faceplate with three slots; in those slots are mounted three (one each slot) little vises. These are used, I assume, for holding thin flat things that you want to do lathe tricks with.
In all of the pictures of these that I have seen the slots are one straight radial, and the other two are curved. Why?
Here's an example: ebay #130098743586
I expect it has to do with it's intended use on watch plates. It's a bugger to try and center up on a hole at the edge of the plate when the three supports are all radial. The two supports swing aside as well as outboard, when the work requires the access.
Cheers Trevor Jones
In order for one to be able to hold ''thin flat things'', off center.
not sure exactly, I would say the curved slots would allow more adjustment in holding odd shaped items, these faceplates or mandrels are used to hold plates, wheels, bridges from watches for things like replacing rubbed in jewel settings, you need to be able to centre something like a train wheel bridge over the hole you want to work on, these bridges are not round or flat so the mandrel would need to be adaptable to hold the part.
I have a couple of these mandrels that have 2 curved slots curving away from each other with one straight radial slot in between them, just one manufactures idea compared to another.
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