This is a centre finder for lathe work. If ever you have had need to align a centre punch mark to centre you will know all too well how tricky it can be and there is precious little that is commercially available to do this. Every person I have shown this to has said very nice what is it, okay I haven't seen one in forty years, and am now beginning to think I invented it way back then. This one I made for my Rivett and the tool holder on that machine takes half inch round hence the shank on this one is made to fit that. It is such a simple thing to make and requires no real precision in its making, it is just a set of gimbles and a sprung loaded rod with a hardened point. Because the rod does not rotate you can measure error with a clock gauge once it is close, prior to that you can do so by eye against the tailstock centre. Of course by using the principle of leverage, the tail (being longer)errors are exaggerated greatly.
The big secret to this is to bore the holder out, so you can park the thing away when not in use. How much trouble you go to, to make one, or make it nice, is of course entirely a personal choice.
Click here for the top row, the instrument itself.
Click here for the bottom row in use on a mock up.