wrote in message
...
Marking rough castings for centerdrilling used to be a must-have
skill. My 1905 Audel's shows one way. You use an oddleg caliper, one
leg is curved like normal outside calipers, the other is straight(and
sharp). Set the thing a little more than half-way to the other side
and draw an arc off the edge. Rotate about 90 degrees and repeat,
repeat 2 more times and and you'll have a rough box shape marked out.
Use a machinist's rule across the diagonal and scribe a line.
Repeat. Centerpunch the intersection and it should be close enough to
center. Drill using a center drill. Repeat on the other end.

***I believe it is called a hermaphrodite caliper. I have one of those but
on small parts it is quite useless. I have gone into this in my reply to Jim
Wilkins. I did like his idea of spinning the part and letting a pencil draw
a circle about the rotational axis the other end. I have not tried it yet.

As far as interrupted cuts, castings were snagged off using chisels
and grinders so they weren't really too far out. With iron castings,
you have to make sure you get under the hard skin on the first cut,
otherwise the tool edge disappears. With brass, you just have to make
sure you aren't cutting sponge with embedded sand.

***That would be the residue of casting?

Thanks

--
Michael Koblic,
Campbell River, BC