"jim rozen" wrote in message
...
In article , Robert Swinney says...

If I may "go off half cocked" and without giving it a lot of thought
(popular reaction lately on RCM): It seems that the 0.0005" might be
absorbed, so to speak, by making the drilled center slightly larger but
with
a correctly shaped 60 deg. cone. In other words, the point of the center
drill would drill larger by the amount of runout and then guide the cone
in
correctly. I know, intuitively, this is not right - but help me out
here.

Bob Swinney

Uh oh. You just pushed Harold's hot button!

I'm pretty sure I know what he's gonna say here....

Jim

:-)

Yep!
When you're looking for that level of precision, the center becomes all
important. That's one of the reasons center laps were made, aside from
re-locating centers when a thou made the difference between a good part and
scrap.

What Bob suggested works, but only to a degree. The biggest problem is it's
not predictable.

What Jim said is on the money. You can't make determinations to tenths when
a test bar has more error than the one you're looking for. Standard
practice in the tool room where I was groomed was to have a tolerance of
only 10% of the product tolerance for tooling. It wasn't uncommon to find
dimensions with only a tenth tolerance. It makes sense if you want proper
results.

Harold