On 2008-02-28, Wes wrote:
wrote:

Sigh...another question from StudentLand, that strange area where,
after reading the textbook, being shown what to do by the teacher, the
results are STILL all over the place......

I always insert the telescoping gage with the points as close to center line
but the handle tilted off bore axis and 'wipe' the bore with the screw
locked. The only way the gage can come out is by the bore forcing the gage
to a more closed position.

Hmm ... with aluminum, at least, you want the gauge only
semi-locked. The aluminum is probably soft enough to deform instead of
forcing the gauge to proper dimensions with it seriously locked. Once
you have the feel right, then try fully locking the gauge and make sure
that it still wipes through with the same drag as before. If it
produces more drag, the gauge has shifted when you fully locked it, and
it is time to try again.

A developed sense of feel is essential along with duplicate measuring to
deal with errors in technique. I use some ancient Starrets where only one
side telescopes. Haven't let me down yet.

They are good ones. Some which I have had from GSA were really
rough. Interestingly enough, a Craftsman set from the early 1970s was
as good as the Starrett. But they shift makers from time to time. At
that time, they were using Scherr Tumico as the maker of their
micrometers.

I assume you have access to a lathe and sand paper. You could make your own
dedicated plug gages to check the fit of the hole. Turn it close, sand to
final diameter.

Indeed.

Enjoy,
DoN.

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