On 2008-02-29, Harold and Susan Vordos wrote:
"Mechanical Magic" wrote in message
...
There is another method for precision measuring of holes. Inside
micrometers.
http://www.shars.com/product_categor...de_Micrometers
I have been using them for years, and wouldn't be without them,
although I only have up to two inch.
Since the barrel travel is not intuitive (to me), I always check the
reading with a caliper to verify, or a calibrated outside micrometer
for precise measurements. The 2" does have some spring, and a good
feel is required to get better than .0005".
Chuckle!
You're joking about using calipers to verify your readings, aren't you?
I don't know for sure, but I think that it may be a case of
using calipers to be sure about the more significant digits. The ones
which I saw at the web site were not what I was thinking of, which was
the short 0.500" range micrometer thimble with screw-on tubular
extensions to allow it to measure the inside of a bore. Since there is
the basic length of the micrometer itself, plus the possible added
length of tubular spacers on either side, it is easy to lose track of
what the zero length should be, so it is easy to be off by 0.500" if you
add the parts incorrectly. Using the calipers to get the approximate
length will help you to be sure that your readings are in the right
ballpark, and is something that I would do to be sure if I did not use
the tubular inside mics often enough to trust how they read. IIRC, the
set which I used at work had a non-intuitive 1.250" length for fully
closed with just the basic end caps and no extensions.
I certainly don't think that he meant that he was using the
calipers to get the final digits right, just the coarser measurements.
I don't give a damn about the type of caliper, or how many places it will
display a reading, they can NOT be relied upon for measurements closer than
+/- .003", particularly if you're attempting a press fit with a pin, using
both inside and outside jaws. They're simply not made that precisely, nor
do they retain their original precision after being used for a period of
time.
Of course.
Where I was trained, QC could not accept or reject any dimension closer than
+/- ,005" with calipers. They were caught way too many times missing the
boat by a thou or two.
Aside from dial bore gauges, or inside mics, there's nothing quite as good
as telescoping gauges in the hands of someone that knows how to use them.
My own favorite is the tri-mics which will be self-centering and
pretty much self squaring as well. I've got a few sets (picked up in
eBay sales) covering from something lke 1/2" up to about 2", plus one or
two much larger ones. They are what I use when I really want to be
sure, and I check them against the ring standards which come with the
sets before use.
Enjoy,
DoN.
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