On 10 Dec 2005 08:43:02 -0800, jim rozen
scribed:

In article , Harold and Susan Vordos says...

... Remember, you are not really measuring with a DTI
under setup conditions, but looking for anomalies so they can be eliminated
or minimized. The indicator, in use, behaves as a comparator. It's not
really "measuring", although it does display in increments that translate
into measurements.

The analog in the world of electrical measurements is a "null detector"

The actual linear sensitivity of a last word type indicator varies
a great deal across its range because of the cosine error between the
angle of the point and the surface it's sweeping. But it will be
most sensitive at one spot and if the needle nulls (never moves)
during the measurement, one can be sure the feature and the machine
axis are lined up. The non-linearity of the detector drops out.

Jim

Yes, Jim. You are right. There are very many cosine errors to be
worried about using a dial lever gage (DTI). Some can be reckoned with
by using a pear shaped contact point that may null out some of the
angular problems. But only through a very, very small range .

Others can be dealt with by using the instrument in a parallel way...
Ie; the more you put the point out of parallel, the more error you get
on broader measurements. Tip here is only use them for very small,
comparative measurements, not as a measuring tool on the bigger
ranges. Keep it close to home.

As Jim says, the indicator is most linear in it's first few thou of
travel, (past null) and if you need more than that, better to get a
good dial gage that reads in tenths (0.0001). They are linear,
accurate and unfortunately, quite expensive.

Best as always,

Freddie