"DoN. Nichols" wrote:


Note that some makers design so the proper reading is given at a
specific angle -- not necessarily 90 or 0 degrees to the work surface.


DoN:

This is true. Interapid test indicators seem to be one brand that is
most accurate when the contact point is at 12 degrees rather than
parallel to the work surface.


http://www.longislandindicator.com/c...tion.html#Test Indicators

An excerpt from the above site:
===============================================
Test indicators can also be calibrated on a surface plate using
certified
gage blocks. The indicator is securely fastened to a stand and the
contact point is brought in contact with a gage block of a given size.
The
contact point must be parallel with the surface of the block for most
manufacturers. Interapid test indicators are an exception and should be
at
a 12-degree angle, approximately.
================================================

Here is a cosine error chart from the same site with a correction
factor to be used for various indicator tip angles. (If it doesn't copy
correctly just go to the site)


================================================
About the cosine error: for test indicators excluding Interapid models.
If
the contact point can not be kept parallel to the work surface then you
will have to make a mathematical adjustment to the dial reading.

contact correction
point angle factor

10° 0.98
15° 0.97
20° 0.94
30° 0.87
40° 0.77
50° 0.64
60° 0.50

From this chart you will notice that a contact point held at a
60-degree angle results in one-half the dial reading. Once you
determine the angle, simply multiply the dial reading by the
corresponding correction factor.
For example, an indicator reading of .0085" at an angle of 30-degrees
is equivalent to .0085" x .87 = .0074"
===============================================

Some indicators used to be made with involute "balls" (sort of pear
shaped tips) that automatically corrected for minor cosine error. But I
haven't seen any in years.



Or you could set it up on a stable surface, then slide a piece
of shim stock under it to deflect it a known amount.

Using shim stock to try and calibrate your indicator is probably not
such a hot idea. Especially with a .0001 indicator. Shim stock not
only varies in thickness but can be bent, or burred. That's what Jo
blocks and Height Masters are for. If you don't have a Height Master or
Jo blocks then in a pinch you could use pin gages. Mic them first to
see what they actually measure.


--
BottleBob
http://home.earthlink.net/~bottlbob