In article ,
BottleBob wrote:
"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.
[ ... ]
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.
O.K. I knew that I had encountered at least one such brand.
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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)
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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
That is simply the cosine of the angle. Something which can be
looked up in tables, or with a scientific calculator, had at the press
of a button -- to finer steps than are given above. (Though getting
that close a measurement of the angle of the tip is a bit problematical. :-)
[ ... ]
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.
I suspect that they wore faster than the spherical ones, and
were more expensive to replace.
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.
But -- shim stocks (or feeler gauges) might be present in a shop
which doesn't have a set of Jo blocks (or equivalent), though a cheap
Chinese set would be good enough for the purpose.
Ideally, the shim or feeler gauge would be placed between a ball
contact and the ball on the indicator, to eliminate the problem with
bent or burred examples.
And all things considered (including the cosine errors), you
really don't want to be depending on such tools for truly accurate
measurements, anyway. A plunger style would be better, as it has no
cosine error if set up square to the surface being measured.
Shim stock would be good enough to get an order-of-magnitude
indication of the sensitivity of the indicator -- for such things as
telling whether you have the right length feeler arm on the indicator.
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.
Again -- I doubt that you would be using that style of indicator
for precision measurements -- unless the access to the point to be
measured precludes anything else.
I would agree to the use of Jo blocks for checking a plunger
style indicator -- especially a tenths-reading one (or more sensitive,
if you are really lucky.)
Enjoy,
DoN.
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