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> Bill,
>
>> With a large flat tip it will
>> measure the high points along the arc and not dip into the teeth.
>
> I see. Then the problem is how to attach it to the arm rigidly.

Yes.
>
>> Easily done if a magnetic base could be used, but alas....
>
> ??? Sextants are not made of iron. They are non-magnetic.

Yes, therefore the "but alas...
>
>> My method would secure the arm and move the frame.
>
> How do you imagine securing the arm? You also need to secure your
> measuring devise, so that it does not move with respect to arm.

I have given it little thought, as it initially would point to one (or a
combination of) of three possible defects, none of which I can affect.  The
best one could do is what you have already done.  Measure the error in a
dynamic rather than static situation, and adjust for it in your
observations.
>
>> case you measure the distance from the pivot point to wherever the tip of
>> the dial indicator is located on the arc.
>
> That is the excentricity.
> Though I am very sceptical. Yoiu have to secure the arm and the
> gauge to the same firm foundation (how?) and be sure that the
> gauge is oriented towards the pivot of the arc.

Not a huge problem, but what is the point?  It is a static measurement.  I
for one use static measurements to set up a tool in theory.  Only after I
get it up and running and make a cut do I know what it will do with all the
little variables included. For example, in a simple device like a table saw,
the blade may not be perfectly flat, they will be arbor runnout due to its
bearing, pulleys, belts etc. Is the miter slot (which I initially aligned
the blade to) perfectly straight and parallel to the other miter slot that I
align the fence to? What are the tolerances in grinding the cast-iron top
etc.?

Point being, if anything in the system moves, static measurement are only a
beginning.  What matters is how it behaves in operating conditions and how
to calibrate/adjust for reality vs. theory.

I have a $29 (Chinese) 9" X 12" X 2" granite surface plate (lapping stone),
certified to be within plus/minus .0001" of flat at 20d Celsius plus/minus
5d C.  This is measured with the Auto Collimator, "which can detect surface
errors optically to 0.000005" per inch."  This my friend is what you need, I
think, to finally satisfy your quest to determine the machining of the
Sno-T.

In the global overview, Formula-1 race cars, fighter jets, and space craft
are machined by some of the best shops in the world.  That doesn't seem to
stop them from crashing and burning--or just plane old blowing up--does it?

>
>> Again, I am not clear this is the measurement you are looking for.  My
>> reading is you want the distance from a given point on one tooth to the
>> next.
>
> That's what I originally wanted. But then again you have to secure the
> gauge to the arm.

I feel measuring tooth-to-tooth would be a different setup, and outside the
scope of a dining room table or garage workshop for most of us.

Bill

   

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