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    Re: Problem with a sextant
    From: Bill B
    Date: 2006 Apr 28, 02:03 -0500

    > 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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