Welcome to the NavList Message Boards.


A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Add Images & Files
    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.
    >> 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
    >> 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
    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
    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.

    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.

    Email Settings

    Posting Code:

    Custom Index

    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site