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    Re: When even a Davis Mark 3 is too much
    From: Brad Morris
    Date: 2017 Jan 15, 18:44 -0500

    With all due respect, it's where you place that ball bearing with respect to both parts, and not the accuracy of the ball bearing ball itself.  

    Please do not confuse resolution with accuracy.  Accuracy is measured against a standard.  Resolution is merely how many times the the arc is divided.

    It's very easy to see the effect.  Let us assume that the index error is adjusted to zero.  However, we will displace the index arm .006 inches towards the arc.  Mind, the reading is zero, yet the center of the axis of rotation is now displaced.  Rotate the index arm through 90° of travel.  The index arm will now indicate the double of 89°59', due to the mirror arrangement.  That is, you will end up with 1 arc minute of error, strictly due to the eccentricity.  If you wish further clarity, I can sketch something for you.

    With all due respect, John, after 30 years if professional leadership in the motion control field, I believe I understand centering error, accuracy and repeatability.

    You mentioned some older octants  and etc.  Please provide proof, wherein the ACCURACY of the device is measured against a standard.  The resolution has nothing to do with it.  It's the accuracy that is the claim and the accuracy that counts



    On Jan 15, 2017 1:00 PM, "John D. Howard" <NoReply_Howard@fer3.com> wrote:


    Rember that the device being made is for navigation, not surveying or astronomy.  Navigation requires the least precision of the three.  Wooden octants were made and used for a very long time, even after the metal sextant - sometimes at the same time.  The extra precision and range of a sextant over the octant was to do lunars.  A ship's navigator could do morning time sights and noon latitude just fine with a wooden octant.

    A small ball bearing used in an electric fan has tolerance of 1/10,000 inch. Small electric motors spin at 8,000 or 9,000 RPM.  Press that bearing into hard wood, then use the bearing as the pivot point to fine tune the outside of the arc.  The result should be well within the acuracity of an 1800s octant.

    John H.

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