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On Wed, 26 Apr 2006, Bill wrote:
>
>> If I understand Joel, what this setup would
>> measure is distance from the arc
>> to the pivot point.
>
Alex replied

> From WHICH point on the arc to the pivot point?
> (The outer part of the arc consists of teeth.)

From whatever point on the arc the probe of the dial indicator is resting
(mostly, to be exact the dial-indicator tip would have to be aligned with
the degree marker, and that is not going to happen with an assembled
sextant--worm gear will be in the way).  With a large flat tip it will
measure the high points along the arc and not dip into the teeth.  With a
small needle tip if will measure the high and low points along the arc (top
of tooth and as far as the tip can get into the bottom of the tooth).

Bill had written

>> I believe he suggests securing the sextant's arm and placing the dial
>> indicator against the arc
>
> Against what part of the arc? The only part of the arc which is relevant
> is covered by teeth.

I misread Joel.  He suggested mounting the dial indicator to the arm.
Easily done if a magnetic base could be used, but alas....

My method would secure the arm and move the frame.  Same deal.  In either
case you measure the distance from the pivot point to wherever the tip of
the dial indicator is located on the arc.

As a crude example, say we take a piece of poster board and a compass and
draw a circle on the poster board.  Then we cut out the circle with a pair
of scissors.  We want to know how close we came to making a perfect circle.

We stick a pin or nail through the hole left by the compass point, and mount
our disk to a board.  Then we take the dial indicator and place the tip
against the outside of the disk.  Then we rotate the disk and record the
measurements at points along the circle.  That tells us how close our
"machining" of the disk came.  Off course if the pin is not *exactly* at the
same location the compass point was, that will throw off our measurements.
Or if we spin the disk enough that the hole enlarges (non uniformly) that
will throw off are measurements as well.

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.

Bill

   

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