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I own a FREIBERGER TROMMELSEXTANT for a while. It was manufactured in 1993
because of serial number 930113 (after the German re-unification in 1990, so a
"non-communist" one ;-) ). I bought it second hand in 1995 in an - I would
dare to say - unused condition. The certificate (issued by FREIBERGER) is
dated 25 Feb 1994 and says:

Deg / Corr.
0 / 0"
10 / +4"
20 / -1"
30 / -2"
40 / -1"
50 / -1"
60 / -7"
70 / -7"
80 / -2"
90 / -6"
100 / -15"
110 / -20"
120 / -25"

Not too bad, I think.

Werner


Am Montag, 16. April 2012, 03:27:36 schrieb Bill Morris:
> Bill B wrote "...the errors along the arc are not a sine--or any
> other wave.  They come and go at their own pace."
>
> The reasons for errors are complex. If the centre of rotation of the index
> arm is not at the centre of the rack, there will indeed be a sine error.

> The teeth of the rack are cut on a hobbing machine. Simply put, a steel copy
> of the worm has longitudinal teeth cut in it and is rotated, so that when
> it is pressed against the proto-rack it tends to make it turn and excavates
> teeth at the same time. Since the worm , or hob, as it is called would then
> be dragging the proto-rack, the teeth spacing would not be accurate, so in
> practice, the hob and the rack are geared together, so that they rotate in
> the correct relationship to produce the correct tooth count and, if
> everything is perfect, perfectly even spacing of the teeth. In practice,
> the teeth are usually excavated to their full depth in one pass.

> Everything seldom is perfect. The hob itself may not be perfectly centred on
> its shaft. The gears in the screw-cutting lathe or grinding machine used to
> make the thread form on the hob, that connect the lathe spindle with the
> cutting tool may be imperfectly centred or not have perfectly regular
> spacing,or there may be errors in the leadscrew bearings. In the hobbing
> machine, the gears that connect the hob to the rack may have similar
> imperfections. All these tend to be reflected in the finished rack,
> producing a very complex picture.

> Add to this recipe various imperfections of the worm which are usually
> reflection of imperfections in the lathe that produced it and it is a minor
> wonder that makers of the SNO-T and C Plath sextants could regularly
> produce sextants that are indeed "free from error for practical purposes",
> which I take to mean errors of less than 12 arcseconds.

> I personally would not be reassured to find that a modern sextant had errors
> in excess of 30 seconds, and 50 or 60 seconds is not acceptable, correction
> table or not. It reflects badly on the goodness of manufacture and implies
> worn out machinery, lack of expertise or lack of care. I suspect such
> sextants are now being produced simply so that the ship manager can say
> that they do indeed carry a sextant as the regulations demand.

> Bill Morris
> Pukenui
> New Zealand
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