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Sextants seem to have followed the practice of other surveying
instruments. The older ones locate a fairly slow taper(about 4.5
degrees included angle) in a matching hole and this accommodates
mainly radial load as well as centring the bearing. A slow taper
cannot accommodate much thrust before it jams(cf. Morse taper shanks,
included angle about 2.5 degrees), but in a sextant or theodolite
there shouldn't be much axial load, just enough to cause the taper to
locate properly. In a sextant, the bearing is lightly loaded and
always moving slowly, so I cannot imagine that significant wear would
ever occur after initial correct fitting. There is really only one
taper, with the middle part on the shaft machined away, so it looks as
though there are two. This is an old trick to improve goodness of fit
when the total area of contact does not need to be large.

The outside of the bearing seat is also tapered. My guess is that this
is used to locate the sextant in a taper hole on a jig prior to
cutting the arc and teeth. Producing the taper shaft is simple taper
turning. If I were producing a tapered hole as a "one-off" I would
drill, taper-bore with a single point tool and ream with a tapered
reamer. The shaft and the hole can both be lapped. The lap has to be
softer than the part being lapped, otherwise the abrasive grains embed
themselves in the workpiece.

Newer instruments such as the USSR SNO-T(Sextan Navigacionnyis
Ostretitelem-T : Tropicalised Sextant for Navigation, with
Illumination) use a large plain parallel bearing with correspondingly
large "thrust" faces. Presumably, this reflects an ability to
manufacture such bearings so that there is no significant play and
hence centring errors. This is a rather severe requirement. Leaving
aside errors in dividing the teeth of a micrometer sextant and errors
in the worm, the maximum allowable eccentricity to keep errors under 6
seconds is about 0.002 mm.

The older sextants have bronze running on bronze. Hardened steel on
bronze would be better if resistance to wear were important, but would
not be ideal in a marine environment. The SNO-T sextant has a bronze
shaft running directly in the frame, but the bearing seat seems to be
very hard. Perhaps it has been locally anodised.

If Bob will send me his e-mail address I will send him(or anyone else
interested) photographs that anatomise the two types of bearings. The
ten photos in pdf format occupy nearly 5 gigabytes.

Bill Morris




On Nov 9, 2:06 am, Robert11  wrote:
> Hello,
>
> I guess I should introduce myself, somewhat.
>
> Am a retired research engineer, and have always been interested in
> navigation, but, unfortunately, more on the theoretical than the
> actual "doing" side.
>
> However, I did complete the two celestial navigation courses given
> many, many, years ago by the U.S. Power Squadron, which included the
> taking of many sextant sites.  Can't remember when i enjoyed any
> courses more.
>
> I don't really have the opportunity to inspect any sextants now
> firsthand, but am quite interested in how these folks did, and
> presently are, designing and fabrication the pivots for the arms.
>
> Are they just a bored, and subsequently honed, hole for a steel pin ?
>
> Is it a cylindrical hole, or tapered ?
>
> Is there any "preload" to minimize wobble or looseness with wear ?
>
> Different mfg's doing it differently ? In the past ?
>
> Any good detailed pix available ?
> etc.
>
> Great Group; have learned much.
>
> Regards,
> Bob   (Sudbury, Mass., USA)

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