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    Re: Construction of Sextant Arm Pivot Questions ?
    From: Bill Morris
    Date: 2007 Nov 14, 23:20 -0800

    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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