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    Re: SNO-T Backlash
    From: Bruce Stark
    Date: 2005 Nov 23, 13:59 EST
    I'm convinced the mystery screw tightens and loosens the connection of the worm screw to the index. I think its main purpose is to adjust backlash.

    But it should be approached with care. As Alex has pointed out, mess up the slot in the head of that tiny screw and you're out of business. You'll want a screwdriver that fits perfectly. I had to buy a set. If the screw doesn't turn easily I'd give it some WD 40 and try again next day. And note that it doesn't go straight in. It's almost tangental to the trommel.

    Before explaining what I did with my SNO-T, and the results I got, I'll say something about the sextant itself. I bought it simply to get an inverting scope with parallel wires for my Tamaya. But I've learned to like the SNO-T a lot. It was made in 1974 and clearly has seen plenty of use. Moreover, the large shipping box it came in looked like it had been dropped five stories onto concrete, several times. Supports inside the sextant box were splintered and had to be glued back together.

    The thing I didn't like about the sextant was that the vernier drum turned too easily. Widely divergent numbers would show up in sets of observations. What was happening, I finally realized, was that the control knob sometimes brushed my sleeve. I suspected the mystery screw might control the problem, but didn't want to risk a shot in the dark.

    Then Alex brought up the question again. I found that backing the screw out did tighten up the drum. But I couldn't get it as tight as I'd have liked without a slight "chatter" occuring when making small adjustments. Hoping to correct that, I put a few touches of clock oil where it might work its way into the connection. At the same time I checked for the problem Clive pointed out a few days ago and, using a lens, examined the gear teeth in the sextant limb.

    Since I'd brushed out the gear teeth before examining them, I applied a touch of clock oil and ran the worm screw from one end of the arc to the other and back, twice. Besides re-oiling the teeth, the idea was to help Nye oil work its way into the connection the mystery screw controls. After that, the screw was backed out to get the resistance I wanted in the micrometer drum. No chatter now. Small adjustments were buttery smooth.

    Next day I got a few minutes with the sun. Measurements of diameter, going from on the arc turning left to on the arc turning right, then off the arc turning right to off the arc turning left, three times around, suggested there's no significant backlash.

    Alex has pointed out that the manual warns against the screw being adjusted by anyone other than a specialist. Besides the likelihood of messing up the screw head, seems to me there's another reason for this. These sextants had be reliable under tropical sun and in arctic cold. What would be the perfect setting of the screw in one climate might allow too much backlash, or else cause the drum to "freeze," in the other. Only a specialist would know the parameters and how to adjust for them.

    Bruce
       
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