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    Re: Plexiglass horizon
    From: Richard M Pisko
    Date: 2003 Jul 15, 11:01 -0600

    On Tue, 15 Jul 2003 15:48:32 +0100, George Huxtable wrote:
    >As for the mechanics of it, I suggest that at each adjustment-point there
    >should be a spring pressing the plate down against an (ideally) knurled nut
    >working below the plate. A couple of domed spacers would help in allowing
    >the plate to tip slightly without binding. Similarly, the holes could be
    >made with a slight "hourglass" shape to allow a bit of tilt. This is easily
    >done; first you drill a straight hole and then with the drill right through
    >the hole, wiggle the drill-bit in a narrow cone so that its flutes enlarge
    >the hole near its ends but not in the middle. It's useful to put a bit of
    >stiffness into the movement of the nut, such as by running a wisp of fibre
    >between the threads, or using a "nyloc" nut.
    I like the idea, but would suggest a two plate method.  The bottom
    plate heavy with threaded friction fit holes, the top with a conical
    pit, a slot, and a flat on the underside.  The top plate would just
    rest on the rounded points of the screws or threaded rod below.  The
    pit anchors one corner of the top plate in all directions but allows
    rotation.  The slot on another corner points to the first corner and
    stops the rotation while allowing distortion free thermal expansion
    problems.  The flat (no treatment is really needed, the underside of
    the top plate is good enough) just allows gravity to remove the last
    degree of freedom.
    A double nut capturing a large area washer, at about the mid point of
    the gap between the two plates, on two of the screws would allow
    finger adjustment of those two screws.  Best if they are of a very
    fine thread pitch, so a slight movement of the fingers results in a
    similar slight but perceptible movement of the movement of a sensitive
    bubble vial.  This gives a "sweet" action.
    Richard ...

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