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    Re: When even a Davis Mark 3 is too much
    From: Brad Morris
    Date: 2017 Jan 15, 18:51 -0500

    A quarter degree (15 minutes) is much more like what I would expect.  

    Even if you attempted to align the arc to the index arm, both ends of the arc must be held to within .006 inches in order to get the stated accuracy of 1'.


    On Jan 15, 2017 4:35 PM, "David Pike" <NoReply_DavidPike@fer3.com> wrote:

    Brad you said:  Whilst a metal sextant, using modern machine tools, can be manufactured to that level of precision, I find it astonishing that a sextant made of wood can be manufactured to that tolerance.  Further, I don't understand how wood, given its proclivities to expand and contract with moisture content, can hold that tolerance.

    I think we have to remember at lot of our activities can be prefaced ‘Just for fun’, so experimenting with wood is worth a try.  Not that it was always ‘Just for fun’.  Tycho Brahe used wood in some of his instruments; he just made everything very big.  There are at least four surviving wooden clocks by John Harrison, one of which is still working, and there were of course the back-staff and the cross-staff plus the early octants.  We can get around many of the problems with wood by using thick plywood, nine-ply in my case, which is remarkably stable and several coats of polyurethane makes it even more stable. 

    When it comes to centring error, I’m reminded of something I read in one of the classic books on woodturning in the UK.  The author said that in the chair making industry around High Wycombe no manufacture would order chair legs from bodgers and then try to make a hole in the seat to fit the legs.  They would send the hole to the bodger and tell him to turn his chair legs to fit the hole.  You can do something similar with a plywood sextant.  First mount the arm.  Then attach a pencil to the arm to mark its swing.  Then glue the scale into position to fit the pencil arc.

    After leaving the RAF, I decided to become a Craft, Design, & Technology teacher and took a Post Graduate Certificate in Education. The final task was to produce a scientific instrument for use in schools, and I chose to make a sextant.  Today, I actually managed to find my notebook from 1982 that I used when making the sextant.  I see that I put quite a bit more thought into it than I probably would today.  I even found, slipped amongst the loose papers, my first ever shot with it.  If I’d lived at Coningsby and not Waddington, I’d have been spot on.  As it was I was about 15nm or ¼ degree out, which admittedly is slightly larger than 1’, but I was using an artificial horizon.  In addition, it was all a bit rushed, because 7th June was probably the hand-in day, and the last coat of varnish was probably still drying.  DaveP


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