NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Sextant calibration in the workshop
From: Bill Morris
Date: 2008 Jan 26, 11:38 -0800
From: Bill Morris
Date: 2008 Jan 26, 11:38 -0800
Fred wrote: "One would expect that some smoothly varying curve passing through the points would describe the errors, so that one could interpolate between points to estimate an error. Unfortunately, this may not necessarily be so. " The varying patterns of errors seen that Fred refers to is likely due to complex interactions. Older sextants had the racks cut by a dividing process with one tooth(more accurately, the space between two teeth) being cut at a time, with the sextant rotated between successive teeth on a dividing engine. The sextant's errors would be a reflection of the dividing engine's errors, which in turn would reflect errors in the lathe that cut its tangent screw(in turn reflecting errors int the gears linking the lathe spindle to its lead screw, in the leadscrew itself, etc.)eccentricity errors of the engine's spindle and of its tangent screw, axial slip in the tangent screw and so on. Differential heating of the engine or of the sextant during cutting of the teeth would add to the confusion. Members may be interested to know that the first dividing engine with any pretence to high accuracy was made and described by Jesse Ramsden in 1777. A number of exact or very close copies were made by Troughton and other instrument makers and one of them was borrowed from the Science Museum in London in June 1942 by D Shackman and Sons to produce "upwards of 2000 sextants" for the US Navy. Shackman's was still active in February 1990, making cameras rather than sextants, and the then Chairman wrote " Mrs Eastwell....recalls visiting the NPL with her father(Mr Reuben Shackman) when he used to take the sextants for inspection." After numerous(and increasingly anxious)requests, the engine was returned to the museum in November 1951. It is not clear whether Shackman's were making Mk II s. Certainly there is at least one sextant they made(in the Smithsonian collection) that closely resembles the Hughes and Son "Mate" sextant, made for Kelvin and Wifred White in 1942. I do not know how the rack of modern sextants is cut. A hobbing process is inherently more accurate, but this is not the place to go into details. Frank wrote: "Since I have a Tamaya clone, can you elaborate on this? I'm not clear which direction you're talking about. " In a C Plath/Tamaya and clones, a "thrust pre-load spring" bears on a bronze insert in the end of the micrometer screw and keeps the right side of a thrust collar in contact with a thrust block. The sight should be made in such a way that this load on the thrust block is always increased, not decreased. A moment's thought will show that doing otherwise will tend to take the right side of the thrust collar away from its bearing surface. Then, when turning of the micrometer screw stops, the spring will return the thrust collar to its rightful position, but with uncertain force, dependent on variable friction from the parallel portions of the micrometer bearings and from the index arm bearing. Turning one way gives a positive final setting. Turning the other gives an uncertain final setting. On sextants like the SNO-T and Frieberger, thrust can be taken at both ends of the screw, so that potential backlash should be less than 12 seconds. On the Mk II that I own, the backlash is variable and can amount to a couple of minutes. Members who would like a photo of my calibration setup and a labelled photo of the micrometer screw thrust arrangements may e-mail me off- group. Bill --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---