NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Construction of Sextant Arm Pivot Questions ?
From: Bill Morris
Date: 2007 Nov 14, 23:20 -0800
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, Robert11wrote: > 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) --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---