Backlash and Micrometer Errors

12 03 2010

Lately on NavList there has been much written about accurate determination of index error, some of it relating to the instruments used and some relating to the observers’ physiology. Recently, Richard Pisko mentioned the subject of backlash, in the context of surveying instruments and I see there was quite extensive (though not always well-informed) discussion of this on the list in 2005. Not for the time being having any new sextants to pull apart, I have devoted a little time to considering aspects of accuracy of a few micrometer sextants of various ages. All are second hand but in good condition and have been completely overhauled.

 Resetting accuracy

 I did a prelimary determination that each sextant could be reset to a given reading within the precision with which I can read the testing instrument, by taking a series of thirty readings, each time resetting the sextant to the initial reading, from the same direction, to avoid backlash. Give or take a second or two,  all met this criterion.

 Backlash or lost motion

 Backlash due to lost axial motion in the micrometer mechanism was dealt with in three ways by different manufacturer.

 C Plath seems to have got things correct from the beginning (in about 1909). Manufacturers, like W Ludolph, Tamaya and Astra who imitated his design principle did well to do so. In this design, a small leaf spring (labelled”pre-load spring” in Figure 1) bears on one end of the worm shaft to hold a shoulder on the shaft against some sort of thrust bearing, and axial clearance is automatically taken up.

Figure 1: General arrangement of Plath micrometer mechanism

 Heath and Co, with their Patent Automatic Clamp arranged for the micrometer shaft to be held between two adjustable, hardened, conical centres (Figure 2). In principal, careful adjustment could remove for practical purposes all backlash while still allowing the shaft to rotate. It also allows for wear to be taken up if necessary. In theory, differential expansion of the shaft and the frame in which it is held might lead to end play developing.

Figure 2 : Micrometer shaft held between centres (Heath and Co)

 The third group, which includes Freibergers, SNO-Ts and pre-WWII Husuns, attempted to eliminate backlash by careful construction at the manufacturing stage and made no provision for user or automatic adjustment for wear, though one would not expect much wear in slow moving surfaces.

 In the Freiberger, the worm shaft runs in plain parallel bearings.  A 1 mm-thick bronze washer, shown in Figures 3 and 4), separates two thrust surfaces at one end of the shaft and if wear develops the solution is to fit a thicker washer. “Thicker” means an increase as little as 2 or 3 thousandths of a millimetre. Six arc seconds was considered the maximum acceptable amount of backlash in its first cousin, the SNO-T.

Figure 3 : Freiberger thrust washer in situ

Figure 4: Freiberger thrust washer exposed

 Husun fitted two opposed conical surfaces on the shaft to matching surfaces in a split bearing. Careful fitting here could again remove backlash for practical purposes and if play did develop, then it could be taken up by closing up the bearing.

 The micrometer mechanism is mounted on a frame to allow the worm to swing out of engagement with the rack. I have chosen to call it the “swing arm chassis” (Figure 1), since noone else seems to have given it a name . The swing arm chassis rotates around a bearing (Figure 1) that is attached to the expanded lower end of the index arm. Here, it is radial, rather than axial, play that can contribute to backlash.

 In the original C Plath, this bearing was a tapered one, similar to the index arm bearing, with provision to reduce clearances by closing up the bearing, but by the second half of the twentieth century, most manufacturers that adopted Plath’s pattern settled for well-fitting plain parallel bearings without provision for adjustment. Provided that it was well made in the first place, this seems to have been satisfactory and little wear was to be expected.

 Heath and later Kelvin and Hughes sextants mounted the swing arm chassis between adjustable centres (one of which is labelled “cone-pointed screw”  in Figure 2).

 Freibergers are altogether more complex. The bearings which carry the worm shaft rotate in a bearing machined in the casting carried on the end of the index arm. This bearing is eccentric, so that as it rotates against spring pressure in the indeex arm casting, the worm swings out of engagement with the rack. Axial backlash of this bearing can be taken up by tightening a nut which has a radial locking screw.

 All micrometer sextants take up lost motion between the worm and the rack by spring loading the contact between the two in various ways, so that the only clearance between the two is occupied by the lubricating oil film. The spring is labelled “swing arm spring” in Figure 1 and a leaf spring lies between the swing arm and the index arm in Figure 2.  Backlash here in a well made, undamaged sextant can arise if the oil is too thick, so that the thickness of the oil film varies with loading, or if it is too thin or absent, when “stick-slip” occurs to give irregular readings. Damage to the teeth of the rack or to the thread of the worm, while a cause of irregular movement and readings, should not in itself cause backlash, as contact is maintained by the spring loading. Rise or fall of the lower end of the index arm from the face of the limb with change in direction of rotation of the micrometer could in theory give rise to backlash, but in practice the keepers that hold the index arm close to the limb are usuallyadequate for their task.

 Finally, incorrect adjustment or wear in the index arm bearing can cause backlash. Most bearings are tapered ones with provision to adjust the clearance by moving the conical bearing surfaces axially against each other by means of a slender screw. If too slack, there is lost movement and if too tight, stick-slip, sometimes called “stiction”.

 This is perhaps a good place to warn inveterate fiddlers about overtightening the screw. The washer beneath the head of the screw usually has a square hole in it that fits on to a square on the shaft, or there is some other means of preventing the washer from rotating on the shaft. This ensures that as the shaft rotates, no rotational forces get transmitted to the screw head. The purpose of the screw is simply to move the shaft axially until there is a little drag indicating that clearance has been taken up. The screw may feel quite slack at this point. If you “firm it up” as one does with most screws, you may introduce stick-slip or, worse, twist its head off. Freiberger, SNO-T and some later C Plath sextants used well-fitting plain parallel index arm bearings that have no provision for adjustment.

The measurements

a) Backlash

 To measure backlash I used an autocollimator to project a light beam on to the index mirror of the sextant and to measure the deviation of the reflected beam after changing the micrometer reading (next photo). The least graduation of the autocollimator is 0.2 arc seconds, and people who used this particular type frequently were in ideal circumstances able to achieve an accuracy approaching 0.3 arcseconds. I find I can get reproducible readings to within 2 or 3 seconds.

Figure 5: Autocollimator set-up

 To measure backlash, I approached a zero micrometer reading alternately increasing or decreasing the reading to zero, and noted the differences between the two readings. I took the mean of ten pairs and calculated the standard deviation (SD). The latter is an estimate of the dispersion of the results around the mean. 1.96 SDs each side of the mean includes 95 percent of the data and a small SD implies that the results are tightly clustered about the mean (thirty readings might have been better, but there are limits to everyone’s patience…).

 b) Worm errors

 These are often neglected and much attention given to  calibration charts that says there is no error over the whole range of the sextant at 15 degree intervals, or that the instrument is “free from error for practical use”, generally meaning that the maximum error at these points does not exceed 6 or 12 seconds. But of course, this tells us nothing of the points between, nor of the errors within each degree

 I used the same  autocollimator set-up to estimate errors of the micrometer worm, by using the autocollimator to measure the error for each 5 minute step around a full rotation of the micrometer drum. Most people will be surprised at the size of some of these errors. It may be that the second-hand sextants that I used had been damaged, but I chose ones from my collection that I had carefully examined for damage during the course of their restoration or overhaul.

 Results

                                     Backlash                                 

 Tamaya 1977      Mean 0.9”      SD 1.1

 SNO-M 1966      Mean 1”         SD 0.8

 USN BuShips  

Mk II (Ajax Engineering)     Mean 1”          SD 0.8  

Freiberger  trommelsextant      Mean 6”          SD 0.7

Ditto, after  adjustment     Mean 2”          SD  2 

Hughes and Son 1938     Mean 6″       SD 2.5                           

Heath Navigational 1977     Mean 1’ 15”  SD 2.3      

                                            

Worm Error

 Discussion

a) Backlash

 The first three sextants all have leaf springs that oppose axial movements of the worm shaft, following the principle established by C Plath’s original design. It is easy to see how effective the method is in removing backlash, and the usual admonishment to always make a measurement turning the drum in the same direction can probably be ignored for this type of instrument.

 The Freiberger trommelsextant had backlash within its design parameters (assuming they were the same as for the SNO-T). However, I made up a washer a little thicker than 1 mm  (1.01 mm) and then hand lapped it to reduce it in thickness until the shaft just turned with a trace of drag. By then it was 0.99 mm thick and non-engineers may be surprised to know that the shaft would not turn at all when it was only 0.02 mm (less than a thousandth of an inch) thicker. The reduction in backlash was obvious.

 I could by no means reduce the backlash of the Heath sextant to less than 75”. The reason for it quite mystifies me and it may be something to do with the way the worm is skewed across the rack in this design of sextant. Making a new worm reduced the maximum worm error somewhat and certainly reduced the individual errors around the circumference. Truly, the heyday of British sextant making had long passed when this instrument was made.

 b) Worm error

Many people, I am sure, will be surprised at the size of some of the errors at first glance at the graphs. Bear in mind that the worst are those that deviate most from a horizontal straight line as, over a full rotation, there are always exactly 360 degrees in a circle and the errors have to sum to zero (in practice there is nearly always a deficit of plus or minus a handful of seconds).

 On this basis, the SNO-T, a 1938 Husun with a new worm, a 1938 Husun with the original worm, an Ajax Engineering US Navy MkII and a Freiberger all perform well, but before deciding that the Tamaya, Heath and SNO-M don’t cut the mustard, consider that they were second-hand instruments and that 10 seconds at the periphery of a radius of 150 mm represents a movement of about 4 thousandths of a millimetre. Invisible particles of dust and fibres and invisible nicks can thus easily lead to variations of this amount. Perhaps the moral of this story is to be careful not to drag the worm across the rack, to keep it well brushed and to oil it regularly.

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A: AN INDEX TO POSTS

6 03 2010

INDEX

 As the pages of the blog posts are not numbered, I have given the dates of the index entries as the easiest way of retrieving the information they contain. Don’t forget that WordPress also allows you to search this site specifically (see search box in top right-hand corner of the home page).

adhesives, industrial       27 Mar 11

Admiralty, British, pattern of sextant     27 Mar 11,  24 June 11

alcohol, filling of A10 A bubble chambers   13 Oct 09

angle, vertical sextant     13 Jul 12

Araldite, for cementing lenses       24 Jan 10

refractive index of                    24 Jan 10 (Comment)

arc, dangers of polishing                                    24 Jan 10

platinum                                                           10 June 10

silver, letting in to limb                               10 June 10

astigmatiser shade             20 Mar 11

autocollimator, principle of                                6 July 09, 13 Feb 11

accuracy of                     12 Mar 09,  13 Feb 11

calibrating dip meter with             5 Apr 2012

setting wires of                13 Feb11

view through eyepiece of      13 Feb 11

backlash     12 Mar 09

beam splitter     13 Feb11

bearing,

index arm           18 Feb 10, 13 Jul 12

adjusting     12 Mar 09

of US Maritime Commission sextant    15 Dec 10

swing arm

conical centres     12 Mar 09

plain     12 Mar 09

of US Maritime Commission sextant     15 Dec 10

Belleville washer                                               18 Feb 10, 11 Aug 09

Blish prism                                                      5 Apr 2012

Bracket, mirror,

bent     20 Oct 16

slotted                1 Oct 14

shade                                14 June15

brass, hammered                                              10 Nov 09

bubble chamber

overhaul, of AN5851                            20 Dec 08

refilling, of AN5851     20 Dec 08

bubble sextant

A  Coutinho  Pattern                   6 May 16

restoration,

A10 series                                            25 Feb 09, 13  Oct 10

screw closure of     13 Oct 10

Mk IX series                                        25 Feb 09

British Admiralty pattern sextant      24 June 11

Broken legs, mending  7 Oct 16

Byrd sextant                                                     30 May 09

Byrd, Commander Richard                               30 May 09

calibration, of C18 sextant                        10 June 10

calibrator, sextant

calibration of           13 Feb11

construction of        13 Feb 11

mounting sextant on    13 Feb 11

Carbonara, Victor             1 Oct 14

Carl Path’s earliest sextant (see also C Plath)       21 April 2017

case

latches                                                      18 Feb 10

keystone                                                  17 Dec 09

furniture, of US Maritime Commission sextant  15 Dec 10

plywood, delamination of                           14 June 15

casting, pressure die-                                     14 July10

C Plath (See also Carl Plath and Plath)

production numbers       14 July 10 (see also Plath)

sextants, eighty years of     13 Nov 12

yachting sextant                    14 June 15

centres, as bearings     12 Mar 09

chassis, swing arm     12 Mar 09

of La Filotecnica sextant 5 Oct 10

CHO-M sextant. See SNO-M sextant

Chronometer, marine    See www.chronometerbook.com

cleaning sextant parts             18 Feb 10, 23 Nov 08

clearance, axial of worm     12 Mar 09

collimating rising piece     2 September 11

collimation, faulty     2 September 11

collimator      13 Deb 11

horizontal     13 Feb 11

Commission, US Maritime   15 Dec 10

Cooke and Son                       28 Apr 10

Coutinho Pattern Bubble sextant   6 May 16

Dip                                        5 Apr 2012

abnormal                  5 Apr 2012

meters                       5 Apr 2012, 23 June 2012

calibration of             5 Apr 2012

Dollond, John                           10 June 10

Dollond, Peter                           10 June 10

double sextant                                                  16 Jul

micrometer mechanism of                     16 Jul 09

handle of                                              16 Jul 09

mirrors of                                             16 Jul 09

optical paths of                         16 Jul 09

telescope of                                          16 Jul 09

distance meter,

Fiske     13 Nov 12

Fleuriais     4 Nov 12

Stuart      13 Nov 12, 13 July 12

drawings, of SNO-T mirror adjusting screw and bush 17 april 10

drill, slot, use of  17 April 10

error, index, adjustment            1 Oct 14

side, adjustment            1 Oct 14

perpendicularity, adjustment            1 Oct 14

errors, of worm     12 Mar 09

Entandrophragma cylindricum, see sapele

eyepiece

micrometer                                           22 Mar 09

monocular, disassembly                        18 Nov 09

positioning                                18 Nov 09

of C Plath’s earliest sextant          April 21 2017

fake C Plath sextants                   14 July 10

La Filotecnica Salmoiraghi        5 Oct 10

Filby, David      13 Nov 12, 21 April 2017

float glasss                                                        11 Feb 09

Florez, Luis de                                      30 May 09

Francis Barker ,sextant, small craft precision     14 June 15

frame,

        aluminium alloy          29 Sept 10, 13 Nov 12, 14 June 15

        bent     20 Mar 11

braced                                                            10 June 10

bronze                    29 Sept 10, 13 Nov 12

         curve bar pattern      28 Jan 10

          Dreikreis                                             24 Jan 10

of C Plath’s earliest sextant    21 April 2017

evolution of                                        29 Sept 10

of Freiberger Skalensextant                                   22 Mar 09

of La Fil0tecnica               5 Oct 10

plastic                                                      29 Sept 10

pressure die cast                                  26 Oct 08,  29 Sept 10

stiffness of Heath sextant                      28 Jan 10

of Plath sextant                        28 Jan 10, 14 June 15

                      of  tulip pattern                                                10 Nov 09

of US Maritime Commission sextant  15 Dec 10

Freiberger , yacht sextant              14 June 15, 10 Aug 11,

skalen sextant 4 Apr 10

drum sextant   22 Mar 09

Gavrisheff dip meter                                         5 Apr 2012

Gilbert and Company                                      10 June 10

glass, float                                                        11 Feb 09, 27 Jan 09

graduations, sextant, refilling                              18 Nov 09

grinding paste, for circular mirror     27 Mar 11

Hall, Chester Moor                                                    10 June 10

handle

detachable            1 Oct 14

kinematic mounting of       20 Mar 11

of double sextant     16 July 09

of Gilbert sextant   10 June 10

of La Filotecnica sextant 5 Oct 10

of Troughton and Simms sextant   28 April 10

of US Maritime commission sextant  15 Dec 10

of WWII C Plath sextant                     14 July 10

Heath curve bar sextant,

frame stiffness                          28 Jan 10

tangent screw                                      28 Jan 10

Heath

micrometer shaft bearings                      24 May 09, 5 May 09

rack                                                     24 May 09

swing arm bearings                               5 May 09

vernier sextant                                      23 Nov 08

Hughes

lamp switch assembly                            18 Feb 10

micrometer mechanism              18 Feb 10

rack                                                     3 Jan 09

telescope mounting                               18 Feb 10

index arm

Ilon Industries Mark III sextant            1 Oct  14           14 Jun 15

index arm, bearing                             18 Feb 10, 22 Nov 08, 14 June 15

C Plath                                          14 July 10, 13 Nov 12, 14 June 15

SNO-M                                           14 July 10

US Maritime Commission    15 Dec 10

index mirror turntable                                10 June 10

sealed     24 June11, 14 Jun 15

joint, corner rebate                                           11 Aug 09

Keeper, index arm                 14 June 15

, swing arm                14 June 15

lacquer, see paint

lamp assembly, making new for Husun  18 Feb 10

latches, of case                                     18 Feb 10, 24 June 11

left-handed sextant                                            2 Dec 08

legs,

broken, mending                           7 Oct 16

turning new                                           24 Jan 10

placement of                                         6 May 09, 26 Apr 09, 2 Dec 08

lens,

auxiliary                                                30 May 09

recementing                                          24 Jan 10

swaging                                                24 Jan 10

achromatic, airspaced                   10 June 10

Leupold and Stevens see US Maritime Commission sextant, 15 Dec 10

level, spirit                                                        30 May 09

sensitivity of                                          30 May 09

light path, in Ilon Mark III sextant              1 Oct 14

lighting, of scale                                                30 May 09

limb, rivetted to frame                                  10 June 10

chromium plated                  5 Oct 10

logo, probable fake Plath                              14 July 10

of La Filotecnica Salmoirhagi       5  Oct 10

lost motion    See backlash

makers names                                                   10 Nov 09

mandrel, test     20 Mar 11

meter, dip, an improvised                 5 Apr 2012

Gavrisheff                                5 Apr 2012

Russian naval                        23 Jun 2012

micrometer

eyepiece                                         22 Mar 09, 13 Feb 11

sextant

bearing, by C Plath     20 Mar 11

bent         20 Mar 11

calibration of                                         6 July 09

drum, making new, dividing       20 Mar 11

numbering       20 Mar 11

evolution of                                           3 Jan 09

examination of                          6 July 09

index, bent          20 Oct 16

mechanism                          18 Feb 10, 14 June 15

of  Husun                                   3 Jan 09

of Ilon Industries Mark III            1 Oct 14

by Carl Plath                            3 Jan 09, 13 Nov 12

shaft, bearings,  by Heath                        24 May 09, 5 May 09

bent, repair of     20 Oct 16

by La Filotecnica     5 Oct 10

by C Plath     20 Mar 11

setting errors of     13 Feb 1

of US Maritime Commission sextant 15 Dec 10

worm, axial play in       14 June 15

Micrometre a reflexion, Fleuriais’     4 Nov 12

mirror,

adjusting screws, of SNO-T    17 April 10

auxiliary                                     30 May 09

backing of brass sheet,  15 Dec 10

blank, preparation of circular     27 Mar 11

bracket, Ilon Mark III sextant            1 Oct 14

bracket, SNO-T, repair of     17 April 10

Observator, repair of     20 Oct 16

bracket, C Plath casting numbers   14 July 10

circular, cutting       27 Mar 11

first surface            1 Oct 14

f latness of                                             27 Jan 09

grinding  edges                          11 Feb 09

horizon, C19 mounting assembly           10 Nov 09

Gilbert method of adjusting     10 June 10

Overcoated    13 Nov 12

Plath and SNO-M  compared  14 July 10

sealed  24 June11

index,              of Troughton and Simms sextant    28 April 10

,         , adjustment   14 June 15

adjustment by turntable      10 June 10

making new                                          11 Feb 09

removing silvering from             11 Feb 09

rounding corners                                   11 Feb 09

sealed                                                   6 May 09,  24 June 11

Mk IX A sextant restoration manual                  17 Nov 08

magnifier, scale, mounting in vernier sextant   24 June 11

monocular

eyepiece disassembly                           18 Nov 09

labyrinth seal of objective lens      20 Mar 11

La Filotecnica                     5 Oct 10

mounting,                                             18 Nov 09

Freiberger, boring                      18 Nov 09

marking out                  18 Nov

milling and shaping        18 Nov 09

squareness                                18 Nov 09

prismatic, in Ilon sextant            1 Oct 14

prismatic, making                                  18 Nov 09

removing objective                                18 Nov 09

removing prisms                                    18 Nov 09

separating halves                                  18 Nov 09

Tamaya etc                                           28 Jan 09

mounting, of Husun telescope                            18 Feb 10

of La Filotecnica telescopes     5 Oct 10

Navistar professional sextant     13 Nov 12

objective, removing from monocular                  18 Nov 09

octant, aeronautical, Mark I Mod 3                   2 Dec 08

O rings, Viton  13 Oct 10

Neoprene 13 Oct 10

paint

defective    15 Dec 10

for sextant finishing                                   24 Jan 10,10 Nov 09

photoluminescent                                  9 Dec 08, 26 Oct 08

wrinkle finish                                         18 Feb 10

Parallax, solar     4 Nov 12

Plath

Dreikreis sextant                          24 Jan 10, 13 Nov 12

Eighty years of

logo                                                      24 Jan 10, 14 July 10

optics of, post WWII     20 Mar 11

sextant frame stiffness                           28 Jan 10

sextant production, WWII                 14 July 10

shades adjusting tool                 6 Oct 13

tangent screw mechanism                      24 Jan 10,  24 June 11

telescope rise and fall mechanism          24 Jan 10, 24 June 11

polish, French                                                   24 Jan 10

Porro, Ignacio                              5 Oct 10

Poulin, Roger              29 June 14, 14 June15

pressure die casting                                           26 Oct 08,

working life of dies                                     14 July 10

prism, eyepiece, of AN5851                             9 Dec 08

Blish                             5 Apr 2012

Roof                            23 Apr 2012

removing from monocular                      18 Nov 09

protractor, three armed. See station pointer

quintant, vernier                                                30 May 09

rack,

Hezzanith                                              3 Jan 09

Hughes                                                 3 Jan 09

Heath                                                   24 May 09

Ilon Industries            1 Oct 14

C Plath Yachting sextant       14 June 15

radium

paint, decay of                                       9 Dec 08

half life of                                             9 Dec 08

release catch

of La Filotecnica sextant     5 Oct 10

of US Maritime Commission sextant   15 Dec 10

of C Plath yachting sextant         14 June 15

Repsold      13 Nov 12

rising piece

bent, repair of    28 Apr 10,      20 Oct 16

collimating,    2 Sept 11

faulty construction of Tamaya    2 Sept 11

structure of, in Admiralty pattern sextant  24 June11

of Carl Plath’s earliest sextant        21 April 2017

in Ilon Industries sextant            1 Oct 14

in La Filotecnica sextant      5 Oct 10

in Troughton and Simms sextant    28 Apr 10

Salmoiraghi, Angelo       5 Oct 10

sapele                                                               24 Jan 10

scales,

glass                                                     22 Mar 09

ivory                                                     10 Nov 09

screwcutting conical threads                              6 July 09

screws,

broken, removal of                    24 May 09, 17 April 10, 20 Oct 16

grub, replacement of                          4 Mar 10

pin    29 Apr 10

tangent, of  Troughton and Simms sextant    28 Apr 10

of C Plath’s earliest sextant      21 April 2017

Service, US Maritime    15 Dec 10

sextant

Admiralty pattern     27 Mar 11

calibration of         13 Feb 11

cleaning of                                            23 Nov 08, 18 Feb 10

double. See double sextant

fake                                             14 July 10

left handed                                            2 Dec 08

small craft            14 June 15

survey. See survey sextant

Yachting             14 June 15

shades,

assembly of horizon                              18 Feb 10

assembly of index                                 18 Feb 10

carrousel of AN5851                            14 May 09

remounting index                                   18 Feb 10

repainting                                              18 Feb 10

shellac, sealing bubble chambers with    20 Dec 08

secondary sealing bubble chambers with   13 Oct 10

Simms, William      29 Apr 10

Skalensextant, Freiberger                                  22 Mar 09

bearing of                                             22 Mar 09

dimensions of                                        22 Mar 09

optical path of                                       22 Mar 09

prism, cleaning of                                4 Mar 10

scale

lighting                                         22 Mar 09

focus screw, access to            4 Mar 10

focus of                                        4 Mar 10

SNO-M sextant (Navigation sextant with illumination, marine)

compared to C Plath                             26 Oct 08

origins of                                              26 Oct 08

telescope of                                          26 Oct 08

SNO-T sextant (Navigation sextant with illumination – tropicalised)

index arm bearing of                             22 Nov 08

instrumental accuracy of                        22 Nov 08, 13 Feb 11

micrometer of                                       22 Nov 08

mirror adjusting screws            17 April 10

use of                                               17 April 10

repair of                                          17 April 10

drawing of                                       17 april 10

origins of                                              22 Nov 08

position of index arm                             22 Nov 08

telescopes of                                        22 Nov 08

spring,

of tangent clamp                                    10 Nov 09

material of                                    10 June 10

of swing arm chassis     12 Mar 09

pre-load, of worm shaft     12 Mar 09,  15 Dec 10

Stanley, W F and Co, merger with Heath          26 Apr 09

station pointer                                                   26 Apr 09

survey sextant                                                   6 May 09, 26 Apr 09, 28 Apr 10

frame of                                                6 May 09

handle of                                              6 May 09

placement of legs                                  6 May 09

telescope mounting of                           6 May 09

use of                                                  26 Apr 09

swaging, of lens                                                24 Jan 10

swing arm

of Observator sextant       20 Oct 16

bearings, Heath                                5 May 09

chassis     12 Mar 09

Ilon Industries            1 Oct 14

keeper       14 June 14

switch, lamp, of Husun                          18 Feb 10

Tamaya, faulty rising piece,   2 Sept 11

tangent screw,

Heath                                                  28 Jan 10

Hezzanith endless                                 3 Jan 09

Hughes Admiralty pattern   24 June 11

of C Plath’s earliest sextant    21 April 2017

taper pin,

removal                                               18 Dec 08

replacement                                         18 Dec 08

sealing bubble unit with      5 Oct 10

telescope,

collimation of     2 Sept 11

making                                                23 Nov 08

Galilean, optics of                                 30 May 09

of C Plath’s earliest sextant     21 April 2017

of C Plath WW II sextant                  14 July 10

of C Plath yachting sextant           14 June 15

of Hughes Admiralty pattern sextant          24 June 11

of La Filotecnica Salmoiraghi sextant     5 Oct 10

of SNO-M sextant                               26 Oct 08

of SNO-T sextant                                 22 Nov 08

of US Martitime Commision sextant   15 Dec 10

Willson bubble                          2 Dec 08

telescope mounting

of survey sextant                                   6 May 09

Husun                                                   24 Jan 10

telescope rise and fall mechanism, Plath 24 Jan 10

Admiralty pattern  24 June11

thread, interrupted of telescope mounting     5 Oct 10, 24 June 11

tool, shades adjusting, Plath           6 Oct 13

Troughton, Edward    28 Apr 10, 10 June 10

Troughton, John     28 Apr 10

US Maritime Commission  15 Dec 10

US Maritime Service   15 Dec 10

US Navy Bureau of Ships   Dec 10

vial, of spirit level                                              30 May 09

washer, Belleville                                              18 Feb 10

lead, for sealing filling hole in A10 bubble unit  13 Oct 10

making                                                 11 Aug 09

Wild, Heinrich                                                  22 Mar 09

Willson bubble telescope                                  2 Dec 08

Wollaston, William                                10 June 10

worm

bent shaft of                                         6 July 09

burrs on                                               6 July 09

cutting new                                          6 July 09

errors     12 Mar 09

Hughes                                                3 Jan 09

Ilon Industries           1 Oct 14

periodic error of             6 July 09

progressive error of                              6 July 09

xylene,

effects on plastic                                   18 Dec 08

refilling bubble chamber with ,              18 Dec 08, 13 Oct 10

resistance of Viton to, 13 Oct 10