27: Fusee chain substitute

13 02 2017

A few weeks ago I received a chronometer which seems to have been heavily damaged by dropping, possibly while out of its bowl . The upper balance pivot was broken, the upper jewel of the third wheel was absent, or rather, it was present in many tiny pieces, and there were burrs raised on a few adjacent teeth of the great wheel. The stop bar of the stop work was stuck and the maintaining power did not work. The hands were absent and so was the fusee chain.

I have covered re-pivoting in posts 6 and 7 and making hole stones in post 26, though when there is no end stone to take into account, the process of making one is a rather simple turning operation. A little careful filing removed the burrs on the great wheel. Dismantling the stop work and maintaining power revealed dried-up grease, so that merely cleaning the parts of all old lubricant and replacing it with new brought these back to life. That left the hands to make and the chain to replace. I will cover making hands in a later post.

To a mariner, “cable” used to mean “A large rope by means of which the Ship is secured to the Anchor.” When it became possible in the nineteenth century to make large chains economically, they began to talk of “chain cable”. While the weight lines in clocks were of gut, this presumably was not strong enough to use as the link between spring barrel and fusee, so chain was used. It may well be that the technology of the eighteenth century was not equal to making wire lines that were both flexible and strong, and since chronometer makers were nothing if not conservative, the use of chain continued to the end of chronometer manufacture. These chains, which look rather like miniature cycle chains, were made by specialists and the chronometer repairer limited himself to repairing a broken link, usually by simply removing it.

Such books as there are that tell of chronometer repair are silent on what to do when the chain is completely absent. I found that I could buy a replacement chain for a trifling US$190, but as readers will perhaps by now have gathered, I am no spendthrift, and I began looking for an alternative to chain. The problem resolved itself into several requirements. The replacement has to be strong enough for its task; it has to be flexible enough; and it has to be possible to attach it to the barrel and fusee without special techniques. It also has to fit in the pre-existing groove of the fusee. These groves are always flat-bottomed so that conversion to a round cable is possible.

Less than a kilometre from my house is a famous game fishing club from which tormenters of large fish set sail. It is considered unsporting of the fish to bite through the trace of the hook and so it is often nowadays made of steel wire. A visit to a local supplier of fishing tackle provided me with a metre of stainless steel wire 0.8 mm in diameter for the sum of NZ$1.20 and I thought my problems were at least partly solved.

I carved hooks out of some scrap roofing iron using a piercing saw and files, and immediately encountered some problems: I could not solder the wire to the hooks, it was not flexible enough to allow me to pass it through a hole and bind it back on itself and when I made a little brass bush and squeezed it on to the end of the wire, it projected enough to foul either the top plate or the ratchet wheel of the maintaining power or the centre wheel arbor. By creative bodging I managed to overcome this, only to find that the wire was almost too stiff to be able to fit it. I then recalled that many years ago a kind person had given me a box of bits and pieces that he had bought at a deceased clock maker’s clearance auction with no clear idea what to do with them. Among the bits and pieces were several pieces of flexible wire and one length of the correct diameter to fit between the cheeks of the fusee groove. It appeared to be of multi-stranded steel, was very flexible; and it could be doubled back on itself with relative ease.

A test of its solderability revealed that it was plastic-coated and when I burned off the coating and scraped the wire clean, its fine structure could be revealed and contrasted with the relatively coarse structure of the stainless steel trace wire ( middle, Figure 1). The figure also shows how I at first anchored the trace wire to a hook with a bush.

ends-001

Figure 1: Stainless steel trace (left and centre) and multi-stranded flexible steel (right)

Attaching the cable to the barrel with a normal-shaped hook was relatively easy, using techniques well known to a mariner setting up standing rigging. In my case, after doubling back the wire on itself I held the bight in a small vice while binding it with some fine nichrome wire that I had bought for another purpose. As Figure 2 shows, the free end  just clears the top plate.

attach-to-barrel

Figure 2: Attachment of cable to the barrel hook.

Attaching the other end to the fusee hook required some modification to the normal shape of the hook, as otherwise the standing end of the wire would not lead into the beginning of the fusee groove. Thus, I made the hook deeper than usual so that the end of the hook could be twisted through 90 degrees and bring the cable in line with the fusee groove (Figure 3).

attach-to-fusee-1

Figure 3: Attachment of cable to modified fusee hook.

I have not been able to find a supply of similar wire. Phosphor bronze wire 1.5 mm in diameter is available but is too big. I tried unravelling  a sample of the wire and using only three of the five strands, but it stubbornly refused to be pulled straight. As it was hard drawn, I tried annealing it in a gas flame while pulling on it, but it broke under a very moderate stress. Clock fusee chain is too wide and pocket watch chain is too short.so I am very glad to have been able to find just the right diameter.

After a thorough overhaul and repair, the chronometer sprang into life with a very strong balance motion. I have gradually regulated it  over three days and it is currently losing at the rate of 1.3 seconds a day. I hope that with a little more tweaking it will do a little better.

If any reader knows where to obtain flexible, multi-stranded steel or phosphor-bronze wire of between 0.8 and 1.0 mm in diameter, I would be glad to hear of it.

 

 





26: Making a hole stone.

24 01 2017

When chronometers get dropped sometimes it is not only the pivots that suffer. The jewels, often known as “stones,”  sometimes get chipped or cracked. Continuing to run a chronometer with a cracked stone is to invite the pivot to develop grooves and eventually the pivot gives way. A half-dead pivot can be seen at Figure 2 of post number 5 of April 2013. When a stone is chipped and the hole in it becomes irregular, often the chronometer will not run at all, or if it does, may have an irregular rate, or only run one way up. Unfortunately, replacement hole stones in chronometer size are hard to come by for the occasional repairer, though occasionally mounted stones turn up for Soviet chronometers on e-bay – at a price.

Early chronometers had few jewels, but ran well with the pivots running in the brass of the plates, or in bushes let into the plates. A few days ago, I became impatient waiting for a replacement to arrive and thought to make a replacement out of brass. Phosphor-bronze might have been better, but I didn’t have any. The main problem is in drilling the tiny hole required, 0.2mm diameter for the hole stones of the Soviet MX6 chronometer’s balance and escape wheel staffs. It is now possible to buy tungsten carbide drills of this size, but they were intended for drilling fibreglass printed circuit boards using specialised machinery. In the amateur or repairer’s workshop, where the tailstock of the lathe or the drill chuck may not be perfectly co-axial with the lathe axis, drilling a tiny hole off centre results in the drill wobbling and then breaking. In what follows, I have tried to show a method that works for me in producing emergency replacement hole stones, but first, a sketch of an MX6 holes stone. I have not dimensioned it fully, as other makes of chronometers may well need different thicknesses and diameters.

hole-stone

Figure 1: Sketch of MX6 chronometer hole stone.

The upper face of the stone has an annulus  excavated for a depth of about 0.3 mm around a central island and the face of the island is relieved by a “whisker”, perhaps 0.05 mm, so that when the end stone is in place, there is a slight gap between them to allow oil to run by capillary action and form a reservoir. The lower face is also relieved by drilling in stages so as to leave a hole 0.20 mm in diameter and of about the same length. The point of a larger drill leaves a conical lead-in to the hole, to help the end of the pivot to find its way and then an end mill gives a flat-bottomed hole surrounding. it.

5-face-blank

Figure 2: Facing end of the blank.

A scrap piece of brass is first turned down to a close fit in the hole in the plate, in this case 5.00 mm diameter and then carefully faced (Figure 2)

2-excavate-end

Figure 3: Excavating annulus.

Then the annulus is turned away with a round-nosed tool (Figure 3). Note that the tool has negative rake, which gives a very fine finish on brass. It doesn’t matter how deep the excavation is, as long as it is much more than a whisker deep! Then the face of the island is relieved by 0.05 mm, as noted above and the blank parted off  a little over-thick (Figure 4).

3-part-off

Figure 4: Parting off.

One must now digress to make a shallow socket in the end of a scrap of brass bar (Figure 5).

4-make-spcket

Figure 5: Drilling a socket

The diameter of the scrap doesn’t matter as long as it is larger than that of the blank, but the size and the depth of the hole does matter. Unless a drill is perfectly sharpened it is likely to drill over-size, so I suggest making a start with a centre drill, followed in this case by a 4.7 mm drill. Then even a 5 mm drill that has slightly un-equal cutting lips will follow the hole so-formed and be of the correct size. The finish of the walls may not be great, but the size will be right. I made mine a mere 1 mm deep. Before proceeding further, measure the thickness of the blank so as to know how much to take off the thickness. As the faces of parted-off material may slightly concave or convex, measure near the edge, as in Figure 6.

measure-003

Figure 6: Measuring thickness of the blank.

Then the socket is heated gently and annointed with flake shellac to leave an even coating inside. While the shellac is still liquid, the blank is firmly seated in the socket and the whole allowed to cool, when the socket and blank can be replaced in the lathe.

1-face-end

Figure 7: Facing underside of blank.

After careful facing and removing metal to the correct thickness (Figure 7), drilling can begin. Here, careful depth control is essential, using the graduated thimble on the tail stock quill rather than a ruler. A dial gauge could also be pressed into service. Carbide drills are usually sharpened by the four facet method so that they are self-centring, but to make sure, make a dimple with the much more rigid centre drill and then feed in the 1 mm drill carefully to a pre-calculated depth (Figure 8).

6-drill-1-mm

Figure 8: Drilling with 1 mm carbide drill.

In the example shown, this leaves only 0.2 mm for the very slender 0.20 mm drill to tackle and there should be no problem of it wandering off centre , since the larger drill leaves a conical lead in.(Figure 9).

7-drill-0-2-mm

Figure 9: Drilling with 0,20 mm carbide drill.

Although tungsten carbide is a very strong and dense material it is also brittle, so the lathe has to run at high speed and the feed has to be slow to avoid breakages. Once the hole has been drilled successfully, it remains only to use a 3 or 4 mm end mill to form the relieving step. The depth of this must also be calculated, as it is important to leave the conical run-in area to guide the pivot into the 0.20 mm hole. The new stone is now separated from the socket by heating and given a soak in alcohol to remove all traces of shellac, after which the thickness can be checked. It is a good idea to brush the floor around the bench when separating the stone, to make it easier to find it when you drop it on the floor.

Figure 10 is a composite photograph to show both sides.

OLYMPUS DIGITAL CAMERA

Figure 10: The finished stone.

I don’t know how long this improvised lower hole stone for a chronometer escape wheel pivot will last, but at the moment, the instrument is running well and heading for a good rate. I have a French drum clock that was one of  my grandmother’s wedding present in which the relatively fine pivots run directly in the plates and there is no sign of wear at the low power end of the train after over a hundred years of continuous running, so I hope that my stones will also last over a hundred years. If they do not, they are easy to replace.





25:Making a Four Orbit Face and Motion Work.

22 01 2017

With the exception of Figure 1, all figures may be enlarged by clicking on them. Return to the text by using the back arrow.

I have long been aware that the Hamilton Model 21 chronometer had a version with four orbits, that is to say that as well as the usual minute hand it had four subsidiary dials: the usual seconds and power reserve dials with two extra ones to show 24 hours and the days of the week, shown in Figure 1 (I have been unable to trace the source of this photo). The movement of the chronometer is the same as standard, but the motion work, the system of pinions and wheels behind the face that drives the hands, is different. Only 27 were made and were designated Model 221. One sold in 1982 for $6,600, underlining its rarity.

hamilton-21

Figure 1: Hamilton Mod. 21 four orbit face.

When navigating in the vicinity of the International Date Line it is surprisingly easy to take out data from the Nautical Almanac from the wrong day of the week, and on long voyages across the Pacific, when tired, it is also possible to get the Greenwich time wrong by twelve hours. At least, I gather that this was the ostensible reason for making a four orbit version. However, seafarers are a conservative lot and the idea never caught on.

Until recently, I was unaware that the Soviet MX6 chronometer, based on the German war-time Einheitschronometer, had also been made in a four orbit version. However, last October I received an e-mail from someone in Queensland, Australia, together with a photograph of his MX6 4 orbit chronometer (Figure 2).

tony-mantons

Figure 2: MX6 four orbit chronometer face (courtesy of Tony Manton).

The power reserve dial has moved from 12 to 9 o’clock to make room for the hours dial, and a new one has appeared at 3 o’clock for the days of the week. The Australian friend put me in touch with an owner in the USA and while I was waiting for him to respond, a friend in Korea made contact with the Moscow Watch Factory where these chronometers were made to ask whether any of the old workers could shed light on them. It turned out that only a few, experimental ones had been made, but there had been little interest shown in them and no more were produced.

Then Tim Schultz, an owner in the USA was kind enough to send me some photographs of his instrument. Figure 3 shows the face of what appears to be a surveying chronometer with a rubber suspension.  German chronometers with a similar suspension were made during WW II. The words at the top say simply ” Made in the USSR”.

mx6-face-in-box-tim-schultz

Figure 3: Surveying chronometer (Courtesy of Tim Schultz).

Figure 4 shows the motion work, from which it is possible to deduce at least some of the tooth counts of the pinions and wheels.

mx6-motion-work

Figure 4: Motion work of MX6 four orbit chronometer (Tim Schultz)

The power reserve modification is simple enough: the 120 tooth wheel has simply been moved to the 9 o’clock position and remains engaged with the 12 tooth pinion that fits on the end of the fusee arbor. It is not possible to count the pinion teeth beneath the hour wheel, but the reductions between the canon pinion and the hour wheel must of course be two factors of 24. The hour wheel appears to drive a pinion immediately beneath a one-tooth pinion that rotates free on its arbor  and this latter pinion drives the 84 tooth wheel  that carries the days hand. The ratio between the hour wheel and the days wheel must of course be 7 to 1.

At about this time, I acquired a damaged MX6 chronometer for about half of the usual price, and as I was waiting for it to arrive I thought that if I could get it going, I might make it into a four orbit instrument. Accordingly, I mused over possible designs, aided by an aging CAD program to get the approximate placing of wheel and pinions, to see whether my design might work. Figure 5 shows the final result of my musings.

layout

Figure 5: Planned motion work.

The first numbers in the figure are the numbers of teeth in the wheels and pinions and the second number is the Metric module of the teeth.This latter is the diameter in millimetres of the pitch circle of the gear, divided by the number of teeth and is a measure of the tooth size. The pitch circles are shown as dotted lines. The outside diameter is the number of teeth-plus-two divided by the module and is shown as full lines. To avoid having to make a new canon pinion, I kept the old minute wheel too, shown in red at about 2 o’clock, but planned a new pinion for it, so that all the other wheels and pinions could be of 0.3 module rather than the somewhat larger 0.35 module of the canon pinion and minute wheel. This would allow me to fit in all the gears without having to excavate into the foundation ring as in the instrument shown in Figure 4 above. The pinion on the old minute wheel would drive both the new hour wheel and the gear cluster that drives the days wheel. I elected simply to move the power reserve wheel to 9 o’clock. I retained the pivot of the old reserve power wheel for the new hours wheel.

There are various ways to cut the teeth of gears (or more accurately, the space between them). Traditionally, the jobbing clock maker would cut the tooth spaces one by one, using some sort of dividing attachment and the teeth would be of cycloidal form, which can be seen by enlarging Figure 4 by clicking on it. While all the wheels teeth can be cut to correct form with one cutter, the pinions require one cutter for each pinion tooth count up to 8 and in steps of two up to 16 teeth, and these little cutters are astonishingly expensive, with a current unit price of £84, about US$ 104. However, if the teeth are cut by a generating process called hobbing, about which more below, one cutter will cut any number of teeth at a cost of US$ 30, so this is the method that I chose. The tooth form would be involute, which is much more tolerant of errors of spacing of meshing gears. There seems to be a belief among some clock makers that involute gears do not run well when the wheel is driving the pinion, but this is not my experience and the two clocks I have made with involute teeth seem to run exceptionally freely. In any case, in the new motion work the pinions will be driving the wheels.

pinion-hob

Figure 6: Hobbing a 14 tooth pinion.

The hobbing cutter seen in Figure 6 is essentially a screw thread with straight sides and flat top with cutting teeth formed in it. The spindle carrying it is geared to the spindle carrying the gear blank in such a way that for each revolution of the cutter the blank rotates through one tooth space, in the process generating a tooth with curved sides of involute form. The blank is advanced gradually into the cutter until a gear of the required length has been generated. In the figure. a pinion of 12 teeth has just been completed. Figure 7 shows the 84 tooth wheels being hobbed as a batch.

gears-wheel-hob

Figure 7: Hobbing a batch of 84 tooth wheels.

Figure 8 shows a 12 tooth pinion that has been drilled through the centre and is being parted off from the parent metal.

pinion-part

Figure 7: Parting off a pinion.

When it came to removing the pivots of the old motion work and re-positioning the power reserve and minute wheels, I discovered that all the pivots were tapered in form, as were the holes in the wheels and pinions, so that the holes in the plates for the pivots and in the hubs of the gears had to be taper reamed to match the standard taper pins I used for the pivots. Figure 8 shows the new pinion for the old hour wheel being reamed.

pinion-ream

Figure 8: Reaming a pinion.

Once the wheels and pinions had been turned to size, hobbed, drilled and reamed the wheels were ready to have hubs made and fitted. Once this was done I could start to “plant” them. Before this can be done, the correct centre distance has to be determined and transferred to the pillar plate. The traditional tool is called a depthing tool and though I have made my own it is rather easier for the occasional user to make and use the improvised device shown in Figure 9.

planting-2

Figure 9:

Two threaded bushes slide in a slot cut into a substantial slab of brass and can be fixed in place by finger nuts. The holes through the centres of the bushes accommodate round rods, one end of which are machined to a running fit in the gears being fitted and the other end of which carries hardened centre points for locating in a hole or scribing.The central rods can be adjusted in height and locked in position too. The gears are placed on the device and the centre distances adjusted until they run sweetly together, at which point the bushes are locked in place.

punch-mark

Figure 10: Marking for centres.

In Figure 10, in the upper right of the picture, one arc has been scribed from the hole for the days pivot and another from the hole for the centre wheel pivot, though the latter is not clearly visible in the photo. At the intersection a sharp fine prick punch is used to make a shallow mark at the intersection, and its position is checked and coaxed to the exact position if necessary. The shallow mark is deepened and then a 90 degree punch mark made to guide the drill point. In the lower centre of the photo one centre has been set in the hole for the centre wheel arbor and the other has scribed an arc for the new minute wheel. The latter has then been meshed with the hour wheel and an arc scribed, centred on the hour wheel hole. At the intersection of the arcs at lower centre, the hole for the pivot has been punched, drilled and reamed to size.

Once the holes were drilled, it was a simple matter to insert taper pins and tap (not hammer) them into place. Then I turned and drilled the hubs for the wheels  and pinions in brass and fitted them in pairs to check for smooth running. I needed to taper ream the holes in the hub until each gear or gear cluster sat on the pillar plate and ran smoothly on its own before checking that it ran with its partner. I could then mark the taper pins for height, tap them out, cut and smooth off the ends and replace them in their individual holes (there are small manufacturing variations betweeen taper pins). For the days wheel to fit, I had to mill away a couple of millimetres from the diagonal bar that carries the jewels for the third and fourth wheels and I also had to turn a groove in the underside of the days wheel as it otherwise fouled the end of the spring barrel pivot. I could of course have turned off the end of the barrel pivot, but turning the brass of the wheel seemed to be an easier task than removing the tough barrel arbor and grinding off the end. It was also necessary to  reduce some of the hubs in length so that the wheels did not interfere with each other or with the underside of the face.

Figure 11 shows the wheels finally fitted in place, pretty well as I had planned it in Figure 5 This left the face to do.

gears-plan

Figure 11: Plan view of new motion work.

It is important that all the holes are in the right place. The hours, minutes and seconds dials retain their previous centres, so I jig drilled these, the three fixing screws and the tiny hole for the locating pin at about 25 minutes past the hour by clamping the old face to a new disc of 1 mm thick brass and using the old holes as a drill guide for the new.  When planning the positions of the reserve power and days wheels, I used my makeshift depthing tool to set out the distance between the centre and power reserve holes in the pillar plate and used the same setting from the centre wheel to the days wheel. I could then mark out their positions on the brass disc. I also carefully measured this distance and transferred it to the drawing of the face.

Computer aided draughting makes drawing clock faces relatively easy (Figure 12). I printed the design on to high quality paper and rested it face down on a sheet of glass lit brightly from below. That way, I could bring the glue-coated brass back exactly into coincidence by lining up the holes with the hole markings on the other side of the paper. Once the glue had dried, the smaller holes could be opened up by poking through with a tapered scriber and the excess paper around the hole was cut off against the sharp edge of the backing by rotating the scriber. I cut out the large centre hole with a fine-pointed knife blade. A brushing with PVA glue as a size finished the face.

blank-face

Figure 12: Face ready to fit.

With a wide range of fonts available, it was tempting to simply make a copy of the MX6 face, except that the days of the week run anti-clockwise in my design, thereby avoiding some complication. I also felt that copying the MX6 might be construed as forgery, and anyway, I think my face looks more elegant without the red numerals. I did however make a concession to colour when it came to the hands and fitted a red hand for the hours to give it some emphasis. Figure 13 shows the final result, with the hands just loosely in place, as, with all the handling the movement has received, I feel a full overhaul would be wise before putting the chronometer into use. I changed my mind about the red hour hand and there seemed to be little point in retaining a gold-coloured minutes hand so I have blacked it in the final version..

face-corrected-001

Figure 13: Face fitted.

If you have enjoyed reading this post, you will probably enjoy reading my “The Mariner’s Chronometer”, available through http://www.amazon.com and its worldwide branches.

Update 31 January 2017: A more careful tooth count in Figure 4 shows that the canon pinion has 14 teeth and the hour wheel 54 teeth, not 12 and 56 respectively. I have made the appropriate changes to the figure.

Update February 6 2017: Sharp eyed readers will have noticed that in Figure 12, the “Up” and “Down” are incorrectly placed. This has been corrected in Figure 13, which shows the final, final version.

Update February 22 2017: After overhaul daily rate is now + 0.22 seconds per day and mean deviation from the mean over ten days is 0.39 seconds.

 

 

 

 

 

 

 

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24:Making a Tungsten Carbide Locking Stone.

27 12 2016

This month, in search of a challenge, I bought a Soviet MX6 chronometer “for spares or repair”. It arrived, beautifully packed, a few days before Christmas.  The previous owner had given an extensive list of problems and described them fairly. I thought I might have to replace or re-pivot the balance staff, as one photograph had shown the balance spring leaning to one side, but happily, the staff and its jewels had survived, though the spring was wrongly located in the upper spring stud. It was obvious from the amount of end play with the escape wheel that all was not well with at least one of its pivots, but this was as nothing compared to the state of the detent (Figure 1)

old-stone-2

Figure 1: Damaged detent.

The detent spring was buckled and the locking stone had been replaced by a piece of steel wire. This led me to look more closely at the balance rollers and I found that the impulse roller too was a piece of steel, crudely shaped and glued into place like the locking stone. Stripping the instrument down completely, revealed that the upper escape wheel pivot was indeed broken, but the stub that remained had been utilised by replacing the hole stone with a tiny bronze bush crudely soldered on to the end stone bush.

I was able to salvage the end stone and I had a spare hole stone, so I made a muff to replace the missing pivot (see Post 7 of July, 2013) and all was well with the escape wheel. Interestingly, A L Rawlings in that fine book, “The Science of Clocks and Watches,” writes of having owned a chronometer which had gone for a hundred years from 1840 without showing any wear on the impulse face of a steel impulse roller, but since I had a spare jewelled impulse roller, I used it. By drawing the buckled detent spring repeatedly between my finger nails I was able to restore it to something like a functional shape, with no warping, and observed in passing that the passing spring was an original fitting.

That left the locking stone. A spare was available for US$ 85 with postage included, but that seemed a lot for a sliver of artificial ruby and, since I had once read of a clock that had been constructed with tungsten carbide pallets to its Graham dead beat escapement, I decided to see if I could make a locking stone from the same material. Tungsten carbide is an extremely dense and hard material, about twice as dense as steel and about as hard as ruby and sapphire. It can take a high polish and generally only tools using cubic boron nitride or diamond can be used to shape and polish it.

My blank was the stem of a solid carbide engraving cutter that I had bought as a job lot of worn cutters about ten years ago, to use as small lathe cutting tools, and I have a Quorn tool and cutter grinder that I constructed about 30 years ago, though it has had relatively little use. A little improvisation to allow the blank to be rotated against the face of a 400 grit diamond grinding wheel resulted in the set up shown in Figure 2.

locking-stone-003

Figure 2: Grinding set-up.

The blank is set up with its axis parallel to the face of the wheel and I fitted a handle to the graduated disc on the left so that I could rotate the blank against the rotation of the wheel. The Quorn has a micrometer feed that allows very small increments to be removed, so eventually I achieved a cylinder of tungsten carbide a mere 0.8 mm in diameter. Then, by locking the tool against rotation I ground a flat about 2 mm long to form the locking face, rotated the blank a further 110 degrees to give clearance behind the locking face and to allow about 12 degrees of draw as well. Cutting off was as simple as using the corner of the wheel to grind a notch and then breaking it off (Figure 3)

cut-off

Figure 3: Cutting off.

The finish left by a 400 grit wheel, although fine by normal standards, won’t do for clock work and has to be lapped and polished. I did this by hand, holding the cylindrical part in a pin vice with just the flat part projecting and resting it on a piece of metal of a thickness such that the handle of the vice lay on the table while the part with the flat was supported by the metal. (Figure 4).

lap-layout

Figure 4: Supporting the work piece.

Any tendency for the lap to dip or rock resulted in it fouling the metal, so it was easy to maintain the orientation of the lap, shown in use in Figure 5.

lap-in-use

Figure 5: Lap in use.

By using progressively finer grades of lap impregnated with diamond grit, 20 microns, 10 microns, 5 microns and finally 1 micron, I has able to give the acting face a high polish. Figure 6 shows the finished locking stone, cemented in place in the detent with 8 to 12 degrees of “draw”.

finished-stone

Figure 7: Finished locking stone in place.

The “proof of the pudding is  in the eating” of course. I had to adjust the depth of engagement of the locking stone with the escape wheel teeth and refit the balance spring in the upper spring stud and after a general overhaul as described in my book “The Mariner’s Chronometer“, the timepiece began easily when wound. I found it necessary to tweak the discharge roller a little to achieve a satisfactory action and it is at the moment gaining at an unacceptable rate, but the new locking stone is plainly doing its job. Perhaps the balance spring is a little too short, or I could perhaps replace the timing weights (Post 21: of April 2016, A Balancing Act). The holiday period is still young…

28 December 2016: The balance spring was indeed a little too short, but increasing it tended to capsize the spring, and there was not enough room to wind out the timing weights to correct the error of about 2 minutes a day, so today I made two sleeves, each weighing 80 milligrams, to fit over the weights (see Post 21). Now the error is within a “tweakable” range.of about 24 seconds/24h.





23: An Improvised Let-down Tool

27 12 2016

A friend from Norway recently asked my advice about a chronometer which had been damaged in transit to him. It seems likely that one or both balance staff pivots have been broken. If so, he will need to access and remove both end stones and hole stones, to remove any bits of the pivots and to check the hole stones for cracks and chips.

The upper stones are easy. He just needs to block the movement and remove the balance cock. However, to access the lower stones, he will have to dismantle the chronometer and to do this the main and maintaining power springs MUST be let down. As someone who pulls apart chronometers and clocks from time to time I have made a proper let down tool that will fit winding squares of various sizes, but it is scarcely worthwhile to make such a tool for one job.

I have viewed a video on the internet which shows a repairer using the chronometer key itself to let down the spring while holding the movement between his knees. He does so successfully, neither bruising his fingers on the key as the spring runs away from him nor dropping the chronometer on the floor. The key can be used with less risk of discomfort and disaster, at the expense of a little woodwork. Figure 1 shows a sketch of a simple key holder. I have not given dimensions, but the octagon is about 40 mm across the flats.

key-holder

Figure 1: Key holder sketch.

Start with a piece of wood about 100 mm long and 40 mm square, and cut a slot about 30 mm deep in it, the same width as that of your key. For the Soviet MX6 chronometer, this is 3 mm, while for most others it is 2 mm.Then drill a hole across the slot. For the MX6, which has a 4 mm hole in it, you can rest the key on a flat and use it as a jig to position the hole for a cross screw. It does not need to be tapped for a screw as a screw passed through the wood and the hole in the key will suffice to keep the key in place. For others, you will need to tap the hole in the wood for a suitable screw to hold the key firmly in place, though if you drill the hole a little larger than tapping size, the screw will usually make its own thread if the wood is fairly soft. This won’t work with hardwoods like mahogany, oak and teak.

Now plane the corners off the square piece of wood  to make a regular octagon. It helps to mark out the octagon, but it does not have to be exact as long as it looks correct. To finish, sand off sharp corners, and stain and varnish the wood. In my stock of stains, I found an old can of stain  labelled “Nordic teak”,  which I thought might be just right for my new Norwegian friend. but when I considered the place of origin of teak, I thought again… Figure 2 shows the finished object with an MX6 key in place.

key-holder-001

Figure 2: Finished let down tool.





0. LIST OF POSTS

8 10 2016

If you are looking for a particular topic, it is probably best to use the search box in the top right hand corner of each opening page. If you simply want to browse, this list may help you. Go to “Archives” on the right, select the date and you will be taken to the most recent post for that month. Scroll down for earlier posts of the month.

“Categories” will probably be of less use to you, especially as it is no longer editable and I cannot easily bring that section up to date.

1. In the beginning.     March 2013

2. Getting started.       March 2013

3. Hamilton 21: getting started.     April 2013

4. Hamilton Model 21 escapement.     April 2013

5. Replacing a balance staff.     April 2013

6. Re-pivoting a balance staff.     June 2013

7. Re-pivoting part 2: an escape wheel arbor.     July 2013

8. Putting a lid on it.     July 2013

9. Chronometer key.     July 2013

10. Dial refinishing.     September 2013

11. Hamilton M22 chronometer watch.     March 2014

12. Hamilton Master Navigational watch.     December 2014

13. Rating a chronometer.     March 2015

14. Repairing a damaged barrel arbor.     May 2015

15. Replacing a locking jewel.     July 2015

16. A Shipwrecked M22 Chronometer watch restored.    August 2015

17. Replacing an impulse jewel.     January 2016

18. Turning a chronometer balance staff.     February 2016

19. Making an upper balance cock.     February 2016

20. A Soviet deck watch.     February 2016

21. A balancing act.     April 2016

22. Making a spring detent for an MX6 soviet chronometer.     June 2016

23. An improvised let-down tool. December 2016

24. Making a tungsten carbide locking stone. December 2016

25. Making a four orbit face and motion work. January 2017

26. Making a hole stone. January 2017

27. Fusee chain substitute. February 2017

 





22: Making a Spring Detent for an MX6 Soviet Chronometer

20 06 2016

Recently, while checking that a part I had made for a friend would fit in an MX6 chronometer, I broke a perfectly good detent. There was a harsh click followed by a “ping”. I expected the chronometer then to run away but whatever shock had broken the detent spring had also broken the lower escape wheel pivot. The latter I could re-pivot relatively easily by means of a “muff” (see post number 7), but the detent was another matter. Some replacement parts for the MX6 appear for sale on e-bay, but not at a price I am prepared to pay unless absolutely desperate , so I set about making a new one for the first time.

I began by seeking guidance in W J Gazeby’s Watch and Clock Making and Repairing (London, 1953) and he reassuringly begins six pages on the subject by saying that “It has always been a job for the specialist…” He describes how to file up the detent for an English chronometer and how to thin the spring and drill the holes, but it is plain from his description that this is not a job for the beginner. A couple of internet sites also give a brief outline of how to file up a detent from the solid. One maker states that he uses up more than a dozen top quality Swiss files to complete the task. Both gloss over on exactly how one files a spring to a thickness of about 0.08 mm (0.003 in). I have done a fair bit of filing in my time, though never at the scale of a detent, so I rashly set out on developing my own method, a task that was to occupy many of my spare daylight hours for about six weeks.

It seemed to me to be a good idea to start with a dimensioned drawing. Enough remained of the broken part for me to be able to piece it together for measurement, using a micrometer to measure thicknesses and a microscope with a moving stage to measure lengths. The latter allows one to measure to 0.1 mm. Lucky owners of a tool maker’s microscope will be able to measure to better than 0.01 mm, but lengths are generally much less critical than thicknesses and in any case I had to make do with what I have.(You can click on figures to enlarge them and use the back arrow to return)

Detent plan

Figure 1: G.A. of detent.

Detent section

Figure 2: Section through detent.

Note that except for the holes in the foot of the detent, longitudinal dimensions  use the tip of the horn as the datum. This makes setting out dimensions on the workpiece much easier and, as will be seen, is essential if machining rather than filing is used. After several attempts at filing, I found it very difficult, even with the help of a swing tool, to get the lengths and thicknesses located as precisely as I would have wished, but finally, at the fourth attempt, I achieved something acceptable, to me if not to expert watchmakers (Figure 3).

Finished

Figure 3: First acceptable attempt by filing.

Admittedly, it looked a bit rough. The pipe was not filed round and the oval hole for the fixing screw was a bit, well, un-oval, but it was true to dimensions. I even got as far as hardening and tempering it, but when I was fixing the unlocking jewel in the pipe, I accidentally knocked over the fixture in which it was being held and the spring broke. In a wakeful moment of that night it came to me that the pipe did not need to be round on the outside as long as the hole through its middle was in the right place and of the right diameter, so that was one hand finishing process that was not needed. Then a barking dog at 2 a.m. reminded me that there is no need to keep a dog and bark oneself. Why should I own a light vertical milling machine and still use a file to produce a complex shape in metal? This is not to say that everything was plane sailing when it came to work out a machining sequence, but once I had done so, and provided I did not make any stupid errors, it became possible to produce an unhardened detent in three hours rather than in two to three days of filing.

This led me to redraw everything from one datum and entering the distances of each change in section and each hole from the datum. I also planned to machine critical surfaces with the end teeth of the 3 mm end mill I used, rather than with the side teeth. With large end mills when cutting with the side teeth, it is the column of the machine that tends to lean away from the cutting forces, while with small ones, it the is flexing of the  mill itself that leads to slightly wedge-shaped parts.

This introduced a slight complication in holding the workpiece. It is not possible to hold something the size of  a detent in a machine vice, so one starts with a large piece, does as much machining as possible and then cuts it off the parent metal for finishing. Thus, I cut off two pieces of 3 mm gauge plate about 30 mm long and milled them square as a pair before reducing one dimension to exactly 26 mm, the over-all length of the detent. I could then hold one of them as a workpiece flat on a pair of parallels with about 5 mm projecting beyond the end of the vice jaws, while the other one occupied the other end of the vice jaws to forestall any tendency of the moving jaw to rotate. Before I did this, however, I milled the 13 x 1 mm notch in one end to form the eventual top of the horn.

As I will recount later, I eventually realised I had to anneal the gauge plate before machining it, by heating it to a red heat for a few minutes and then allowing it to cool slowly while covered in dry sand. This is something the books did not tell me.

The maximum thickness of the detent, 1.25 mm, is at the pipe, so the first job was to reduce the thickness of the 3 mm plate to this dimension by milling 0.87 mm off each side for a width of about 3.5 mm. I used a large end mill to do this, to save the wear on my one and only sharp 3 mm end mill. It was then necessary to establish the centre line of the milling spindle in relation to the datum. Many years ago I made an edge finder and I wonder how I ever did without it (Figure 4). A true, hardened cylinder on the end of a spring loaded shaft is held in the chuck and rotated at a moderate speed. It wobbles all over the place but is roughly trued with a finger nail and then the workpiece is brought slowly up to it, using the calibrated screw of the machine table. As it begins to make contact, the wobbling reduces until none is discernable and shortly afterwards, as the workpiece is advanced, it suddenly flicks along the edge of the workpiece. At this point, the edge is half the diameter of the cylinder away from the centre line of the machine, and as the diameter of my cylinder is 5 mm, I have only to advance the table 2.5 mm to align the centre line with the edge.

Edge find

Figure 4: Using edge finder to establish datum.

Referring to the drawings, to make the first cut to form the horn the table now needs to be advanced 3.87 mm less the semi-diameter of the end mill, or 2.4 mm, and machined to take off 0.5 mm in thickness to make the first shoulder of the pipe. When the part is eventually turned over to do the other side, the horn will then be the desired 0.25 mm thick (1.25 – 2 x 0.5). The other shoulder of the pipe is machined by advancing the table 5.13 mm plus the semi-diameter of the cutter and taking slightly less off the thickness to give an eventual thickness of 0.3 mm. This surface is extended right to the end of the part so that there is a long flat surface to stabilise it when finishing the spring. The latter’s edges are located in a similar way to those of the pipe and the spring on this side machined to a depth of 0.30 mm, the finished depth, so that only the other side of the spring will need to be filed.

Figure 5 is a posed view of the milling process as it nears completion. Beginners will need to be advised that the workpiece is seated firmly on the parallel by striking it firmly with a soft hammer (I have a large hunk of copper for the purpose) until the parallels cannot be moved beneath it or under the packing piece at the other end of the jaws. In this figure, one can see that I have extended cuts for the various shoulder about half a mill diameter into the parent stock, to ensure  full-height shoulders when the detent is cut off.

Milling

Figure 5: Milling nearing completion.

I then turned the piece over, re-set the datum and machined the other side in a similar way. When it came to the spring, I took off only 0.2mm thickness, taking very light cuts, leaving just under 0.2 mm to be removed by filing and polishing. At this point, I used the same datum to locate the holes to be drilled, and used the edge finder to locate the longitudinal centre line to establish drilling co-ordinates. While Gazeby and others seem to locate the holes by measurement and to start them by centre-punching, this will not do here, and any attempt to start a small hole without guidance will lead to wobbling and near-certain breakage. I use the smallest of centre drills to make a starting dimple and the rigidity of the centre drill ensures good centring (Figure 6)

Hole drill

Figure 6: Drilling holes for guide pins.

When buying small drills it pays to buy quality. I was seduced by the very low price of a very large set of small drills, 10 of each in steps of 0.1 mm. When the 0.5 mm drills failed to cut rather than buckle and break I looked at the cutting edges under high magnification and found that they had either no relief behind the cutting edges or very unequal lip angles. The same applied to the majority of drills in this “bargain” set. Happily, I discovered Element 14 (au.element14.com/) who although supplying mainly electronics, stock a wide range of small diameter drills of high quality and who deal with orders the same day. My drilling problems were solved instantly, though drilling with such small drills requires a soft touch with frequent withdrawal of the drill to clear swarf from the holes, and extra care when the drill is about to break through..

This left the hole for the pipe to be drilled to a little over 2 mm deep (Figure 7).

Pipe drill

Figure 7: Drilling 0.8 mm hole in pipe.

The workpiece can now be transferred to a small vice to convert the two 1 mm holes into a continuous oval. This could of course be done using a slot drill, an end mill with only two cutting edges, designed specifically for this purpose, but I have never seen one as small as 1 mm diameter, and if they exist they are likely to be both expensive and fragile. I used a tiny, round file to join up the holes and make a fairly oval oval. This is also the moment to tap the hole for the screw that attaches the passing spring. Mine is larger than usual at 1 mm, but as I was to make my own screw and passing spring this did not much matter. At this point, the proto-detent can be marked out for separation from the parent metal. I suppose this could be done with a milling machine using a fine slitting saw, but I can do it in less than half the time using a piercing saw, taking great care not to let the delicate blade, having about 25 teeth per cm,  jam and break (Figure 8).

Sawn out 2

Figure 8: Detent separated from parent metal.

From this point on, the detent needs to be handled carefully,  as it is all too easy to catch it on something and bend it beyond redemption. When hard, it is extremely brittle and even after tempering it remains somewhat delicate. The first task is to file up the lower edge of the detent to the scribed line, including the lower edge of the pipe. The drawings show that the foot and the block for the passing spring hole are of the same thickness, so this part can be held in a vice and filed with a certain exuberance, but the pipe is thicker and does not take kindly to being squeezed hard in a vice, so a gentler touch is needed for the horn and the pipe.

While still in a fairly strong state it makes sense to draw file and polish as much as possible. A check with the section drawing shows that everything between the pipe and the end of the detent, excepting the spring , is on one level, so it can be rested on this surface to deal with the other side, provided that the pipe is rested in some sort of depression, say a groove in a piece of scrap metal, wood or cork, to prevent a bend forming. While the spring is still relatively strong its back can be gently cleaned up to remove machining marks and given a preliminary polish.The back of the tip of the horn needs to have a bevel for the escape wheel teeth to slide off and the front needs to have a square seat of good finish for the tip of the passing spring.

The spring, which I have discovered to be the pons asinorum of detent making, can be next. The old (and some modern) accounts suggest resting it on a block of cork with a slip of cork behind the surface being filed to support it, but a swing tool allows everything to be kept flat and to be filed to a more-or-less precise thickness. Unfortunately, such tools cost about 800 Euros new, so I made my own last year and described how to do it in “Model Engineers’ Workshop”. As the back of the spring has been reduced by 0.30 mm, positive support is given by cutting a 5 mm wide slip of 0.30 mm feeler gauge and sliding it beneath the spring and the support surface. The piece of ground-flat stock on which the detent rests in Figure 9 has a variety of grooves cut into it to accommodate the pipe.

Detent 2 012

Figure 9: Detent held in swing tool to file spring.

In my swing tool, the top of which is shown in Figure 9, a closely fitting square plunger slides up or down inside an outer casing, the top of which is provided with a dead hard surface. The plunger’s position is set by means of a calibrated screw, so that the workpiece may be raised in steps of 0.01 mm  if necessary, and when the file starts to slide across the hardened area instead of cutting the workpiece, a further adjustment can be made to bring the part to the desired thickness. The whole tool swings between trunnions so that if there is any tendency for one end of the file to lift, the tool swings to correct it. The business surface of the file should be checked to see that it is flat. I use a fairly coarse “four square” swiss file to remove most of the metal and mark the flattest of the four cutting surfaces. As the spring approaches the desired 0.08 mm thickness, as measured by means of a micrometer with clean measuring faces, I switch to finer files and leave 0.01 mm to allow for polishing and bringing it to its final thickness.

By the time I had managed to machine and file an acceptable detent I had to turn my mind to hardening and tempering it by my usual method of surrounding it with case hardening compound (Kasenit, Cherry Red, etc), bringing it to red heat and decanting it into water. The compound not only prevents the thin spring from burning up in the flame, but also refines the surface grain of the material, so reducing the tendency to crack. The gauge plate is already alloyed to maintain a fine grain structure, so the compound simply protects the detent from burning. The traditional method involves wrapping it with fine iron wire and soft soap – if one can find either nowadays.

In the hardened state, the detent spring is in an extremely brittle state and easily broken as I twice accidentally discoved, so one should waste no time in tempering it. This can be done by heating it on a bed of dry sand with a piece of polished feeler gauge or a clean old ball bearing alongside so that the tempering colour can be seen and the detent removed from the sand as soon as it passes dark brown and reaches a purple-blue colour, but much easier is to put it in an electric oven set at 250 Celsius. My first two detents came out of the process with a slight longitudinal bend and one broke when I tried to correct this. I softened the other one by heating it, corrected the bend and re-hardened and re-tempered it, only for the bend to reappear, but this one did not break when I straightened it. On a trial fitting it became obvious that the horn of the dentent was canted upwards slightly and the pipe axis was tilted sideways, so a longitudinal twist had been added to a bend.

This led me to make the little jig shown in Figure 10, in which tiny dogs hold the foot rigidly in place while the other two dogs bear lightly on the passing spring block and the horn of the detent. A slip o 0.30 mm feeler gauge guards against buckling of the spring

Harden jig

Figure 10: Jig to prevent distortion.

The whole could then be hardened and tempered before removing the detent from the jig (Figure 11)

OLYMPUS DIGITAL CAMERA

Figure 11: Hardening jig with tempering tell-tale.

This unfotunately did not prevent the upward cant of the horn, so I cut out yet another detent and let my brain think about the problem during the small hours of the morning. Professional engineers will have immediately divined the problem, but it took me several days and three detents to realise what was happening.

Steel plate is made by rolling at red heat and so-called “hot-rolled” steel is recognisable by being covered with a black scale and being only of nominal dimensions. If the scale is then removed chemically and the steel rolled cold between smooth rollers, it can be brought close to nominal dimensions, but the rolling leaves stresses locked up in the steel and these stresses can be released by machining and by heating, both of which my proto-detents had suffered. Once I had annealed the workpiece before machining it, my distortion problems disappeared. I also abandoned my little jig and case hardening compound in favour of placing the detent inside a small steel spring to shield it from the flame (Figure 12).

Harden device

Figure 12: Hardening container.

The lower end of the device has a short steel plug to stop the detent from dropping out and the spring is prevented from extending at red heat by two wire bridles which hold the coils together. The whole is heated to a “bright cherry red” and then dunked into water, taking care  that it remains upright until completely submerged and cooled, as to let one side of the workpiece cool before the other is to invite severe distortion, even with previously annealed material.

This left only polishing to finish the detent itself and fitting the guide pins, locking jewel and passing spring. I am in two minds about the necessity of polishing and perhaps readers will let me have their views. It seems to be important from a cosmetic point of view in chronometers, as witnessed by the engine turning and other beautiful patterns to be found on chronometer plates, which would normally be seen only by clockmakers at overhaul. It can also be argued that a high polish tends to prevent dust from sticking to form a nucleus for rust. In the case of a spring, a fine finish is desirable, as scratches and machining marks may form an area of increased stress, from which cracks may propagate. On the other hand, an oxide film forms a protective layer against rust, one of the reason why screws and hands are often blued or blackened . I polished my spring and the tip of the horn very carefully and was less attentive to the rest, reasoning that the longer I spent at the task the longer the spring would be at risk.

The guide pins of the original detent were tiny taper pins of about 0.45 mm diameter. The smallest taper pins I have are a little larger and in any case I did not have any 0.45 mm drills, so I settled for 0.5 mm holes and larger pins, which I drove home (Figure 13 shows this for an early, distorted detent) before filing the business ends to fit in the guide slot. I melted a little flake shellac around each pin to make sure that they do not work loose.

Pins drive

Figure 13: Driving home taper pins.

 

I have covered fitting the locking jewel in post number 15, and it presented no more than the usual problems of locating a tiny sliver of ruby in its correct orientation in a hole only 0.8 mm in diameter and 2 mm long.

I was surprised to find that the passing spring presented very few problems that could not be overcome by trying again. I made it out of 0.05 mm brass shim stock, which is easily cut to shape with small nail scissors, first drilling a hole to form a datum for everything else. Brass work hardens and becomes springy, and as the shim is produced by rolling, it is work hardened to some extent, so that as the spring is cut out, it tends to curl up. This is not necessarily a disadvantage, since the process of straightening it to the desired shape continues the hardening process. I found that it was relatively easy to correct bends and twists by drawing it between two finger nails. Although the spring appears to be very weak and vulnerable, its elastic limit is very unlikely to be exceeded in normal use. Once fitted and its screw driven home, I surrounded the base with a little melted shellac to ensure that it stayed where it was. I then cut the spring to length. About 0.5 mm seems to be about right.

Figure 14 shows the finished detent fitted to its support block. The polish is better than it looks in the photograph.

On carrier

Figure 14: Finished detent in support block

When fitting the detent, it is best to remove the balance wheel so that the engagement of the escape wheel teeth with the locking jewel can be clearly seen. The movement should of course be blocked, but enough free movement of the escape wheel is possible to check that a tooth covers about one third of the face of the locking jewel when engaged and that the pipe rests against the face of the banking screw when a tooth is not engaged, while the lower end of the pipe should not drag across its threads.

Before fitting the balance wheel, it is wise to slacken off the adjusting screw and to slide the detent back on its block so that the foot of the detent rests against the screw head. This reduces the chance of the unlocking jewel striking the back of the horn and possibly breaking the jewel. Once the balance wheel is fitted, check that the horn is not fouling the underside of the unlocking roller, though this will usually be obvious because the lower balance pivot will refuse to sit in its hole. Obtain a good view of the roller and, rotating the balance with a delicate finger, observe how the jewel interacts with the passing spring, advancing the detent  a little at a time until the jewel lifts the passing spring off the horn one way and the locking jewel off the escape wheel teeth the other. The unlocking jewel should unlock the teeth of the escape wheel plus an allowance for shake in the pivots (which should be negligible in a good chronometer), plus a little more, the amount to be decided by the behaviour of the balance. At this point, the movement can be unblocked  and the balance again moved by hand to watch the interaction of the detent and escape wheel.

The balance can then be set in motion and with any luck, the instrument will spring into life again and build up to a healthy motion of the balance wheel. Some tweaking of the detent position may be needed to prevent tripping (skipping of teeth) and to ensure that the balance receives adequate impulse before locking occurs, but it is always wise to block the movement every time before making any adjustment, bearing in mind that the range of adjustment is tiny, no more than the overlap of the passing spring on the horn of the detent. In fact, the angle between the unlocking and impulse stones is a rather more important factor in preventing skipping and achieving good acceleration, but if the chronometer was running before the detent broke, this should not need adjustment. Altering the depth of engagement of the locking jewel with the escape wheel teeth seems best left at one third.

After a couple of days to settle down, my chronometer now has a very respectable rate of less than half a second a day with a strong action, so my weeks of experimentation seem not to have been wasted. Some more details on chronometer adjustment may be found in Appendix 1 of my book The Mariner’s Chronometer available for purchase through Amazon.com.