The Micrometer

A two-monthly review of the engineering of time

Issue 13   September - October 2018

Dust

     It is rare for a clock that I have serviced to return to me, but a few weeks ago an English fusee clock from some seven years ago did just that.  It had stopped, and the reason was not hard to see: hanging in a cluttered and carpeted room that would have made a whole programme in itself for Kim and Aggie*, I have attempted to photograph the cause of its stoppage.  The upper right hand inset also gives an idea of the first, third and fifth pegging out of just one of the pivot holes (the back centre arbor).

     Of course it is easy to criticise the owner, but should one do so?  Is it the owner's fault that the design of the clock case so poorly protected the movement from dust ingress?  And even if the room was dusted on a daily basis (which generally means re-distributed), would it have prevented the accumulation in the movement?  I suspect it might have helped a little if a vacuum cleaner were used more regularly, but what of the first half of the clock's life before Mr Hoover's invention?  Then (I guess) carpets - were they fitted at all - were beaten outdoors on a regular basis.

 

* Kim Woodburn and Aggie MacKenzie: co-presenters of a television programme How clean is your house? which ran in the UK from 2003 to 2009.

Rating issues

     The owner of the late 19th Century recoil escapement fusee dial clock shown on the Home page of this issue of The Micrometer was concerned that his clock would not keep good time.  After a thorough service including attention to a bent rating screw that did not permit the bob to fall as the rating nut was unscrewed, I could not get it better than about ± 2 minutes over the 7 day run even at a constant temperature.

     To live up to this accuracy does require the clock to behave between fully wound and fully unwound, so for this clock (rate = 8825 beats per hour) I took readings from the timing machine averaged over at least ten full rotations of the escape wheel, the count period set at twice the number of escape wheel teeth (33T x 2).  I got the following beats per hour all at a fairly constant 17ºC:

          Fully wound:      Minute hand on the 'uphill' side = 8832, 'downhill' = 8832;   Average = 8832.0, error per day = +70 secs

          Half wound:       Minute hand on the 'uphill' side = 8813, 'downhill' = 8818;   Average = 8815.5, error per day = -95 secs

          Fully unwound:  Minute hand on the 'uphill' side = 8822,  'downhill' = 8828;   Average = 8825.0, error per day =    0 secs

     Hmmm, tricky, especially as this was the second time I had adjusted the set-up in an attempt to improve the uniformity of the rate.  Over a seven day period and after correction for a steady drift, the best rate I could theoretically get is shown in the chart.   In practise and installed over his Aga cooker it would be worse.

     The chart essentially confirms what my spot timing machine results suggested, the ambient temperature being pretty much the same for each of the seven days of the test.  In short, even with healthy pallet drops this 'best rate' is unlikely to be bettered.  Noting the timing machine results suggested a sensitivity to the weight of the minute hand and hence the torque reaching the escapement*, I can but subscribe this to a mismatched fusee and mainspring, which is surprising as the mainspring appeared to be of the same vintage as the clock.

 

     * As an aside, I have never found a basic fusee clock obeys the circular error theory that says a larger arc of swing decreases the rate.  In my experience, the greater the torque (and hence arc of swing) the faster the rate which suggests (a) the impulse tends to be before the dead centre ('Airy's law'), and (b) the point of impulse and not the arc of swing dominates the rate of a typical fusee clock.

 

     Anyway, the upshot was that I felt that I had failed, so I waived my fee even though it included a set of new dial pegs and a winding key that actually  fitted the winding square.

     From a more commercial viewpoint, perhaps I had failed to manage my customer's expectations; based on his apparent age, his only frame of reference was likely to be quartz clock and watch accuracy.  If so, it does suggest that at the outset one should make it crystal clear what accuracy constitutes success for an old clock that may have had non-obvious replacement components of the incorrect size (perhaps a replacement mainspring?), though I do not recall ever seeing a table of what is a typical rate for the various types of clock.  Sounds like a possible area for research by one of the professional bodies, which surely they must have an opinion on if they are ever to adjudicate on complaints from the public under their Codes of Practice?

Securing a seconds hand

     Sometimes a seconds or other hand that is a push fit on a tapered arbor works its way off, and one way I have found to make things more secure is to turn to an engineering solution.  One method I have found to be successful is shown below; essentially the split hand pipe is grooved at its outer end and a single coil from a commercial helical compression (or tension) spring is slipped into a groove.

    The photograph shows the seconds hand from a Wilding regulator, and the pipe diameter has been reduced to allow for the pipe complete with spring to be inserted through the hole in the dial.  After cutting off from a 4 mm outside diameter x 0.70 mm thick (5/32 inch x 22 gauge) spring, the ring of spring was flattened in the vice and a groove in the form of a shallow arc machined in the pipe.  The groove depth is such that there is a slight closure of the bifurcation when the spring ring is fitted; as the groove is most easily cut before slitting the pipe (the required groove depth being estimated by inserting a drill shank of known diameter into the spring), some slight adjustment of the spring after slitting might be needed in order to get the perfect friction grip.

     Not only does this method provide adequate friction to prevent the hand from working its way out, but it also helps to prevent one leaf of the bifurcation cracking at its root as a result of less than careful insertion or removal.

Pictures at an exhibition

     The 2018 Model Engineering and Model Making Exhibition of the The Bristol Society of Model Engineers at which the Society of Model and Experimental Engineers (the SMEE) had a stand has just ended.  It was the first time for many years that I had served as a SMEE stand exhibitor, and the exhibition was to be well attended.  I enjoyed the three days enormously not only in answering visitors questions and responding to their enquiries but also in being able to talk 'in the margins' to the highly knowledgeable fellow members of the SMEE.

     The SMEE put on two major demonstrations (engraving by Jake Sutton and wire erosion by the Digital Group's Dr Brian Neale) that ran pretty much continuously throughout the three days.  What was apparent was that these demonstrations attracted by far the most interest, with Adrian Garner's superb partly-constructed and ticking astronomical clock coming a close third.  I took along a couple of my motorized workshop lathe attachments which have been described in previous issues of The Micrometer, and I was pleased to be able to run (and open) them up to show and describe their inner workings to those with a specialist interest in workshop accessories.

     But what impressed me most of all was the gentle discipline of the stand members, and it marked the SMEE out as decidedly more professional than other club or society stands I visited.  All four of us wore uniform work coats (far better than a lapel badge), and the focus was always to engage with visitors who stopped to look.  Leaving the stand unattended, sitting back passively or talking among ourselves was decidedly frowned upon, while taking refreshments or lunch was definitely something to be done in the exhibition rest areas rather than on the stand.  I liked it, and felt proud to be a SMEE member.

Photos:  A visitor tries her hand at engraving brass sheet, while (right) SMEE members pose for the team photo.

One advantage of 35 mm film photography

     With their sensitivity to low ambient light conditions and the instant nature of the images, the development of digital cameras has revolutionised the taking of photographs.  But 35 mm film photography had at least one advantage in that the film was supplied in very useful plastic pots (cassettes) that made such lovely storage containers.  So when it came to making a cover for a 25 mm diameter 4μF motor run capacitor for a 60 watt induction motor, it was fortunate that I still had a few in stock.

     Being a perfect size, the job would not have been necessary had the supplier's illustration been correct (it showed a capped, wire-ended capacitor), but it was not.  Faced with incorrect supply or inaccurate advertisement (not as uncommon as it should be), returning wrongly-supplied low-value items is not worth the time and money, which is where my 35 mm film cassette came in.  Although the capacitor was fitted with spade terminals, I chose to solder wire ends to the terminals and, with the addition of a grommet and plastic sleeving, my solution is shown in the photographs above.

The magnetic chuck

     Not uncommonly found on workshop surface grinders, the magnetic chuck is a less commonly found accessory for the milling machine.  But on occasion it is invaluable for holding ferro-magnetic materials requiring a light skim, and one such application recently undertaken in my workshop is illustrated below.

     A block of rough-machined cast iron otherwise devoid of any features that would allow it to be gripped onto a work table or faceplate was in need of squaring-up and finish skimming.  Bolted to the work table of my Westbury light vertical milling machine, the face of the Eclipse permanent magnet chuck is approximately 7 inches by 4 inches (175 mm x 100 mm) and is 'switched on' by operating the lever to slide internal high strength permanent magnets from the 'keep' position to the energised position.  A useful diagram is included on the Eclipse web-site along with other interesting information.

     Although my chuck is over thirty years old and pre-dates the 1980's discovery of neodymium 'rare-earth' super-magnets now used, by being stored in the 'keeper' position the Alnico(?) magnets still have considerable gripping strength*.  In the photograph, a first 15 thou (0.4 mm) skim is being taken, the fences to the left and rear of the table preventing any tendency to slide under impact loading such as when the fly-cutter contacts the edge of the cast iron block.

     One essential accessory for the chuck is a demagnetiser (degausser) for removing any residual magnetism from the work piece, while a pair of magnetic chuck blocks can prove useful for smaller or irregular workpieces, or to provide additional side restraint.  It being several years since I last used my chuck, I was pleased to see that it still held work firmly, so I went on to use the chuck to mill the locating groove for a tee slot key strip and locate the holes for the clamping bolts for use in subsequent machining operations (illustrated below).

     There are a few points that are perhaps worth commenting upon in the above photograph.  Starting from the right, the partially machined block now fitted with its key strip is shown as it was demagnetised, the block being simply slid over the two demagnetiser poles (separated by the non-magnetic strip).  Being such a large piece of metal, this was repeated for each face and in each direction.  To the left in the photograph the 'fuzz' of the cast iron swarf from several cuts 25 thou deep using a 6 mm diameter milling cutter can be clearly seen.  While the fuzz is easy to remove when the magnetic chuck is switched off, some small residual magnetism is induced in the workpiece and toolholder - which is where the demagnetiser** comes in.

     Also of note in the left hand photograph is the chuck block which is simply a stack of soft iron and copper riveted together.  As this is here being used to provide additional lateral restraint to the workpiece, the strips are oriented transversely to the poles of the magnetic chuck itself.   Were it being used to provide a vertical grip, the strips in the chuck block would need to be oriented in line with the poles in the magnetic chuck, additional care being taken to ensure the work piece provides a magnetic circuit between north and south poles.

* While the magnetic strength of neodymium magnets is greater than Alnico magnets, perhaps the most important benefit they offer is their resistance to losing their magnetism with time (age).  If a keeper is used, the longevity of Alnico is good (as my magnetic chuck demonstrates), but they do suffer from age if no keeper is used.

     Permanent magnet electric motors are perhaps a good example as no magnet 'keeper' can be fitted.  When recently clearing out the box room, the Alnico magnet of a childhood Hornby-Dublo model railway locomotive was found to have lost nearly all of its magnetism over twenty years.  In contrast, one source suggests that, even with no keeper fitted, neodymium magnets lose less than 1% of their strength in ten years, and I suspect it is this property that has primarily led to the recent meteoric rise in permanent magnet motors in preference to DC shunt wound motors.

 

** A word of caution: it is wise not wear a quartz wrist watch or carry a debit card, USB stick or any other device susceptible to damage by magnetic fields when the demagnetiser is switched on.

A salutary tale

     A recent visit to Herne Bay in Kent saw me inspecting the fine clock tower on the promenade which bore a sad memorial to a scaffolder taking his tea break at the time of the accident.  Built in 1837, the clock tower gets a very interesting entry in Wikipedia which those that would like to know more might wish to refer: https://en.wikipedia.org/wiki/Clock_Tower,_Herne_Bay

     This link also includes a photograph of the original movement, apparently now replaced by a synchronous electric movement.  It is one way of eliminating the problem of a falling weight ("eliminate the hazard"), another perhaps more conservation-friendly method being by enclosing the weight in a chute.  For those engaged in tower clock repair or who might own or be responsible for a tower ("turret") clock, Chris McKay’s most informative publications offer guidance:   http://www.turretclock.force9.co.uk/Flyer%20TCKH%20NRV.pdf

     The often acerbic Pevsner (well, in this case, John Newman who in 2013 updated the Buildings of England: Kent: North East and East) does not disappoint: "The inept design... thus debased... ripe for attack... absurdly too big..." before ending with a wholly unnecessary attack on the town's benefactor "... the gift of a Mrs Thwaites (sic), a satisfied holiday-maker."  Personally I find it rather pleasing; after all, why should Mrs Thwaytes satisfying donkey-ride-and-ice-cream holiday resort be constrained by the tasteful views of a teacher of architecture?

The burst water pipe
     One consequence of an avid clockmaker's passion for regular mugs of tea is a need for a sometimes urgent recourse to a convenient 'rest room', but ours fell into the category of - let me be reasonably polite - a back dunny.  So it was time for a refurbishment, and in the process of removing the suspended floor to re-route the pipe to the WC cistern I managed to burst open the heavily rotted sixty year old iron feed pipe where it passed under the foundations of the house.  Worthy of the fountains at Versailles, the cascade was truly impressive, so it was fortunate that I both knew the location of the main stopcock in the lane and had an extended key to turn off the supply.  A few hours later I had got the flow under control with rags and jubilee clips, and after filling every available bucket with water, it was time to engage an emergency plumber.

     Wrong.  Plumbers in the City of Bath do not do underground work, especially in this case where the outside path needed some heavy concrete breaking to gain access.  The following day I did manage to find a West Country firm specialising in this sort of work - Aquamain - who came straight round and very efficiently fitted a length of replacement plastic water main piping.

     Once the blue plastic pipe had been fitted I could continue with my works.  A  brass pipe clip was made and fitted to give rigid support to the internal stopcock (inset, left) and the whole of the pipework - routed through a water meter - run neatly with ball-o-stop valves in the copper supply pipes to the wash hand basin (pipes, top right) and lavatory (lagged pipe, bottom).  Feed pipes to and from the cold and hot water tanks are the pipes bottom right.  The cold water pipework was lagged using pre-formed pipe lagging and cable ties; within the pipework casing not so much to prevent freezing but to eliminate condensation and the consequent formation of mould.  The completed installation is shown in the right hand photograph.

     One thing I like in wet rooms is a lower row of (in this case black) vitreous tiles instead of a wooden skirting board as they are much more suitable.  Also not apparent in the right hand photograph is the lift-off lid to my slim pipework casing so the meter can be easily read, space being saved by making the wall support from ¾ inch aluminium angle (middle photograph).  The front is secured by three M6 stainless steel screws tapped into the space-saving ¼ inch thick aluminium angle to the back wall, while the forward three fit into M6 thread inserts fitted in the side wall.  Machine screws are often more suitable than the more common wood screws which, after a few removals, can easily lose their grip in softwood battens or plastic wall plugs.

 

CAUTION:  If replacing an underground metal water pipe with a plastic water pipe either partially or completely, one must make sure that the water pipe was not used as the main electrical earthing (grounding) conductor.  If it was, an alternative arrangement must be provided for the obvious reason that plastic does not conduct electricity.  Earth bonding to the copper pipes inside the house is still necessary, though it is not easily visible in these photographs.

Next time

     Some simple statistics are used to calculate the dimensions of Myford tee bolts and nuts that will have a high probability of fitting all tee slots.

Guy Gibbons

Copyright (c) 2018  G E Gibbons

The Micrometer

A two-monthly review of the engineering of time

Issue 13   September - October 2018

The Micrometer

A two-monthly review of the engineering of time

Issue 13   September - October 2018

Copyright (c) 2018  G E Gibbons