Skele­ton-clock maker

Clocks, steam en­gines, tools — David Curry has more skills than most

The Shed - - Contents - By Jon Ad­di­son Pho­to­graphs: Adam Croy

If David Curry were to be summed up in a sin­gle word it would almost cer­tainly be ‘pre­ci­sion’. But that wouldn’t be enough; it would be ab­so­lutely vi­tal to add ‘artistry’. Putting them to­gether amounts to a search for per­fec­tion, so per­haps that’s the word to use.

Here’s an ex­am­ple: David is at present build­ing a daunt­ingly com­pli­cated per­pet­ual cal­en­dar skele­ton clock, which has 38 gears, one with 165 teeth, all of which have to be in­di­vid­u­ally ma­chined. Clearly pre­ci­sion is para­mount. But the levers and some parts of the frame look a lit­tle util­i­tar­ian, so he’s re­design­ing them with a few artis­tic flour­ishes. For in­spi­ra­tion he looks to a skele­ton clock that was built in 1855.

“The en­gi­neer­ing has to be right so that the parts work prop­erly, but they also have to look right,” he says.

The clock tells the time; has a sec­ond hand; notes the day, date, and month; and knows whether the month has 30 or 31 days. David is adding a sec­ond chim­ing sys­tem so that the half hour, three-quar­ter hour, and quar­ter hour chimes have dif­fer­ent tones from the full-hour strike.

Mak­ing tools

Clock gear teeth can be pretty small so David knocked up a lit­tle pro­jec­tor from a torch and a lens to en­able him to project mag­ni­fied im­ages of the teeth to check that their shape is right. Al­though at a glance the teeth on a small clock gear may ap­pear to be tri­an­gu­lar, in fact the form re­sults from two arcs drawn from the gear cen­tre, so the sides are gen­tly curved.

Need­less to say, the tool­ing to achieve this isn’t ex­actly sim­ple. And while it’s pos­si­ble to buy gear cut­ters for the larger gears, they are ex­pen­sive, so David makes all his own from scratch.

“I prob­a­bly spend as much time mak­ing the tools as I do mak­ing the clocks,” he reck­ons. “And mak­ing the tools is the most chal­leng­ing part of the whole project.”

The four-tooth gear cut­ters are ma­chined

“The en­gi­neer­ing has to be right so that the parts work prop­erly, but they also have to look right”

from flat-ground plate tool steel, and then tem­pered. It takes con­sid­er­able time to get the cor­rect arc on the sides of the teeth. “They have to be vis­ually cor­rect as well as work­ing prop­erly,” David points out. “It took me two months of re­search to work out how to make them be­fore I even started on the lathe.”

He has a di­vid­ing head to en­sure that the teeth are per­fectly spaced around each gear.

Dig­i­tal mea­sur­ing sys­tem

He has also built what he calls a ‘depthing tool’, which has a pair of shafts cul­mi­nat­ing in fine points. The shafts can be ad­justed to­gether and apart. David fits a gear to each shaft and ad­justs their en­gage­ment (or depth) un­til they mesh per­fectly. Then he uses the points to mark the clock frame for the fi­nal shaft lo­ca­tions. He says that he’s “never seen one for sale”.

David bought his first lathe, a My­ford ML7, in 1961 when he was just 16 years old. “It cost £110 but I was only earn­ing £310 a year,” he re­mem­bers. The lathe in his Orakei work­shop in Auck­land now is an Aus­trian-made EMCO Max­i­mat V10, which was orig­i­nally his fa­ther’s. It’s fit­ted with a small ver­ti­cal mill. He has added a Ger­man-made dig­i­tal-dis­play mea­sur­ing sys­tem, ac­cu­rate to five dec­i­mal places. That’s pre­ci­sion!

“I couldn’t work with­out it,” David says. As well as giv­ing him the pre­ci­sion he re­quires, it also en­ables him to switch from Im­pe­rial to met­ric mea­sure­ments at the touch of a but­ton. Clock plans are in­vari­ably in Im­pe­rial, but he prefers to work in met­rics.

Most clocks have lantern pin­ions — the type that look like two discs with rods be­tween them to en­gage the gear

A close look at the long bolts that clamp the heads onto the cylin­ders of David’s model steam en­gines will re­veal that they are ma­chined with a slight ta­per to­wards the top: “They just look bet­ter that way,” he says. He started build­ing steam en­gines in 2014 when he in­her­ited a partly built kit­set from his fa­ther. After com­plet­ing it he de­cided that he could do bet­ter, so made an­other four.

“The last one I made is an os­cil­lat­ing steam en­gine, some­thing only a clockmaker could build,” David says. “I thought I wouldn’t be able to im­prove on it, so I haven’t made any more.” Most of the lit­tle steam en­gines are mod­els of real ones built for dis­play at shows and can be run on com­pressed air, chuff­ing away just like the real thing.

David buys kits to get the ma­jor cast com­po­nents but ma­chines all the other parts him­self. All, that is, apart from the wooden bases, which are made for him by a friend. “I’m no good at wood­work,” he says.

The work is sur­pris­ingly de­mand­ing. For ex­am­ple, del­i­cate 2mm di­am­e­ter op­er­at­ing rods are ma­chined from ¼ inch (6.35mm) square bar so there are no joins at the square ends where they pivot. Such a fine piece is flex­i­ble in the lathe so re­quires a care­ful and ex­pe­ri­enced touch.

For the painted parts David uses au­to­mo­tive spray cans, which can be found in prac­ti­cally any colour.

He also made a com­pound steam en­gine, in which steam is first ex­panded in one cylin­der then ex­hausted in to a sec­ond cylin­der to in­crease the en­ergy that can be ex­tracted, and thus the ef­fi­ciency, of the en­gine.

“Be­fore I started it I didn’t even know how a com­pound en­gine worked, so there was a bit of learn­ing to do,” he says.

“I thought I wouldn’t be able to im­prove on it, so I haven’t made any more”

teeth. But David feels this is not el­e­gant en­gi­neer­ing, so he ma­chines tiny pin­ions from steel bar, us­ing cut­ters that he makes on his lathe.

Al­though pre­ci­sion is re­quired in mak­ing the clock levers and frames, this is achieved by painstak­ingly hand saw­ing them from brass sheet and fin­ish­ing each part by hand. A fret­saw with a wire-type blade is used for the cut­ting. The only parts of the clock that he doesn’t make from scratch are the main springs.

“I prob­a­bly spend as much time mak­ing the tools as I do mak­ing the clocks”

Restor­ing clocks

David es­ti­mates it will take about 12 months to com­plete the per­pet­ual cal­en­dar clock, but says, “Once I’ve made all the tool­ing it ac­tu­ally comes to­gether quite quickly.”

It will be the sec­ond clock that he’s built from scratch, but he’s also worked on or re­stored parts of old fam­i­ly­heir­loom clocks, in­clud­ing a pair of English grand­fa­ther clocks, one from 1680 and the other made in 1720. He says the 1680 clock prob­a­bly came from a stately home, as when it was made

most houses still had dirt floors, so the clock cab­i­net feet would rot, but the feet on this clock are as orig­i­nal.

David had worked on vin­tage cars, rac­ing cars, and built model steam en­gines, and was scratch­ing around looking for a new project when he spot­ted an ar­ti­cle in The Shed about build­ing a skele­ton clock. His back­ground in clock­mak­ing goes back much fur­ther, though.

“School bored me be­cause I tended to be­lieve what I could see with my own eyes, so I left at 15 for a job at the post of­fice work­shops in Welling­ton, and started in the clock work­shop,” he ex­plains. “They had wind-up clocks in ev­ery post of­fice in those days.

“How­ever, after a few months a watch­maker from Ra­hotu joined the work­shop. It was sug­gested that windup clocks were be­com­ing a thing of the past and I was trans­ferred to the elec­tri­cal in­stru­ment sec­tion.”

David says many of the post of­fice staff had come through the De­pres­sion when they had to make do with what­ever they could lay their hands on, and were very skilled and will­ing to pass on ev­ery­thing they knew. “They gave me the start-up skills, which I have con­tin­ued.”

He then got a job at the Depart­ment of Sci­en­tific and In­dus­trial Re­search (DSIR) physics-en­gi­neer­ing lab­o­ra­tory, where he made test equip­ment from the ground up for sci­en­tists and in­dus­try, which re­quired me­chan­i­cal and elec­tronic skills.

It was a ca­reer that equipped him per­fectly for his ex­act­ing hob­bies later in life — and at 73 years old, he’s still learn­ing. 

“School bored me be­cause I tended to be­lieve what I could see with my own eyes”

Plenty of shed­dies work on or re­store vin­tage and classic cars, but David went a step fur­ther, de­sign­ing and build­ing a car part that he sold around the world. The part was an elec­tronic ig­ni­tion sys­tem for V12 Jaguar and V8 As­ton Martin en­gines from 1971 to 1981. “In those days most en­gines used me­chan­i­cal con­tact breaker points, but they couldn’t keep up with the 600 sparks a sec­ond needed for the V12 at 6000rpm,” David ex­plains. “So Jaguar used an elec­tronic ig­ni­tion sys­tem, orig­i­nally de­signed for the Cos­worth For­mula 1 en­gine, that wasn’t very re­li­able.”

So he de­vel­oped a more mod­ern sys­tem us­ing the ‘Hall ef­fect’ pickup — es­sen­tially a mag­netic pickup. Th­ese days such a sys­tem is com­mon­place, but David had to learn how to make and use ny­lon moulds to cast high-tem­per­a­ture epoxy fit­tings to lo­cate the pickup in the dis­trib­u­tor, and to de­velop parts such as an an­gled grom­met to pro­tect the out­put wiring so as to fit within the orig­i­nal me­chan­i­cal com­po­nen­try. Soon after­wards the in­ter­net “changed my life” by pro­vid­ing a chan­nel to mar­ket the ig­ni­tion sys­tems to Jaguar V12 own­ers all over the world. It be­came a suc­cess­ful small busi­ness un­til David tired of sit­ting at his work­bench mak­ing the parts, as­sem­bling the units, and pack­ag­ing them for dis­patch.

His in­ter­est in cars has been life­long — his fa­ther owned vin­tage cars and even had Bu­gatti parts in his garage. Along with the skills David learned at the post of­fice, he com­pleted a panel beat­ing course to pre­pare for restora­tion work.

David’s first car was a 1930s Ri­ley 9 Kestrel. “I had to re­build the wooden body. I owned a few 1930s Ri­leys, which were well bal­anced and nice to drive,” he re­calls. “On their skinny tyres they could go side­ways ev­ery­where.” He grad­u­ated to a more ad­vanced Alvis Speed 25, one of just three that came to New Zealand, and then to an As­ton Martin DB4. He paid $5K for it and kept it so orig­i­nal that it still had maps used by the first owner in the glove­box. Three E-Types fol­lowed, the first a very early model that David didn’t like at all, then one of the last six-cylin­der ver­sions. Fi­nally he bought a V12 coupé, which he liked so much that he kept it for 30 years. He also flirted briefly with mo­tor rac­ing, restor­ing two Brab­ham BT18 sin­gle­seaters, one ex–Graeme Lawrence and the other ex–Roly Le­vis. David raced in classic events be­fore sell­ing the cars. “I con­sider my­self a very lucky in­di­vid­ual,” he says. “My fa­ther took me to mo­tor rac­ing meet­ings, which aroused my in­ter­est, and I ended up owning cars [that] I loved.”

Pol­ish that high-qual­ity an­tique clock face too much and it will soon be ru­ined, as the sil­very fin­ish is a very thin coat­ing over brass plate.

Many old clocks have faces printed on pa­per, but bet­ter ones have sil­vered brass faces, with the nu­mer­als, mark­ings, and dec­o­ra­tions hand-etched and filled with en­graver’s wax.

The 1720 grand­fa­ther clock in David’s liv­ing room orig­i­nally had a sil­vered face, but it had been pol­ished off. Some own­ers feel that that’s a nice patina, he says, but there comes a point at which patina turns into dam­age — and this clock was near that.

So David re­sorted to the sil­ver­ing process used when the clock was first made and re­stored its face to its orig­i­nal state. The en­graver’s wax, which is mixed with shel­lac, is quite durable, but if it is be­yond sal­vage it is still pos­si­ble to buy the wax, which can be melted into the en­grav­ings after they have been com­pletely cleaned. In David’s clock the wax was as orig­i­nal.

The first step is to re­move the dam­aged sil­ver­ing, tak­ing the face back to the brass us­ing fine sand­pa­per. Great care has to be taken to re­move as lit­tle ma­te­rial as pos­si­ble be­cause some of the fine en­grav­ing will be quite shal­low. Once the brass is ex­posed it has to be kept per­fectly clean — a sin­gle fin­ger mark can spoil the sil­ver­ing.

The coat­ing is achieved by first cleaning and then grain­ing the dial, then rub­bing the sil­ver­ing pow­der, fol­lowed by a fin­ish­ing pow­der, into the sur­face of the brass, wash­ing away any ex­cess. Th­ese days many re­stor­ers will fin­ish with a coat of clear lac­quer to pro­tect the sil­ver­ing, which, over time, will bear the marks of fin­gers touch­ing it dur­ing spring wind­ing or other han­dling. With or with­out the lac­quer, the face needs just an oc­ca­sional wipe — never a pol­ish.

Clock­wise from top left: Skele­ton-clock parts; the Ger­man dig­i­tal dis­play that David added to his lathe; a home-made cut­ting tool; a lamp that David made to project mag­ni­fied im­ages of gears; a disc and the cut­ter that will turn it into a gear wheel; the jig that David made to en­sure per­fect gear mesh­ing

A gear wheel set up for the tooth cut­ter, with the di­vid­ing head at left Left: A com­pleted skele­ton clock Right: The home-made tooth cut­ter at work

David’s col­lec­tion of model steam en­gines shows the pre­ci­sion and work­man­ship in the huge va­ri­ety of parts re­quired

Above: A se­lec­tion of tools that David has made to ma­chine clock partsRight: A gear-tooth cut­ting tool ready for workBe­low: De­tails of skele­ton clocks give an idea of the pa­tience re­quired to build one

David Curry in his home work­shop The in­ner work­ing of a skele­ton clock that David has made from scratch

Classic cars — the red As­ton Martin and two Brab­ham BT18s that David re­stored, along with the ig­ni­tion-sys­tem pick­ups and am­pli­fier cir­cuit board he made to fit 1970s V12 Jaguars and V8 As­ton Martins

The Curry fam­ily Grove grand­fa­ther clock with the orig­i­nal un­re­stored face above and the re­stored face left

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