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Holiday
Three years ago my family and I were conducting our yearly pilgrimage to the forests of Western Australian. 

Forrest

The southwest forests of Australia contain some of the tallest and oldest hard woods in the world. Most of the ancient forest has been turned into wood chips for Japan, railway sleepers for England and fence posts for Australia. 

Inspiration Clock

A small amount of the wood is turned into high quality furniture and art pieces. We visited an art gallery in Pemberton and on display was a beautiful wooden grandfather clock.

 

 

Clock Artisan

The Inspiration Clock was made out of Western Australian Sheoak and Jarrah by a recently deceased Electrical Engineer. He had seen a wooden wall clock in the same gallery, built by a recently deceased artisan and decided that he would continue the tradition of wooden clock building. The background to his story is very sad and highlights some of the most appalling aspects of human behavior. The clocks were mass-produced by gluing quarter section gears together to form a single gear. The quarter gear sections were manufactured by routing slots into a quarter rounded blocks. The blocks were cut into thin slices to form the gear sections. From the photographs you can see that the clock has been manufactured out of the finest furniture grade Sheoak.

 

The Clock

The Inspiration Clock was impressive but even the deceased estate price was unaffordable. I embarked on a three year project to build a clock even though I knew nothing about clocks, had never build anything before and possessed basic hand tools. My ignorance of clock was highlighted when people were teaching me the names of the clock components after the clock was built. I spent half an hour talking to my friend Rob about escarpments and I photocopied several pages out his escapement book. Most of the escapement designs we too exotic so a basic club escapement was used. The wood used to build the clock is considered by furniture manufactures as scrap. High grade Sheoak is unavailable to the public. A small quality of reject sheoak is available through a single retail outlet. The inspiration clock had a hour/minute and seconds dial with a short run time. What I had in mind was a clock with a symmetrical dial and a day of the month dial. The symmetrical dial was achieved with addition of a Day/Nick dial and a day of the week to balance out the second hand dial. The month dial required the addition of two extra gears. The run time was increased to two days by maximizing the weight run length and minimizing the bush bearing friction. The original run time design was seven days but could not be achieved with wooden bush bearings.

Clock Size

The size of the clock was determined by the largest piece of wood that could be purchased and fed through a $50 Taiwanese band saw. The size of every clock component is a ratio of the escapement diameter. The escapement took three months to build. The 180mm diameter by 4mm thick escapement required a dimensional stability of less than 0.5mm. In an attempt to minimize warping the first escapement was laminated out of two 2mm sheets of Sheoak. This only doubled the effect of warping. The warping problem was solved by polishing Sheoak with a round sanding disk and heating the wood until the resin melted and the moisture driven out. Heating the wood also increased the hardness and the contrast in the wood grain. By clamping the wood and allowing it to cool over several days the wood will stabilize and stops warping. This procedure must be repeated in order to thickness the escapement down from 8mm to 4mm in 1mm steps. To relieve internal stresses the escapement must be roughed out before thicknessing. Once the required thickness has been achieved then the escapement hammers and ribs can be finished off.

Escapement

Three Component Clock

One of the few lessons learnt from building complex engineering system is that you build the most difficult bit first. If you cannot build a clock with only a second hand then there was no point continuing with the project. A three component clock was assembled from the escapement, simple rocker and a pendulum rod. The clock worked well but ran fast. Calculations indicated that a two second time constant required a 1 meter pendulum. What I failed to understand is that the time constant of a pendulum is a function of the distance between the pivot point and the center of gravity. The mass of pendulum rod shifts the center of gravity closer to the pivot point. The only piece of the clock that had to be rebuilt was the pendulum rod whose length had to be increased to 1.25 meters.

 

Clock Gears

The quarter section gears used in the Inspiration clock were mass produced products and required equipment and skill to manufacture. Since equipment and skill were unavailable the gears were made out of a solid piece of wood. As a result the clock gears took over one year to manufacture. A computer laser printer was used to generate gear templates that were printed on adhesive paper. The center of each gear teeth were drilled out using standard drill bits and finished off by hand. Sheoak has a tiger skin grain with a non-uniform density across stripe boundaries. Drilling straight hole across the grain boundaries is difficult. The hole centers were marked with a fine cross on the laser printer templates. The centers of the holes were punched with a pin. The gear was clamped to a cross feed vice and then drilled with a 3mm lathe centering drill bit. The fine drill bit was then swapped for the correct size drill bit. This process produced precision holes but was very labor intensive.

Dial Side View 

Clock Finish

To minimize gear wear the teeth were coated with cyanoacrylate. Large bottles of cyanoacrylate can be purchased from model aircraft shops in three different grades. The fine grade cyanoacrylate soaks into the wood and forms a hard transparent casing. A small amount of cyanoacrylate spilt onto the front of the escapement. The contrast in the grain was enhanced and it looked so good the entire clock was coated with cyanoacrylate. A problem with cyanoacrylate is that it hardens to a rough finish and requires hours of hand polishing to obtain a matt finish. As a result the clock is hard as a rock and completely waterproof.

 

Drive Shafts and Round Things

The drive shafts were turned on an old metal working lathe that belonged to my dad. Woodworking lathes lack the precision required to build the center drive shaft assemble (i.e. Three concentric shafts). The pendulum adjusting nut and bolt were machined out of sheoak. The threads were cut one layer at a time, coated with cyanoacrylate and allowed to dry before the next cut. The threads took over one week to machine.

Center Drive Shaft

 

Dials and Frame

Large pieces of high quality sheoak were unavailable and the clock dials has to be manufactured by laminating 12 pieces of wood. This increased the complexity and manufacturing period to one year.

Front Dial Inside Assembly

 

Tuning

After the clock was assembled it was time to adjust the clock time constant. This process took six months. The clearance between the gear teeth had to be adjusted to allow the clock to run free. The clock drive weight was 1.6Kg which transfer's a load of 0.5 gram to the escapement. Wood does not have a uniform density and the large gears were so unbalanced that the gears loaded the escapement by more than 0.2 grams. This would cause the clock to stop at certain times of the day. The gears were statically balanced by spinning the gears in the clock frame. If the gear stopped in the same place then wood was removed from the bottom of the gear. The density of the 60 x 8 gear was so uneven that a slot had to be machined at the junction of a rib and the teeth. The escapement, 60 x 8 gear, 64 x 10 gear and x 64 gear were prone to rubbing together. If the drive shafts were not at right angles to the gear then the clock would stop every few hours. The time constant of the clock was set by adjusting the pendulum weight screw. Since the clock has a two-day run time a wristwatch is more than adequate to set the clock. Once the clock time constant is set the clock will run for two days with an accuracy of better than five seconds per day. 

 

Pendulum Parts

The Pendulum Sage

The clock can be adjusted to an accuracy of better than 5 seconds in 48 hours. This requires a 0.03mm pendulum weight position accuracy for a 1.2 meter pendulum. As the seasons change the clock accuracy will degrade to less than 30 seconds per day. This translates to a 0.4mm change in the length of the pendulum rod. The clock accuracy is dominated by the thermal expansion of the wooden pendulum rod. An improvement in the clock accuracy can be achieved by using a pendulum rod with a coefficient of expansion that is 10 times better than the current Jarrah rod. There is a old saying from the Australian bush 'You know when wood is hard because when you hit it with an axe sparks fly'. Pine has a hardness index of 2.6 and a good quality hard wood has a hardness index of 7 to 9.  There is a Western Australian wood called Wandoo that has a hardness index of 15! After a long search a source of  Wandoo was found from an old road bridge. There hundreds of old road bridges in Western Australia that use Wandoo log stringers. The Main Roads Department did not know how long the bridges would last. A local university was tasked with testing the strength of the Wandoo stringers and I was able source one of those logs. The next task was to extract  1.5m x 0.035m x 0.015m strips. I contacted several commercial wood milling companies and they refused to touch Wandoo. Access to a circular saw obtained and in the process of extracting the pendulum rod blanks the following was destroyed: (Two circular saw blades, two band saw blades, three hack saw blades, two router bits, lots of sanding disks). A tungsten circular saw blade was required extract the pendulum rod blanks from the log. I think that aged Wandoo has a hardness index greater than 15! I have a few spare pendulum rod blanks availiable for those who have built a clock.

 

Next Project

The garden shed has been cleaned up and the key thrown away. I still make the occasional mathematical shape and puzzles but there are no plans to build another clock. My wife wants me to make her coffee tables but I am incapable of cutting a straight lines. I am teaching my 12 year old son programming and we may embark on some sort of software project. Probably a game.

Overcrowded Garden Shed (Standing room only!).


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