The Woodshop Shed

A Clock of my own design

Furnished content.
(from WoodNet.net)


I have gone down the rabbit hole, an obsession my wife would say, to builda clock of my own design. My wife also hates the tick-tock of typical mechanical clocks with pendulums, or similar timing mechanisms, so my primary objective was that whatever I build had to be very quiet. During my investigations into different escapement mechanisms I found that one called the grasshopper is exceeding quiet. It was developed by John Harrison, a brilliant English clock maker who developed the first clock that could keep time accurately at see, which solved the longitude problem that had plagued sailers since man started sailing across the open ocean. Harrison started as a woodworker, so there's hope for the rest of us wood butchers.

If you see a clock with a grasshopper movement you'll see it's very unique. It's also not obvious how to design andmake that movement. Fortunately, Harrison wrote it down, more or less, and others have studied and interpreted his work so that us mortals can draw or calculatethe key dimensions required for any particular size and number of teeth on the escape wheel. I built a grasshopper movement and escape wheel just to see if my design actually worked. I was amazed that it worked just as it's supposed to.


With that confidence boost, I set about to design the rest of the drive train of the clock. I built a prototype, and then a second prototype to work out both the design and some technical challenges. I had planned to use a typical weight drive and rewind system, but some what turned out to be unrelated problems, pushed me towards using an endless rope (remontoire, for those who might know about clocks) mechanism. With the design I developed the remontoire weight drops nearly 12"/hour, so it would need to be rewound at least every 6 hours. Well that's no good, so I incorporated an automatic rewind system that lifts the weight about once per hour. This is done with a small motor, a relay, and two limit switches tripped by the weight, one to turn the motor on, the other to turn it off.

I built the clock from walnut plywood that I made from shop sawn veneer. The wheels are machined from cast acrylic and the pinions from cherry plywood. The grasshopper itself was solid cherry. Most parts were cut on my CNC, some several times to get them right. Lots of handwork to fit shafts, bushings, alignment pins, and all manner of other pieces. All in I must have somewhere around 300 hours in this, making it the longest woodworking related project ever.

I used acrylic for the escape wheel because the one I made from plywood would always chip out on the tips of some of the teeth, and precise trips are key to the grasshopper working.

The acrylic machines so beautifully that I decided to use it for all the wheels.

Thanks for your patience. Here's the clock.


What a great project. And, yes, it's incredibly quiet, any background noise makes it silent.

John

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