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Thread: A Test Clock using John Harrison's Grasshopper Escapement

  1. #1
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    A Test Clock using John Harrison's Grasshopper Escapement

    I wanted to build a tall, wood gear pendulum clock that would meet my wife's requirement of no tick-tock. The only movement I know of that does that is the Grasshopper movement developed by John Harrison in 1722. He didn't develop it because it was silent; he did it because by indexing the pallets on the tip of the tooth it eliminates sliding friction, the enemy of accuracy with mechanical clocks. But that action also makes it almost silent. As a side note, Harrison was the guy who invented a clock that could keep time accurately enough for navigation at sea, which allowed sailors to accurately know longitude any time of day, in any weather. The guy was a dogged genius who started out as a carpenter, so there's hope for the rest of us.

    There are a few sources on Harrison's grasshopper which explains how the geometry works, and even gives critical dimensions for some versions of it, but nowhere could I find an accurate drawing of one. I finally found a rendering of one online, copied that, pulled it into my CNC software, and scaled it so the dimensions matched those I needed. I didn't know if this would really work, so I built a test rig just to evaluate the grasshopper on the escape wheel that I designed go with it. Harrison was very adamant about many aspects of clock building. He claimed that a 200 tooth escape wheel to be best, though I don't think he offered any evidence why, that the pendulum should swing in an arc of over 10 degrees, and it should weigh 3 lbs. Most clocks at that time had pendulums that only swung about 3 degrees and often weighed close to 50 lbs. I went through the calculations to see if Harrison was correct when he said both would give the same force. He was right. I couldn't imagine what the structure would have to be to be stable with a 50 lb weight swinging back and forth, which made Harrison's design even more attractive.



    To my delight, it worked immediately, and it was, indeed, almost silent, with the only noise coming when the pallets release and strike the brass composer that guides it back to the tip of the tooth on the next half cycle. With confidence it would work, I constructed the going train (wheels and pinions) to go with it. The wooden escape wheel above looks very nice but many of the teeth chipped away during machining. Since the pallets rely on catching precisely on the tip of the tooth, unlike other mechanisms which ride on the sides, this wouldn't do, so I machined one from 1/4" cast acrylic. It was about as perfect as I could expect, so I decided to make all the wheels out of acrylic, and just the pinions and connectors out of wood.

    Clock wheel and pinion gear ratios is pretty straight forward. You can do them by hand, but I created a simple spreadsheet to do it. Once you have the number of teeth (ratios) for each wheel and pinion figured out, you need to determine the size of the wheels and the center to center distances. You can do that manually, too, or with a spreadsheet which I did, but then you have to draw the wheels in order to machine them. Doing that manually, as I had to for the escape wheel, is very tedious, but I found a calculator by Matthias Wandal at reasonable cost that works very well and draws the wheels and pinion sets for you after you input a range of variables. Best of all, it can output files in several formats, including those who would be cutting the wheels by hand, or a dxf and other formats for CAD use, which is the format I used. Once I had all the wheels and pinions into my CAD/CAM software (VCarve Pro) I could machine them on my CNC.

    To those who think cutting the wheels and other parts on a CNC is cheating, I'm fine with that. My hat's off to anyone who can do this level of precision by hand. I'm a very patient guy, but I do have limits. Even after the parts are cut, there is a lot of hand work needed to fit the arbors, bushings, connectors, what not, together to assemble a workable clock. Hours upon hours of work.

    I assembled the clock and soon found it wouldn't run with the typical gravity weight drive system, even with 11 lbs of weight, so I decided to use a form of remontoire drive/rewind system instead, using the outside of the center wheel to drive the clock. That reduces the weight dramatically, only about 1/3 lb was required in a test I did.

    More to follow.

    John

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    This is the center wheel with the remontoire wheel attached to it. Some of the other wheels can be seen as well. The center wheel makes one full rotation per hour. A couple of other wheels sit in front of it to make the hour wheel make one full rotation every 12 hours. The trickery of having both hands on the same arbor is to use a tube over a tube (or solid arbor, depending upon what else is going on).



    Here are the little teeth on the remontoire wheel that engage the belt so that it drives the wheel rather than slips. Those little teeth are cut with a 1 mm end mill. You can see the quality of the cut on the teeth, too. The blue stuff is from the tape I use to hold the parts down for machining. I need to clean that off before assembling the final clock.



    A small DC motor drives the remontoire. It sits in a small frame that slides onto the lower dowels, between the front and back face.



    Here's a photo of the wheels and motor assembly installed. I hosed up on the location of the motor/idlers on the right side, so I had to run the belt over one of the dowels temporarily.



    With everything installed it looks like this.




    The motor is triggered by two limit switches engaged by the primary weight, on the right side. A relay locks when the lower limit switch is triggered, and is released by the upper limit switch. The limit switches are about 12" apart at the moment. The center wheel has a perimeter of about 24", so the motor is engaged about once/hour.



    Video to follow.

    John

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    Here's a video of the clock running.

    https://photos.app.goo.gl/5VFGApz57FLpvsz79

    Now that I have a working prototype, it's time to design a more attractive, refined version.

    John

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    Very impressive !

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    Oh John, you are having WAY TOO MUCH fun. Fascinating project. Thanks for sharing with us mere mortals with ham fisted hands . . . and no CNC

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    Quote Originally Posted by Patrick McCarthy View Post
    Oh John, you are having WAY TOO MUCH fun. Fascinating project. Thanks for sharing with us mere mortals with ham fisted hands . . . and no CNC
    I am having fun, thanks. It's possible to make clock gears by hand with nothing more than a bandsaw or scroll saw. There are several videos and articles on how to go about it, so you could build a clock if you have the motivation. Clayton Boyer sells plans that can be made either by hand or with a CNC. I've built 2 of his clocks using my CNC, but I could see doing it by hand being possible, just a lot more tedious shaping the teetch on the wheels. The CNC makes it a lot easier, for sure. I bought the CNC not really knowing how I'd use it with my woodworking, and mostly to keep my brain engaged. How true that has turned out to be. Little did I know at the time that clock making would be such a fascinating field of study.

    Here's one of Clayton Boyer's clocks I built with a friend. Very fascinating to watch.

    https://photos.app.goo.gl/h4fqDFGWUs5CJKa57

    John

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    John, absolutely fascinating. I have always planned on doing a tall clock case, but never before considered tackling the mechanism . . . until now . . maybe . . . . . .

  8. Fantastic project, John!
    Independently from you, I have chosen a fairly similar approach to build a Harrison inspired grasshopper clock mainly from wood:
    https://sawmillcreek.org/showthread....n-s-principles

    Regards, Norbert

  9. #9
    Thank you John and Norbert for posting, great work!

  10. #10
    I've got nothing to add but to say you definitely have a keen mind, patience and a dogged determination to build this. Bravo.

  11. #11
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    Would you care to show how the teeth are cut on the gears, the actual cutting of them? I'm impressed with the profile of the teeth.

    When I was in the Navy we cut a spur gear and a gear rack. But we had a profile cutter that was the exact shape of the teeth being cut.
    Confidence: The feeling you experience before you fully understand the situation

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    There is a show on Youtube called, Longitude or something like that. It's about nautical navigation in the age of sail and John Harrison figures prominently. There was a substantial prize offered for a clock that would keep accurate time on a pitching ship. Latitude had been figured out for some time using celestial features - sun and stars. Calculating accurate longitude required accurate time keeping and Harrison figured it out.
    Last edited by Curt Harms; 01-06-2024 at 11:18 AM.

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    Yes, the book is called Longitude. It's a pretty good read. Now there was a man, and his son, with dogged determination over several decades.

    https://www.amazon.com/Longitude-Gen...s%2C116&sr=8-1

    John

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    Quote Originally Posted by Jerry Bruette View Post
    Would you care to show how the teeth are cut on the gears, the actual cutting of them? I'm impressed with the profile of the teeth.

    When I was in the Navy we cut a spur gear and a gear rack. But we had a profile cutter that was the exact shape of the teeth being cut.
    Jerry I cut the wheels on my CNC. This photo is of some wheels and pinions for the oscillator clock.





    The design is from Clayton Boyer. When you buy the plans you get both PDF files that can be printed out full size for those who wish to cut them by hand, and also a dxf file for those who use a laser or CNC. I used the dxf files. You might think that you just pull that file into your CNC, chuck in a bit, and press go. It's nothing like that. The vectors in Boyers file aren't continuous, so there's a lot of work to make them so. And there is even more toolpath and simulation work required to see which bits are needed, and many other parameters. Holding the small parts on the CNC for machining can get pretty interesting, too. There are many parts much smaller than even the smallest one in the photo.

    Here's a photo of the escape wheel for the grasshopper clock. The teeth were cut with a 1.5 mm diameter endmill, and took about 1.5 hours I think. I first made this wheel out of wood, but could not keep the ends of the teeth from chipping out, so I switched to cast acrylic to solve that problem. It cuts so nicely that I used it for all the wheels.






    For my grasshopper clock, I designed all the wheels and pinions from scratch which was a new and "interesting" experience. For both clocks, and another, I made my own plywood from shop sawn veneer. That allows me to have high quality material in any thickness and species I want. A bit more tedium, but well worthwhile.

    John

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    Thanks for the explanation John. Makes sense how you got the perfect profile using the 1.5mm cutter.

    Nice that you can plug your information into a program and have it do the calculations for you. We had to memorize 12 different calculations for the spur gear and do the math to four decimals without the help of a calculator.
    Confidence: The feeling you experience before you fully understand the situation

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