Introduction: Training Chopsticks (Chindogu)
This was done as a joke for a friend who owns a Japanese restaurant. It also gave me the opportunity to experiment with the tolerances of my 3D printer. The hinge is over-sized for the task. Both as a way to focus in on the humor of it, and increase the mating area of the working surfaces. The logic being, if these don't weld together, smaller working things won't either.
After the print, the sticks will be solid. Hold the sticks close to the hinge and with a firm, back and forth pressure, work the hinge. In short order, the parts should break free and the sticks will work as intended. If you decide to use them, you'll probably want to sand the ends to make them smooth.
I've removed the restaurant's name from the stl file, so you'll have "plain" training sticks. My buddy doesn't require free international advertising. :-)
You'll need the attached stl file, access to a 3D printer and about 50¢ worth of plastic.
Have fun... And be careful... Don't poke an eye out.
The hinge is the part I was most interested in exploring. My printer has a .27mm "resolution", which means, theoretically, such things are possible. Very little has been published on the internet about this, so I figured it was up to me to experiment on my own. My first attempt was a dismal failure. The tolerances were way too close. I drew them up as I would parts going to injection molding. First, to my knowledge, there's no mass production technique available that can produce this part as one piece. Secondly, the 3D printer uses extruders to lay plastic down. The plastic leaves the nozzle as a thin cord, but as soon as it hits something, it spreads out, closing the space between it and the previous line. That's why my first attempt was permanently welded together.
The pintle (the round part) diameter is just over 3/8" in diameter. It's attached to one of the sticks via a center mounted support. The other stick holds 2 gudgeons (the parts with the holes) that lay flush with the surface of the stick. The spaces between these components are drawn much larger than a comparable injection molded product, allowing for the spread of the plastic. This spread will vary depending on the speed of your printer, the type of plastic, the speed of the extruder and your head and bed temperatures.
There is no play in the sticks I make, so the following dimensions are good for me.
I suggest you start out with these numbers and if they don't work, increase or decrease by a couple of thousandths until you hit the the one that works. Here's my dimensions for the hinge:
Pintle Diameter: .395"
Pintle support thickness: .084"
Horizontal gaps: .015"
Vertical gaps: .020"
Gudgeon thickness: .062"
One note: Try to make the area surrounding the hole in the gudgeon as wide as the diameter of the opening (in my case, .435"). Unfortunately, I was confronted by the width of the chopsticks and the size of the pintle, so the overall gudgeon diameter is as large as I could make fit.
OK. Let's try them out. Practice using the images below until you feel comfortable using them, put them in your pocket and then visit a Chinese or Japanese restaurant. When the server comes over, pull the sticks out of your pocket and say something like; "I'm learning, so I brought my own". I've done this twice, and both times, the server (both Oriental) just "had" to have them, so I couldn't refuse. I'll need to print another pair.
One final point: The question came up; wondering how these sticks create more problems than they solve. There already are "training" chopsticks on the market, but the only one's I've seen are hinged at the top and sprung, like a giant pair of tweezers. They are extremely easy for beginners to use, since they don't require the chopstick "grip" and spring open by themselves. Mine are hinged toward the center, where an actual pair of sticks would pivot and have no spring action to keep them open. Therefore, the person using these must already be adept at holding and using chopsticks... Thus, "training sticks" is an oxymoron, as their action makes them nearly impossible for beginners to use effectively and restrictive for experts. Yet, they work as advertised. Did that make sense?
Anyway, Enjoy... No talking with your hands while holding these in close company. :-)
Update: I was able to visit an oriental restaurant without the waitperson wanting to take my sticks before I had the chance to use them. I have lived in Japan, so I am very proficient using sticks and I have to say... These are more difficult to use than regular sticks (which allow subtle movements that these don't), so any question about their Chindoguness is moot... These are totally useless for anyone. Beginners or experts.
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Ain't technology great? :-)