#HTE

Prototyping a Martial Arts Training Aid

Upon request, I’ve been prototyping a martial arts training aid for my instructor. One of the arts he teaches is called Baguazhang. This is a Chinese internal martial art, similar to Tai Chi, but whereby the practitioner moves in circular patterns.

My instructor explained that for certain exercises, it is helpful to have a visual reference point for the center of the circle one is following. Some practitioners will draw a circle on the ground using chalk. Others, if practicing outdoors, will use a tree.

Not having a tree in his apartment, my instructor asked if I could make him something that would hold a 6.5-foot pole vertically that he could use in his apartment or while teaching seminars and classes. He travels internationally for this.

First I thought about what this object should look like. The symbol of Bagua is called the trigram and it looks like this:

So that seemed an obvious shape for the base. I drew the plans up in CAD and cut it on a ShopBot.

I added through-holes to each layer so I could align them using dowels. I’m using ¾-inch plywood and figured the three circular layers would provide enough support to hold a pole vertical, and if not I could add more. I wasn’t sure about sizing but was mindful that my instructor would occasionally carry this around, so I didn’t want it to be too large. I also didn’t want it to be too small or it might tip over. I settled on eight inches in width since an octagon has eight sides, and I am a bit of a simpleton.

That was the first version. Here I’ll walk you through how it works and how I arrived at the fourth version a week later.

In the photo below, on the left is version 1. On the right is version 4. Versions 2 and 3 were mechanical models that I did not bother to cut the trigram into, I’ll get into those in a minute.

So for the first prototype I sized the hole precisely to a particular pole for a friction fit. But then I realized there was two, actually three, problems to this approach.

The first problem is that wood expands and contracts with changes in humidity, which is why doors or drawers in your house that close well in winter can start to stick in summer. So this particular pole fits this particular hole perfectly right now, but in summer the pole will swell and no longer fit. The second problem is that my instructor travels internationally to teach. I’d like this pole stand to be something he can throw into a bag to bring with him, but getting a 6.5-foot pole onto an airplane would be a hassle.

It would be better if he could simply throw the base into a bag, then use whatever pole was handy at his destination. My thinking was that he could use a pole or spear someone else had brought, or even a mop handle, or just go into a local hardware store and buy a dowel for a couple of bucks or Euros. But wooden dowels are inconsistent. For example both of these, below, are sold in my local hardware store as 1 ¼" diameter, but my caliper says one of them is closer to 1 1/8", and as you can probably see in the photo, neither of them is actually round. That could make for a sloppy fit.

So the third problem is one of size. What I needed was to make a stand that would securely hold an inconsistent variety of dowel diameters. I say “securely” because I’m thinking about how my instructor would actually use this thing: At some point he might interrupt the circle-walking drill to demonstrate something with a partner, at which point he’d probably grab the pole and move it to the side. It would be annoying if he grabbed the pole and it popped out of the stand, and he’d then have to lean the pole against something and kick the base aside to get it out of the way. So the connection should be firm enough that once inserted, the pole stays inserted and it can be picked up without disconnecting.

So I made a couple of mechanical models. At lower right you can see the cutouts for version 2, a failed test that used a rotating cam to lock or unlock the pole into place. That one was a failure, being so fiddly and inconsistent that I didn’t even bother to photograph it before trashing it. At top right you can see version 3, where I struck upon the idea of a wedge. That worked so I incorporated the wedge into version 4.

Here I’ve placed a much smaller dowel (¾" in diameter) into the base of version 4. You can see that it doesn’t fit well and sits at an angle. Stretch that angle up 6.5 feet and you’ve got a non-vertical pole.

That’s why I made version 4 with a series of steps cut into this notch, which forms a 20-degree angle traveling downwards. I’ve lightened the photo in an effort to show the steps.

With this design I can insert this wedge that I cut to 20 degrees…

…which then securely holds even this narrow ¾" dowel in place, and dead-vertical.

It will also hold a variety of sizes up to a 1 ¼" dowel. To lock the wedge in place you simply step on it with your foot. The connection is secure enough that you can pick the stand up by the pole, and in fact is so tight that you cannot remove the wedge with your fingers. Instead you twist the pole out to remove it, then the wedge comes loose and can easily be removed.

The next problem was how to store the wedge during transport. While it fits inside the base like this…

…it makes the shape bulkier and less bag-friendly, and I figured the wedge could fall out in transport and get lost, rendering the entire object useless.

So I inserted a couple of neodymium magnets into both the wedge…

…and the base.

Then the wedge just sticks to the base for transport. It’s not exactly elegant-looking, but the connection is strong enough that the parts will not shake apart in a bag.

It irks me that the wedge does not harmonize visually with the rest of the design, and obscures the trigram when stored. For a future version I’ll have to come up with a better way to store the wedge. I thought about recessing it into a cavity in the underside of the base with magnets but that raises some other problems that need to be solved first.

Anyways, this version works well enough so I’ll call it done for now. I’m giving it to my instructor on Wednesday and hopefully I’ll have some feedback in a few weeks’ time, and can then work on improving the design.

http://www.core77.com/posts/71026/Prototyping-a-Martial-Arts-Training-Aid