Iron Man 2 Swinging Sticks Aka Double Pendulum

Like many of you, after watching Iron Man 2 (which was awesome, by the way) I was very interested in the “Swinging Sticks” sculpture on Pepper Potts’ desk. A little bit of searching turned them up for sale. They could be mine for just under $300. If they were a whole lot under $300, I would have bought them. Instead I set out to make my own version.



Unfortunately, the one for sale runs on batteries, so this is not an exact replica of it.  This is more like a modified double pendulum.  Double pendulums are pretty cool by themselves.  Much has been written on them, including some pretty cool flash simulations

This is an entry in the Kinetic Sculpture Contest, so please rate it and vote on it when the time comes.

Step 1: Materials

At the minimum you're going to need:

Rods for the main beams - I used 3/8" diameter steel.
Bearings - sized to fit the rod
Rods for the axles - I used 1/8" diameter steel.

Drill press
Drill bits
Reamers

Step 2: Sizing Things Up

I did my size estimations by using a pictures of the commercially available version and counting the pixels using Paint. From there, I used a ratio and based it on 12" length.

Short Arm
Length – 12”
End to Pivot – 5-1/8”
Pivot to other end – 6 7/8”

Long Arm
Length – 15.75”
End to Pivot – ¼”
Pivot to Pivot – 5”
Pivot to other end – 10-1/2”

The pivot point will be the center point of the ¼” bearing or the 1/8” pin.

After I made this, it didn’t seem to move the way I liked, so I started cutting pieces off to change the balance.  Your mileage may vary, so feel free to play around with it.  The size ended up being:

Short Arm
Length – 10-1/4"”
End to Pivot – 4-1/4"”
Pivot to other end – 6"

Long Arm
Length – 13"
End to Pivot – ¼”
Pivot to Pivot – 5”
Pivot to other end – 8-7/8"

I went back to see where I went wrong, watching the youtube vid of it in action. It turns out that the commercial version runs on batteries. I suspect it uses magnetism to keep the bars in motion. Without the magnetism, this reverts back to being a double pendulum. It doesn’t run for hours on end, but it does do some nice chaotic motion when you get it going.

Step 3: It's a Set Up!

It’s a Set Up!
Set up is always the most important step in work like this. For the model I made, I needed a ¼” hole in a 3/8” diameter rod. That leaves 1/16” on either side of the hole. Not much room for error. There are a few ways to find the center of a round object. If you have a good x-y table with digital readouts and an edge finder, then I probably don’t have to tell you how to use it!

The item I use is a center finder I got from McMaster-Carr (their part number 22025A11, $12). You chuck this into your drill press and drop it onto your round bar. There are two tick marks to indicate when you are at top dead center. This probably is only accurate 1/32” or so, but it seems to be good enough for this.

Another less accurate method would be to chuck up a piece of round bar the same diameter as the piece you’re drilling. Use that to line up the fixed jaw of the vise with the chuck.

Once you have the drill press centered for one piece, it should stay centered for all the subsequent pieces.

If anyone has any other tricks for this setup, I welcome your comments.

Step 4: Getting Reamed Out

To get the bearings to make a nice tight fit, we need our hole dimension to be accurate. The bearings I bought are ABEC 5 which have a tolerance of + 0.000” and – 0.002”. That means my hole needs to be no larger than 0.250”. That seems easy enough, a ¼” drill bit measures 0.250”. True, but a ¼” drill bit never drills a precise ¼” hole. That’s pretty much true of any size drill bit. If a precise hole is needed it must be reamed.

Starting a hole in round stock can be difficult if you have a thin bit, or if you’re slightly off center. To combat this, I started with a ¼” center cutting endmill and made a flat on the round bar. Next, use a starting drill bit (spotting bit) to start the hole.

For reaming the hole, I drilled a hole using a 15/64” bit, which is 1/64” under. After drilling, I used a 0.250” reamer to finish the hole to the correct dimension. Note that some reamers are meant to be used by hand and some can be chucked in a machine. The one I used can be chucked in a machine. When reaming, you want to turn the tool very slowly. I suggest you do some googlin’ on the subject to gain some knowledge on the process before beginning.

For the 1/8” holes, I drilled them with an 1/8” drill bit about 1/4" deep. This was a mistake as it made the hole oversized for the 1/8” pin I had. Instead of re-drilling them, I tack welded the pin in place.  You want the pin to be a tight fit and not move. 

After reaming the ¼” holes, the bearings were a press fit.

Step 5: Bearing Down on You

I ended up using 2 bearings in each hole. The bearings allowed more angular variation than I would have liked. Putting 2 bearings on and supporting it at 2 places seemed to do the trick. The bearings I used were McMaster-Carr part number 57155K365, app. $5 each. Some of the double pendulum building guides use skateboard bearings because of their low cost – on ebay for under $1 each. They are metric, usually with a 22mm OD and 8mm ID. That causes me two problems. First, my 3/8” rod is roughly 9.5mm so it’s pretty hard to drill a 22mm hole in a 9.5mm rod. Second, even if I went with something larger, I’d need to use a metric sized pin and tools, which I don’t have. Choose your bearings based on the size of your project.

For the axles, I used 1/8” rod. It’s slightly undersized, but allows me to easily slide the pieces apart to modify the lengths. The length of the rod is just under 1".

Step 6: The Stand

Since this was my first attempt at this, I opted for a simple single pole for a stand. I happened to have a piece of 1-3/4” diameter round bar in the shop and it looked massive enough to work as the base. I put some grooves in it so it would look, well, groovy! 

Most of the time, it works just fine. If you spin it too fast, it can get off balance and fall over. The A frame and wide base would be a better choice for this, I guess that’s why the manufacturer chose that. Make sure that the height of the stand is enough that your rod won’t hit the bottom at its maximum lowest point.

Step 7: Go for a Spin

To assemble, slide the pins into the bearings and that's it.  Mine are a little loose, so I can take them apart easily.  I usually leave about 3/16" space between each bar to make sure they don't hit each other.

When it’s all together, give it a spin and watch it go. It doesn’t move with the slow hypnotic action of Pepper’s desk sculpture, but it is pretty neat to watch. It is definitely chaotic motion. I completed this in time for our Halloween party and the guests were pretty impressed with it.

Hope you got something useful out of this.  If you make your own, please post a picture or video.  If you hit the magic ratio for the lengths, don't be shy with them!