Magnetic Key

The goal of this project was to make a wind-up doll key for a Halloween costume, which would rotate at a reasonable speed. I also wanted to make it so that the key could be attached to an hidden harness without damaging or altering the back of a costume. In the end I ran out of time and encountered some major technical hurdles, namely the fact that no matter how light you make the key you will need an absurdly strong magnet to attach it, and the motor I chose was not nearly strong enough. Meh, it makes an interesting Steam Punk artifact.

Over all, I spent about $100 on this project, but it would have been less than half that cost if I had gotten the brass at someplace besides McMaster-Carr. That place is EXPENSIVE!



... and yes Virginia, The Dresden Dolls rock.

Step 1: Tools and Materials:

- brass plate
0.010" thick 8"x12" brass
mcmaster.com 9011K7 $5.59 (x2)

- brass sheet
0.010" thick brass shim stock 6"x60"
mcmaster.com 9504K24 $10.96

- brass tube
2" outer diameter 0.032" wall 3' length
mcmaster.com 8950K842 $52.37 (seriously WTF?)

- 1/2" balsa
3"x36"x1/2" balsa
pearl art $5.00ish

- magnet
k&j magnetics $7.00
http://kjmagnetics.com/proddetail.asp?prod=MM%2DB%2D42

- nut + bolt
scrounged

- disco ball spinner
amazon $9.95
http://www.amazon.com/gp/product/B000HZJXCE

- toggle switch
scrounged

- 2 cell AA battery box
scrounged

- epoxy
- dremel/ drill
- band saw
- disc, belt, drum sander
- soldering iron + solder
- pliers

Step 2: Motor Modification:

The motor for this project needed to be geared down really slow, so that it would look like a clock spring, not a fan, and an almost ideal solution was a disco ball spinner. The only problem is that they're not designed to exert any significant torque, so the key must be as light as possible. To make the motor fit inside the shaft tube, I cut off the gearbox housings screw tabs and epoxied it shut, being very careful not to gum up the gears. Next I discovered that by doubling the voltage I could increase the power and speed a bit, So I wired up a 2 cell AA battery box to a switch and soldered it onto the motor contacts. In order to fit the battery box, I had to yank out the existing spring contacts and re-fit (squish) the hinge rivets. The magnet is attached via a nut, bolt and washer through a hole in the bottom of the battery bucket.

To attach the motor to the key I glued a disk of balsa wood onto the hook and drive axle. The wood has enough friction to grab the inside wall of the tube, but still be loose enough to remove the motor.

Step 3: Brass Key Construction:

The key "handle" is a lamination of thin brass sheet around a balsa wood core. I wanted the look and feel of solid brass, but that would have been prohibitively expensive and heavy. I also wanted the surface to age like brass and possibly be etched with a pattern at a later date, so I
decided against spray paint or other fakery. If you don't care about the "feel" of brass, this project would work much better with cardboard, foam, and spray paint. Building upon the motor diameter I was able to model a simple clock winding key in SolidWorks and scale it to fit the tube. I also made the thickness of the key a round number that was proportionate to the shaft, in this case 1/2". I have provided a 1:2 scale drawing of my key as a PDF below.

Start by creating a plank of balsa wide enough to cut the base key from. Ignore the shaft for now. I just stacked 2 3" wide strips going long ways across the key to give it some strength. Just epoxy and clamp, but make the piece is flat. Once cured, trace the key onto the wood and cut using an x-acto or band saw. Use the sanders to smooth the surface and finalize the shape.

Then create 3 strips of brass from the shim stock roll a little more than 1/2" wide. You will need 2 shorter strips for the inside of the holes and one long piece to wrap around the edge. (I didn't have tin snips handy, so I cut the strips on the band saw, which is pretty dangerous - the brass is already sharp, springy, and soft enough to get pulled into the throat of the saw and jam.) Neatly cut the ends of the strips square to the edge, wrap them where they will go, and trim them to length. Now epoxy the strips in place, making sure to have some extra hanging over each side. Once it had cured, I took the brass down flush with the wood on the belt sander, being careful to hold it so the brass would not get ripped away.

Now that we have a brass edged form, we can glue on the front and back brass plates. I pre-cut the 2 plates to a rough shape so I could see the core when doing the final shaping. Just slather on epoxy and pile weight on the sandwich. Once that is cured, I used the drum sander to take it down to the final shape making sure not to go through the thin edge banding.

For the shaft, I just cut the brass tube down to about 8" long and glued a brass covered wooden disk into one end. To assemble the key and shaft, I placed the shaft on the edge, where I wanted it to slot into the key and traced out a circle. Where that circle met the edge, I drew up 2 lines perpendicular to the bottom and used them to cut a 3" notch into the flat key handle. Once you sand out the notch a bit, the shaft should slide very snuggly into the notch - Just epoxy to hold it in place.

Step 4: Assembly:

Now that we've got the 2 parts, all you do is push the motor assembly into the tube, making sure to turn on the motor before it's all the way in, and then stick it to whatever you want. You need to make sure that the axis of the battery bucket hinge is vertical though, otherwise it will bend and the motor will not have the power to straighten it out.

I was planning on creating a harness with a steel plate on the back, so that I could wear the key without destroying the clothes it attaches through, but didn't get my magnets in time for Halloween :( It would be pretty cute on the back of a Yaris, VW, or Vespa, but I would not risk driving with the key attached. It's far to fragile as it is made right now, doesn't have a super strong method of attachment, and is still beefy enough to cause some serious damage if it flew off on the freeway.

Also, remember not to go anywhere near hard drives, credit cards, TVs and computer monitors with the magnet - It's ridiculously strong and will mess up electronics, and possibly pinch fingers.