Introduction: Making a Moving Mechanical Pumpkin

About: Geek, Developer, Maker, Tinkerer, Dad

I've been enjoying making mechanical contraptions lately, and decided to see if I could extend this to my Halloween pumpkin. I wanted a mechanism that could stay fairly well hidden, not be a fire hazard with a candle, and make some of the elements of my pumpkin move by turning a crank. Overall I think it turned out well for a first attempt. Keep reading to see how I went "behind the rind" to bring this pumpkin to life.

Step 1: Making the Sandwiched-cams

The source of the motion in this mechanism is a simple offset/eccentric cam... basically a circle rotating off-center.

For this pumpkin, I wanted a side-to-side motion and an up-and-down motion. To do this I used two 1-inch diameter washers, sandwiched between 1 1/4" washers. The larger washers would keep the pieces from moving off their 1-inch cams.

To assemble this, I used a grinder to rough-up the metal on the washers to get better adhesion from the JB Weld epoxy I used to glue them together. These were clamped and allowed to cure for at least 24 hours. The extra epoxy "squeeze-out" was then cleaned out by spinning the washer sandwich in my drill press, and running a hack saw blade in the grooves.

I then drilled a hole 1/4" from the center, and tapped it so it could thread onto some threaded rod. I happened to use 8-32 threads, but you may have better luck finding 10-24 hardware in the local hardware store.

Step 2: Making the Eye Slider

The two pumpkin eyes will be connected to one piece of metal. For this I used a thin sheet of aluminum flashing (found in the roofing section of the home store). I cut out a long piece with four "uprights" sticking up.

Since this mechanism will likely be curved to fit the shape of the pumpkin, I chose this thin aluminum because it would flex around a curve easily. I also don't have to worry about rust in the relatively wet environment inside of the pumpkin.

The two uprights in the middle have a 1-inch gap between them to fit around the 1-inch cam. This will allow the piece to get pushed left and right as the cam spins.

The outer uprights will eventually get bent so they stick out the eye-holes, and the eye pieces will be attached. I sketched this out on paper at full size (not pictured) to get my spacing right.

I then used a thicker sheet of aluminum to create a more rigid frame for the slider to move in. This frame is also where the cam shaft is mounted. I bent over parts of the frame to lock the slider in place so it could only move left and right.

The end of the cam shaft is held in using two nuts tightened up against each other. The cam itself is attached in a similar fashion. The threads in the cam make it act like a nut, and I tightened another nut up against it to hold it in place.

Step 3: Selecting and Prepping the Pumpkin

Once I had my mechanism built, I took note of how wide it was, and looked for a pumpkin big enough, with a wide enough "flat side." Most pumpkins have a flat side where they were sitting on the ground. This is typically also the dirtiest side. I found one that was in fairly good condition and only needed a little washing/brushing to remove some dirt. This would allow me to fit the mechanism inside without having to bend/curve it too much.

I cut out my design with the eyes spaced to match my mechanism. I also made sure to cut a large opening on the top of the pumpkin (more oval shaped, extending toward the back) so I could get the mechanism in and out, and work inside with both hands if necessary.

Using the cut-outs from the eyes and mouth, I carved two pupils and a tongue, which will be our moving pieces.

Step 4: Mounting the Mechanism

The pumpkin rind isn't the strongest material to mount hardware to. The threads on traditional screws are in my opinion too small to securely grip into pumpkin. We need something with large threads that is short enough so it doesn't go all the way through the pumpkin.

To solve this I used a plastic wall anchor for mounting things to sheetrock. I cut the tip off leaving only the threaded part behind. Using a drill bit I pre-drilled the mounting holes in the pumpkin, then turned the wall anchor into the pumpkin with a screwdriver, inserting it through the slots I cut into each end of the mechanism.

I was really impressed with how tightly it held. This turned out to be a great fastening method with soft materials. I had done some tests using styrofoam earlier with similar results.

Step 5: Attaching the Drive Shaft

Since the mechanism already has a threaded rod coming out the back, I just attached a longer one to it using a threaded rod coupler. The longer rod then extends out the back of the pumpkin. To prevent the spinning rod from tearing up the pumpkin, I ran it through another wall anchor, with a clearance hole drilled through the center.

At this point, with a rod out the back, you could theoretically hook up a motor to run it. In my case I attached a very simple hand crank.

Step 6: Installing the Tongue and Pupils

To mount the tongue I inserted two fairly stiff wires into the back of the tongue. They were bent outward to spread out wider for stability, and inserted into the back side of the pumpkin. This will allow the tongue to move up and down and stay in position in the mouth. Spreading the wires apart also allows room for the candle to sit later on.

Once the eye mechanism was mounted in place, the outer uprights were bent outward through the eye holes, and inserted into each pupil.

Step 7: Attaching the Tongue to the Mechanism

As you may recall, the cam has two grooves, one for the eyes to move sideways and one for the tongue to move up and down. The tongue is held up with a loop of clear fishing line. Originally I tried hanging the fishing line directly on the cam, but it tended to get caught.

To solve this, I cut a u-shaped piece of aluminum from the same thin material I used for the eyes, with the ends bent up to hook the fishing line (see photo). This worked out better for assembly, as I could tie the loop outside of the pumpkin, put it around the wires behind the tongue, and lift it up and into the hooks. It took some trial and error to get the loop at the right size, but it worked. The edges of the u-shaped piece were a bit rough from cutting so it got caught at times, but this problem went away after I sanded the edges smooth.

Step 8: Light It Up!

Light the candle and enjoy. You could connect a motor to the back, or maybe you have friends or guests that would like to turn it themselves. Either way it's a delight to watch, and with most of the mechanics well-hidden, it's sure to impress and delight.

Halloween Decor Contest 2015

Participated in the
Halloween Decor Contest 2015