At the heart of this automaton is the cam follower mechanism. In the diagram below we have named the basic parts of the system. For a more in depth view, take a look at this excellent page by automata builder Dug North.
Here's how the mechanism works -- the axle or shaft rotates, as does the cam which is attached to it. Gravity keeps the follower in contact with the cam as it rotates, however the rod that is attached to the follower is constrained by the bearing, so it can only move up and down.
This has the end result of converting rotation to linear motion.
In this animation we can see how an oval shaped cam will create a regularly repeating up and down motion for the follower and rod. The regular, smooth, symmetrical shape of this motion is similar to a sine wave if we to plot the motion on the y-axis over time.
Irregular Pear Cam
This cam has a somewhat irregular pear shape. Note how this produces a more abrupt rise and fall, with a longer period in the down position, more like a jump motion.
The basic construction of our automaton is fairly simple -- we'll use a small box as the frame, two dowels (or pencils!) poked through holes in the box for the cam's drive shaft and follower rod, and cardboard shapes for the cam and follower. A piece of drinking straw will act as our rod bearing.
Instead of cranking the drive axle by hand, we'll turn it with a DC motor driven by the Crickit and Circuit Playground Express. Since the DC motor spins too fast to connect to the cam shaft, we'll need to gear it down by building a large cardboard wheel which the motor will drive indirectly with a smaller wheel and rubber tire.
Start with a small cardboard box. From it, remove a long, rectangular section from the front of the box. This will be a guide for the follower so that the rod (and automaton) doesn't spin around on it's long axis.
Cam Shaft Holes
Next, poke a hole through the centers of both sides of the box -- this will hold the cam shaft as it rotates.
- Measure the spot at the center of the sides so that the axle is level
- One way to do this is by using a ruler to draw and 'x' from corner to diagonal corner -- the center of the 'x' is the center of the rectangle!
- Use a sharp, round object to poke the holes through so the dowel will fit and turn without much friction
Now, poke a hole through the center of the top of the box for the rod dowel.
To help the rod move up and down, we'll add a short length of drinking straw to act as a bearing. The plastic is pretty low friction, and the length of the straw will prevent the rod from rotating.
- Mark the center of the top of the box
- Poke a hole for the bearing
- Cut a short length of drinking straw -- make sure the straw's internal diameter is large enough to fit the dowel
- Place the straw piece in the hole
- Optionally, use a dab of hot glue to hold the straw in place
Cut out an identical rectangle from the back of the box as you did for the front -- this will act as the other half of the guide for the follower to prevent it from rotating.
Now, we're ready to make a cam. You can experiment with different sizes and shapes of cams for your automata. For this one, we'll make a type of jumping motion by using an irregular pear shaped cam.
- Measure the distance from the center to the top of your follower guide rectangle you cut from the front face of the box. This is the maximum radius our cam can be before it would get stuck during rotation
- Draw a circle of that measured radius on a piece of cardboard using a compass. Alternatively, you can use an appropriately sized lid or can to measure and draw the circle
- Draw a smaller circle centered on the same spot as the first. This will be the lowest point the follower can drop
- Connect the shapes as shown to create the irregular cam -- this doesn't need to be exactly the same as shown here
- Cut out the cam using a knife or scissors
- Transfer the shape to create two more cam pieces, then poke their center holes out to the same diameter as your dowels
- Glue the cam pieces together
Make the Follower
Now, we'll make a simple rectangular follower that will be able to ride the edge of the cam and transfer the motion to the rod and automaton above.
- Measure a piece of cardboard that will protrude from the front and back of the box and is a bit more narrow than the rectangular follower guides you cut from the front and back of the box
- Cut four identical rectangles
- Poke holes in the centers of three of the rectangles for the rod to fit
- Glue the pieces together in a stack with the piece that has not hole at the bottom -- this will contact the cam so we want it to be smooth
Insert Rod and Follower
Slide the rod into the bearing and then attach it to the follower by pressing it into the hole. You can add a bit of hot glue for a secure fit.
Add the Cam and Shaft
We'll add the cam shaft, cam and a couple of rubber bands to prevent the shaft from moving left or right.
- Push the cam shaft through one side of the box
- Wind a rubber band around it. This will help prevent the shaft from moving around
- Slide on the cam
- Add the second rubber band
- Push the shaft through the other side of the box
- Add hot glue to secure the cam to the cam shaft -- these two parts need to move as one
Test the Motion
Close the box lid and try rotating the cam shaft -- you'll see the follower has no choice but to move up and down!
Cam Shaft Wheel
Since the DC motor we're using spins faster than we want, we will gear it down. To do this, we'll use a small wheel on the DC motor shaft that will spin multiple revolutions to turn a much larger wheel it is "riding on". This is similar to using toothed gears to accomplish the same task, but we'll be able to get away with just the friction of the rubber tire to do the turning.
We'll make a wheel of the exact size we need out of cardboard! The wheel needs to have a diameter a bit smaller than the height of the box so that it won't touch the surface upon which the automaton is resting.
Draw out a circle on a piece of cardboard that is a bit smaller than the box height. You can use a compass, or find a large can or lid to trace.
- Cut out two circular discs for the wheel's sides
- Mark the centers of the discs, so we can poke holes for the cam shaft later
For the center tread of the wheel, we'll cut a long strip of cardboard from an unfolded shipping box.
- Flatten the box and cut of the flaps
- Cut one edge of the rectangle to create a single, long strip
- To determine the length needed, mark a start point on the strip and a disc and then roll it along the tread strip one revolution until the wheel mark hits the strip again-- this circumference is the length needed so mark the strip for cutting
- Cut the strip a bit longer than measured to account for material thickness (I didn't do this very well so you can see in some pictures where I added a bit of cardboard to fix it!)
Pre-Bend the Tread
You can roll the tread tightly around a cylindrical object a bit smaller than the discs in order to form the tread before attempting to glue it to the discs. This will give the tread a bit of a 'memory' of the shape, making things easier.
Attach the Wheel
Now we'll slide the large wheel onto the cam axle.
- Slide the wheel onto the cam axle dowel
- Push it close to the box, but not touching
- Add a bit of hot glue to fix it to the axle
Your basic automaton mechanism is complete! Give the wheel a spin (be careful to note which directly allows the follower to smoothly glide over the edge of the cam, it may get stuck going the other way due to the flat side of the cam) and watch the rod go up and down.
Next we'll program the Crickit and Circuit Playground Express in MakeCode to run the DC motor, which we'll later connect to the mechanism.