Baby rocking machine

My original intention was to build some sort of baby rocking machine that would use a large cam to make more complicated rocking motions to soothe our baby.

First challenge was how to power this thing. A gearhead motor would be ideal. Maybe a cordless drill where the battery was long gone, but they are loud and wear out fast. Then I thought of using an AC gearhead motor (at right), but this one is meant only for temporary duty. Running it at 60 volts instead of 120 keeps it from overheating, but I'd need an extra transformer to get that voltage.

But thinking of drills again, I realized I had kept the gearbox from a big impact drill that I was unable to fix. I could put a pulley on the motor shaft and power it with a belt. And the gearbox has two speeds, which could come in handy.

Initially I wanted to use an O-ring for the belt, but I didn't have one long enough. But I did have a flat belt from an old 8-track player.

Flat belts need crowned pulleys to track. I made my pulleys out of pieces of 18 mm birch plywood, first cutting a circle on the bandsaw, then turning it against the belt sander. I tilted the table a few degrees and sanded from both sides to give the pulley a bit of a "crown" (the middle of the pulley is slightly larger). I used a screwdriver as a central pin to turn it on to make sure the outside is perfectly round and concentric with the hole.

Initially I was thinking of using this small 1750 RPM induction motor. But it draws about 50 watts, so I figured it would get fairly hot.

Then I tried this 1750 RPM motor that came from an old record player. It only draws 8 watts and is super quiet, but I wasn't sure if it was powerful enough.

I made some wooden brackets to hold the drill gearbox. I'm holding it upright so the drill's chuck is higher above the base. This would leave more room for a cam wheel to mount to the drill's chuck.

With further experimentation, I was pretty sure the record player motor wasn't powerful enough. So I switched to this shaded pole fan motor, which draws 25 watts. But this motor is a 3500 RPM motor, so I needed a much smaller pulley to get the same belt speed. I again used the belt sander, but I shaped it by spinning it on a drill against the running sanding belt.

I also found a bushing that had the same diameter as the drill's motor shaft and made a block to hold that on there. This gives the shaft a bit more support. Here I'm applying some grease to make it run smoother.

I was still thinking of using a cam, but I was keen to try out something for the time being. So I made a small crank to go in the drill's chuck, and a long arm to attach to the bassinet for rocking it.

The bassinet is on wheels and I guess I could have made my machine roll it back and forth, but rockers would be much better.

To get the round curve, I clamped a strip of wood to a bigger piece of wood so that it's bent at an angle. I clamped it in such a way that the bending moment is applied at the ends as opposed to bending the strip by pushing it with a block in the middle. With the bending moment applied at the ends, there is a constant bending moment along the length of the strip of wood, which will make for an arc that's much closer to circular.

With the curves marked on the piece of wood, I cut the rockers out on the bandsaw.

Then sanding to get a smoother curve. This took some tweaking. Even a very slight flat spot on the rocker will cause very noticeable bumpiness in the rocking motion.

I cut some indentations on the ends of the rockers for the bassinet's wheels to fit into, just by drilling a partial large hole. I had to jig this up with clamps. I tried drilling the first one without clamping the rocker to my sacrificial piece, but wasn't able to hold it firm enough.

Placing the bassinet on the rockers, and attaching the actuator arm to the frame.

Testing it out. Surprisingly, it actually helped calm our little baby Harriet down. For a while, we kept her in a cardboard box in the bassinet, which made it easier to move her around without waking her. But she soon came to hate that box so we stopped using it.

We ended up using the rocking machine quite often, so I refined the design a bit. Without air flow, the fan motor got very hot and we could smell it. So I added a fan blade to the motor shaft, with a protective shroud around it. I also added a support to the end of the motor shaft.

Also, the device kept sliding around a bit, so I added an extension that hooked to a knock-down screw in one of the rockers.

I also shortened the arm that connects to the bassinet and rearranged the parts so that the machine partly fits underneath the bassinet. With a mobile attached to one side of the bassinet with a wooden bracket, it was slightly off balance, so I added a wooden block as a counterweight, which you can see in this picture. This cuts down on the force needed on the actuator lever to keep it vertical.

We use it with a relatively small crank throw and the gearbox in low gear, but if I change it to a large crank throw and high gear, it's quite the wild ride! We haven't tried it in that mode yet with Harriet.