Motorising a hand-cranked coffee grinder

We go through periods of drinking coffee in the mornings. Lately, with the 9-month old waking up a few times in the night and getting up at 6, we are in a coffee drinking phase.

Rachel has this hand-cranked coffee grinder, which works very well, but is a lot of work to crank. Holding it while cranking it has aggravated a condition called "mommy thumb", so she made a complicated bracket to help hold it. But lately, we have been leaning towards just buying a nice (burr style) coffee grinder for a few hundred dollars. But before we do that, I figured I should try motorizing this one.

I happen to have this gear head motor that runs at about 120 RPM that would be perfect for the job. It came from an ancient mechanical cash register that was given to me in the 80s. It still worked perfectly until I wanted to see if it could subtract if I turned the motor backward. Then I took it apart. Yes, I know, today one of these would be worth quite a bit, but 30 years ago nobody knew that.

I just needed to make some coupling piece to turn the grinder from the motor. My first thought was to just have an arm that sticks out and turns the crank. This would be the simplest, but perhaps a bit dangerous with that crank flailing around.

So instead I made a coupling piece to go between the motor and the top of the grinder.

I started by drilling two concentric holes in a piece of maple...

... then cut a slot in it to fit around a bracket that is just below the crank on the grinder.

Checking how part of the grinder will fit into the bracket I just made.

I then scratched a circle around it and cut that out round on the bandsaw.

And then cut it conical on the other end and sanded the whole thing smooth.

I also drilled a hole for a pin to lock it to the motor shaft.

Checking the fit. I neglected to take into account the knob that locks the crank in place (which I wanted to keep in place), so I had to make the whole coupling piece again, this time with room for that knob.

Locking it in place with a piece of copper wire. I figured the softer copper would be better, as it would shear off if things went badly wrong.

I decided to re-use these brackets that were part of the contraption that Rachel made two years ago to help hold the coffee grinder. These clamp around the top of the grinder with two bolts.

I attached the motor with two metal angle brackets. These had just the right height to hold the motor.

Using my beam compass to mark a semicircle in the base, for the grinder to slide into (this saves me about 18 mm of height for the whole unit).

The base just attaches to the bottom of the main beam of the contraption using six long screws. Long wood screws actually hold surprisingly well in end grain.

I then wired things up temporarily with clip leads. The motor is a capacitor run motor. I'm surprised that 60-year old capacitor still works. So far so good. I then put some coffee beans in it and let it grind.

It was very satisfying watching and listening to it grind. I set the grind to as fine as I could without having the burrs grinding against each other. But it turns out, ground this fine, it was impossible to press the hot water through the grounds with an aero press. The coffee grains just sealed up! So I have to adjust the setting.

With it proven to work, I set out to finish it up. I made a sturdier bracket for the motor, which was cut from a bracket that used to hold the washing machine motor that I used for this belt grinder.

I hollowed out the back of the main beam to embed the capacitor and switch inside it, then covered that with a sheet metal plate.

I also attached the crank to the back of it, for "emergency use". The crank is the only part I removed from the coffee grinder.

And here is how the grinder is attached using two 4" jig bolts and threaded knobs.

Finished motorized coffee grinder. Quite the contraption.

We actually also have two $30 coffee grinders, but these don't appear to last very long, which is why we have been considering a fancy one.

The grinding burr in this hand cranked one is really good, so motorising it is a good option. It's the manly way to grind coffee!

So far we have been using it for a week.