Motorize Your Grain Mill

Like a lot of homebrewers, I started out with extract batches, eventually moved on to extract with specialty grains, then partial mash, and finally all-grain. Up to the partial mash stage, I was happy zipping my grain into sealed bags and whacking it with a rubber mallet. Once I reached all-grain brewing, though, the mallet method wasn't going to cut it, and I was not eager to keep relying on the local homebrew shop for a proper crush. It was time to invest in a mill of my own.

I went with a Monster Mill 2 and began drafting plans, scrapping them, and starting over again. After roughly five years of on-and-off tinkering, I finally got the rig finished and ran grain through it for the very first time. What follows is the build that worked for me.

Monster Brewing Hardware, the makers of the Monster Mill, recommend running the mill at around 150 rpm. My first instinct was to follow the path of countless brewers: a standard 1725 rpm farm-duty motor (easy enough to source from any major retailer) paired with a 12+ inch driven pulley on the motor and a large sheave on the mill itself. The trouble is, a setup like that demands either a huge pulley or a jackshaft to bring the speed down, and I didn't have the space for either. Belt drives work on the same principle and run into the same headaches. Screw-driven openers, I should note up front, simply won't work for this project.

Then it hit me: I already owned a quiet, geared-down, low-RPM motor sitting right above the cars every evening. A garage door opener.

Why a Garage Door Opener?

Chain-drive garage door openers typically run in the ½ to 1 horsepower range, and they use a worm gear on the motor shaft that turns a secondary shaft attached to the chain sprocket. That combination of torque and low RPM is essentially perfect for milling grain. As far as I can tell, belt-drive openers operate the same way, so those should be fair game too — but again, screw-drive units are a non-starter.

The first real hurdle was figuring out how to attach the opener's output shaft to the mill. The opener shaft and the mill shaft have different diameters, so I chose a Lovejoy coupling. Lovejoy connectors are removable, they tolerate small misalignments between shafts, and the rubber spider in the middle will absorb a sudden torque spike if a rock or pebble finds its way into the grain.

The second hurdle was rewiring the opener to run continuously instead of stopping at preset limits like it would on a garage door. I pulled the main control board and wired the motor directly to relays — the details are in the step-by-step below.

So far I've milled roughly 36 pounds of grain without a hiccup. The Lovejoy spider is plastic, so I'll keep an eye on wear over time, but replacement spiders are easy to find and run under twenty dollars.

Step by Step

I picked a 5-gallon (19 L) bucket to catch the milled grain. With the mill mounted flush on top of the box, I only had to make sure the box sat a little taller than the mill's mounting bolts, while keeping the distance between the mill top and the top of the box short enough to cut down on dust. I framed the box out of 2×2 and 2×1 lumber with an Orientation Strand Board (OSB) skin, and painted the inside with leftover house paint to make cleanup easier. Casters or wheels aren't required, but they make the whole unit much easier to roll around the brewery — now is the moment to add them if you want them.

The main circuit board has to come out. It's mounted vertically at the top of the opener housing. A smaller board carrying the limit switches also needs to come out, along with the small gear attached to the drive shaft — in most openers you'll find the grey, yellow, and purple wires running to it. The limit switches, the RPM sensor at the end of the motor shaft, incoming power, and the motor itself are all wired back to the main PCB through a single connector. I cut the connector off and kept only the wires I needed. I left the RPM sensor wires in place in case I decide to add a tachometer readout down the road.

I wanted to keep all the high-voltage wiring inside the motor's own enclosure, so instead of using switches I drove everything through relays. I used a relay board running on 5VDC. The opener already had a terminal block where shore power entered the unit, so I ran an extension cord out to that block and plugged in a cell phone charger to feed the relays with 5VDC.

The motor wiring uses two relays — one for On/Off, the second for Forward/Reverse. Incoming power passes through the On/Off relay first; when that relay closes, power flows through the second relay, which decides whether the motor spins forward or in reverse. For safety, the leads running back to the relays are soldered on the underside of the board, and I added a plastic sheet between the relay board and the metal opener frame so the mounting screws can't short anything out.

I mounted the switches themselves in a small project enclosure on the front of the mill. The wire ran through the factory safety-sensor channel and was secured at every contact point so nothing rubs or chafes during vibration.

The flanges on the opener were about 1¼ inches deep, so I fastened the opener onto 2×2 lumber (actually 1.5 inches square). That gave me enough play to slide the opener up and down and line its output shaft up with the mill shaft. Once the opener was positioned, I marked and cut the drop hole for the crushed grain to fall through into the bucket below.

I used Lovejoy couplers to join the mill shaft to the opener output. They come in a range of bore sizes, so they'll handle a little misalignment. I had to grind a small flat on each shaft to give the set screws something to bite into. Once the mill was coupled, I bolted it down, double-checked every fastener, and installed the hopper on top.

To pull heat off the motor, I picked up a 115VAC 120mm muffin fan — the same kind used to cool computer cases — and cut a hole in the bottom plastic panel of the opener. I set up the fan to push air upward, because heat rises and because grain dust will fall. Dust filters went on both the intake and exhaust openings to keep as much flour as possible out of the motor. The fan is wired straight into the incoming power block, so it runs whenever the unit is plugged in.