DIY Coffee Roaster
coffeeI’ve been roasting coffee beans for my own consumption for a few years by now, and I wanted something better than doing it in the pan.
Here is a picture of the beans roasting in my oven. They are rotated in a cylinder for uniform roasting. Previous attempts at roasting in the oven, with the beans just spread out flat, did roast but quite badly: beans at the top were over-roasted and almost burned black, whereas beans below were under-roasted.
Move the beans around #
The challenge was to move the beans uniformly while roasting, and I took inspiration from commercial roasters to use a rotating motion. The only problem with that is: my motor cannot survive the inside of an oven.
Motor cooks in the oven? #
Instead of trying to make some kind of oven-safe high-temperature motor (which is going to be quite complicated), I pondered on the problem and arrived at a simple idea: the motor can be outside the oven, and the rotation is transmitted inside with two spools: one inside the oven unspools a metal wire and one outside turned, by the motor, winds it up.
Outside motor assembly #
Mounting on the oven door
Outside spool, with motor and gearbox
The idea with the metal wire spooling up was the key to this whole thing working. It does have the disadvantage that I need to “charge up” the inside spool (just a used food can, it has to fit over the cylinder), making sure there is enough wire length on the inside, to be pulled out over the entire duration of the roast (about 1h30 to 2h).
3D printed in purple is the reduction gearbox, to achieve low speeds and high torque from a nema stepper motor. The model is freely available here (Creative Commons licensed).
All metal frame #
Here is the frame for the cylinder inside, I had access to a cnc plasmacutter to cut it out, and then bent it.
The cylinder itself also needs an axis to pivot about. Because it needs to be all metal, I opted for just M5 screws as they are readily accessible. They certainly aren’t providing any precision, but that’s not an issue here.
Also the entire frame can be bent which turns out to be an advantage: to fill the cylinder up at the start, I need to get it out anyways. Just bending it out of the frame is very fast and convenient.
Home roasted coffee #
The result tastes great, and I can also control how light or dark the roast goes by playing around with time inside the oven and temperature.
Usually, I go for about 1h20 to 1h40 and a temperature starting at 190°C and going to 220°C at the end. This is called a temperature curve (temperature as time progresses), but I mostly eyeball it. A future project could be about using a timer and temperature probe to do this precisely and repeatably.