If you have an old washing machine that you want to put to new uses, you could try this washing machine drum coffee roaster project from We Make Things.
The drum in this project required some modification, as it had plastic parts that melted, not all do – but keep in mind that you may have to be creative. The project’s builder, rxdtxd, used a 4.0W, 975 RPM motor, with a reduction gearbox to bring the speed down to 10 RPM. A little simple welding was required to couple the motor to the drum. The actual roasting comes by way of a propane stove that sits underneath the drum. When the beans are finished roasting, they are released into a large pan to cool.
The washing machine drum coffee roaster is capable of working through 1.5kg of green beans at a time. To fully roast them takes about an hour. Given the capacity, that is pretty efficient. Most home-style roasters can only work through a few hundred grams at a time.
It makes for a fairly ambitious DIY project, but if you like your coffee and have the skills, it would definitely be a worthwhile build. Check out the completed build here.