Inspired by my friend Dominick’s attempt at a Turbo Oven-powered Drum Roaster I thought I’d build my own. While I appreciate the simplicity of my Stir Crazy base I’ve never been happy with the consistency of roasts; by its very nature it just can’t stir the beans around to evenly roast them.
In his version, Dominick talked about a long roast time of about 20 minutes. I figured that was due to heat loss through the sides of the enamel pot used for the base. So in my version I’ve made an insulated base which looks to have solved that problem.
Here’s a video I shot going over the roaster, it also includes some clips of it running.
The base is made out of two aluminum walls fastened onto a wooden base. I turned the base on my lathe, making a tenon around the circumference that the two walls seat against. There is about 1″ is space between the walls.
The inside diameter is about 9.5″, enough to house the drum I’m using. The height of the walls needs to be high enough to not only house the drum (plus a reasonable gap on the bottom for clearance) but also high enough to clear the “nose” of the turbo oven. I forgot this second consideration and was only able to use the smaller of the two drums I bought. The walls are long strips of 0.025″ aluminum riveted to make loops. I used small #4 wood screws to secure them to the base.
Insulation is provided by glass fiber insulation. It doesn’t seem to mind the heat from coffee roasting.
Bearings for the drum rod are two flanged bronze sleeve bushings, 3/8″ in inner diameter. Because I’m using 1/4″ square rod, I needed something with a slightly bigger inner diameter to house the rod. And nicely enough, 0.25 x 1.414 is roughly equal to 0.325. I made sure to buy bearings with a flange to make them easier to mount in the walls. I only then have to secure one end.
Doesn’t the wood catch on fire?
I asked myself the same question! But no it has not, it has darkened a bit over 5+ roasts and oozed some sap. And after each roast I can smell pine resin, but that aroma has not made its way to the beans at all. The aluminum foil cover seems to help because it reflects the radiant heat produced by the Turbo Oven’s halogen light.
But with that said, when it comes time to re-make the base I will attempt an all-metal construction. And I have moved a fire extinguisher to my coffee roasting station just in case!
Drum and Rod
I bought two coffee drums off of eBay for this prototype because I didn’t know what size would work the best. Here is the listing, but in case that stops working they are each 18cm long with diameters of 14cm and 12cm. I’ve only ever used the 12cm drum because the 14cm drum doesn’t fit underneath the turbo oven.
The drums have a 0.6cm^2 square drive hole, which is pretty darn close to 0.25″ (6.35mm). The rod fit in there right away without any filing. However the holes were out of axial alignment so I had to file one of them on the corners to “rotate” it a bit to allow the shaft to slide all the way through.
Speaking of the shaft, it is a 1/4″ square solid stainless steel rod from Home Depot. I can’t find it on their site right now or I’d provide a link. Here is a similar product from Speedy Metals.
This prototype has already seen three iterations of the motor!
Ice Cream Maker Motor
The first was off of an ice cream maker. It worked OK but was quite loud making it difficult to hear the cracks. (I also have not-great hearing, picking out details from background noise is difficult.) However, one positive for this motor was that its drive shaft was designed to accept a 1/4″ square rod! Another good thing about the motor is that it came from a $10 thrift store find so it didn’t require any eBay’ing.
4W Gear Motor
Version two has me using a 4W, 35RPM synchronous gear motor. These things are all over eBay and about about $10; here’s one listing. Be sure that the one you’re looking at is rated for 110V or 120V, there are also 12V models out there.
I have two of these little motors, one fixed to go clockwise and another which will randomly choose a direction when started (this is normal). We’ll call the former CW and the latter CW/CCW, because that’s what their labels have.
In a test run with 16oz of greens the CW/CCW motor had great difficulty starting out and needed a push. Not much more was needed to stall it, either. The CW motor worked a lot better, still needing a push to get started but it would not stall out on its own. As this writing I have a 14W motor on its way and will try that out and update this post when it arrives.
My first real roast with the 4W, CW motor worked really well but I only used 14oz of beans. It stuttered a bit while starting but once going worked the whole time. Roast #2 used 16 oz of greens and the motor would not start spinning until given a little help; but it at least ran the whole time without stopping.
14W Gear Motor
The third motor is a lot more powerful and should be the final one for this roaster. Here’s an eBay link, but if that doesn’t work for you look for a “14W 30RPM 110V synchronous gear motor”. It should be about $15 shipped.
While the motor’s description said it has a 6mm output shaft it’s actually 7mm so it immediately worked with the flexible coupler mentioned below.
The motor is reversible, so it has three electrical connections. This video shows a little more detail into how to wire it up, but the gist is that one AC lead goes into the middle and the other lead goes into one of the side connections. To change rotational direction use the opposite side. There’s no polarity with AC.
This motor, like the 4W, is close to silent in operation but has tons of power. It easily turned 16oz of greens, and I was unable to stop the drum with my hand.
It has four threaded bolt holes in its flange, M5-0.8.
Drive Shaft Coupling
While using a square bar for the drive shaft made it easy to mount onto the drum it made it a lot less easy to attach to the motor.
So I gripped the bar in my chuck’s pin jaws and filed the end of the square bar down to round.
Now I can couple the shaft to the motor. The motor has a 7mm shaft, and I now have a 6.35mm shaft, so I used a flexible coupler to join the two. I went with a flexible one because I know that none of my work is close to perfectly square or aligned. This lets the motor and shaft rotate in peace with the flexible coupler taking out the slack.
The motor is simply screwed to a T-shaped plywood stand. The above photos show the 4W motor but I used the same stand for the 14W motor.
- Pre-heat the unit with the drum in place, slid onto the shaft and in the vessel. Make sure drum door is pointed upward.
- Add beans and replace the turbo oven, set for perhaps 460F.
- When 1C comes around, lower the heat to around 440 in order to avoid a rolling 2C
- The end of the roast is still kind of awkward. Turn off drum, remove turbo oven, and then pull out the motor and shaft leaving the drum in hand. Now open the drum door and dump beans into cooling vessel
It’s not too different of a process as compared to my Stir Crazy, but is definitely less user-friendly when it comes to removing the beans from the device.
I’ve been getting very good, consistent batches out of this machine. The drum’s agitation of the beans also does a better job of removing chaff than the stir crazy, my beans are very clean. I did struggle with over-roasting when I used the loud ice cream maker motor because the light 2C snaps were hard to pick out. But my last batch with the quiet synchronous motor was right on target because those first 2C snaps were easy to hear.