This is a failed project, I am no longer developing it, but have chosen to upload my design documents and what I've learned in case anyone else is interested in picking it up! It is an attempt to make a single-dosing, zero-or-low-retention coffee grinder capable of making consistent espresso grind for specialty coffee, but in the price range of a home enthusiast.
I wanted a single-dosing, zero-retention grinder for making espresso with, but those are outside my price range for what I'm willing to pay for a coffee grinder. I attempted to transform a Rancillio Rocky grinder into a low-retention grinder costing under $400, after significant frustration with a grinder that I received broken, twice in a row. I have sent it back to the distributor after determining that there are far too many flaws with the device for it to be worth its price tag, and thus I cannot continue this project. There are three main improvements I wanted to make:
Single Dosing Hopper
I wanted a hopper that would accept a reasonable quantity of beans (about 30g) and feed them vertically, straight down into the burrs. The Rocky uses its bean hopper as the grind adjustment, which is popular in many grinder types. Thus, in order to replace it, I had to make an indexing ring on the outside of whatever hopper I chose to create. I made the hopper conical so that the beans would feed straight down into the opening of the Rocky's burrs, and steeply, so that a small number of beans would not end up causing "bridging" in the hopper and prevent the full dose of beans from falling through.
This was the only part of this project which was mostly successful. The hopper would have gone together beautifully, but I chose to cut it out of Delrin, a plastic which machines nicely, but for which I have no glues. I should have opted for aluminum or perhaps just some nice oak hardwood, which would have accented the black metal and the stainless steel of the Rocky very nicely.
I got the inspiration for this from the Niche Zero, which is the grinder I should have purchased in the first place. The idea is that beans can fall in through a rotating disc with a hole in the side of it, but that they cannot fly back out when touching the rapidly-rotating burrs and thus unevenly grind or take a long time to finish grinding.
The Rocky's burrs rotate at 1725 RPM, which is way too fast for this kind of thing to work. I did end up trying the screen, but it actually made the popcorning problem worse. The Rocky was just not made to grind coffee without a full hopper.
Ground Ejecting Sweepers
I noticed there is significant space on the bottom of the grinding chamber that the sweepers built into the lower burr holder assembly do not do anything about. Seeing as grounds can become trapped there and thus be retained in the grinder, I sought to extend the sweepers so that they would eject more of the grounds out of the dispenser chute.
This was a real disappointment. I made the sweepers, but there is insufficient clearance for them. The final height that I made them was 1.91 mm thick, not the 3.175 mm that they are in the design file. I realized that it would be impossible to make these without gluing onto the bottom of the lower burr assembly, which I did not think would be strong enough for reliable use. Also note that I had to knock the corners off the design with a bandsaw because it did not fit by a few tenths of a millimeter in the grinding chamber. I could not measure what the removal amount was supposed to be with the tools available to me, unfortunately.
The OpenSCAD files for all of these are included in this project! I hope someone can find them useful. I have not updated them to fix the problems I mentioned above with each.