This week I had to take apart my Hario Slim coffee grinder for cleaning, as the taste of my morning coffees have started to suffer from build-up of stale grounds in the grinder. I took the opportunity to analyze the mechanism using some concepts we learned in class. Here is some casual analysis I did:
Coffee nerds (like me) want to produce grounds with consistent particle sizes that can be adjusted with high resolution. For a conical burr grinder, the sensitive directions are:
- translation along the “power” axis (controls grind size)
- rotation about the two orthogonal axes (determines burr wobble, and controls grind consistency)
Since this grinder is driven by a hand crank, there is a significant cyclical parasitic load that comes from the user bearing down on the handle while rotating it. This load contributes to shaft wobble by taking up radial clearances in the 2 bearings, as well as at the interface between shaft and burr. For what I imagine to be ease-of-manufacturing, the steel shaft is coupled to the ceramic burr via a snap-fit plastic part. There is significant clearance at this interface, and I think this may be the biggest contributor to wobble.
I wonder whether this coupling can be improved through the use of elastic averaging, which would provide a relatively precise connection while accommodating the inherently loose tolerances achievable in a cheap sintered ceramic part.
Speaking of elastic averaging, I have found that the grinder produces more consistent grounds when adjusted for fine grinds (e.g. for espresso) than when it is adjusted for coarser grinds (e.g. for a French press). I have a theory for why this is so. In operation, the conical burr is subjected to a collection of random (technically just chaotic) forces from the interactions between coffee beans and the burr. When the burrs are closer together for a fine grind, the tight space between the two burrs is filled by a large number of smaller particles. Conversely, when the burrs are further apart for a coarse grind, the space between them is typically occupied by a smaller number of larger particles. This means that the burr experiences a much more uniform force distribution when grinding finely, since it is essentially averaging over a large number.