Back to 2016: and that original challenge to fit a Knock quality grinder into the compact space of the Aeropress plunger gave rise to some design parameters that could not be breached.
The outside diameter (OD) of the grinder is the obvious one, but to then fit in a version of our classic top dial adjustment whilst still retaining enough space to load the grinder with actual beans (!) and still be able to fit fingers in and around to adjust the dial made it clear very quickly that the Feldgrind's beloved nut-dial would simply not fit.
The idea of bringing the dial into the lid design was the solution that presented most opportunity for innovation and, when combined with our bottle opener pointer, it gave rise to our design patented lid adjustment. Attempting to hit a price point for the grinder's launch led us to go with a moulded plastic lid for the first iteration; replacing it with a metal lid in 2018 we also made incremental improvements to the visibility and edge knurling to make adjustment more intuitive and comfortable.
One of the less obvious parameters was around fitting the crank arm into the compact packed form of the pairing. To achieve this we had a custom crank arm band made, sized to cope with the stretch needed for getting around the Aeropress' outer chamber OD. This is why the crank arm band sits comparatively loosely on the Aergrind - a perfect fit here would result in a split band on the Aeropress.
The Aergrind's crank arm too was set to a length that would fit around the body of the Aeropress when packed together, just as you can see in the image above. Any longer and it would protrude beyond the package and snag or break free of the band while bumping around in a backpack. To further ensure compactness the current wood knob followed the design parameters to be as minimally intrusive as possible. With the knob's new magnetic attachment facilitating storage off the crank arm that trim packed profile is maintained.
We'd recommend that if you intend to use the crank arm band on the Aeropress regularly that you pick up a spare, one for the grinder and one to keep on the brewer. That way any stretch that the Aeropress puts into the band will not mean a slack band on the grinder when travelling.
Those crank arm bands also make for great charging cable tidy on your back up phone battery!