title_body: How can I get a more even grind with a rotary grinder?
upvoted_answer: "Using more or less beans in a single grinding session" will yield the most bang for buck, usually.  However, this is highly dependent on your particular rotary grinder.  Rotary grinders are like miniature blenders.  Just like in a full size blender trying to make a milkshake or something, if you over- or under-fill the blender, then the blade won't be able to mix the ingredients as it cuts through them.  Cheap grinders won't be designed to mix very well as they grind, so with terrible grinders, you probably won't see much benefit to your strides.
  
  You need to experiment with your particular blade-style rotary grinder to see how much you can add without prohibiting movement once the beans are fully chopped up, and without leaving too much movement where they just bounce around inside.  Too much room will create friction from the grinder chamber sides, and will start to warm up the coffee oils.  Too little room will result in the same bean being over-ground (too much friction from the blade), and you'll get coffee paste in the inner center of the grinder chamber.
  
  Achieving consistency and repeatability to the level of a fast-food chain will be impossible, as rotary grinders are way to sensitive to how many beans you put in, as well as size and shape of the bean, how hard the bean is (age of bean), lunar phase, etc.  Maybe this is why all the big coffee chains use more expensive burr grinders in-store (not that the coffee is better, which it arguably is, its just more predictable).  Some fancy rotary grinders have timers and knobs and levers to set so you don't have to count to yourself before you let go of the grind-button.  That helps, but is far from a silver bullet.