Here is a couple links to some more info.

Basic fact sheet.
Hammer mill blog articles

I have been an organic gardener for quit a while but more seriously only for the past few years. My experience so far has me thinking of trying to come up with a multi-surface processor. For instance for gravel paths, I have found a harrow works well to bring the larger rocks back to the surface and gives the path a fresh uniform look. Although, I would like to pick up the over-sized rocks and either crush them and redeposit them, or collect them for later processing. Also, I would like to remove the organics such as leaves and twigs and rock dust so that the path height doesn't get higher every fall. I seem to recall Eliot Coleman mentions in his book
http://www.fourseasonfarm.com/books/index.html#grower

that a device exists that is used with a tractor and plow or tiller that crushes rock as the soil is tilled. He also covers using crushed rock powders for adding nutrients to plots used for growing.

I also do composting of home and yard wastes. Common tasks are to sort the various size and type of material, then crush or shred the material to go to compost, worms or mulch. It would be a big help to have the surface processor do this kind of work, although any sort of automation would be good since I am doing it manually now. For my situation, having a multi-purpose machine would be better than one or more machines that would be more efficient on one process or have bigger capacity.

I have a friend that has rebuilt small rock crushers that basically have a wedge shaped cavity that has the wide part pointing up, and the narrow outlet facing down. One wall of the wedge oscillates such that the input material is either crushed or pops back up to the top. When the material becomes small enough, it can fall out the bottom. I'd like to make one of these for myself to see if it can also process materials other than rock, although I don't think these can create powders.

Anyway that is what I have on my mind that might apply to this thread. I'll be watching for progress on this.

Oops, I just saw the hammer mill fact sheet linked above. This reminds me, I have a home shredder/chipper that has a hammer mill section with a chipper blade on one side of the mill. I haven't used it much because it has a number of problems, even though it was considered a high quality unit. Mostly the engine won't run very long due to a poorly designed carburetor (too small, so there is very little venturi left to give a good vacuum signal, the engine stalls when the load changes). The chipper dulls quickly on hard wood or flexes and strings up the wood and jams on soft green wood. I have avoided letting any rocks get into the mill, but small rocks have gotten in on a couple of occasions. They where crushed pretty quickly but made an awful noise doing it, so I'm not sure it is a good idea to let it happen again. Another bother is that the output screen has to be changed in order to change the output material size. The loose screens are easy to misplace. In other words, I think an effective hammer mill will need to be very well built, and therefore be fairly expensive.

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Kirk Wallace
http://www.wallacecompany.com/machine_shop/
http://www.wallacecompany.com/E45/index.html
California, USA