I have always wanted to set up a proportioning mixer, basically it is a set of bins, with short augers, Usually a main auger, and then some smaller additional augers, and they by size and by by speed, (I suppose one could put slides above them as well) to control the amount they auger, and you then run them all at once making a on the go mix, with them all running at once, (mix mill is a company that make a unit),
they use a special, (on I saw use one way drives, like a Bendix on a starter), and a crank and an adjust able arm to work the auger), but if one could collect a set of different toothed of sprockets and then drive the augers by one shaft, changing out the gears to get the proportions you want, and fine tuning with with a slide gate,
since most people seem to feed a similar mix year after year, most would not need any fast or major changes often, the gears and such would work easly for most I would think.
say one took and made a ply wood or steel bins, and then narrowed them down to the feeds, with a small HP motor and possibly some reduction, to run augers, and the slides, an auger to refill the bins, (either a swinging drop tube to reach the top of the mixer bins, a small arc to slide the auger from side to side, to fill), you may want a small bin with a small auger for adding mineral or supplement to the mix at the base,
I know it would take some experimentation but I think it could or would be easly doable
here is one drive that would be simple to make, (the company uses a series of retractable pins for drive teeth, one could make some thing similar with screw in dowel pins, and like there literature say with 4 different auger sizes, the rations are about 100 different rates, per auger,
Fritsch Equipment
yes there would or may be some wear but the cost of replacement pins would be near nothing, (even bolts with the head cut off), (they have some system to slide the drive off to the side to by pass that auger if not wanted, one could do the same either by removing the pins or with a PTO pin in the shaft and two holes,