I think I've seen info on home-built threshing machines in older issues of The Mother Earth News, and possibly an issue of Back Home. The latest issue of TMEN has a photo of a homesteader threshing by hand, and she is not looking particularly pleased about the whole process. I can see why you want to build a machine!

We got to discussing this over on IRC and since the threshing machine was first invented in the early 1800's we can't figure out where you got the idea there was such a thing as an ancient machine?

Prior to that it was all done by hand near as we can tell. If you have other information please share it with us.

Cheers.

TomW

Having worked on a LOT of old combines and thrashing machines,  I would say that the chain flail would workk great for grain, and would be a snap to build.  Suggestion: don't weld the chain flails to the shaft, bolt them.  They WILL eventually wear out.  Wheat and other grains are surprisingly abrasive.

The article mentions a sieve to separate the wheat from the chaff.  I would suggest building it with two layers of screen.  Even better would be two layers of sheet metal perforated with 1/2" holes.  To adjust the size, you just slide one screen so the hole size is larger or smaller.  If you are getting too much chaff through the sieve, close the holes a LITTLE bit (like a 1/16").  Also, it's not going to work too well without a fan blowing air up underneath it.  Think of building a large box, with a blower pushing air up through the sieve set over the top.  You can shake the sieve yourself or you can rig up some sort of shaking device with a motor and linkage.  The air blows the lighter chaff up and into your face :( while the heavier grain falls down into the box.  When it's getting full, stop and empty it.