Jamieson, Fausset and Brown on Isaiah 28:27

27. The husbandman uses the same discretion in threshing. The dill ("fitches") and cummin, leguminous and tender grains, are beaten out, not as wheat, &c., with the heavy corn-drag ("threshing instrument"), but with "a staff"; heavy instruments would crush and injure the seed.

cart wheel--two iron wheels armed with iron teeth, like a saw, joined together by a wooden axle. The "corn-drag" was made of three or four wooden cylinders, armed with iron teeth or flint stones fixed underneath, and joined like a sledge. Both instruments cut the straw for fodder as well as separated the corn.

staff--used also where they had but a small quantity of corn; the flail (Ru 2:17).