A heavy, curved metal grate positioned directly beneath the rotating threshing cylinder or rotor in a combine. The crop is pulled between the spinning cylinder and the stationary concave, rubbing the grain out of the seed heads. The operator adjusts the clearance gap between them based on the crop size and moisture to prevent grain damage.
We need to tighten up the concave clearance, we're leaving too many unthreshed kernels in the wheat heads.
Running the concave too tight will crack the grain, while running it too loose will send unthreshed crop out the back of the machine.
Related terms
Combine harvester
A large self-propelled machine that reaps, threshes, and cleans grain crops in one pass.
Thresh
To mechanically shatter harvested crop heads to separate the edible grain from the stalk and chaff.
Chaffer
The upper adjustable sieve in a combine harvester that separates grain from heavier plant debris.