Broken kernels and cracked cobs are the quiet cost of harvest season: they cut the price at the buying station and shorten how long the grain keeps in storage. Most damage is not the machine's fault alone — it is a matching problem between the thresher, its settings and the crop in front of it.
Three adjustments decide most of the damage: rotor speed, concave clearance, and how evenly the crop is fed. Spin too fast, and kernels meet the drum with more energy than they need. Set the concave too tight, and the crop cannot pass through without being crushed. Feed in uneven fistfuls, and the machine has to hunt for a setting that never quite exists.
Maize is not maize. Moisture is the first variable: kernels that are too dry crack like glass, kernels that are too wet clog the screens and leave grain on the cob. Dent and flint types break differently, cobs differ in size and strength, and mixed plantings make every load a small surprise. Adjust the machine to the crop; do not wait for the crop to suit the machine.
The best thresher is the one whose settings you understand on the second day of harvest.