In this report, three important principles for designing rotary knives are analyzed and explained with the conceptional experiment data of rotary knives.
　One of them is angle β₁, which is the rake angle between the rotor radius direction and the tangential line of tip-outside surface of the knife. β₁ can be designed with the value of more than 45° to less than 80°, which gives throwing action to the clods, depending on soil and machine conditions.
　The other is the edge curve of the knife. This has influences on the entwining of grass and straw around the blades and rotary axle, and the tilling resistance. The angle Φ₁ between the radius direction and the tangential line to the edge curve is important to know its characteristics. Less entwining occurs when Φ₁ is more than 57° and greater entwining occurs when Φ₁ is less than 57°.
　The third point is the section shape of the knife. This is related to the value of the frictional resistance between the knife and the untilled soil.