In the conditions of farms, it becomes necessary to grind feed that has a different degree of moisture. Therefore, the choice of universal working bodies is the object of research, and the theoretical determination of the average size of pre-crushed feed particles is of great scientific and practical importance and is a problem that needs to be solved. Proceeding from this, a constructive-technological scheme of a grinder with a universal grinding working body is proposed. The grinding apparatus is equipped with a hammer working body which has cutting edges. In this case, hammers with cutting edges crush the wet mass, and also create an air flow and work as flow accelerators at high speed. As a result of theoretical studies, the steps for arranging these working bodies were determined. Analytical expressions are obtained for determining the average length of pre-crushed feed particles depending on the distance between the faces of replaceable knife working bodies, i.e. from the step of arranging them in rows. At the same time, the calculation results showed that with a distance between the faces of replaceable hammers of 20 mm, the value of the average length of the grinded particles was 38.38 mm. The average size of pre-crushed particles from feed raw materials intended for farm animals was 37.64 mm, i. e. the difference between theoretical and actual value is only 2.0 %. This proves the reliability of the obtained analytical expression, which provides the determination of the main parameter of the grinding working body, i. e. spacing of radial knives in rows. The proposed method of determining the average length of crushed particles allows theoretically finding and planning the required particle size