The effect of knife geometry on cutting force and fracture in sugar beet topping

Abstract

This study arose out of attempts to reduce the occurrence of beet fracture and overturning when topping with blunted knives and to reduce the loss of working time in replacing blunted knives. The investigation is concerned with the effect of knife edge geometry on the cutting force and fracture in sugar beet topping. The cutting force in laboratory tests with a pendulum impact device increases with increasing wedge angle for symmetric and asymmetric edged knives and with diameter for semi-cylindrical edged knives and wires. In the same tests, wires and semi-cylindrical edged knives of diameter less than 2 mm and symmetric wedge-edged knives of angle less than 45° caused the least fracture. These latter results are in agreement with theoretical predictions of the likelihood of cracks deviating from the cutting place. The theory also shows the likelihood of cracks deviating is reduced when the coefficient of friction between the beet and knife is low, as it is under high loads because of the lubricating effect of beet juices. In agreement with the laboratory results, a 2 mm wire and a low angle symmetric wedge-edged knife, with 0·5 mm radius tip, caused less fracture than a conventional 20° asymmetric wedge-edged knife when topping beet in the field. We recommend the use of reduced width and thickness knives, with low angle symmetric wedge-edges or semi-cylindrical edges of less than 2 mm diameter, and of wires of less than 2 mm diameter. The resulting reduction in fracture may account for a saving of up to 5% of the volume of each topped root. For knives, the width and thickness must provide adequate stiffness and limit permanent distortion during use. Less permanent distortion may occur if high tensile steels are used. For wires, the problem of trash collection remains to be overcome.