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Abstract. Size reduction processes are very important in crop production agriculture, especially during harvesting operations such as during maize chopping. The required energy in such chopping can be very high. To minimize the energy consumption and to guarantee a high chopping quality, it is necessary to periodically grind or replace the chopping knives. Grinding or replacing the knives should only be performed if the knife is dull or worn out because these operations are time intensive and grinding reduces the knife life. But there is currently no practical method to accurately determine the knife sharpness or dullness. The objective of this research project was to develop a hand-held measuring device to determine the sharpness of the knives in various agricultural machines with a high accuracy.

The method developed is based upon the measurement of the force which is necessary for the knife being tested to cut through the insulation layer on a wire. The insulated wire has an electrical voltage within it and the knife causes an electrical short in the wire as soon as the knife passes through the insulation. The force required to do the cutting is transmitted via Bluetooth to a smartphone with an app for handling the data. The developed app contains calibration models for a wide range of different kinds of knives and employs statistical methods for calculating the desired sharpness information. It also guides the measurement by informing the user when there have been a sufficient number of measurements to accurately measure knife sharpness.

A user starts a measurement by selecting the corresponding type of knife from a pull-down menu on a smartphone. Next, the user presses the sharpness tester containing the wire on several spots along the knife until a sufficient number of measurements have been collected. The smartphone app then presents high accuracy information about the sharpness status of the knife and a recommendation about whether sharpening or replacement of the knife is necessary.

This mobile, hand-held, and low-cost device is designed so that it can be easily used by operators of various kinds of agricultural machines with knives, such as forage harvesters, balers, self-loading trailers, shredders, or mowers. Spools on the device hold the unused and used wire so that the device can be used for many measurements and there is no introduction of foreign material into the agricultural environment.