The authors show that first and second differencing of a time averaged resultant force is extremely effective in the recognition of tool breakage in milling. In order to be useful in production, however, such a system needs to have a knowledge of allowable levels of the first difference. It is shown that these levels, (thresholds), are extremely dependent on the ratio of cutter radius to width of cut, (immersion ratio). A method of on-line identification of the immersion ratio and threshold tuning is presented with experimental verification.

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The paper describes a rather simple and efficient algorithm for processing the milling force signal to detect cutter breakage. Using sampling synchronized with cutter teeth the basic variation per tooth is removed by calculating average forces per tooth. The first difference of these forces detects both breakage and some sudden changes in cutting conditions (cornering, milling over a slot). The second difference distinguishes between the two. The algorithm is illustrated by computational simulations as well as by measurements in milling tests.

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