The dynamics of chip formation during high-speed orthogonal machining (planing) is examined. Merchant’s vector diagram of the forces acting upon the continuous chip free body is expanded to include inertial force components. Expressions are developed for cutting force and pressure. Energy balances are used to show that Merchant’s classical equation relating shear angle φ to rake angle α and friction angle τ applies, independent of cutting speed. Apparent differences between experimental observations of shear angle φ and Merchant’s prediction are attributed to workpiece material anisotropies, tool wear, built-up edge, and inaccurate measurement of the friction coefficient at the tool–chip interface. It is shown that good experimental values of the shear angle and friction coefficient may be obtained by measuring cutting pressure, utilizing dynamic material properties data, and invoking Merchant’s relation to resolve the energy balance. Continuous and segmented chip formation are contrasted. Melting in the tool–chip interface is verified.

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