How to construct productivity charts

Productivity of a milling operation is directly proportional to the axial depth of cut, radial width of cut, spindle speed and chip load. Traditionally, the industry uses machinability data handbooks as a reference to select cutting conditions leading to a safe but conservative operating point. The most optimum cutting condition; however, can only be obtained by determining the limiting cutting conditions based on workpiece material properties, machine rigidity, cutting tool properties, rigidity of tool/work holding system and spindle capacity. One extremely powerful method to tune cutting conditions for optimum production is dynamic analysis.


Like any other structure, a machine tool combined with a tool holder and a cutting tool is also a dynamic structure. Each component in this chain introduces its own dynamics, which are called modes or weak-spots. When the machine tool is operational, depending on the operating point, the effect of each mode in the form of structural vibrations is seen anywhere along the chain. The most critical point in this chain is; however, the tool tip due to its direct impact on the part being formed. A measurement method to capture machine tool dynamics is called "tap testing" In tap testing, an accelerometer is attached at the tool tip to measure vibrations when the tool is excited with an instrumented hammer. MALTF developed by MAL Inc is used to collect, filter and analyze this data. Later, measured dynamics is used in process simulation software, CUTPRO, and stability lobes are generated.



Stability lobes are very powerful as they show the exact location of "sweet spots" The horizontal axis of the stability diagram is the axial depth of cut (ap) whereas the vertical one is the spindle speed. Any cutting condition selected below the curve will result in a stable, chatter-free cutting.