All explained above makes it clear why it is so hard and sometimes even impossible to give precise forecast for tool life. It is not always possible to predict the influence that numerous factors and their combinations may have. If we take, for example, the expression for Pz(shear force) given above

where the following parameters are included: feed, cutting depth, hardness and a number of experimentally determined coefficients … up to kh, thus illustrating the complexity of the problem. If we assume that the __main factor __influencing tool life “T” is the applied __load __expressed by the applied __forces__, __temperature __(heat), __shocks__, __vibrations__, then the applied load is in turn influenced by:

- the cutting conditions: speed, feed, depth, cooling, type of processing
- material type: Al, Fe….., size, geometry, quality: porosity, abrasive inclusions, oxides
- the machine-tool-workpiece system: stability, power, vibration resistance characteristics, type
- tool type and characteristics: ceramics, Figure 1: A machine-tool-workpiece system
- high-speed steels, carbides; geometry, shape, angles, grooves, chipping,
- cooling: intensity, coolant type
- other factors: operator, operator’s faults, low qualification, processing technique (incorrect processing technology).

When cutting long workpieces the change in the angle j on the tool holder can sometimes cause __unexpected __vibrations, tool __wear __or __breaking off __due to operator’s fault (poor qualification).

The complexity of the problem can also be demonstrated using the expression for T = C^{7} V^{7} d^{-1} f^{-4}.