7. A CBN tool is used in dry turning of P20 mold steel with 34Rc hardness. The cutting edge has a 15 chamfer edge, which acts like a negative rake angle. A number of turning tests are conducted at different feeds and speeds. Each test is continued until a cutting length of lm is reached, and the average flank wear (VB) is recorded. By the use of a least-squares method, the following relationship was identified from an actual cutting test data: VB = 0.0023568V1.330.742310.796 m where the units are flank wear VB [mm], cutting speed V [m/min], feed rate c [mm/rev], and machining length lm [m]. Predict the tool change time when the cutting conditions are selected as V = 240 m/min and c = 0.06 mm/rev and the maximum allowable wear is VB = 0.2 mm. The diameter of the shaft is 100 mm.