A fundamental mathematical model for profit obtained in unit time interval of machining operation was built. It was found from this model analysis that the general formula expressing the cutting speed for the maximum profit per unit time interval derived from the model coincides with one obtained from the break-even analysis with linearity assumption. It was concluded that this optimal cutting speed for the maximum profit per unit time interval exists between the cutting seeds for the minimum cost per unit piece produced and for the maximum production rate per unit time interval. It was also found that the optimal combination of feed rate and cutting speed for the maximum-profit criterion is not determined uniquely under normal conditions. The procedure of numerical computation for obtaining the maximum-profit cutting speed and its computer program were developed and used to solve typical examples of economical machining.