Theoretical determination of surface roughness Ra parameter for methods of machining tool with undefined geometry is complex. Paper presents an analysis of possible relationships to calculate the roughness parameters Ra finishing with rotating parts. Based on the selection of variables involved, wanted variables and factors to establish differential equation. The equation contains in addition of the factors also the constant, which must be established experimentally. For this case, it is appropriate to use the method of planned experiment, which would be extended to a new method for determining of various mathematical relations for Ra. The presented methodology can be applied to the determination of Ra with other methods of finishing machining, especially with the grinding.
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