Interpretation of “Rz = 4×Ra” and other roughness parameters in the evaluation of machined surfaces

The above mentioned roughness calculation formula is considered as a starting point in the nowadays industrial practice and in certain parts of the current curriculum, used in the training of the technician experts of secondary and higher education. Our present work examines some areas in connection with this formula. It introduces that the different machining operations result in different surface microgeometries and in different Rz/Ra-ratios. It points out that the wear development and any change in the edge condition cause significant differences in the arithmetical, average values and in the form of the surface microgeometry. With the help of examples it demonstrates the uncertainties of the surface roughness measurement technology during the evaluation of the real and filtered surface profiles. In the design process of machines we can meet calculating methodes, developed several decades ago. The revision of the connection between tolerance and surface roughness has not taken place parallel with the essential development of surface microgeometry characterization. The long-standing Ra-Rz recalculations are generally used in this case as well, although the determination of the surface roughness is one of those tasks, causing an immediate improvement in the working efficiency of improvement in the working efficiency of machines. Possessing the new knowledge it requires a careful consideration to apply the surface roughness parameters their numerical values in an appropriate way.