Modeling the cost of varying surface finish demands during longitudinal turning operations

Tolerances, including geometry, dimension, and surface roughness, are an important part of production where the desire to manufacture quality products have to be weighed against the increase of manufacturing costs. The desired tolerance will influence the choice of both manufacturing method and machine tool. Given that machining is an adequate production method, variation of the required tolerances will imply a variation of the part cost which needs to be taken into account during production planning. Thus, the term “tolerance cost” is introduced. The paper presents a model for evaluating the tolerance cost in respect to the surface roughness during longitudinal turning operations, enabling a better comparison between different production alternatives. Through knowledge of the required surface roughness, it is possible to estimate appropriate cutting conditions. Knowledge of the cutting conditions and the part geometry then makes it possible to calculate the cycle time, information which in turn may be used for calculating the corresponding part cost. Through using experimental data, it is proven that the required surface roughness has a significant influence on the attained manufacturing cost. For instance, while longitudinally turning AISI 4140, it was shown that an improvement of the surface roughness from Ra = 3.2 μm to Ra = 1.6 μm will entail an increase of the part cost by roughly 20 %. Similarly, a decrease of the required surface quality (larger Ra value) will imply a significantly reduced part cost.