Surface Characterization and the Functionality of Mating Parts

The surface of a machined component is the only part of that item which interacts with its surrounding or mating parts. Therefore, the surface must be made suitable to perform the function for which that component was designed. Most designs merely specify the average surface roughness Ra. This parameter does not reflect the suitability of the surface to perform its designed function. If the surface is to be either a bearing/lubricating surface, as opposed to a locking surface used for painting or coating, then the Ra value is somewhat misleading. The parameters that have to be employed are those of Skewness (Rsk) and Kurtosis (Rku). These can be directly measured with currently available surface analysis systems. When these surface measurements are compared to each other then the functionality of the surface can be determined on a basis of being either a bearing or locking surface. All manufacturing operations will produce surfaces which, dependent on the operation and selected parameters will show a particular functionality characteristic for the area measured. This study examines the suitability of high speed milling techniques in producing either bearing or locking type surfaces. The study uses specially designed face and end mills to machine three grades of stainless steel, two austenitic and one martensitic. It is interesting to note that when employing 12mm diameter endmills with differing axial rake angles with cutting speeds between 100 and 750 m/min, the surface tends to be generally bearing in nature. However, this is not true for a cutting speed of 500 m/min where the surface topography skews to become locking and then once the speed is increased it reverts to bearing. This interesting phenomenon, which is repeatable, is currently being investigated by the authors. Later work will discuss observations using the custom designed facemill.