Understanding Surface Quality: Beyond Average Roughness (Ra)

Design of machine parts routinely focus on the dimensional and form tolerances. In applications where surface quality is critical and requires a characterizing indicator, surface roughness parameters, Ra (roughness average) is predominantly used. Traditionally, surface texture has been used more as an index of the variation in the process due to tool wear, machine tool vibration, damaged machine elements, etc., than as a measure of the performance of the component. There are many reasons that contribute to this tendency: average roughness remains so easy to calculate, it is well understood, and vast amount of published literature explains it, and historical part data is based upon it. It has been seen that Ra, typically, proves too general to describe surface’s true functional nature. Additionally, the push for complex geometry, coupled with the emerging technological advances in establishing new limits in manufacturing tolerances and better understanding of the tribological phenomena, implies the need for surface characterization to correlate surface quality with desirable function of the surface. In turn, the surface quality over the entire area, not just the 2D Ra parameter, dictates the performance and reliability of the part. Both ISO and ASME current standards on surface texture have a range of 3D surface quality parameters. This is further aided by the availability of modern equipment to accurately measure them. Despite these advances, design and quality professionals continue to specify surface finish based solely on the value of Ra. The same outlook trails in graduate and undergraduate education and their textbooks. This article explores how these multitudes of 2D and 3D surface quality parameters are to be understood in the design and development of high performance surfaces, and the strong need for them to be incorporated into graduate undergraduate engineering curriculum, and be taught as an improved toolkit to the aspiring engineers, process engineers and quality control professionals. Included case studies can be used to captivate the attention of the students (target audience would include industry professionals as well) and route their inquisitiveness into why they need to think beyond Ra in this era of advanced manufacturing.