Meaning
Topographic irregularities of a manufactured surface measure the height and spacing of peaks and valleys left by machining tools. Surface roughness determines the friction, wear, and sealing performance of machined parts in assemblies. Quality managers monitor this finish using stylus or optical profilometers to ensure conformance to dimensional tolerances.
Ignoring surface finish requirements leads to rapid seal wear and fluid leaks during high speed operation.
Asperity Profile
A stylus traces the surface to record the heights of microscopic peaks and valleys. In evaluating surface roughness, parameters like Ra and Rz quantify the average deviation and the maximum peak to valley height. Machining parameters like feed rate and tool radius determine these profile dimensions during cutting.
Coarse profiles increase the risk of localized stress concentration and fatigue failure in loaded parts.
Friction Component
Rough surface peaks lock together and increase the resistance to sliding between parts. The surface roughness of mating components governs the transition from boundary to hydrodynamic lubrication. Laboratory friction rigs test different finishes to optimize the torque requirement of the system.
Excessive roughness increases start up friction and speeds up material wear.
Finish Criterion
Manufacturing specifications define the maximum allowed deviation for critical contact surfaces. Maintaining low surface roughness requires fine grinding or polishing steps that increase production time and cost.
