Which of the processes gives the least surface roughness?

Explanation:

The surface roughness has a vital role in influencing functional characteristics like wear resistance, fatigue strength, corrosion resistance and power loss due to friction. Unfortunately, normal machining methods like turning, milling or even classical grinding cannot meet this stringent requirement.

Therefore, superfinishing processes like lapping, honing, polishing, burnishing are being employed to achieve and improve the above-mentioned functional properties in the machine component.

Lapping Process:

In the lapping process, small amounts of material are removed by rubbing the work against a lap charged with a lapping compound. Lapping is a precision finishing operation carried out using fine abrasive materials. 

Filing:

Filing is one of the methods of removing small amounts of material from the surface of a metal part. A file is a hardened steel tool, having slant parallel rows of cutting edges or teeth on its surfaces. On the faces the teeth are usually diagonal to the edge. One end of the file is shaped to fit into a wooden handle. Figure shows the parts of a hand file.

Milling:

Milling is a process of producing flat and complex shapes with the use of multi-point (or multi-tooth) cutting tool. The axis of rotation of the cutting tool is perpendicular to the direction of feed, either parallel or perpendicular to the machined surface.