What is Ra Value?

Answer 1

Surface finish, by definition, is the allowable deviation from a perfectly flat surface that is made by some manufacturing process. Whenever any process is used to manufacture a part, there will be some roughness on the surface. This roughness can be caused by a cutting tool making tiny grooves on the surface or by the individual grains of the grinding wheel each cutting it's own groove on the surface. It is affected by the choice of tool, speed of the tool, environmental conditions, and definitely by what material you are working with. Even when there is no machining involved, as in casting/injection molding, the surface of the mold will have surface deviation, which in turn will be transferred onto the part. Even if you could create a mold which was perfectly flat, the cooling process and thermal properties of the material would cause surface imperfections. So, like everything else in the manufacturing world, we have had to make a compromise between function and cost of manufacturing. If you don't need a mirror finish all over the brake drum, then you just cast it and worry about machining the surfaces that need to be (relatively) flat for the function of the part. I have included at the end of the report a chart that details the surface finishes that can be obtained by many different processing methods.
Surface finish may be denoted by a roughness grade number. Here is a table that specifies the Ra values for roughness grade numbers. (ISO standard - 1992) The surface finish symbol, as denoted on drawings, can specify many more things than general surface roughness (Ra, usually in μin or μm). Roughness values RaRoughness Grade Numbers mm min 50 2000 N12 25 1000 N11 12.5 500 N10 6.3 250 N9 3.2 125 N8 1.6 63 N7 0.8 32 N6 0.4 16 N5 0.2 8 N4 0.1 4 N3 0.05 2 N2 0.025 1 N1