what is diffrence between Ra and Rt in terms of surface finish?
The surface roughness is evaluated by the height, Rt and mean roughness index Ra of the micro-irregularities. Surface roughness number Represents the average departure of the surface from perfection over a prescribed sampling length, (usually selected as 0.8 mm) Surface roughness number (Ra) is expressed in microns.he surface roughness is evaluated by the height, Rt and mean roughness index Ra of the micro-irregularities. Surface roughness number Represents the average departure of the surface from perfection over a prescribed sampling length, (usually selected as 0.8 mm) Surface roughness number (Ra) is expressed in microns.