Surface Roughness Comparison Chart. Composite Set of Roughness Specimens Rubert 130. This set consists of 30 comparison specimens, covering six commonly-used machining methods (6 turned, 6 end-milled, 6 horizontally milled, 6 surface-ground, 3 lapped, 3 reamed/drilled. A label gives the Ra and Rz values of each specimen in both metric and imperial units