- This set consists of 30 samples of metal surfaces produced by 6 different machining methods widely used in industry | The label shows the Ra values of each sample according to both the metric and imperial system | (Label for Rz available on request) | Nominal values of samples - Ra µm 0.05 - 0.1 - 0.2 for lapping and boring - Ra µm 0.4 - 0.8 - 1.6 for grinding - Ra µm 0.4 - 0.8 - 1.6 - 3.2 - 6.3 - 12.5 for horizontal and vertical milling, turning