Different Surface Roughness and Processing Methods

Ra200~400: There are obvious visible knife marks on the surface. Processing methods: rough turning, boring, planing, drilling.
Ra100~200: Visible knife mark. Processing methods: Rough turning, planing, milling, drilling.
Ra50~100: Visible machining marks. Processing methods: Turning, boring, planing, drilling, milling, filing, grinding, rough reaming, milling teeth.
Ra25~50: Slightly visible machining marks. Processing methods: Turning, boring, planing, milling, scraping 1~2 points /cm^2, pulling, grinding, filing, rolling, milling teeth.
Ra12.5~25: Can’t see the machining marks. Processing methods: Turning, boring, planing, milling, reaming, pulling, grinding, rolling, scraping 1~2 points /cm^2, milling teeth.
Ra6.3~12.5: Can discerned the machining marks direction. Processing methods: Turning, boring, pulling, grinding, vertical milling, scraping 3~10 points /cm^2, rolling.
Ra3.2~6.3: Barely discernible of the machining marks direction. Processing methods: Reaming, grinding, boring, pulling, scraping 3~10 points /cm^2, rolling.
Ra1.6~3.2: The direction of machining marks is not discernible. Processing methods: Cloth wheel grinding, grinding, superfinish.
Ra0.8~1.6: Dark glossy surface. Processing methods: superfinish