Surface finish is an apparent witness of tool marks or - lack of same - on the machined surface of a work piece. These two videos show fine, medium, and rough surface finishes for the lathe and the mill.

Surface finish is a characteristic of any machined surface. It is sometimes called surface texture or roughness. The design engineer is usually the person that decides what the surface finish of a work piece should be. They base their reasoning on what the work piece is supposed to do. Here are a few examples that the engineer considers when applying a surface finish spec:

• Good surface finishes achieve high efficiency. High quality surface finishes coupled with millionths of inch fit will produce less friction. For example a 75HP piston engine can loose up to 5HP to the friction of the connecting rods, crank, and piston unless high quality finishes are applied..
  
• Good surface finishes increase the wear resistance of two work pieces in an assembly
  
• Good surface finishes reduce the friction between two work pieces in an assembly
  
• Good surface finishes have a cosmetic effect and make your parts "look good".
• Good surface finishes are the norm in several industries, as in the micro processor industry.
  
• Good surface finished permits the proper function of static, and dynamic O-ring seals in hydraulic and pneumatic equipment.

The design engineer communicates their desires by using a blueprint. The blueprint is the communication medium that the machinist will use to make the work piece. The surface finish is defined and measured as a surface profile. The main components of a surface profile are waviness and roughness.