text: what does the machinability rating of a metal mean?   there is a specific meaning:  how many surface feet per minute can a cutting tool operate with good life, while removing metal, in reference to a particular standard.  the machinability rating per se specifically means how machinable is a particular metal relative to a reference material, such as, a low carbon steel.  you'll see a note saying something like "relative speed based on c-1212", with a machinability rating of 28% (28% as fast as machining c1212, or low carbon steel).  it gives you a starting point for how fast to machine the material.  the actual cutting speed will involve the cutting tool (whether high speed steel or carbide) and the material (whether aluminum or steel for example).  the rpm for the cutter or lathe spindle is then adjusted to render the correct surface feet per minute (sfm).