Sources and scope for this page: Industry convention — no single governing standard (conventional usage, definitions vary between sources), covering the definition of machinability rating. No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. How sources, methods and updates are recorded.
Definition
The rating folds chip formation, surface finish, tool wear and power consumption into one number. It is a shop-floor comparison, not a physical property, and published values differ by roughly ±10% because each was measured under different conditions.
A high rating does not mean soft: gray cast iron rates 70% and cuts easily because the graphite breaks chips, while pure copper rates 20% and is soft but gummy. Use the rating to compare two candidate materials, not to predict a cutting speed.
Common misuse
Using the rating to estimate a cutting speed. It ranks materials; it does not give you an RPM.