How do I know if a cutting tool (bit, end mill, insert, blade) is dull?

How a tool cuts and what it looks like can signal whether it is dull. If the tool is cutting with high resistance or creating significant noise or heat when cutting, it may be dull. Tools that have visible chips along their cutting edges should also be replaced. Learn what a new tool looks like and be able to compare that to the tool you are using. Understand the correct geometry needed in order to create a cutting tool.