A thread chaser is a tool used to cut or clear threads, the spiral patterns that allow bolts or screws to fix into place. They are used in metalwork and woodwork, and may be internal or external.
A thread chaser can be a cylindrical bolt-like item with a sharp thread or can be shaped like a fine, curving knife with a tapered point. Thread chasers are often shaped so that they can be fitted into a power tool.
You use internal thread chasers to make internal threads in holes for threaded items like bolts or screws. You use external thread chasers to make external threads such as you see on bolts and screws. As well as making threads, you can use thread chasers to deepen existing threads or to remove blockages.
For some purposes, you may be able to make your own inexpensive thread chasers. Thread chasers for woodwork or for use with softer metals are relatively easy to construct. For more demanding tasks using harder metals or requiring more accurate measurements, you can purchase more sophisticated thread chasers.