Once a threading tap gets too dull to cut a clean thread, hit the tips of the cutting edges with a grinding wheel to blunt them a little and make a thread chaser. Trying to chase threads with a new, sharp tap is a good way to oversize the female thread and create a sloppy fit. I usually keep a set of worn taps in a separate tool box drawer, modified into thread chasers to avoid that problem. Dipping the shank of the tap into some cold gun bluing solution to blacken them makes it easy to tell a chaser from a regular tap at a glance.
Jerry
