A few things about taps:
Always buy and use high speed steel taps. They're ground shiny all over and marked HSS. They're tough and strong. Carbon steel taps are cheaper, but they are hard as glass and brittle and break easily.
Taps do get dull after some use. When this occurs, don't use that tap to cut new threads, especially in aluminum or brass. You can use it to clean gasket sealer and dirt out of a threaded hole. Don't do this with a sharp tap if you want it to remain sharp.
Always use a tapping fluid. Kerosene works well for aluminum. I like Rapid-Tap for ferrous metals. Stainless steel needs a different lube. I've used Anchorlube for many years. It comes in a squeeze bottle and looks like green snot.
Always use a proper tap wrench so no side loading is placed on the tap. Back the tap up every rotation to break the chips off.
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