Two tap geometries: one pushes chips ahead, the other pulls them back.
A spiral point (gun) tap has a left-hand helix that pushes chips forward through the hole — correct for through holes, and stronger because the flutes are shallower.
A spiral flute tap has a right-hand helix that pulls chips back out of the hole — necessary for blind holes, where pushed chips would pack at the bottom and break the tap. It is weaker and more expensive.
Using a spiral point tap in a blind hole. The chips have nowhere to go, pack at the bottom, and the tap breaks.
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