Which type of tap is best suited for finish tapping a blind hole?

The best choice for finish tapping a blind hole is the bottoming tap. This type of tap has a very short chamfer, which allows it to create thread profiles that are close to the bottom of the hole without needing to remove significant material.

When finish tapping a blind hole—where the tap must reach the bottom without breaching through the other end—a bottoming tap is particularly effective. It is designed for this purpose, ensuring that the threads are fully formed at the deepest part of the hole, providing a good fit for whatever fastener will be used.

In contrast, a taper tap has a more extended chamfer, making it easier to start the tap in the hole, but it cannot finish the threads to the bottom effectively. A plug tap, while it has a shorter taper than a taper tap, still doesn't allow for the complete thread formation at the bottom due to its longer chamfer compared to a bottoming tap. Spiral taps are generally used for faster drilling and are not specifically designed for the depth requirements of blind holes.