Which type of tap is used to cut full thread to the bottom of a closed hole?

The type of tap used to cut full thread to the bottom of a closed hole is known as a bottoming tap. This specialized tap has a nearly flat end that allows it to reach deep into the closed hole and create threads all the way to the bottom.

Bottoming taps are particularly effective for this purpose because they provide minimal lead-in taper, which means they can start cutting threads closer to the bottom of the hole. This characteristic is essential when a full thread engagement is necessary, such as in applications where retaining strength or engagement depth is crucial.

Other tap types, such as taper and plug taps, have a tapered lead-in that helps guide the tap into the material but do not allow for threading all the way to the bottom of a closed hole. Taper taps are typically used for starting threads, while plug taps can be used in both through holes and blind holes but may not cut as deeply as the bottoming taps. Closed tap isn't a widely recognized term in tapping tools and may refer to the concept of a tap used in specific closed hole applications, but it does not pertain directly to cutting threads to the very bottom.

Thus, the bottoming tap is specifically engineered to achieve the full-thread capability required in the context provided.