6- Threads, Taps, and Tapping - Taper, Plug, Bottoming, and Pipe Taps

Principle features of taps are:

The most common hand taps are called taper, plug, and bottoming taps.

All three tools are identical except for the bevel angle at the tip. The bevel at the tip serves two purposes: it guides the tap into the hole and it ramp cuts the undeveloped first threads.

Taper taps have the longest bevel angle with 8 to 10 undeveloped threads.

Plug taps are the most popular of the three and have 3 to 5 undeveloped threads on the bevel.

Bottoming taps have only 1 to 1 1/2 undeveloped threads at the tip and should be used only when full amount of threads specified on the drawing can not be achieved by the other two taps.

The number of flutes and the geometry of the cutting edge will depend on the material tapped.

With the exception of the tip, the three hand taps will form thread to specifications and can be used independently.

The bottom tap should only chase the threads originally cut by the taper or plug tap (for hand tapping) The bottom tap is only forming the last few threads. In the past this chasing of the bottom tap has caused some confusion with serial hand taps.

There is considerable difference. Serial taps come in sets of three. The first two taps cut under size and the third completes the threading.

Serial taps work well in tough metals such as stainless steel or nickel and are recommended for tapping deep holes, open or blind. Also by working the thread in three passes a smoother thread is produced.

Pipe threads are unique in that they are tapered. The taper provides a method of sealing off liquids and gases. When torqued tightly the threads merge together laterally which provides the seal.

Hand tapping may be a thing of the past. At least some students tell me so. One student told me that to tap a flat bottom hole that I just needed a flat bottom tap and that tapping first with a taper or plug tap was a huge waste of time. He said that he could just tap it first time out with a flat bottom. As it turns out he used a CNC machine with rigid tapping function to accomplish this. I would like to think that not all machining is done with CNC these days but some people are convinced otherwise. R.S