3. Tools for internal threading (tapping)

Internal threads are cut by serial taps or by nut taps.

Serial taps:

They consist of two or three tools the distinguishing feature being the design of the cutting part.

The entering tap (1st pass) has a long chamfer and trapeziform cutting edges; it does approximately 60 % of the cutting work.

Figure 3 - Serial taps

1 - Entering tap
2 - Plug tap
3 - Third tap

The plug tap (2nd pass) has a short chamfer and trapeziform cutting edges which are deeper; it does approximately 30 % of the cutting work.

The (finishing) third tap (3rd pass) has a short chamfer and cutting edges which create the final sharp form of the thread groove; it cuts the thread to nominal size and finishes the thread flanks.

The individual tools are additionally distinguished by marks in the form of engraved rings at the shank below the square. Modern versions of serial taps consist of entering tap and finishing tap only.

Nut taps:

The nut tap (also called single-pass hand tap) in its cutting part unites the cutting parts of the three serial taps. Consequently, it has a long chamfer representing about 70 % of the total length of the cutting part; the cutting edges are initially trapeziform and become sharp only towards the end. The cutting part is not essentially longer than that of the serial tap.

Yet this cutting part has to do all the cutting operation. The shank with the square is relatively long.

Figure 4 - Nut tap

3.1. Application of the tools

- Serial taps divide the cutting operation into several passes and thus enable easy but time-consuming threading; the third tap - due to its short chamfer - is able to almost completely cut out blind holes.

Conclusion:

Serial taps are mainly used for tapping blind holes.

Figure 5 - Technology of internal thread cutting (tapping)

1 - Drilling
2 - Countersinking
3 - Rough-cutting by entering tap
4 - Finish-cutting by plug tap
5 - Finish-cutting by third tap
6 - Checking

- Nut taps enable quick threading which, however, involves a great cutting power (tearing of the thread may occur). Due to its long chamfer, this tool cannot be used for blind holes.

Conclusion:

Nut taps are used for tapping short through holes.

What are the distinguishing features for the use of serial taps and nut taps?
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