Question

Aim and objectives of internal tread

Answer 1

Thrilling is the process of threading and drilling (accomplished in the reverse order) internal threads using a specialized cutting tool on a CNC mill. The cutting tool tip is shaped like a drill or center-cutting endmill, while the body has a thread-shaped form with a countersink cutter form near the shank.

Answer 2

1. Purpose of thread cutting

Thread cutting is cutting of helical turns of threads out of the tapping-size hole or bolt in order to create screwed connections.


Figure 1 - Thread cutting

A complete screwed connection requires an internal thread and a matching external thread as a counterpart. With single-piece manufacture or with repair work, manual thread cutting is a necessary working technique, because - due to technical and economical reasons - machines cannot be used in every case.

2. Kinds of threads

Fastening screw threads:

metric threads (V-shaped threads)


Whitworth threads (V-shaped threads)

Power-transmission screw threads:

knuckle threads, acme threads, saw-tooth threads


Figure 2 - Kinds of threads

1 - Vee thread
2 - Knuckle thread
3 - Acme thread
4 - Saw-tooth thread

Sealing threads:

tapered threads (metric and Whitworth)

Pipe threads:

Whitworth pipe threads

By manual thread cutting, only fastening screw threads and partially pipe threads are made. These kinds of threads are made as left-hand or right-hand threads as well as coarse screw threads and fine screw threads..

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.