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Threading the tapping needle: modern tools and the right control software can produce inside threads with or without chips. Finding the right method for a job means knowing the material, the machine, and the means.

During tapping, threads are cut manually into predrilled holes with screw taps or by a machine. This procedure can be used for almost all areas of application and materials.


Cold-forming taps are used for the chipless machining of standard threads. Unlike tapping, the thread is created by cold forming, not by cutting. The material is pressed into the required profile without interrupting the fiber orientation of the material inside the thread.

Thread milling was introduced with CNC machine tools. In that process, the thread is milled into the hole through the specific ciruclar and feed motions of the milling tool, with precision-executed pivoting.

Tapping: The Classic Method

In the tapping process, the material is removed step-by-step in a continous cut. Tapping reaches its limits with materials greater than 60 H[R.sub.c] hardness. It is also problematic for deep threads, in which issues arise in dimensional accuracy and tool breakage because of poor chip removal.

Chipless Cold-form Tapping

Cold-form tapping is used for materials with a strength of less than 1,200 N/m[m.sup.2] and a stretch at break of more than eight percent. In the chipless process, the thread is created through a step-by-step reforming process and, because of cold-forming, achieves higher static and dynamic strength in combination with good surface quality. The disadvantages of cold-form tapping is high torque compared to conventional tapping and the need for high-quality lubricants.

Thread Milling

Thread milling is suitable for almost all materials and offers flexibility and productivity. The thread flanks are machined cleanly with no axial miscut. For hard-to-machine materials, thread milling is the best option. One reason for its preference is that when the tool breaks it can be removed easily. There are restrictions relating to the thread depth, which is generally no more than three times the thread diameter.

Whatever method is used, the thread cycles in advanced CNC software modules offer quick and easy programming.

Inside thread issues also require closer examination.

Inside Thread Cycle Production

Programming inside threads is often complex because of the large number of math values needed. Advanced CNC thread cycle programming offers the most efficiency.

In these cycles, the required tool, die gradient, thread depth, cutting speed, and the procedure-specific technology parameters are defined within the program.

The procedure-specific parameters take into account the individual characteristics of the manufacturing process. For example, in conventional tapping, the thread is produced in a single cut or via chipbreaking. The characteristics must be selected on the screen and in the program code for the reversal movement and change in direction of rotation for chip breaking. The software then executes the process.

As a result, programming can be carried out quickly and accurately and without expert knowledge of conventional or cold-form tapping, thread milling, and drill thread milling cycles.

Work Step Linking

As a rule, separate, individually-programmed work steps are needed for the production of inside threads. First, the hole is centered and pre-drilling, milling, or cold-form tapping. If a workpiece contains several identical inside threads, the work steps must be reprogrammed for each position. To reduce programming, sub-programs can be created, which are recalled for various positions.


With the proper CNC software, programming can be carried out more efficiently. All work steps are programmed once and then linked via a position pattern. This means the programming for centering, drilling, and tapping is created once and then applied to all positions of the pattern. Configurable cycles for lines, hole circles, grids, frames, and special positions are available as position patterns.

Randy Pearson is the Siemens sales support manager for U.S. dealers and OEMs. His special interest is the training aspect on CNC machine tools, through the various seminars and classes the company conducts at votech schools and on-site at shops, as well as the Siemens training facilities around the country.

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Randy Pearson

Sales Support Manager

Siemens Energy & Automation
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Author:Pearson, Randy
Publication:Modern Applications News
Date:Dec 1, 2008
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