And this is how it functions: A specially designed punch-tap tool with a twisted series of teeth arranged opposing one another is punched in a fraction of second into a predrilled hole with simultaneous helical motion. This operation creates two spiral grooves. The spindle then rotates through an additional 180 degrees while the axial feed axis is simultaneously withdrawn by half a thread pitch. And it is precisely this motion that forms the thread. The tool can then be quite simply withdrawn helically as the series of teeth are now again in the previously formed grooves. And that completes the operation. Numerous tests were carried out, and it was clearly verified that when compared to other techniques, this innovative process reduces thread machining times by 75 percent.
The geometry and holding forces of the internal threads punched in aluminum for thread size M6 – which is very frequently required – is comparable to conventionally machined threads. Engineer Hechtle stated that the helical groove form is decisive, and: "The thread depth must be at least twice that of the diameter."
A high quality machining center with a stable and high-performance spindle is also a crucial factor. This must provide a very high torque and must also be able to withstand enormous pulling and pushing loads. Emuge Franken engaged Siemens as technology partner early on in the development project to identify just which forces occur at which locations of the machine when punching threads.
Within the scope of mechatronic support, the engineers in the Technology and Application Center (TAC) were able to calculate the relevant forces using a finite element model of the production process. Based on these values, they defined the main data that the machining center must comply with in order to be suitable for the punch-tap technique.
In the meantime, several machining centers that satisfy all criteria have been investigated. If users are interested, but the planned machine has not been evaluated in the relevant detail, then Emuge urgently recommends that it should be thoroughly tested to evaluate its suitability. Both the OEM as well as SIEMENS offers this service.