Until now, the Forbach-based machine manufacturer PemTec has be known for its electrochemical machining of many electrically conductive components made from metal. “Our showcase parts are valve plates, gearings and forming punches, which our three PECM machine models (400, 800 and 800 S) can easily machine with surface qualities up to Ra 0.03 µm.” The special features, says PemTec Managing Director Hans Kuhn, is that this process runs very precisely and contact-free.
“We have now succeeded in enhancing this wonderful process also usable for hard metal parts. A new control unit, generators, coordinated pulse sequences and an adaptes electrolyte allow us to machine hard metal with the new PEM400.TC very effectively and without microstructural change.” The hard metal which is frequently used in the USA, contains a high amount of nickel and can be easily machined according to Hans Kuhn. “To be honest, with our new 400 series, we achieve a brilliant surface on components containing nickel. Especially ultra/fine grain materials with low binder content are ideal for our pemming process." In Hans Kuhn’s experience, however, the PEM400.TC can be adjusted to other binders and grain sizes, too. “Since there are so many hard metal variations, the machine's concept had to be flexible.” The typical three types of machining are front and side gap as well as side gap with internal or external machining. The machine will be able to be set individually to any material.
Modifications for homogeneous material removal and smooth surfaces
With the electrochemical PECM process, metal parts are machined without contact and precisely. In this process an electrically conductive electrolyte, salt water, is flushed between a negatively polarised tool electrode (cathode) and a positively polarised workpiece (anode). The electrolyte in the machining gap (space between the electrode and workpiece) removes the ionised material anodically. The electrode is mirrored on the workpiece.
“The synchronised current pulse, the oscillating tool electrode and the constant, very small machining gap are the requirements for high-precision and economic production”, explains Hans Kuhn. This process is absolutely contact-free and without thermal or mechanical influences, resulting in workpieces with a reproduction accuracy and repeatability within the lower micrometre range and a surface quality of up to Ra 0.03 µm.
The machining of hard metal with a standard PECM machine was not possible. “Therefore we had to develop a completely new machine and also a new process on the basis of our 400 series”, says Hans Kuhn.
The new process began with the electrolyte, which has to ensure homogeneous removal of the material even in conventional PECM machining. Working with universities and research facilities, PemTec researched and implemented the recipe for special electrolytes to machine hard metals.
“The same motivations, such as consistent metal removal and a smooth surface were the challenge for the electric current generation as well as for the control, in order to ultimately obtain representative and repeatable results”, explains Hans Kuhn.
The improvement in the control also had effects on the flushing behaviour during the machining process. This was accompanied by changes made to the axis diagram and control. “In the end, all these modifications meant that we are now able to present the first PECM hard metal machine in the world by PemTec”, reports Hans Kuhn. The development phase is almost completed. The last fine adjustments are currently being made to the prototype machine before it progresses to implementation of the first series/standard machine.
First hard metal punch
According to Hans Kuhn, PemTec was able to achieve very good results in the task of producing a hard metal die for indexable inserts (10% cobalt binder, fine grain) with the PEM400.TC prototype machine. The target was to accomplish a reproduction accuracy of ± 5 µm. Attention was focused on the edges and side faces as well as the removal rate. “The PECM process is naturally ideal for the series production of such components, because several parts can be produced with one tool electrode.
In fact, with our process we were not only able to meet the requirements of the target reproduction accuracy. We even improve on the targeted mark up to ± 3 µm.” Two other positive aspects were found, says Hans Kuhn, on the one hand, thanks to the contact-free machining, absolutely no surface damage occurred. On the other hand, the process time for the punch production was reduced to 49 min., because several production and preparation steps were no longer necessary. That means almost 5 hours’ time saving per part compared to the EDM process.
Front and side gap machining
Another very successful example put forward by PemTec was the side gap machining of a cutting punch in hard metal (Extramet EMT100 with 6% cobalt binder, fine grain). The electrode was made of stainless steel. In this case the objective was to reproduce the geometry with a good surface quality in the side-gap.” Therefore, says Hans Kuhn, the feed rate was designed for 40 µm. With a machining depth of 5 mm, the total machining time was 125 min. “With a surface quality of Ra 0.14µm, the punch is within the specified target. Up to three stamps were produced simultaneously by using a multiple tool – and this was achieved in just over two hours”, said Hans Kuhn, emphasising the outstanding results. As a third, remarkable Workpiece, PemTec machined a hard metal forming punch for stop teeth in the front and sides. “The challenge was, on the one hand, the contour definition of the geometry as well as the surface. To this end, we had chosen a hard metal with higher binder content and different grain size, specifically the material K40 (by Ceratizit, CF-H40S+, with 12% binder content, fine to medium grain).” The results, says Hans Kuhn, were apparently brilliant: The machining depth was 1.45 mm. A surface of Ra 0.095 µm was achieved within a machining time of 54 min.
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