Precision Mould Making Graphite electrode production: When the robot takes over the night shift

Editor: MA Alexander Stark

Italy — Bologna based company Vetimec supplies the international automotive industry and foundries with moulds, including injection moulds. Since die-sinking EDM dominates the production of these moulds, efficiency depends on the production of the graphite electrodes. With the changeover to special milling tools from Hufschmied, Vetimec can now machine graphite 30 percent faster and 20 percent more economically.

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Vetimec uses its new robot cell for the fully automatic, unmanned production of graphite electrodes.
Vetimec uses its new robot cell for the fully automatic, unmanned production of graphite electrodes.
(Source: Hufschmied Zerspanungssysteme)

Founded in 1962, the Italian Vetimec group of companies supplies the automotive industry with its moulds. For the production of mechanical and structural components, the mould maker designs, develops and manufactures complex and large die-casting, low-pressure and gravity die-casting moulds as well as core boxes used in the automotive industry. The manufacturer’s equipment is used to produce structural parts and engine components from aluminium and magnesium alloys. When it comes to manufacturing injection moulds, Vetimec’s own tool production is a point of leverage for achieving more efficient processes. For this purpose, the company, together with its partners Exeron and PCam, has installed a high-performance manufacturing cell consisting of two Exeron HSC600/5 5-axis CNC milling centres, four Exeron EDM316 eroding machines and a coordinate measuring machine from Zeiss with a handling system from PCam.

The mould maker’s requirement for this robot cell is the fully automatic, unmanned production of graphite electrodes, their robot-assisted measurement and subsequent eroding. Exeron adds its experience in this field to the cooperation. Tests on automated electrode production were run in their showroom and an interesting recommendation was made.

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Process reliability depends on the tool

“If you want to manufacture overnight without personnel, your process needs to be reliable and maintenance-free. There was one aspect that worried us: So far, we’ve not had any milling tools with sufficient tool life that we could, with a clear conscience, leave working unattended all night,” explains Massimo Lolli, Operations Manager at Vetimec. “At Exeron, we were made aware of the special tool manufacturer Hufschmied. We wanted to test their Graftor series.”

So that’s how Hufschmied Zerspanungssysteme entered the scene, scoring points with their process-specific tools, especially with materials that cause problems such as glass and carbon fibre composites, plastics, ceramics, hardened steels up to 72 HRC as well as graphite. “We advised Vetimec and also programmed milling strategies for their existing process parameters so that the company could obtain meaningful results from its milling tests,” reports Stefano Barbagallo, Sales Director Europe at Hufschmied Zerspanungssysteme.

To optimise milling processes, you always have to look at the overall situation, that is, the material, machine, tool, machining strategy and the quality requirements that have been set.

Stefano Barbagallo

Thoroughly tested

“In our milling tests, we compared tool life and milling results and we also evaluated tool costs, the number of milling tool variants required and the programming effort,” says Massimo Lolli. “The results show that Hufschmied’s Graftor strategy actually offers us clear advantages in all aspects, both in terms of cost-effectiveness and process reliability. We run a fully automated 24/7 operation and can guarantee the required manufacturing tolerances of ±0.010 mm while ensuring process reliability.” Vetimec followed up with a long-term test, and an annual framework contract was agreed six months after the initial meeting. A decision was made to fully switch the machining of graphite to the Graftor series.

The tool series, which is available in diameters from 0,5 to 12 mm, allows roughing and finishing to be integrated into a single milling operation. “The patented four-cutting edge geometry results in a significant minimisation of cutting force due to two opposing roughing and finishing cutting edges that are designed to push and pull,” explains Stefano Barbagallo. “The main advantage is that finished surface qualities can be achieved with high feed rates and infeeds. Finishing is possible into the full rest material while utilising the complete cutting edge length.” The Graftor can handle depth infeeds (ap) of up to 3 x D with a side infeed of up to 0.65 x D (ae) and can therefore tolerate significantly larger steps during finishing. This means that fewer machining steps and a smaller number of tool dimensions are required to create the finished final contour. As a result of the low cutting pressure, filigree contours can be produced, for example, conical pins with a diameter of 0.080 mm.

All electrodes are measured in the PCam cell automatically and assisted by robots on a CNC coordinate measuring machine from Zeiss. As the electrodes are measured together with the holders, the zero point is always defined.
All electrodes are measured in the PCam cell automatically and assisted by robots on a CNC coordinate measuring machine from Zeiss. As the electrodes are measured together with the holders, the zero point is always defined.
(Source: Hufschmied Zerspanungssysteme)

The graphite milling cutters are very geometrically accurate (±0.005 mm) and are supplied with gauge block marking on the tool shaft. The tool series features a nanocrystalline diamond coating (DIP) that was developed and patented by Hufschmied to ensure maximum abrasion resistance. This patented tool has allowed the supplier to master the special challenges of the material, that is, that graphite cannot be machined in the proper sense as the material is not plastically deformable. Machining does not produce chips, but knocks grains out of the composite. This results in the unpopular graphite dust that has a highly abrasive effect on tool cutting edges. As the filigree graphite structures of electrodes break out easily, the material-optimised geometry of the milling tool must not only ensure optimal removal of the material, but also remove vibrations from the machining process and reduce forces.

Non-productive time also reduced

The Graftor tools allow Vetimec to machine graphite 30 percent faster. One interesting finding was that savings are not only made due to the longer tool life, but also due to the fact that the roughing and finishing tools can be used more versatilely. This means that fewer tool changes are necessary. Reducing the number of tools also significantly reduces the effort required for NC programming and tool presettings. Massimo Lolli says: “The tools from Hufschmied have played an important role in the process optimisation of our electrode production. We are 20 per cent more economical in this area. Our customers also benefit because we can now deliver high-precision injection moulds even faster.”