The many advantages that graphite electrodes offer have made them the mainstay of spark erosion – no manual reworking, no deburring, hardly any burn-off, even intricate geometries are possible and the electrode comes out of the milling machine ready for eroding. Nevertheless, its manufacture frequently continues to be plagued by graphite dust. Electrode milling in conjunction with cooling lubricant offers a good alternative to dry working. An extensive comparative test of Zecha’s SEAGULL® tools demonstrates the numerous advantages of wet milling: cleaner working, more flexible machine utilization, improved dimensioning and lower tool wear.
Can graphite, copper and hard machining take place on one milling machine? So far, this has hardly been worthy of consideration due to the stubbornness of graphite dust. Dry working even creates additional financial hurdles, especially for small and medium-sized companies, as an additional milling machine had to be purchased besides the eroding machine. Wet processing of graphite might be the solution in so far as cooling lubricant permits a flexible utilisation of the milling machine: The graphite dust, generated during the milling process, is flushed with the cooling emulsion and in following filtered out of the emulsion by means of a special system. Pipes and machines remain clean and free of graphite deposits.
What are the effects on the tools, the electrodes and, last but not least, the end component? A comparative test of SEAGULL® mill cutters produced amazing results.
Wet or dry– SEAGULL® flying high
SEAGULL® mill cutters produce superbly fine ribs and the most intricate of electrode geometries: Patented short flutes – just 0.6 mm at a diameter of 2 mm – as well as a tailored combination of high quality hard metal, diamond coating and special geometry with the tightest of tolerances of just 5 µm ensure minimum flute pressure and cutting forces. This enables excellent surface qualities and maximum life cycles.
Zecha offers the SEAGULL® mill cutters in two quality categories: The quality line – subdivided into Series 568 (ball nose end mill) and Series 578 (end mill with corner radius) – offers optimum value for money for standard applications and is pitched at the price-conscious user, who still requires high quality machining results – even during wet milling. These tools come with the tried-and-tested diamond coating, with a concentricity of 0.005 mm and diameter tolerances of 0/-0.015 mm. The right choice for superb surface qualities and tightest tolerances (diameter of 0/-0.010 mm and concentricity of 0.003 mm) is the high-end line with ball nose end mills (series 567) and end mills with corner radius (series 577). Thanks to a 10 µm thick high-performance diamond coating, these tools also boast a long service life. Whether high-end or quality line, every single tool is measured separately and has its actual dimensions documented on the packaging. Thus consistently narrow tolerances and highest process capability are guaranteed when intricate graphite electrodes are manufactured. All SEAGULL® mill cutters are available in diameters from 0.3 mm up to 12 mm.
“The SEAGULL® tools have had a fundamental influence on further improving dry graphite processing. We now wanted to move forward and experience what could be achieved by wet processing,” says Arndt Fielen, Sales Director at Zecha Hartmetall-Werkzeugfabrikation GmbH.
Longer life cycles and greater dimensional accuracy
To attain these features, electrodes specifically designed for graphite, have been manufactured by SEAGULL® ball cutters (B-2-20-60) on two micron Mill S400 machines of GF Machining Solutions. Cooling has been provided directly at the milling point, one time by means of compressed air, one time with the aid of emulsion.
Z-Mike laser device 1210 produced results of before and after status measurements to verify concentricity and diameters of these cutters.
Contours and surface were optically examined by C-View of CIMTRODE at 1,000X magnification.
“The results distinctly tend to wet processing: After 290 minutes of milling the tools used with cooling, lubricant displayed 30 per cent less wear than those used for dry processing. After 490 minutes the results even underscored the advantages of wet processing: 40 per cent less wear,” said an impressed Daniel Gruber, Managing Director of CIMTRODE GmbH. The wear with cooling lubricant was a maximum of 0.0052 mm and without up to 0.0087 mm.
Wet processing ensured longer life cycles for the tools and thereby increased significantly the dimensional accuracy of the electrodes. After 490 minutes the dimensional accuracy of the wet process electrodes was still in the 0.005 mm tolerance range. Large dimensional deviations had been observed for dry processing.
Good eroding results
“The excellent life cycle results during milling and high dimensional accuracy of the electrodes should also match to eroding,” says Arndt Fielen.
A series of tests compared the eroding time, surface roughness of the end components as well as the electrodes’ burn-off. One roughing and one finishing electrode were used in the test set-up. Arndt Fielen explains: “Although we were unable to record significant improvements in this case, the results were definitely comparable – whether using the dry or the wet milled electrodes.” The burn-off ranged between 3 and 6 µm and the surface roughness of the component between Ra 0.82 and 0.87. All the electrodes were in use for approximately 40 min.
In addition, wet processing keeps the electrodes washed and clean and benefits quality measurements as well as eroding in highest accuracy.
Manufacturing graphite electrodes with cooling lubricant offers an alternative to dry milling that is well worth considering. It enables tool and mould makers to machine copper, graphite and steel on one and the same machine with utmost cleanliness. It is precisely the short flute of the SEAGULL® tools that is, both, suitable for producing astonishing dry working results, and ideal for wet processing of graphite. Life cycles are prolonged, tool costs reduced and dimensional accuracy increase.
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