Precision Cutting Tools Milling reflective surfaces without wear
The core competence of RSD Polytec includes the machining of tools for lens systems. The company has recently been using new EPDBEH ball end mills for fine finishing. Now, only a single cutting tool is needed to mill even a large reflective surface with virtually no wear.
Precision machining has a long tradition in tool making at what is now RSD Polytec. Founded in 1964, the mould and tool-making company has been based at its current location in Bad Säckingen, Germany, for about four years. Over the decades, the company has developed into a high-tech manufacturer of high-precision injection moulds of small and medium-size as well as blanking and bending tools. In 2014, the tool shop became part of the RSD Group.
Over five years ago, RSD Polytec began to switch to HSC milling and, in particular, hard machining on solid carbide (SCM) cutting tools from the Japanese manufacturer Moldino. More and more cavity areas were then machined with these high-precision milling tools, which had previously been vertically eroded by the mould and toolmaker. Initially, only the lens tools were still erosion-polished. The HSC machines from the earlier years were not precise enough. A breakthrough finally came two years ago with the purchase of the new 5-axis Exeron HSC MP11/5, which can fully demonstrate its precision and dynamics in the shop, which is temperature-controlled to within +/- 2°C.
“The catalyst for this investment came from the customer,” recalls Managing Director Jürgen Klausmann looking back. “Extreme precision was required for the lens tools, which the previous EDM polishing of the lens tools was unable to achieve.” Milling was thus required here as well. It was now possible to do this with the high-precision MP11/5. To mill mirror-like surfaces, the final finishing process requires the machine to operate at maximum speeds and with high dynamics. For large surfaces especially, in the past, it had been inevitable to work with sister cutting tools because the service lives of the milling tools were insufficient. However, a slight offset occurs because the milling tool can no longer cut the material properly at the end of its tool life. In the past, this transition had to be homogenized manually, and then the entire surfaces polished to a high gloss. Due to the longer tool life of the milling tool, large surfaces can now be milled with one tool, thus reducing the costs for the time-consuming manual polishing. Nonetheless, a trade-off is always to be made between very long machine run times and manual polishing work. It appears that for smaller surfaces, the entire surface is milled to a high gloss; for larger surfaces, the polishing work is reduced to a minimum in favor of shorter machine runtimes.
As a process optimizer at Moldino, Johannes Zimmermann was familiar with the problem: His solution at the start of 2021 was to introduce the new EPDBEH (Epoch Deep Ball Evolution Hard) series of ball milling tools, which cover a wide range of diameters from 0.1 to 12 mm, with effective lengths of up to a maximum of 20 x D. Other important factors for the machining result and process reliability are the excellent concentricities of the cutting tools and extremely low manufacturing tolerances, which are in the single-digit μ range. Based on a multi-cavity die, the aim was to explore whether the new ball end mills could deliver their promise in practice. The cutting tool is used to inject a transparent plate for an automotive component. This involved a cavity measuring approximately 120 mm x 100 mm and 30 mm deep with a tolerance of +/- 0.01 mm and a mirror-like surface. The material used was 1.2344 with 52 HRC.
Working together, the specialists from RSD Polytec and Moldino fine-tuned the machining steps, the cutting data, and the selection of cutting tools. The tests were performed on the Exeron HSC MP11/5 with a minimum amount of lubrication. Before this, roughing, pre-finishing and finishing were done in Bad Säckingen — in other words, three operations. With the new end mills, a fourth process was added, fine finishing. As previously, milling was performed in 3+1 axes with the B-axis in the upright position, and the Baden-based company chose a relatively small diameter of 0.4 mm balls. The finishing strategy in Topsolid 7 was still used, the infeed was reduced slightly in favor of the surface quality, as was the feed rate. On the other hand, the milling speed was increased significantly to 51 m/min thanks to the TH3 coating. Moreover, the tolerance setting on the Heidenhain CNC was adjusted and synchronized with that in CAM.
The result was more than convincing: After finishing with a single EPDBEH milling tool, the RSD Polytec polisher only had to refinish with the 3-μm and 1-μm paste — and the perfect, dimensionally accurate high-gloss surface was ready. After just under two hours of milling, no wear was visible on the die. This eliminated five polishing passes involving grinding with different grits. Before, the polisher needed three hours per cavity; now one hour is all it takes. The cost analysis prepared for polishing in this project showed savings of almost 30 percent.
Meanwhile, all mirror surfaces in Bad Säckingen are milled using the EPDBEH series. In a current project — a cutting tool for a low-profile light guide — the polishing effort could even be cut in half for its comparatively large mirror surface. The 4-mm ball end mill from the EPDBEH series took seven hours to mill the mirror surface of the cavity, which measures approximately 600 mm x 300 mm and is 60 mm deep. Despite a milling speed of 214 m/min, the milling tool showed no wear. This is all the more noteworthy given that the material in question is 1.2343 ESU from the service provider Contura. This alloy is produced from a powdery raw material, including near-contour cooling channels using the SLS (selective laser melting) process. “This material is known for being relatively difficult to machine,” emphasizes Jürgen Klausmann. “The project would not have been feasible without the new mills from Moldino — at least not in this quality.”