Milling success story Precision surface milling: Meeting the demanding requirements of mirror-like facets

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The complex injection moulding tool for a car headlight's LED fiber optic cable proved to be a real challenge for the milling professionals at Color Metal in Heitersheim. They tackled this task with flying colors using the new roughing and finishing cutters from Moldino. And they did so in a significantly shorter time.

“Our production runs without any personnel during the second and third shifts, on weekends, and quite often in autonomous 24/7 operation,” explains the Owner and Managing Director of Color Metal, Otmar Gutmann. “We are also increasingly milling high-gloss surfaces to final dimensions, which minimises the amount of polishing required and increases accuracy.” This was what we call the lead into the topic: after all, it's about the milling backstory of a 10-ton injection mould with complicated geometries, sophisticated demoulding technology, and exacting surfaces. A single component mould for the production of the light optic for a 'stylish' shaped LED headlight, designed in-house by Color Metal based on customer specifications.

The task was to manufacture a tool with a base area of approximately 1,500 mm x 1,000 mm from 1.2343 and 1.2738HH. There were also roughing challenges, such as deep cavities, overhanging slopes, and other wild geometries. This is why the forming plate was not clamped horizontally but at an angle to look like it was hanging in the machine. This is particularly true since Heitersheim has developed a very high-quality milling level over many years, not least with the tools from Moldino and thanks to their process-engineering support. That is why Color Metal contacted Moldino about the milling challenges involved in this project. The challenges already started with soft roughing: the mold plate is about 600 mm thick. Its mold depth must be more than 400 mm for the headlight to fit inside. This required milling out approximately six tons of raw material.

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A new development from Moldino was used: the indexable milling cutter TR4F (short for: Turbo roughing four corner fast-feed). It is a high feed cutter in the 50 to 100 mm diameter range with an uneven pitch. Among other things, the inserts' unique shape is as much as ten percent thicker, providing 35 percent more contact surface during milling and helping to boost performance. Wide-shaped pockets on the tool holder ensure better chip removal. This means a tooth feed fz of 2 mm or more is possible during soft roughing. The result was compelling from the start: working at a feed rate of 9,300 mm/min and a depth infeed of 1 mm, a metal removal rate of 307 cm³/min was achieved using the TR4F4052BM-5-22 (52 mm diameter).

EHHRE and EHHBE Solid Carbide Cutters with TH3 Nano-Coating

The process optimiser from Moldino had even more optimisation recommendations at the ready: they concerned the subsequent machining steps after hardening to 52 HRC. As a result, they suggested using the new EHHRE and EHHBE solid carbide cutters with the newly developed TH3 nano-coating for roughing and residual material machining. The EHHRE tool has four or six cutting edges with polygonal face geometry, used exclusively for Z-constant milling. The mouldings were pre-milled to an allowance of 0.15 mm after hardening using the 6-fluted EHHRE-6060-TH3, which features a diameter of 6 mm. Often with feed rates reaching 7,000 mm/min. The torus milling cutter, previously used here by another manufacturer, only managed half the rate in the best scenario. The EHHRE's positive impacts on profitability are beyond question, as the tool life has also increased by 35 percent compared to the past.

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The new EHHBE, which features four flutes, was deployed in the areas which had to be roughened and pre-finished with a ball when hardened. It is the counterpart of the EHHRE as a ball cutter, which also features a TH3 nanolayer. The EHHBE-4040-S6 features a cutting-edge diameter of 4 mm (shank diameter 6 mm). It delivers feed rates of 4,100 mm/min for all roughing and residual material processing operations while maintaining a depth infeed of 0.3 mm and a side infeed of 0.8 mm. The new EHHBE easily doubled the feed rate compared to the previous version.

One of the unique challenges involved in this project included the production of the facets. The customer's specifications required virtually mirror-like surfaces and extremely high accuracy. After all, deviations in the geometry affect the light scattering. Manual processing or polishing was not allowed in the areas where the light is deflected. Color Metal, therefore, decided to mill the facets. Producing high-gloss surfaces in hard steel always requires high speed and feed rates. The lateral infeed rate, in this case, is critical for the surface quality. The nano-coated ball end mills of type EPDBEH-TH3 were deployed: a diameter of 0.4 mm was selected, to be more specific the EPDBEH-2004-1.5-TH3. The finishing operations were done with an identical side step but with a higher speed and an increased feed rate. And the result was more than convincing: after approximately 150 hours, all facets of the eight tool inserts were machined to their final dimensions. “All this, with a uniform, almost glossy finish and true-to-size precision,” emphasizes Otmar Gutmann. “We now only needed four balls for the finishing process instead of eight tools. In addition, there were still no signs of wear on the number four cutter.”

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