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Horn Blog

Mirror-finish machining in mould making

June 2021
Author: paulhorn
Company: Hartmetall-Werkzeugfabrik Paul Horn GmbH
Mirror-finish machining in mould making

The range of applications for mirror-finish milling is particularly wide in tool and mould making. The process saves polishing while increasing quality in terms of flatness and surface finish. It is used where the high quality surface of the mould is reflected in the parts to be produced, as in the following practical example.

The sine wave is a well-known term in mathematics and physics. It is represented in two dimensions on a piece of paper or as moving waves on the screen of an electronic measuring instrument. Milling a sine wave three-dimensionally from solid material using current CAD-CAM technology does not present a challenge to experienced users. But to finish the surface so that you can see your face in it requires an extra degree of expertise. Therefore, application engineers from DMG MORI took advantage of the support offered by HORN when they had the idea of manufacturing a mirror-finish sine wave as a demo workpiece.

The exercise focused primarily on high surface quality with long-term thermal accuracy and a demonstration of the machine and cooling concept.

Machining involved three tools tipped with monocrystalline diamond (MCD): a face milling cutter for the flat surfaces and a peripheral milling cutter for the flanks, both of 12 mm (0.472") diameter. For the convex surfaces, a ball nose end mill of 10 mm (0.393") diameter was used. Due to the requirement for extreme precision in terms of concentricity and axial run-out, all tools for mirror-finish machining are designed with one cutting edge. To produce surface finishes of less than Rz 0.1, the quality of the insert edge plays a decisive role. It is polished by hand and is absolutely flawless at 200x magnification.

Apart from the MCD tools, the machine used must also be able to cope with the high requirements placed on it with respect to precision, thermal growth and spindle quality. DMG MORI's application engineers opted for the HSC 70 linear. Thanks to the thermosymmetrical portal construction with active cooling, this machine ensures the necessary precision with very low thermal movement. In addition, the spindle is fitted with shaft cooling, allowing tool expansion to be reduced by 70 per cent. Developed for the tool and mould-making industry, the machine satisfies the demanding requirements of mirror-finish machining.

The finishing strategy is as follows: the aluminium sine wave with dimensions of 500 x 220 x 120 mm (19.685" x 8.661" x 4.724") is pre-finished after rough machining with an equidistant allowance of 0.05 mm (0.0019"). Finish machining with MCD tools starts with the concave radii and involves a machining time of approximately six hours. The tool profiled the contour in two axes. This was followed by machining of the faces, which took 46 minutes, and of the flanks, which took another 33 minutes. The total of around 7.5 hours sounds a long time but the surfaces produced with MCD tools do not require any subsequent polishing.

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