In this sector HSM is located exclusively in the superfinish operation of the matrizes pursuing a specific object, i.e. reduction of maximum roughness ( Rt) of surfaces with values of 10 microns or less. As feeds can reach 5 to 10 times higher than conventional machining, it offers the possibility of increasing the number of passes to the same extent for the same finish time. The result is a better quality surface, reducing subsequent manual polishing tasks, which might imply almost 30% of the total mould manufactuer.
Machines used are gate type with 5 axes. However, they do not usually machine with the 5 axes simultaneously, but simply orientate headstock and machine after that. As they are superfinishing operations with ball-end tools and over-thicknesses of 0,2 mm we would be talking about a machining process similar to that of tempered steels regardig their physical nature. Superfinishing times with HSM are very long, 39 o 40 hours a big die. This is why it is important for the process to be highly reliable and the tool unlikely to break or wear during the operation. The iron castings used like the globular type GGG70, are easy to machine. Cutting speed reaches 400 m/min with coated hard metal tools and up to 1000 m/min with the PCBN. In Fig. 10 we can see a machine in progress and a partially mechanised boot.
Fig. Milling a die and aspect of this die prior to the operation.
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