Technology

Why mould makers can’t treat cutting tool choice in isolation

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First up: Machine tools and milling techniques

Fast and powerful machine tools with increased programming capabilities are essential when it comes to taking full advantage of the latest cutting tools and to get the most (and best) from highly-effective milling techniques. High-feed milling and high-speed milling are two popular milling techniques that require certain types of cutters for operational success.

High-speed milling is becoming increasingly popular. It is also particularly effective when using solid carbide cutting tools. This milling method employs an optimised roughing approach that combines large cutting depths with relatively small radial engagements when cutting steel. This technique is also effective when machining materials 60 HRC and harder.

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For high-feed milling, specially-designed large indexable insert cutters are recommended to remove the majority of material. Essentially, the process starts with relatively large indexable insert cutters for roughing, and works down to smaller-diameter indexable ball nose cutters and solid carbide cutters as the mould approaches its near complete shape.

Size and geometry play a role, too.

The selection of the most effective cutter diameters is determined by mould features – such as corner radii. High-feed indexable insert cutter diameters are usually 15 mm and larger. If smaller cutters are needed, solid carbide end mills are used.

Inserts for high-feed indexable cutters can vary depending on work-piece material, but most applications will dictate that a PVD-coated or CBN-coated insert is used.

As for insert geometry, Trigon-style inserts provide the lowest possible lead angle over round or square inserts. Low lead angles produce a much thinner chip, which in turn requires higher feed rates to maintain proper chip thickness for the insert geometry. The lower lead angle also directs the cutting forces in the axial direction, pushing up into the spindle, which is more stable and easier on the machine. Higher lead angles create thicker chips requiring fewer adjustments to feed rates. In addition, they also produce more radial force, causing vibration and stress on the spindle bearings. Solid-carbide cutters used for high-speed milling are typically four-flute shoulder end mills with long cutting edges and built-in chip splitters. The chip splitters break up chips into small, manageable sizes – resulting in improved evacuation from the cutting zone, as well as from the machine. Additionally, full cutting lengths and chip splitters - when combined with high-speed milling - generate increased levels of productivity and significantly higher tool life due to consistent loads.

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