Tooling up for mould machining

As milling techniques and tooling technology continue to advance, more mould manufacturers are taking advantage of the developments for optimising machining operations, reducing cycle times and producing far superior mould surface finishes. However, factors such as machine tool capability, intended milling techniques, programming, work-holding and toolholding are a necessary part of the equation during cutting tool selection. Equally critical is the ability to analyse worn cutting inserts in an effort to maximise tool life and predict tool usage in mould machining applications.

Today’s fast and powerful machine tools with increased programming capabilities are essential when it comes to taking full advantage of the latest cutting tools and incorporating 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.

Milling techniques

High-speed (or dynamic) milling techniques are becoming increasingly popular. They are also particularly effective when using solid carbide cutting tools. This mill-ing method is 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.

For high-feed milling, large indexable insert cutters designed for that purpose 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 near net shape.

Appropriate cutter diameters are determined based on mould features—such as corner radii. High-feed indexable insert cutter diameters are usually 15 mm (0.62”) and larger. If smaller cutters are needed, solid carbide end mills should be used.

Inserts for high-feed indexable cutters can vary depending on workpiece material, but most applications will dictate a PVD-coated or CBN-coated insert. 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 less adjustment in feed rate. 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 themachine. 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.

During high-feed operations, cutters should be run at full diameter engagement, but no less than half their insert width. Full diameter engagement is possible because high-feed cutters effectively direct
cutting forces at the machine spindle in the axial direction to create balance. Cutters engaged at less than half their insert width will experience push and increased vibrations because the cut is unbalancedAir Precision 2017