Smart approach to milling tools

Recently-developed milling tools provide the operational flexibility required to machine increasingly complex components.

Solid carbide cutters have traditionally provided high metal removal rates and excellent accuracy. However, the one-piece nature of the cutters requires they be reground when worn. Changing tools for regrinding interrupts production, and costs time and money.

As a cost-efficient alternative to solid cutters, toolmakers have developed milling cutters with indexable, replaceable inserts.

When cutting edges wear, inserts are simply indexed or rotated. Worn out inserts can be quickly replaced with the tool body still in the machine, and different insert geometries can be fitted to the same cutter body to create a variety of contours.

The geometries of indexable inserts have evolved as well, with many having cutting edges on both top and bottom sides. Turning such a double-sided insert over after indexing doubles the number of available cutting edges.

However, the design of double-sided inserts does not include space behind the cutting edge for a clearance angle, and thereby does not permit the tools to be applied in a positive-rake approach.

In a positive rake tool, the bulk of the insert body is tilted back from the cutting edge, enabling the edge to shear the workpiece material. On the other hand, in a negative-rake tool the bulk of the insert body tilts toward the workpiece and the tool essentially pushes the material being cut. Because of its design, the cutting edge of a double-sided insert must approach the workpiece from a negative angle.

Negative-rake cutting tools do offer certain benefits. However, because negative rake tools push the work material instead of shearing it, the cutting forces generated by a negative-rake tool can distort a thin-walled part.

Conversely, a positiverake tool is free cutting, which minimises cutting pressure, and can cut tight contours that a negative tool cannot reach. Positive insert geometries provide a large degree of flexibility to perform a variety of operations including slotting, contouring, and helical interpolation and ramping.

The tools can help stabilise machining on older and/ or less rigid machines. And minimising cutting forces is crucial when machining tough materials such as titanium, Inconel, and many stainless steels.

To provide indexable inserts that can cut in the positive mode, tool manufacturers offer one-sided inserts with clearance angles behind the cutting edges. Although the inserts cannot be turned over they can be rotated in the holder, providing multiple useful cutting edges.

The free cutting nature of positive-rake tools reduces cutting forces and heat generated in the cut. That is important because a onesided insert must have three times more tool life per edge to be cost effective, compared with double-sided inserts.

Among the available high-performance, onesided, positive-rake inserts developed for milling are the XO.X10 tools developed for the Turbo 10 series of milling cutters from Seco Tools. The inserts are engineered to minimise cutting forces and control chips while providing the flexibility to cut a wide variety of part contours and materials.

For example, up sharp, uncoated polished edges work well in softer materials such as free-machining aluminium alloys. For more difficult-tomachine materials such as steels, cast iron, or stainless steels, honing or chamfering the tool edge is necessary to protect it from chipping.

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