A titanium machining boost!

The physical, chemical and thermal properties of titanium and its alloys make it a uniquely demanding material to machine. Characteristics include: risk of tool wear due to the cutting edge being exposed to higher temperatures as titanium is a poor conductor of heat; tool wear/breakdown due to the ‘smearing' tendency of titanium, where the chip welds to the insert, causing edge-line frittering when re-entering the cut; material deflection/chatter due to material elasticity; and risk of rapid tool wear because of localised high pressure in combination with heat at the small contact surface.

There are a few general rules for machining titanium that can help in overcoming these factors, such as relatively low cutting speeds, sharp cutting edges, optimised feed rates and avoiding idling in cut. Furthermore, the use of large volumes of coolant, preferably at high pressure through the spindle and tool will also help, as will the replacement of cutting edges at the first sign of any wear, and employing climb (down) milling wherever possible.

It pays to plan ahead

Among the initial determining factors is selecting a suitable cutting tool. Indexable insert cutters remove material most efficiently and are today seen as first choice for roughing as well as unbeatable when it comes to finishing large flat faces. Solid carbide cutters remain the preferred choice for semi-finishing and finishing operations, and when radii, cavities and slots are too small for indexable inserts.

When selecting a dedicated milling cutter for titanium it's important to include a comparatively positive rake with a sharp but strong cutting edge, and a cemented carbide grade that can withstand the thermal and chemical demands of the material. Indexable insert technology has come a long way regarding geometry and tool material, and is taking over as a more cost-effective solution from solid carbide and high speed steel tools, even for medium and large size tools. Until recently, progress in machining titanium seems not to have been dramatic but now a few breakthrough developments have improved milling performance.

Airframe solutions

Increasingly common applications for titanium include airframe components. Radial milling dominates here partly because of the presence of features such as cavities, profiles, shoulders, slots and edges. But there is another reason: large radial depth of cuts (Ae) result in considerable reductions in tool life while large axial depths of cut (Ap) have a relatively slight influence on tool life. A close-pitch, long-edge milling cutter with a radial engagement of 30% and as much axial engagement as the application allows is the most effective way of removing titanium.
An indexable insert, long-edge cutter is made up of multiple rows of inserts that imitate the continuous, helical edge of solid cutters. Absolutely fixed cutting edges are paramount for milling titanium; any movement even in roughing operations can lead to uneven wear and put the cutting edge at risk. A slight tool wear indication or dulling of the edge in titanium quickly escalates to breakdown.

With this background, it was a priority for Sandvik Coromant's new CoroMill 690 long edge milling cutter to have a special insert design. The precise location and fixed locking of the insert provide a combination of capability for high metal removal rate with spacious chip flutes. Inserts for the 690 cutter have been optimised for titanium via a new insert manufacturing process that provides a sharper, direct-pressed insert, leading to lighter cutting, ample engagement, lower power needs and capacity for higher feed rates.

Increasing the pressure

Coolant applied at high pressure through spindle and tool during titanium machining affects distribution of heat, chip formation, tool wear and surface integrity. Coolant jet at pressure from nozzles plays a crucial part in controlling temperature as they can be aimed directly at the part of the insert in contact with the finished surface, creating a hydraulic ‘wedge' effect. High pressure coolant can be adapted easily to the machine tool by way of the Coromant Capto toolholding system, an ISO standard.

When the application involves deep narrow cavities, a solid carbide end mill in an extended chuck does not represent optimum stability as it will limit cutting data and can be a risk to component quality. The concept of exchangeable head cutters, however, provides the advantages of both indexability and high finishing capability. The coupling between the head and shank is a key factor for this type of tool concept as performance relies on strength, stability, accuracy, repeatability and ease of handling.

A generous axial support face, a tapered radial support face and a specially developed thread and support of the screw are integral features of Sandvik Coromant's new CoroMill 316 exchangeable head cutter. The design provides the coupling needed between head and shank, and the basis for high performance at long tool overhangs.

Facing up to the challenge

When face milling titanium, it is essential to consider the cutter position in relation to the workpiece, along with cutter diameter to workpiece width. The cutter should remain on a path that allows full contact rather than multiple passes when milling large faces and, if possible, interrupted cuts should be avoided.

As regards face mill type, a round insert cutter, such as CoroMill 300, is often first choice because of the strength and geometry of the cutting edge. The size used is a balance between the depth of cut required, the feature to be machined and machine tool capability. This is a very effective and reliable roughing and semi-finishing cutter, capable also of machining cavities through helical interpolation. High metal removal rates, long tool life and good security are potential advantages of this type of tool.

A milling cutter with a very small entering angle can provide a reduced chip thickness effect, thus facilitating high feed levels. Combined with a small depth of cut, high feed milling can be a very effective machining method and does not impose high levels of power and torque.

A 10° high feed cutter such as the CoroMill 210 with a square insert also has another use that is advantageous for machining titanium – plunge milling. The cutter is fed axially, making continual plunges into the material. The dominant cutting force is directed upwards, into the machine spindle and therefore well countered. With this degree of stability, the cutter is then also suitable for long tool overhangs.

In conclusion, axial depth of cut is the best route for optimising metal removal rate in rough milling titanium while feed rate best optimises finishing. Cutting speed is always limited when machining titanium, although to different levels, in both types of operations. Armed with these basic facts, along with correctly selected and applied cutting tools, plenty can be done regarding the optimisation of titanium machining to make it a more competitive and reliable process.

www.coromant.sandvik.com