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February 2011 eNews

Milling Developments Help to Cut Away the Challenges of Titanium Machining

Titanium as a work piece material is finding its way into most machine shops with milling being the dominant machining method particularly in structural parts. Material removal rate is becoming an increasingly competitive factor for jobs but is not as easy to achieve in this material having challenging machinability. But new tool and process developments are providing this increasingly useful design material with new possibilities to improve machining costs.

Why has titanium got challenging machinability?

The physical, chemical and thermal properties of titanium make it a uniquely demanding material to machine. There are a number of titanium alloys with different properties and machinability varies considerably - from the traditional Ti6Al4V to stronger alloys like Ti10-2-3 and now Ti5553. There are a number of titanium alloys with different properties and machinability varies considerably. The main characteristics are:

• Risk of tool wear due to the cutting edge being exposed to higher temperatures and having to absorb more heat as titanium is a poor conductor of heat and more heat is generated in machining titanium
• Tool wear/breakdown due to the smearing tendency of titanium, which is reactive with tool materials and where the chip welds to the insert, causing edge-line frittering when re-entering the cut
• Material deflection/chatter tendency due to the elasticity of titanium
• Risk of rapid tool wear because of the localized high pressure in combination with heat at the small contact surface

There are a few general rules for machining titanium to help in overcoming these demands:

• Use relatively low cutting speeds
• Use sharp cutting edges
• Optimize feed rates and avoid idling while in cut
• Use large volumes of coolant, preferably at high pressure through spindle and tool
• Replace cutting edges at first sign of any wear
• Employ climb (down) milling wherever possible

For machining any of the titanium alloys...

... there is the common need for more thorough planning, from selecting the machine tool for the job to programming details of cuts and roughing separately to finishing. Size and shape of component features vary and so do the demands on selection of machine, fixturing, coolant supply, tools, method and cutting data.

The first determining factor is size and shape of configurations and suitable tool size. Indexable insert cutters remove material most efficiently and can today be seen as more of a first choice for roughing many configurations as well as unbeatable when it comes to finishing large flat faces. Solid carbide cutters form the solution for semi-finishing and finishing operations and when radii, cavities and slots are too small for indexable insert tools. They have the advantage of a high number of flutes and high axial cutting capability.

The selection of a dedicated milling cutter for titanium needs to be based on the programming options that are necessary. The tool basics are always to include a comparatively positive rake with a sharp but strong enough cutting edge and for uncoated or coated cemented carbide grade which stands up to the particular thermal and chemical demands of titanium. Indexable insert technology has come a long way regarding geometry and tool material and are taking over as a more cost-effective solution from the vast amount of solid carbide and high speed steel tool options available even for medium and large size tools. Until more recently, progress in machining titanium seems not to have been dramatic but now a few breakthrough developments have improved the performance of milling.

Because of the nature of most of the components involved, radial milling is a very suitable machining method for titanium. There are a lot of shoulders, edges, profiles and cavities to be machined often from billets. But there is also another reason: large radial depth of cuts result in considerable reductions in tool-life while large axial depths of cut have a relatively slight influence on cutting temperature and hence tool-life. Therefore, a close-pitch, long-edge milling cutter with a radial engagement of about 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 which take after the continuous, helical edge of solid cutters. Accommodating the indexable inserts to make up a row from the bottom of the cutter rising along the periphery has up to now presented a limitation to achieving acceptable machining capability and security in titanium. Generous flutes for effective chip evacuation are necessary and, in combination with building effective rows of positive, sharp inserts on the cutter, have created pitfalls for the indexable insert long edge cutter.

Cutting edges that are accurately and firmly fixed in position, to resist the axial forces created by the helix, are paramount for milling titanium. Any movement even in roughing operations can lead to uneven wear and put the cutting edge risk or even screw breakage, leading to catastrophic failure. Especially axial support for inserts is difficult to achieve along the row of closely-positioned successive inserts, which lead to over-reliance on the insert screw. The best solution therefore, to arrive at new outstanding performance levels with long-edge milling, is to have an uncompromising interface between insert and tool body, such as that in the CoroMill 690 long edge cutter. The insert seat has to have a definitive support and locking facility, especially regarding axial and rotational forces. Such an interface location of the insert can then provide the capability for high metal removal rate and allow for spacious chip flutes. Furthermore, a range of tooth capability can then be provided for the same tool diameter by having a choice of insert sizes to tackle various operations. A closer insert pitch provides variables for improving productivity through feed rate.

Coolant applied at high pressure, through spindle and tool, during titanium machining affects the distribution of heat, chip formation, welding on edges, tool wear and surface integrity and as such makes a clear difference to performance. The application of high-pressure coolant, ranging from a standard 70 to 100 bar pressure, has shown to provide very clear advantages in titanium milling. It can be easily adapted to the machine tool by way of the Coromant Capto holding tool system, an ISO standard. Coolant at pressure is a standard on many of today’s machines and as such, it is a potential resource to optimize titanium milling.

When the application involves narrow cavities…

... that is deep and has to have longer tool reach - and for these not to become machining bottle-necks - requires a solution for small-tool capability as well as operational flexibility. Holding a solid carbide end mill in an extended chuck to machine deep into a cavity does not represent optimum stability today as this 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 finishing capability of solid cutters. The coupling between the head and shank is a key factor for this type of tool concept. Performance relies on the strength, stability, accuracy, repeatability and ease of handling.

From performance- and result-capability, tool-cost perspective and flexibility requirements, the exchangeable head concept offers an advantage for the 10 to 25 mm tool diameter area. Flexibility is high with this concept and the reduced tool inventory it offers. The finishing ability is better than indexable insert cutters, and it represents a substantially lower tool-cost than a complete solid cutter and does not need re-grinding with loss of size. Being able to select combinations of different heads and different shanks offers a high degree of flexibility and optimization possibilities.

A generous axial support face, a tapered radial support face and a specially developed thread and support of the screw, such as with the CoroMill 316 exchangeable head cutter provides the coupling needed between head and shank and the basis for the performance at long tool overhangs.

When face milling titanium …

… some of the general rules of face milling are recommended concerning cutter positioning in relation to the workpiece, cutter diameter to workpiece-width, preferred climb (down) milling, with a thin-to-thick-chip but also more care being taken for the cutter to enter and exit the workpiece. The cutter should be kept on a path entailing full contact rather than multiple passes when milling large faces and, if possible, interrupted cuts should be avoided, with holes and cavities made after the face milling.

As far as the type of face mill, a round insert cutter, such as the CoroMill 300, is often a first choice because of the strength and geometry of the cutting edge. Cutters with insert size up to 25.4 mm/1 in. iC is available providing very high metal removal rates. The size used is a balance between the depth of cut required, the feature to be machined and the 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 rate, long tool-life and good security are the potential advantage with this type of cutter. The cutting edge is extra-long, distributing the wear which often leads to a longer tool-life.

The entering angle of the round-insert cutter is variable, from zero to the angle at the depth of cut, and, like the 45-degree face mill, giving a chip-thinning effect which is beneficial for raising the feed-rate. The feed-per-tooth can be generous with higher table feeds as a result. However, power and torque needs extra attention when machining titanium because of the higher cutting forces involved. The tensile-strength property of the alloy, the cutter engagement, feed rate and the number of teeth in cut are especially relevant when it comes to face mill roughing in titanium.

Further options

A milling cutter with a really small entering angle – 10 degrees - can provide an even higher chip-thinning effect. As such, it gives the potential for an even higher feed-level. Combined with a small depth of cut, high-feed milling can be a very effective machining method and does not impose such high levels of power and torque. High metal removal rates can be achieved on smaller, weaker machines.

A 10-degree, high-feed cutter, such as the CoroMill 210, with a square insert also has another use which 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. This makes this type of cutter a versatile roughing tool, capable of high metal removal rate for machining, faces, shoulders and cavities especially when instability prevails.

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Sandvik

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