Machining titanium is something of a double edged sword. Whilst it’s hugely popular in aerospace circles because of its desirable mechanical properties, machining it can be challenging to say the least. Fortunately, tooling specialist Iscar has been busy developing a range of tools that purport to make the task of machining complex shapes from titanium a whole lot easier.

Since the 1980s when titanium alloys were pioneered in military programs, the advanced material’s use within military airframes has continued to grow rapidly and to migrate into the global commercial aircraft sector. Now, titanium alloys effectively compete globally with aluminium, nickel and ferrous alloys in both commercial and military airframes.

Applications cover a wide range of airframe structural parts; from massive highly stressed, forged wing structures, through landing gear components, to small critical fasteners, springs and hydraulic tubing. But the increasing complexity of aerospace parts and the burgeoning demand for ever more efficient manufacturing methods has prompted the development of new machining technologies and innovative tooling solutions aimed at assisting SMEs to compete more effectively by increasing productively.

Why titanium is special – and its limitations

The selection of titanium for aerospace applications results from the specific properties associated with the metal: strength to weight ratio; reliability; corrosion resistance; outstanding mechanical properties; and a low coefficient of thermal expansion.

Titanium 6AL-4V is the most popular material within the aerospace sector due to its light weight and high strength ratio. Machining titanium alloys requires cutting forces considerably higher than those required for machining steels but titanium alloys presents metallurgical characteristics that make machining tasks more difficult than machining steels of equivalent hardness.

Titanium is known to generate a work hardening characteristic which leads to a high shear angle during machining that causes a thin chip to contact a relatively small area on the cutting tool face. In addition, the high bearing forces produced by machining, combined with the friction developed by the chip as it travels results in a significant increase of heat on a localised area of the cutting tool.

Due to its poor conductivity, heat generated by cutting titanium does not dissipate quickly into the air and therefore a considerable amount of heat becomes locked between the cutting edge and the tool face. This combination of high bearing forces and heat produces crater wear mechanisms within the proximity of the cutting edge, resulting in rapid tool breakdown.

With its relatively low modulus of elasticity, titanium has more ‘springiness’ than steel so work tends to move away from cutting tools unless heavy cuts are maintained or proper backup is employed. Thin-walled parts tend to deflect under tool pressures causing chatter, tool rubbing and dimensional tolerance problems. The key solution to these elasticity problems is ensuring the rigidity of the entire system and the use of sharp cutting tools with the correct geometric characteristics.

An additional problem is that titanium alloys have a strong tendency to alloy with, or to react chemically with the materials in cutting tools at tool operating temperatures.

Trochoidal milling – high metal removal

A strategy that delivers high metal removal when machining titanium is trochoidal milling. Machining either deep or shallow pockets in titanium is known to be a difficult task and as a large proportion of the tool is continuously engaged with the workpiece, cutting forces and heat levels are elevated. Another common problem is uneven chip load per tooth, for example high loads at the point where the cutter has advanced furthest into the workpiece and lower loads in other areas. Titanium pocket machining also creates a range of issues related to chip evacuation – as the cutter fills most of the machined slot width, little room remains for chip evacuation so the chance of re-cutting chips is high.

Iscar’s R&D centre has recognised the potential of trochoidal milling in recent years and has promoted the development of solid carbide endmill cutters in addition to extended flute milling cutters for high productivity when using indexable inserts. Now trochoidal milling can easily be applied using a large variety of Iscar milling tools. In order to gain maximum performance in this area Iscar recommends the use of its new Chatterfree endmills and equally Helido or Helimill for indexable inserts.

The titanium machining challenge is even greater when the slots to be cut are relatively deep in relation to their width as this machining task increases the difficulty of chip evacuation. Also, when the machined slots are curved rather than straight, chip evacuation becomes even more difficult.

Typically, titanium slot machining difficulties make it necessary to run at low feed rates and depth of cut to avoid vibrations and premature cutting tool failure. These enforced conditions lead to unwanted deficiencies in productivity. Even at reduced feed rates, tool life tends to be short when cutting slots.

A potential solution

Trochoidal, or spiral milling provides a potential solution to this problem. In essence the theory is to program the cutter to move in a circular pattern with each circle advancing into the cut. A key advantage of trochoidal milling is that only a small area of the cutting tool is engaged at one time and the feed rate is always constant. In addition, trochoidal milling makes it possible to apply an endmill with a diameter that is smaller than the pocket’s width, allowing sufficient room for effective chip evacuation.

Iscar’s Chatterfree cutters, set on variable pitch flute configurations, not only eliminate harmonic vibrations during trochoidal milling, but they are also effective in opening up full slots and cavities in terms of rapid metal removal rates. Due to the reduction in chatter, Chatterfree also delivers an added bonus in terms of extended edge life. Chatterfree is able to handle full slot machining of up to 2xD with four or five flutes even for low power machines with ISO40 or BT40 adaptations, without compromising on high productivity.

Iscar’s IC900 substrate grade coated with advance PVD TiAlN coating provides desirable mechanical properties that make this solid carbide endmill cutter an ideal choice for trochoidal milling.
An alternative option that can be utilised is indexable inserts, such as Iscar’s Helido or Helimill products attached to an extended flute milling cutter. Once the multi-tooth cutter is engaged in trochoidal milling, each individual cutting edge digs into the workpiece material with minimum generated heat and associated stresses. The advantage of using an extended flute milling cutter is the feed rate per tooth that can be applied, resulting in higher productivity.

Despite its potential, trochoidal milling also presents a range of challenges. The cutter must undergo a complex motion that is beyond the capabilities of most conventional CNC software programming systems. In addition, the machine tool must be sufficiently rigid and fast enough to accommodate trochoidal cutting. The cutter likewise must be able to operate at high speeds and strong enough for the material in use. Machine rigidity determines how aggressive the trochoidal cut can be. Other factors include the cutting tool’s size and workpiece material.

The basic idea of trochoidal machining involves substantially increasing the cutting speeds and feed rates. Chips are cut to their maximum thickness at the initial engagement of the cutter’s teeth with the workpiece and decrease in thickness at the end of engagement (climb milling). The toolpath is optimised based on the results of previous machine cycles, eliminating air cutting and minimising retract movements.

Cost-effective benefits

One of the benefits is that a slot width larger than the cutting diameter of the tool can be machined. This means that several slot widths can be used with the same tool diameter in an efficient way. Since a small radial depth of cut is used, cutters with fine pitch configuration can be applied, leading to higher feed and cutting speed than with conventional slot milling applications.

Programming challenges

Iscar reports that conventional CNC programming software typically cannot generate a program to perform trochoidal milling. Previously, the only way to perform trochoidal milling was for a programmer to manually code the complex tool motions involved. The programmer however cannot visually check the program without running it on a machine. For this reason, trochoidal milling is seldom employed.

CAM software developers recently added routine sections for trochoidal milling, substantially reducing the amount of time required to produce a CNC program for this machining operation. These new features also give the programmer access to other capabilities such as graphically simulated machining. By using these software advancements, tests have shown that it can be much faster to use the trochoidal method instead of the ordinary slot milling method, since much higher cutting conditions can be achieved.
An example of how trochoidal milling with an Iscar Chatterfree tool can improve productivity, reduce cycle times and significantly lower associated costs can be understood from the following case study.

Case Study of Titanium Aircraft Part, Using Trochoidal Milling

Trochoidal milling substantially improved productivity of slot milling operations. Normal practice when machining a slot is to feed at a rate of about 20% of the rate used in normal side milling. Use of trochoidal milling enabled an increase in the feed rate up to approximately 80% of the normal side milling feed.

The intention here is to clearly demonstrate how trochoidal milling reduces the stress and heat involved in cutting. Despite that fact the case study presents much higher speeds and feeds than in normal slot milling, the wear on the cutting tool was identically developed.

Raw material: Titanium Ti-6Al-4V (Grade 5), annealed:

Trochoidal milling
Iscar endmill cutter: ECH160B32-6C16
Carbide grade: IC900
Tool diameter: 16mm
Vc = 115m/minute
Fz = 0.12mm/t
Ap = 22mm
Ae= 1-1.5mm
Emulsion coolant
Time to manufacture one part: 33 minutes
Tool life: four pieces
Adaptation: BT40

Conventional milling
Iscar endmill cutter: EFS-B44 16-34W16-92
Carbide grade: IC900
Tool diameter: 16 mm
Vc = 45m/minute
Fz = 0.04mm/t
Ae = 12mm
Ap = 12mm
Emulsion Coolant

Time to manufacture one part: 55 minutes
Tool life: four pieces
Adaptation: BT40

Remarks: Substantial reduction in machine load using trochoidal milling technique. Machine demonstrated easy smooth operation and appeared to be free machining.

Iscar UK
www.iscar.co.uk