Machining of titanium presents a number of challenges, explains Iscar (0121 422 8585). It requires cutting forces considerably higher than those required for machining steels, amplified by the fact that it has a tendency to move away from the tool (springiness), which requires that heavy cuts are maintained or proper backup employed. Thin-walled parts tend to deflect under tool pressures, causing chatter, tool rubbing and dimensional tolerance problems. The key solution to the elasticity problem is ensuring the rigidity of the entire system and the use of sharp cutting tools having the correct geometric characteristics.
Image: Trochoidal milling offers better a solution to milling titanium
But the material also work hardens, 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.
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 titanium's poor heat conductivity, 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, highlights Iscar.
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.
Machining either deep or shallow pockets in titanium is known to be a difficult task, since a large proportion of the tool is continuously engaged with the workpiece, and cutting forces and heat levels are thus elevated.
Another common problem is uneven chip load per tooth; for example, high loads at the point where the cutter has advanced farthest 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.
While the titanium challenge is already high, it is even greater when the slots to be cut are relatively deep in relation to their width, Iscar explains, 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.
To meet the challenges, Iscar's R&D centre has recognised the potential of trochoidal milling in recent years and promotes the development of solid carbide endmill cutters, as well as extended flute milling cutters for high productivity, when using indexable inserts. Following this development, trochoidal milling can easily be applied, using a variety of Iscar's milling tools, says the company. In particular, Iscar Chatterfree variable pitch, PVD TiAlN-coated solid carbide endmills, and equally Helido and Helimill indexable insert systems.
Trochoidal milling sees the cutter 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.
This also means that several slot widths can be tackled by the same tool. Since a small radial depth of cut is used, cutters with a fine pitch configuration can be applied, leading to higher feed and cutting speed than with ordinary slot milling applications.
But, despite its great potential, trochoidal milling also presents a range of challenges. The cutter must undergo a complicated motion that, says Iscar, 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.
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.
An example of the benefits of trochoidal milling with an Iscar Chatterfree tool is given by way of slot milling in a titanium aircraft part (Ti-6Al-4V, Grade 5, annealed). 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 Chatterfree tool delivered a cycle time of 33 mins versus 55 mins for a conventional approach, while parts per tool both numbered four, with the operation carried out on a BT40 spindle machine.
More tooling developments in brief
 Air Turbine Spindles' double turbine 602X DT (ITC), launched at MACH 2012, the 602X DT constant high speed spindle can autoload from the Fanuc Robodrill and other drill tap machines' magazines (video)
 Sandvik Coromant introduces the CoroDrill 870 (video) exchangeable tip drill
 Horn 713 12-tooth milling cutters are smoother groovers
 Titanium nitride coated taps from Dormer Tools
 Walter Tiger.Tec Silver range turns to steel
 Sumitomo SMD Multi-Drill features detachable and regrindable replacement drill heads
 Seco aims TK1001 and TK2001 turning duo at cast iron
 Mapal introduces world's first MQL tool inspection system
 Floyd offers Hommel & Keller on-turning-centre component roll marking
First published in Machinery, May 2012