THE NEW ECONOMICS OF TITANIUM

Titanium possesses many desirable properties, including low density, high strength, light weight, elasticity, and good fatigue and corrosion resistance. These properties make it ideal for a wide variety of applications in nearly every industry.

This is the reason titanium applications are increasing throughout multiple markets, especially for structural components in the aerospace market and particularly with those composites used in the latest commercial aircraft that share its coefficient of thermal expansion. With its promises of weight savings and improved fuel economy and performance, titanium has become a prevalent material in the next generation of aircraft designs.

MACHINABILITY

However, this material also has limitations to its production, its use, and its machinability. Because titanium is a poor conductor of heat, heat can concentrate at the cutting edge. Titanium possesses an alloying tendency (or chemical reactivity) that can contribute to galling or welding in certain cutting tools. It has a low modulus of elasticity that allows the material to spring away from the cut. It also exhibits certain properties that can shorten tool life and lengthen machining time compared to other standard materials.

One big reason many industries don?t use more titanium comes down to costs: costs to acquire the material as well as the cost to machine it. Also, many job shops need the actual process knowledge to go along with the equipment necessary to effectively machine titanium. These factors are driving the development of new methods and techniques and new machines that will help pull all the needed elements together for the efficient production of titanium components across these markets.

Machinability directly impacts the use of titanium. As the cost of machining and other production costs decline, the use of titanium will increase due to its weight/strength advantages. In fact, many major producers of titanium have already announced plans to increase their production capabilities and bring more capacity on-stream to address the new aircraft demands of the aerospace industry, such as the Boeing 787.

This means job shops, contract manufacturers and other precision machining operations that supply the aerospace industry must become aware of this significant growth in the use of the material and prepare their manufacturing operations to enable titanium machining.

In the past, only a few shops were willing or able to tackle this machining objective. The older machine tools they previously employed in the cutting of titanium, particularly in aerospace applications, enabled the cutting of the material but also experienced some inherent process limitations that included vibration, poor chip removal, slow acceleration and deceleration, slow feed rates, limited tooling options, limited 3D machining capabilities without multiple setups, programming issues, limited workpiece access and the lack of automation capability.

But in the future, more people must be mindful of the changing condition of titanium applications. Today?s parts feature much more complex geometries and, frequently, much tighter tolerances and accuracy requirements that allow for part mating with composite panels and other desired features that aircraft designers are applying today.

MACHINING ECONOMICS
Several elements are involved in calculating the cost to machine titanium. Clearly, tool costs are an important factor to consider, with the metal removal rate being the dominant variable. Increasing the metal removal rate by five or ten times can have a dramatic impact on costs, because the cost to remove one cubic inch of material ? including cycle time and tooling costs ? is significant.

Two traditional approaches are used to cut titanium. One is the heavy cutting technique, where high power, high torque provides good metal removal rates. This approach is typically good for roughing operations, but the rigidity requirements of the part and fixturing as well as the machine itself must be considered because they are critical to this approach. The other technique is the high-speed cutting approach, which is good for finishing or even moderate roughing.

The ability to interchange these cutting techniques faster and control each more rigidly enabled one manufacturer to replace a number of geared-head spindle machines with two new horizontal machining centers that substantially decreased cycle times and increased part quality. Many employees in this shop refused to believe that two HMCs could outperform five geared-head spindle machines, but since their installation the company has reported higher metal removal rates, improved accuracies, greater throughput and reduced setup times, with an overall cycle time reduction of 25 to 50 percent in projects that were run previously on the geared-head spindles.

The increase in metal removal rates has been substantial. On one part, where an 800 lb titanium block is machined down to 90 lb in four different operations, the total machining time on a geared-head spindle machine was over 200 hours, but in the new horizontal machining center it takes only 100 hours. The HMC and its automation system also occupy less floor space than the previous machine and achieve, in this case, twice the metal removal rate.

In other words, this shop now spends half the money per cubic inch of metal removed on their titanium tooling. Not only does the HMC improve that production characteristic, it is also able to hold a true position tolerance of .003 in and improve the dimensional tolerance down to .0006 in. It maintains surface finishes as fine as 32 microinches. The bottom line here is that all of these tolerances were previously unobtainable with the old geared-head spindle machines.

Tool changeover time (in terms of chip-to-chip time) is another important factor to consider, specifically the ability of modern machining technology to reduce this non-cut time. Out-of-cut time can be lowered with fast, automatic tool changes, fast workpiece changeovers, and automation capabilities that permit efficient cellular systems to speed the production of part chip sets and lower part quantities that are frequently encountered in today?s environment.

Creative tooling designs and innovative cutting strategies realized by newer machining centers can also reduce non-cutting costs by extending tool life and affecting other tooling variables. Pallet changing equipment or related material handing automation can further decrease the overall cycle time and its associated labor costs.

Other considerations can impact the costs associated with machining titanium, too. Chip removal or cleanup, for instance, can have a greater impact than many would expect. A machine tool with highly efficient chip removal can lower labor costs, extend tool life, increase part accuracies and reduce downtime from cleanup.

Another critical titanium machining requirement is coolant management, both of the workpiece and the cutting area. Good thermal control of these areas can reduce tool deformation, the ability of the tool to alloy with the titanium workpiece, and the potential for scrapped parts due to inaccuracy.

NEXT STEPS
We are now seeing some new machine tool designs that are ideal for addressing the demanding characteristics of machining titanium used for structural aircraft components. In fact, the newest five-axis horizontal machining centers integrate several key technologies that are specifically designed to handle the heavy forces (up to 20,000 Newtons) associated with the efficient, high production machining of titanium.

One of these, for example, is an active dampening system that uses hydraulic technology to balance the frictional forces associated with the movement of the column, the spindle and components and suppress vibrations as necessary to cut the titanium effectively. This balance produces a more stable zone of cutting that provides geometric flexibility for the cutting strategies associated with these types of aircraft component parts. A large stable area of rpm and feed rate enable superior tool wear and superior metal removal rate. This active dampening system helps these machine tools perform deeper cuts, achieve higher metal removal rates, and improve tool wear.

These HMCs contain other numerous design features that positively impact the elements and variables of machining titanium and the other factors associated with real world production, like the ability to do rapid pallet change and unattended operations. Another example is an extremely rigid construction for enhanced performance with a high-torque, high powered interval drive spindle, and a high pressure, high flow coolant system for increased speed and productivity.

Beyond HMCs, some unique titanium booster technology for electrical discharge machining has been released recently that is designed to aid sinker EDMs in machining titanium components for aerospace applications with little or no HAZ (the layer of heat affected zone) and to better control the micro-cracking typically associated with sinker EDM processing.

This titanium booster technology utilizes a 60 amp power booster to create new machining conditions that can increase roughing speeds up to 80 percent while reducing electrode wear to nearly one-third that of previous and existing EDM technologies. Graphite-based electrode material is used for processing to eliminate the need for high cost copper and copper tungsten materials.

This new sinker EDM technology enhances the productivity and capability of the roughing processes used on titanium and titanium alloys. It addresses several issues previously associated with rough machining or ?hogging? large pockets or cavities in titanium, including the high wear rates of expensive specialty cutting tools, the previous slow EDM machining speeds, and high electrode wear. With increased EDM speeds, manufacturers can extend sinker EDM usage during titanium machining for reduced tooling costs and greater part accuracies.

As titanium applications expand, particularly in the aerospace market, these are some of the new technologies being worked on today that will improve the economics of machining titanium for tomorrow.

– – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – –

Mark Rentschler is the marketing manager of Makino, Inc., 7680 Innovation Way, Mason, OH 45040-8003, 800-552-3288, Fax 513-573-456, www.makino.com.