Advances in Titanium Machining
Results in higher metal removal rate, longer tool life and lower cost per cubic inch of metal removed
There is one key challenge in the machining of titanium today—fast metal removal rates (MRR) with reasonable tool life. Fortunately, recent advances in machine tool and tooling technology have made the goal of high MRR with longer tool life possible. As a result, manufacturers can now produce high-quality titanium components with shorter cycle times, increased tool life, and higher shop productivity.
Many factors are important in determining the manufacturing of titanium and what MRR can be achieved. These include the tooling, spindle speed, depth of cut, feedrate, total cut time, the investment in the machine itself, and tool life. All of these factors contribute to cost of manufacturing and, in turn, the ability to profitably and efficiently machine titanium.
Before these factors are considered, the technique of machining must first be established. Two cutting philosophies have prevailed in titanium—heavy and high-speed cutting.
Heavy cutting involves chunking out large volumes of metal, requiring high horsepower and torque, and resulting in high metal removal rates. This method is often applied for roughing operations. While MRR is high, heavy cutting sacrifices speed in finishing operations and tool life when using a high horsepower geared spindle.
The second approach is high speed. This is generally used in moderate roughing conditions and finishing, particularly to achieve final part accuracy and a good surface finish. Typically, the goal in high-speed machining is not achieving a high metal removal rate; it is achieving an acceptable accuracy and a fast finishing speed. The most influential component to determine the MRR in high-speed machining is the geometry and configuration of the part being machined.
Beyond the technique applied, the other factors of tooling, spindle speed, depth of cut, feedrate, cut time, and the machine tool all come into play.
So the question becomes, is there a way to boost MRR while maintaining satisfactory long tool life, lowering the cost per cubic inch of titanium removed? First, we must examine what makes titanium so difficult to machine.
Hard on Machine Tools and Tooling
Titanium is a popular material for the aerospace, marine, and medical industries. Titanium parts are stronger than their steel counterparts but weigh half as much. Furthermore, titanium components have twice the elasticity as steel parts, which make them ideal for applications that require flexible components that won’t crack or disintegrate under extreme forces.
Titanium components also resist corrosion better than those manufactured from stainless steels, and, like steel, titanium offers manufacturing flexibility because it can be readily cast or forged into various shapes.
While titanium offers advantages compared to other materials, it does create several manufacturing challenges. Because titanium is a poor thermal conductor, heat generated during the cutting process doesn’t dissipate through the part or machine table. Instead, the heat intensifies at the cutting area, sometimes reaching 2,000 degrees Fahrenheit, which can quickly dull cutting tools. Using dull cutting edges can generate even more heat, further shortening tool life.
Titanium’s elasticity creates additional manufacturing challenges. Under cutting pressure, the material’s elasticity makes it spring away from the cutting tool, which causes edges to rub together (instead of cut), increasing friction and further raising the temperature at the cutting area.
While using the right tool can help mitigate these manufacturing challenges, using the wrong tool will only compound them. Since titanium tends to work harden during normal cutting, those tools that can’t cut through the depth of hardened material will actually accelerate the hardening process. Instead of cutting, the wrong tool will push against it, straining the material. As the material reaches a higher level of hardness, cutting speeds that were appropriate at the start of the cut become excessive, wearing down the cutting tool quicker than normal.
The irony is that titanium’s physical properties are the reason it is so popular in the aerospace and medical industries. Unfortunately, many machine shops avoid titanium because of its reputation as a tool killer that is unreasonably expensive to machine. While there is little anyone can do to alter the material’s physical properties, there are techniques to simplify the machining process, prolong tool life, increase metal removal rates, decrease the cost of metal removed, and reduce the intimidation factor.
Test Cut: Landing Gear Bracket
Makino has recently performed a test cut on a landing gear bracket, using the a81M horizontal machining center with a high-torque integral drive spindle and a two-inch diameter by four-inch-long Kennametal HARVI® inserted carbide endmill in Ti 6-2-4-2.
The test cut included two operations, totaling two hours and 30 minutes. At any given time, two inches of the 36 inserts of the tool were in the cut. Due to the cutter geometry, this provided 18 effective inserts engaged in the cut. Makino was able to achieve three hours of tool life on those 18 inserts prior to indexing, with two cutting edges available per insert. One set of carbide inserts on the tool costs about $270. That means about $45 per hour of cutters were used during the test cut.
The result of this test was a titanium removal rate of 20 cubic inches per minute (1,200 per hour), three hours of tool life, and a cost of only $0.0375 per cubic inch of metal removed (insert cost).
Makino has learned through extensive test cuts that the ingredients needed for results like these include the use of high-pressure coolant, a machine tool up to the task, and the proper tooling.
Coolant for Titanium
When coolant is not appropriately applied in titanium, tools tend to wear quickly or fail. There are many reasons for this, including chip issues, poor lubrication, and the phenomenon of super-heated steam.
When machining at high speeds, proper chip evacuation is always a potential point of failure. If chips fall back into the chip/tool interface or don’t clear the cutting area, the result will inevitability cause damage to the tool or workpiece, often recutting the chips. Beyond evacuation, super-heated chips tend to not break into smaller pieces, compounding the problem of poor evacuation by clogging the tool flute. So not only do the chips have to be flushed out of the machining zone, they also have to be cooled so they are easily broken into smaller, more manageable pieces. Because many standard coolant delivery systems spray coolant over a large area in an attempt to cover the entire machining area, the coolant often does not properly flush or cool the chips, especially of a hardened material being machined at high speeds.
Another common coolant issue in titanium is poor lubrication. This is often caused by coolant pressure not being great enough to push through the tool tip while machining at high speeds or being vaporized by the high heat of the cut. Poor lubricity is one of the most common reasons for tool failure and accelerated tool wear in titanium, raising the cost of metal removal and extending cycle times.
Finally, during the machining of titanium, the tool tip can reach temperatures of 2,000 degrees Fahrenheit or more. This tremendous heat often causes super-heated steam to form in water-based coolants, making the coolant vaporize before it even touches the workpiece. This leads to all the problems listed above, plus additional heating of the workpiece instead of cooling, making accuracy even more challenging.
Makino has dealt with these common coolant problems in titanium by implementing a high-flow, high-pressure through-spindle coolant delivery system in the a81M. It pushes out 20 GPM of coolant at 1000 PSI, creating localized pressure to eliminate the potential for super-heated steam and improper chip evacuation while cooling the workpiece and chips. This system eliminates potential coolant problems by focusing the stream of coolant into the machining area at high pressures, allowing for faster feedrates and greatly extended tool life.
Beyond the coolant delivery system, the proper machine must also be utilized to achieve metal removal rates like those in our test.
One of the primary ingredients to successfully machining titanium is a stiff and rigid machine tool and workholder. It is critical that the machine tool and fixture are capable of securing the work-piece as it begins to vibrate, as is the tendency with any tough material, reducing the potential for chatter. As a result, only those fixtures that have been designed with stability as its primary function should be used in titanium machining.
Machines capable of efficiently machining titanium must have some basic characteristics, including stability, a high-torque spindle, high-pressure through-spindle coolant, and fast acceleration and deceleration.
Makino’s horizontal machining centers have three-point leveling, as well as rigid column and bed characteristics so the machine can handle the force required to generate high metal removal rates. A slant-style bed and column design is one example of a machine characteristic designed to resist Z- and X-axis force, while creating superior stiffness. This design also allows for high Y-axis machining without deflection and with the ability to utilize the full work zone of the machine.
Having a stiff and rigid machine design allows the machine tool to counter the high-torque, high-horsepower forces of the spindle and produce consistent part quality with less effort.
In addition to using stiff and rigid machine tools and fixtures, the second requirement for successfully machining titanium is high torque. When cutting titanium, machine tools should be able to produce torque in the 1,000Nm range, and spindle speeds up to 8,000 rpm. The key is the ability to create high torque at low rpms, while still having the ability to go to higher rpms for drilling operations and finishing. Both speed and torque are needed to reduce your cycle times of machining titanium.
Makino a81M Is Built Specifically for Cutting Hard Materials
Makino has developed a machine specifically for use with hard materials, such as titanium. It has a quick tool-to-tool time of only 1.7 seconds, with a chip-to-chip time of 4.2 seconds.
The a81M is particularly suited for long-reach and large-diameter-boring operations that require a great deal of torque, particularly at low rpm. For applications such as tapping, where a significant amount of spindle stopping, starting, and reversal occurs, the high-torque spindle on the a81M is significantly faster and has less idle time.
The Makino high-torque, integral-drive spindle employs a standard CAT 50-taper toolholder, with an HSK-A100 option. The spindle on the a81M has an available 744 foot pounds (1,009 Nm) peak torque rating. The a81M spindle has no corresponding loss of acceleration and deceleration, unlike other high-torque spindles using gear-type heads.
Many variables must be considered in cutting tool evaluation, all of which can have a profound effect on relative costs. The diameter, coating, tool type, SFM, number of teeth, rpm the tool is designed to run at, and IPM of performance all come into play. For example, during test-cutting, Makino found that some tools that cost as little as $75 could yield a better cost per cubic inch of metal removed than others that cost $175.
With that being said, a more expensive tool could actually be the lower-cost tool because it could generate the maximum amount of metal removed at the lowest overall total cost, based on the tool life that the particular cutter can generate. So if you examine all the variables in the decision, what it boils down to is the amount of total cost to generate a cubic inch of metal removed. It is important to not focus on the cost of the tool, but rather to focus on the cost to remove a cubic inch of metal.
One example of a tooling advance that could lower the cost of metal removal is variable rake tooling like Kennametal’s HARVI (helical, axial rake, variable, indexable) indexable insert endmill, used in the test cut mentioned earlier. According to the manufacturer, HARVI tools enable chatter-free machining due to a patented flute form with unequal flute division. The variable rake design assists in producing a chip that is easily removed when the proper coolant flow is applied.
Tool coatings are also important since the temperatures in the cut of titanium are higher than typical steels. The HARVI cutter’s inserts have a TiAlN coating, designed specifically for working in hardened materials.
Whatever the manufacturer, a tool must be chosen that is designed to handle the heat and stresses of titanium machining. This combination of advanced machines, such as the Makino a81M, and proper coolant pressure and volume allow modern tooling to last longer and cut faster.
Producing Quality Parts at the Lowest Cost
With the right ingredients, fast metal removal rates and prolonged tool life can be achieved in titanium. The right combination of machine, speed, power, tool holder, and tool must be found for each application.
As a result of new machines and tools developed specifically for hard metals, it is now possible to prolong tool life and reduce tooling costs, while decreasing cycle times, ultimately resulting in a lower cost per part.