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ADVANCESIN TITANIUM MACHININGT here is one key challenge in the machining of titanium today fast metal removable rates(MRR) with reasonable tool life.Fortunately, recent advances in machine tool and

tooling technology have made the goal of high MRRwith longer tool life possible. As a result, manufac-turers can now produce high-quality titanium com-ponents with shorter cycle times, increased tool lifeand higher shop productivity.

Many factors are important in determining themanufacturing of titanium and what MRR can beachieved. These include the tooling, spindle speed,depth of cut, feedrate, total cut time, the investmentin the machine itself and tool life.

All of these factors contribute to cost of manufac-turing, and in turn, the ability to profitably and effi-ciently machine titanium.

Before these factors are considered, the techniqueof machining must first be established.

Two PhilosophiesTwo cutting philosophies have prevailed in titani-

um, heavy and high-speed cutting.Heavy cutting involves chunking out large vol-

umes of metal, requiring high horsepower and torqueand resulting in high metal removal rates. Thismethod is often applied for roughing operations.

While MRR is high, heavy cutting sacrifices speedin finishing operations and tool life when using ahigh horsepower geared spindle.

The second approach is high speed.This is generally used in moderate roughing con-

ditions and finishing, particularly to achieve finalpart accuracy and a good surface finish.

Typically the goal in high speed machining is notachieving a high metal removal rate; it is achievingan acceptable accuracy and a fast finishing speed.

The most influential component to determine theMRR in high speed machining is the geometry andconfiguration of the part being machined.

Beyond the technique applied, the other factors of

tooling, spindle speed, depth of cut, feederate, cuttime and the machine tool all come into play.

So the question becomes, is there a way to boostMRR while maintaining satisfactory long tool life,lowering the cost per cubic inch of titaniumremoved? First, we must examine what makes titani-um so difficult to machine.

Machine Tools and ToolingTitanium parts are stronger than their steel coun-

terparts, but weigh half as much. Furthermore, tita-nium components have twice the elasticity as steelparts, which make them ideal for applications thatrequire flexible components that wont crack or dis-integrate under extreme forces.

Titanium components also resist corrosion betterthan those manufactured from stainless steels, and

like steel, titanium offers manufacturing flexibilitybecause it can be readily cast or forged into variousshapes.

While titanium offers advantages compared toother materials, it does create several manufacturingchallenges. Because titanium is a poor thermal con-ductor, heat generated during the cutting processdoesnt dissipate through the part or machine table.

Instead, the heat intensifies at the cutting area,sometimes reaching 2,000 F, which can quickly dullcutting tools. Using dull cutting edges can generateeven more heat, further shortening tool life.

Titaniums elasticity creates additional manufac-turing challenges. Under cutting pressure the materi-als elasticity causes it to spring away from thecutting tool, which causes edges to rub together(instead of cut), increasing friction and further rais-ing the temperature at the cutting area.

While using the right tool can help mitigate thesemanufacturing challenges, using the wrong tool willonly compound them.

Since titanium tends to work harden during nor-mal cutting, those tools that cant cut through the

depth of hardened material will actually accelerate

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ADVANCESIN TITANIUM MACHINING

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the hardening process. Instead of cutting, the wrongtool 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 cuttingtool quicker than normal.

The irony is that titaniums physical properties arethe reason it is so popular in the aerospace and med-ical industries. Unfortunately, many machine shopsavoid titanium because of its reputation as a toolkiller that is unreasonably expensive to machine.

Coolant for TitaniumWhen coolant is not appropriately applied in titani-

um, tools tend to wear quickly or fail. There are manyreasons for this, including chip issues, poor lubrica-tion and the phenomenon of super-heated steam.

When machining at high speeds, proper chip evac-uation is always a potential point of failure. If chipsfall back into the chip/tool interface or dont clear thecutting area, the result will inevitability cause damageto the tool or workpiece, often re-cutting the chips.

Beyond evacuation, super-heated chips tend to notbreak into smaller pieces, compounding the problemof poor evacuation by clogging the tool flute. So notonly do the chips have to be flushed out of the machin-ing zone, they also have to be cooled so they are easi-ly broken into smaller, more manageable pieces.

Because many standard coolant delivery systems

spray coolant over a large area in an attempt to coverthe entire machining area, the coolant often does notproperly flush or cool the chips, especially of a hard-ened material being machined at high speeds.

Another common coolant issue in titanium is poorlubrication.

This is often caused by coolant pressure not beinggreat enough to push through the tool tip whilemachining at high speeds or is vaporized by the highheat of the cut. Poor lubricity is the one of the mostcommon reasons for tool failure and accelerated toolwear in titanium, raising the cost of metal removaland extending cycle times.

Finally, during the machining of titanium the tooltip can reach temperatures of 2,000 degreesFahrenheit or more. This tremendous heat oftencauses super-heated steam to form in water-basedcoolants, making the coolant vaporize before it eventouches the workpiece. This leads to all the problemslisted above, plus additional heating of the work-piece instead of cooling, making accuracy even morechallenging.

Makino has dealt with these common coolant

problems in titanium by implementing a high-flow,

high-pressure through-spindle coolant delivery sys-tem in the a81M horizontal machining center.

It pushes out 20 GPM of coolant at 1,000 PSI, cre-ating localized pressure to eliminate the potential forsuper-heated steam and improper chip evacuationwhile cooling the workpiece and chips.

This system eliminates potential coolant problemsby focusing the stream of coolant into the machiningarea at high pressures, allowing for faster feedratesand greatly extended tool life.

Beyond the coolant delivery system, the propermachine must also be utilized to achieve metal

removal rates like those in our test.

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Test Cut: Landing Gear BracketMakino has recently performed a test cut on a land-

ing gear bracket, using the a81M horizontal machin-ing center with a high-torque integral drive spindleand a two inch diameter by four inch long KennametalHarvi inserted carbide endmill in Ti 6-2-4-2.

The test cut included two operations, totaling twohours and 30 minutes. At any given time, two inchesor 36 inserts of the tool were in the cut.

Due to the cutter geometry this provided 18 effec-tive inserts engaged in the cut. Makino was able toachieve three hours of tool life on those 18 insertsprior to indexing, and there are two cutting edgesavailable per insert.

One set of carbide inserts on the tool costs aboutUS $270. That means about US $45 per hour of cut-

ters were used during our test cut.The result of this test was a titanium removal rate

of 20 cubic inches per minute (1,200 per hour), threehours of tool life, and a cost of only US $0.0375 percubic inch of metal removed (insert cost).

Makino has learned through extensive test-cutsthat the ingredients needed for results like theseinclude the use high-pressure coolant, a machine toolup to the task, and the proper tooling.

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Machine ToolOne of the primary ingredients to successfully

machining titanium is a stiff and rigid machine tooland workholder.

It is critical that the machine tool and fixture arecapable of securing the workpiece as it begins tovibrate, 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 beused in titanium machining.

Machines capable of efficiently machining titani-um must have some basic characteristics, includingstability, a high torque spindle, high-pressurethrough-spindle coolant and fast acceleration anddeceleration.

Makinos horizontal machining centers havethree-point leveling, as well as rigid column and bedcharacteristics so the machine can handle the forcerequired to generate high metal removal rates.

A slant-style bed and column design is one exam-ple of a machine characteristic designed to resist Z-and X-axis force while creating superior stiffness.This design also allows for high Y-axis machiningwithout deflection and 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-horse-power forces of the spindle and produce consistentpart quality with less effort.

In addition to using stiff and rigid machine toolsand fixtures, the second requirement for successful-ly machining titanium is high torque.

When cutting titanium, machine tools should beable to produce torque in the 1000Nm range, andspindle speeds up to 8,000 RPM.

The key is the ability to create high torque at lowRPMs, while still having the ability to go to higherRPMs for drilling operations and finishing. Bothspeed and torque are needed to reduce your cycletimes of machining titanium.

The Makino a81M is particularly suited for longreach and large diameter boring operations whichrequire a great deal of torque, particularly at lowRPM. For applications such as tapping, where a sig-nificant amount of spindle stopping, starting andreversal occurs, the high-torque spindle on the a81Mis significantly faster and has less idle time.

For more information, visit www.makino.com.s

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Many variables must be con-sidered in cutting tool evaluation,all of which have can have a pro-found effect on relative costs.

The diameter, coating, tooltype, SFM, number of teeth,RPM the tool is designed to runat and IPM of performance allcome into play.

What it boils down to is theamount of total cost to generatea cubic inch of metal removed. Itis important to not focus on thecost of the tool, but to focus onthe cost to remove a cubic inch

of metal.One example of such an

advance is variable rake toolinglike Kennametals Harvi (helical,axial rake, variable, indexable)indexable insert endmill, used inthe test cut mentioned earlier.

According to the manufacturer,Harvi end mills enable chatter-free machining due to a patentedflute form with unequal flutedivision. The variable rakedesign assists to produce a chipthat is easily removed when theproper coolant flow is applied.

Tool coatings are also importantsince the temperatures in the cut of titanium are higher than typicalsteels. The Harvi cutters insertshave a TiAlN coating, designedspecifically for working in hard-ened materials.

Tooling