Consider all elements and adhere to principles of good machining
Demand for machining titanium for aerospace applications won’t abate any time soon. It is driving OEMs and the supply chain in the commercial airplane market to find ways to dramatically increase machining output. Whatever date you pick from now until 2030, there’s a sufficient backlog of commercial airliners for both structural and jet engine applications to keep spindles humming around the clock cutting titanium. Titanium and its alloys have all the attractive engineering properties that go along with high strength-to-weight ratio, but with the challenge that hardness and high heat resistance bring to machining. Chips don’t efficiently carry heat away, and workpieces and cutting tools must be protected from its quality robbing and possibly catastrophic consequences. The answers can be found in adherence to the principles of good machining practice, which apply in all machining environments certainly, but perhaps most critically in machining titanium. In the long run, adopters of new advanced machine technology who can improve output will be the winners.
Materials-Centered Manufacturing Takes Center Stage
“There has been a decided shift to materials-centered manufacturing,” said consultant Bert Erdel, PhD. “Titanium, undeniably, is the material of choice for aircraft structures and engine parts—be it as a single alloy or a stacked material (Ti-Composites) for advanced fan and skin designs or the alternative material for the engine hot zone as an Intermetallic (Ti-Al) or the alternative material for advanced landing gears. Of the many different titanium alloys, 6 Al-4 V is still the most common and its machining characteristics are pretty well known,” said Erdel.
Because of its hardness and high heat resistance, every element of the machining process has to be optimized, including cutting tool, cutting tool materials, coolant, machining data, machine and machine spindle, toolpath, toolholding, and part fixturing. “It is the holistic approach that yields Best Process,” said Erdel. “Because titanium is expensive, it’s what you do in the machining process that really counts. Double your feed rate, for example, and you can get more parts out the door and thus can compensate for the higher acquisition cost of the material itself,” said Erdel who points to the fact that Boeing owns the material and under its Revert program reclaims and recycles titanium chips from its suppliers so as to assure a ready (secondary) supply stream.
Erdel said development of new tools can make a big difference. He cites the robust machining of titanium with a Z Carb HPR solid-carbide end mill with M coating from SGS Tool Co. (Munroe Falls, OH) as an example. “The Z Carb HPR gives 40–45% better tool life than any other tool in the market by virtue of its patent pending, innovative design and coating. This true high-performance tool dramatically reduces the radial load on the machine spindle, shields the tool from premature failure due to temperature build-up and secures a smooth chip flow away from the cutting area during machining. Featuring special five-flute design, variable indexing geometry and the Ti-Namite M-coating plus coolant exit from the middle of the tool, the Z Carb HPR-series allow for extreme high metal removal rates and excellent surface finishes,” said Erdel.
Big Target to Shoot For: Twice the Production in Same Space
According to Scott Walker, president, Mitsui Seiki USA (Franklin Lakes, NJ), Boeing has set a goal of doubling production in the same amount of floorspace, whether in assembly operations, or machining titanium. Translated into machining titanium terms that means that what they would like to see is a machine tool capable of getting 2000 ft-lb (2711 N•m) of torque at 500 rpm compared with today’s 2000 ft-lb at 100 rpm on a geared spindle machine.
“That machine in a horizontal trunnion platform that best suits FMS strategies doesn’t exist at this time, but I think that what’s going to happen is that machine tools are going to evolve with totally new designs to provide torque within the 300–500 rpm range at high-torque values like 2000 ft-lb or more,” said Walker. “Cutting edges are getting better, but new tools will have to be developed. Coolant volumes that are running as high as 50 gpm [189 L/min] through the spindle at 1500 psi [10 MPa] will be improved. New machine designs must consider the volume of chips that have to be evacuated as they will be increased by a factor of two or three.”
Mitsui Seiki’s current designs popular in the aerospace industry are its HU 100 series four and five-axis horizontal machining centers with three-speed gear box that develop 2000 ft-lb at 100 rpm. “Market demand for our products is in the 630-mm to 2.5-m sizes for structural components and jet engine parts,” said Walker. “In titanium blade machining what they are looking for is to be able to run at 90 ipm [2.2 m/min] and hold blade to blade profiles 0.001″ [0.03-mm] tolerance over the blade surfaces, blade to blade, and leading edge to trailing edge. They are pushing 120 ipm [3.0 m/min] and would like to get to 160–180 ipm [4–4.5 m/min]. Cutting tools are getting better, but titanium is tough on end mills and aerospace-type parts. Today, we’re getting 60–90 minutes per cutting edge which is pretty much standard in cutting 6 Al- 4V. Ten years ago it was 10 minutes,” Walker said.
“Newer developments affect literally every aspect of the machining process,” said Walker. “They include determining the right spindle connection [Kennametal’s KM4X], adopting the latest cutting toolpath strategy [trochoidal and constant chip volume approaches], and CNC controls with tool point center control with smoothing algorithms [Siemens 840 D and Fanuc i31 CNC].”
Testing Leads to Robust Titanium Machine Design
According to Mark Larson, manager of process R&D, Makino Inc. (Mason, OH) testing inserted tools for roughing, measuring cutting forces on the machine and tool wear, looking for ways to improve lubricity through coolants, and using the latest toolpaths for milling are essential to optimize the titanium machining process. The Titanium Process Development group at Makino, has focused on testing machines designed specifically to address titanium machining challenges including high hardness, high strength, heat resistance, and harmonic issues.“On our T series machines we can maintain 787 ft-lb [1067 N•m] of torque all the way up to 980 rpm. That’s a lot different from previous machines. Most of them especially the gear driven spindles start to fall off around 230 rpm, depending on the size of the cutter and how fast you’re running and what your tool life is. That 230 rpm might be okay in some cases, but if you want to be more productive you want to be able to run at a higher speed and remove metal faster. We like having that high torque at higher rpm,” said Larson.
“Our T series machines routinely use 1000 psi [6.89 MPa] with 52 gpm [196 L/min] through the spindle. Let’s say we have a 3″ [76-mm] roughing tool with as many as 30 inserts on that tool. We want coolant going to each insert. There are a lot of orifices to feed coolant through and we want to make sure we have enough flow so we don’t lose pressure for each of the inserts. We want a very good flow rate and high pressure to prevent chips from being recut by efficiently moving them out of the way,” said Larson.
“We see many more customers interested in five-axis machining because even if they don’t need five-axis simultaneous motion, it reduces the number of setups and allows them to keep the part fixtured once, for more accuracy. There’s a definite trend for shrink-fit holders to use anti pullout types of adapters like the Haimer Safe-Lock to prevent solid tools from being pulled out,” said Larson. “Right now for the tool/spindle interface we prefer the HSK 125, which is a 5″ [127-mm] connection and we’ve been working with Kennametal to evaluate their KM4X 125 connection’s performance.”
Faster Metal Removal Lifts Capacity Constraints
Michael Standridge, aerospace industry specialist, Sandvik Coromant (Fair Lawn, NJ) said, “our customers are asking us to find new ways to increase productivity, especially in milling structural components without sacrificing process security. As a result we’re targeting applications with heavy roughing to exceed 20 in.3 [327.7 cm3] of metal removal. High-pressure coolant systems that enhance flow rate at the cutting edge are extremely important in machining titanium. With all the heat and the need for lubricity, coatings must withstand high temperatures to protect the insert from breaking down prematurely. Better adhesion to the substrate and less variation in our coating structure is limiting chipping and crack propagation using a mix of CVD and PVD coatings.”
“We offer our customers multiple solutions from machining strategy and process development for a particular material or component feature, to cutting tool applications and design, as well as full turnkey services that include complete and verified CAM programming. At our facility we can demonstrate the latest techniques that show how to get the most productivity out of cutting tools and processes, developing solutions outside of a customers’ facility that can then be seamlessly introduced into their production,” said Standridge.
Many factors affect tool performance, robust machine and spindle interface being two of them. The further away the cutting tool is from the spindle and machine, the greater the potential for issues like vibration, radial run out and deflection. Many aerospace component designs require long tool assemblies. Typically, these assemblies are lacking rigidity and can potentially reduce productivity. “This all too common scenario is where aerospace manufacturers can take advantage of Capto. Originally developed by Sandvik Coromant and now an ISO standard for the industry, Capto combines rigidity and modularity and is the only system designed to work effectively in all machining phases (turning, milling, drilling/boring). Its polygon-shaped coupling with face and taper contact provides the most deflection resistance and highest torque transmission up to 100-mm flange diameter. Deflection resistance and torque transmission are two key factors that need to be robust to increase productivity in machining hard metals,” said Standridge.
Coolant Concentration Makes A Big Cutting Difference
The P290 indexable milling cutter from Iscar Metals Inc. (Arlington, TX) features wavy serrations on the cutting edge to shred chips into small pieces and is designed for machining deep shouldering applications where choices were limited in the past. “The P290 gives the customer an option of using an indexable milling product with replaceable inserts,” said Terry Carrington, national milling product manager. “The serrations on the cutting edges of the P290 inserts are designed to provide an overlapping effect between all adjacent pockets, resulting in improved wall finish and reducing the work hardening in materials like stainless steels, titanium, and high-temperature alloys. Cutters are available in diameter 1.25″ up to 4.0″ [31.75–101.6 mm] with extended flute lengths up to 4.0″, said Carrington.
“High-pressure coolant in this environment is always important, but equally important is the coolant concentration. We have achieved the best results while machining titanium with concentration levels of 12–14%. Titanium is an insulator of heat as opposed to a conductor of heat, thus, the majority of the heat remains in the cutting zone—as opposed to being exhausted with the chip. Ground cutting edges typically perform the best.”
“There are certain principles that should be applied to every machining environment. You may or may not notice any difference if you don’t apply them when machining aluminum. Failure to apply these principles while machining alloy steel may negatively affect tool life as well as surface finish and dimensional control. Failure to apply these principles while machining titanium or a high-temperature ally could result in catastrophic failure, dramatically increasing production costs,” said Carrington.
Iscar’s FTP high-feed milling cutter with a 12.5° approach angle continues the trend toward improving metal removal rates by transferring more of the machining forces axially as opposed to radially. This allows for higher metal removal rates on lighter duty machines as well as long overhangs. “Axial chip thinning allows us to increase programmed chip loads from a more conventional .005-.008 inch per tooth to rates as high as 0.040–0.060 inch per tooth. Due to the 12.5° approach angle, the actual chip thickness is approximately 20% of the programmed load. It is important to create enough thickness in the chip to carry heat away from the cutting zone, providing better metal removal rates and extending tool life.”
New Family of Titanium Cutters Set to Debut
“Titanium machining is one of our main focuses right now,” said Kevin Maples, aerospace solution engineer, Walter USA LLC (Waukesha, WI). “The titanium we are generally working with are 555-3, 10-2-3, 6 Al-4V, and Beta 21s, the latter of which is a fairly recent material development. Each has their own characteristics with 6 Al-4V accounting for about 70% of the aerospace market. To give a basis of machinability, if the 6 Al-4V is factor of 100, Triple 5-3 is around 55-60, 10-2-3 is around 60-65, and Beta 21s is 50-55.”
Walter has been engaged in an ongoing program to develop new technologies in carbide insert cutting geometries and coatings as well as a new configuration of tool bodies. Targets for these inserts and tools are structural machining applications like spars, pylons, landing gear, flat tracks, and other internal structure parts, principally in titanium. “In testing, we have achieved more than an hour and a half of intensive cutting in titanium with remarkably low insert cutting edge wear. The industry prefers less than 0.015″ [0.38-mm] wear. In our test environment we were down between 0.004 and 0.006″ [0.102, 0.152 mm],” said Maples. “Walter’s goal was to achieve 20 in.3 [328 cm3] or more material removed per minute for a minimum of 60 minutes. They have been able to achieve over 24 in.3 [393 cm3] of metal removal per minute up to 90 minutes. Higher material removal rates were achieved, but the insert edge life drops off.”
The new family of standard products will feature tangentially mounted inserts, with four cutting edges on the periphery, two specified radii on the front stations. The periphery inserts are designed to be interlocked, providing a more rigid tool and permitting additional tool body cutting rows. Anticipated product offering will range from 1.75″ (44.4 mm) to 3–4″ (76.2–101.6 mm). The new product grouping which is already in testing with customers around the world will be introduced around the end of the first quarter.
Walter has developed a new process for its coatings aimed at removing stress factors out of the coating to eliminate chipping or flaking. To remove heat from the insert and part more effectively, Walter’s process blasts the top surface of the insert to remove high spots so that the transition from the edge to the chip enables heat to go into the chip so that thermal cracking, notching, and chipping are eliminated.
Tapping, Threading Products for Titanium
According to Marlon Blandon, thread mills product manager, Emuge Corp. (West Boylston, MA) offers a complete solution for today’s titanium challenges in the aerospace industry, ranging from drills, taps, and thread mills to end mills such as TiNox-Cut, designed specifically for machining titanium.
Emuge has developed a specific tap with an advanced High Relief Geometry (HRG) ideal for aerospace applications. This geometry with a 15° flute angle, increases space between the contact surfaces of the tool and the part, for enhanced lubrication and reduced torque load in both forward and reverse direction. Emuge has added an NT2 (nitride and steam treatment) surface finish and now also a TiCN Multi-layer PVD coating for edge hardening and surface lubricity on these tools. Emuge Tap selection also includes specific tools for STI threads, which are widely used in jet engine components. The Ti taps are offered for either through or blind holes. When combined with an Emuge Softsynchro Holder, which is a collet-type holder system with minimum length compensation on tension and compression, Emuge is able to substantially increase tool life and thread quality in these aerospace applications.
An Emuge thread mill option is a solid carbide cutter with a PVD coating, either TiCN or TiALN. “We recommend it for applications with limited bottom clearances that leave no room for chamfered tools such as taps, and as a reliable threading process for hardened titanium alloy parts that require high precision and accuracy. With thread milling, users are able to easily control pitch diameters, and the feed rate can be adjusted on the CNC programming instruction to optimize the cutting data,” said Blandon. With a small CNC program modification, the same thread milling cutter can also be used to produce right-hand or left-hand threads, through or blind holes and STI threads. Due to this and the interrupted cut process for milling threads, these tools are a great solution for achieving excellent thread finish and gaging. The tooling range comes in a variety of styles, full milling sections, single plane cutters or indexable inserts.
This article was first published in the January 2016 edition of Manufacturing Engineering magazine.