The Heat Is On Difficult-to-Machine Metals

The Heat Is On Difficult-to-Machine Metals

Jan 13, 2015

By Jim Lorincz, Manufacturing Engineering

Successful machining depends on making the right engineered tooling choices

Cutting tool manufacturers call on a lot of tribal knowledge to help their customers find the most productive machining solutions for processing difficult-to-machine materials. Manufacturers in industries that are big users of titanium, stainless steels, Inconels, and other high-temperature heat-resistant superalloys (HRSA) are increasingly seeking machining solutions that extend tool life, improve productivity, deliver consistent part quality, and take advantage of the latest advances in machine tool technology—all at the same time. In machining these materials, heat, as well as the unique processing characteristics of the metals, is the enemy. Failure is not an option for workpieces that are expensive and production that is costly and time-consuming. In a real sense, cutting tool suppliers have become an invaluable engineering resource for their customers. Here’s how.

Advanced Tooling Solutions Trump Material Challenges

According to Michael Standridge, aerospace industry specialist at Sandvik Coromant (Fair Lawn, NJ), aerospace manufacturers are faced with critical issues affecting capacity, cycle time, cutter security, tool life, repeatability, longevity, and part quality. “There’s a lot of pressure on the supply chain to deliver quality parts on time to meet increasing monthly production rates for commercial aircraft. With the strong forecast and large volumes of aircraft to produce over the next 25 years we are seeing an increase in the quantity of all major component groups within the engine, frame structures, and landing gear,” said Standridge.

“At our Aerospace Application Center, we offer many solutions for our customers: from machining strategy and process development on a particular material or component feature, to cutting tool application and design, as well as full turnkey services that includes CAM programming. CAM programs are complete and verified. The real benefits of our center comes from having the ability to show our customers the latest techniques that demonstrate how to get the most productivity out of our cutting tools, plus it gives us the ability to develop solutions outside of their facility, which can then integrate into their facility,” said Standridge. 

“Titanium, particularly 6AL4V titanium, has been used in aircraft for a long time and the industry has a pretty good handle on machining it. What we’re looking for are ways to machine it faster with reliability and a cost-effective process. To do this we are investigating advanced manufacturing methods, for example, the use of cryogenic coolant to help increase tool life and cutting speed, as well as the use of laser technology softening the titanium material prior to cutting. The industry is always seeking new materials to aid the design of aircraft. Beta 21 titanium is something we are starting to see more and more of particularly in smaller components. Beta 21 is not difficult to cut, but challenging because it’s very sensitive to cutting forces and vibration. Another Beta titanium, 5553 titanium, which was a new material just a few years ago, has mostly been applied towards components that require high-strength with less weight. Typically we see it being used in components for landing gear and certain structural parts, but it doesn’t seem to be expanding or replacing titanium 6AL4V applications like we thought it might,” said Standridge.

 “The investment that we are seeing in machine tool technology, particularly in heavy-duty machines optimized for high-strength alloys has been tremendous. These hard metal machines feature high-horsepower, high-torque designs with high-pressure coolant systems built in with extreme flow rates up to 50 gpm [189 L/min]. High flow rates are very important to maximize the pressure exiting the tool. In addition, the machining industry is moving toward optimized spindle technology that is less prone to deflection. Our CAPTO C10 spindle connection, for example, is being offered on a machining center line of at least one builder today. The C10 features a robust design with a 100-mm flange that requires high pull force to actuate the clamping in the machine,” he said.

“The higher the pull force, the greater the rigidity and stiffness that the tool assembly has. This rigidity is extremely beneficial for aerospace applications that require long reach both on structural parts or for reaching down into tubular barrel-type components like engine casings.”

Optimizing Machining Parameters Is Always Critical

“When I think about difficult-to-machine materials, titanium comes to mind, so does Inconel, and so do stainless steels, which are also very difficult to machine. The ductility of the materials is much harder, the yield strength and tensile strength are closer in a way to each other, making it difficult to shear and break the chip,” said Scott Etling, director, global product management, indexable milling, Kennametal Inc. (Latrobe, PA).

Optimizing machining parameters is critical in machining titanium where heat and forces are the enemies of carbide. “You have to have the right PVD coating, a tough substrate, the right geometry, right edge prep, and high shear geometries, not big T lands on the edge of the insert. The heat doesn’t go into the chip like it does in steel. The heat has to go somewhere, so for most titanium applications you have to use an enormous amount of coolant,” said Etling.

Kennametal’s customers are now able to benefit from the advanced titanium cutting technology of Stellram, which has been fully integrated into the Kennametal family. “Stellram’s 7792VX high-feed milling cutter and X-grade carbide are specifically designed for machining nickel, cobalt, and titanium-based alloys. The insert angle in the cutter body is optimized so that the cutting forces are pushing into the spindle so there isn’t a lot of deflection when you’re machining these tough materials and the cutting edge is very sharp with an optimized geometry,” said Etling. “In addition, the rigidity of tool and spindle connection so important for vibration-free and deflection-free machining of these metals is provided by our second-generation KM4X spindle connection. The KM4X has a four-ball clamping mechanism and is capable of improving metal removal rate by increasing depth of cut and feed rate. We are working closely with machine tool builders to offer KM4X to the market,” said Etling.

Long Extension in Milling Creates Challenges Regardless of Material

“Long extension, or overhang, while milling creates many challenges, especially when combined with difficult to cut materials such as titanium or high temperature alloys. In virtually every industry, there are machining environments which require extensions that exceed normal reliable diameter to length ratios. In this arena, deflection compromises productivity and adversely affects tool life. Many of today’s lighter duty machines with smaller spindle adaptation suffer even more,” said Terry Carrington, national product manager-milling, Iscar Metals Inc. (Arlington, TX).

“To address this, lighter radial engagement combined with longer axial length of cut allows higher metal removal rates than heavy radial engagement with lighter depths of cut. Deflection can be further reduced when using cutting tools capable of segmenting, or splitting the chips into smaller pieces. One such example is Iscar’s P290 milling system. The P290 insert shreds material into fine chips similar to traditional HSS roughing cutters, dramatically reducing radial cutting forces. When applied with an extended flute milling cutter, the P290 system can efficiently cut 6″ [152 mm] or more axially in any material with up to .35 times diameter radial engagement. With ports supplying coolant at every insert location, this system is perfectly suited for long reach environments,” said Carrington.

“One of the most significant improvements for turning lately is the development of high-pressure coolant tools,” said Randy Hudgins, Iscar’s national product manager ISO turning and threading. “Our JetHP tooling line combined with 1000 psi [6.89 MPa] high-pressure coolant tooling gives better tool life and better chip control. Even at lower pressures, you can see improvements because of the pinpoint direction of the coolant. Our tooling delivers coolant from two different directions, one on the top of the insert which helps chip control and one from the bottom of the tool.” Iscar’s JetHP coolant is offered on its grooving, threading, and boring lines. 

Innovative Taps for Titanium, Nickel Alloys

“Tapping titanium alloys such as titanium 6AL4V is more difficult than tapping most alloyed materials, but certainly doable with appropriate taps and thread mills featuring innovative geometry and coatings to promote maximum machinability and chip evacuation,” said Mark Hatch, product director, Emuge Corp. (West Boylston, MA). “Nickel alloys, commonly used in aerospace, power-gen and oil field applications, exhibit high hardness and extreme elastic memory and are also difficult to thread. And stainless steels such as 304, 316, etc., can also present challenges due to their high ductility, low yield stress and relatively high ultimate tensile strength.” 

Emuge has just introduced a comprehensive line of high-performance tools for threading demanding alloyed titanium materials, ranging from taps with unique, new geometry designs to reliable solid carbide thread mills. “The new Ti threading line incorporates advanced high relief geometry technology to increase space between the friction surfaces for enhanced lubrication and reduced torque load in both forward and reverse direction. This new technology counteracts the high compressive forces produced by the extreme elastic memory of titanium,” said Hatch. 

New Emuge C-Ti Taps with advanced left-hand helical flute form and chamfer geometry combine to optimize chip evacuation in the forward direction and add strength to the cutting teeth for enhanced tool life and process security. And new Emuge Spiral Flute Bottoming Taps for threading nickel alloys have also been recently introduced. These taps are designed with a 10° Variable Helix Correction, a specially ground relief geometry in the primary cutting zone, which generates smaller, tightly rolled chips to prevent chip jamming in the tap teeth during machining. 

For stainless and alloy steel challenges, Emuge’s new Z-Taps are designed with an advanced chamfer geometry and rake/flute form and are coated with GLT-1, a newly developed multilayered coating that produces a consistent, controllable chip formation that is released smoothly for fast and efficient chip removal. GLT-1 is uniquely structured with advanced heat resistance characteristics, along with an outer anti-friction layer.

“For best results, high-performance tap holders should be utilized for providing a rigid setup. Ideally, we recommend the use of rigid/synchronous tapping cycles with Softsynchro collet-type tap holders. Our KSN/HD Softsynchro design features patented constructional separation of the transmission of torque and axial force which improves machining and thread quality while increasing tool life,” said Hatch.

Partner Knowledge Resolves Machining Conundrum

Ken Bellinger, CET (Component Engineered Tooling) manager, Seco Tools LLC (Troy, MI), said that knowledge from technology partners was instrumental in solving a production challenge for an older aerospace component from one of its customers. “The titanium part was challenging because the prints were 20 years old. We found a 3D scanning company, scanned the part, built models around the print based on the original specs, and found that the features they were making weren’t necessarily on the prints. The workpiece was a milled part with a shaft that had to be turned and threaded. Our solution eliminated the need for the lathe and a whole series of secondary processes, cut the cycle time in half, and eliminated a manual drilling operation. The net result was the component became manageable and they could make money on a production run of 500 per year.”

“The CET group acts as an engineering resource for our customers, drawing on the experience of 12 hubs around the world and meeting regularly to discuss common machining projects encountered in the automotive, power generation, medical device, and large equipment manufacturing industries. CET process engineers work with our knowledge partners including machine tool builders, workholding companies, gaging, and CAD/CAM systems to supply engineered tooling solutions. Our solutions can include inventory management, training programs, engineering resources, custom tooling and design resources as well as providing training,” said Bellinger.

Solutions Include Standard, Special Tools as Needed

“The challenge with machining titanium and Inconel is extending tool life and reducing cycle time as much as possible,” said Yourik Gharibians, aerospace specialist, Walter USA LLC (Waukesha, WI). “What I really see out there most of the time, people are not employing the tool in the right parameter, not using the right tool. You need to know the material and choose the best geometries to machine it. About 60% of the business in aerospace is in specials, but we have a lot of standard inserts that are used in combination with special cutters. High-temperature materials don’t like to be pushed. They aren’t like steel or cast iron, so the geometry of the insert or the end mill has to be specific for that material and tools have to be sharper,” said Gharibians.

“At first, it might be difficult to get customers to run more aggressively with our new tools. With our geometries, we’re taking the heat off the part and tool and sending it out with the chips, so the part stays cold and the chips hot. In most cases in high-feed applications, they were using old-style cobalt end mills running at 3 ipm [76 mm/min]. When I give them my cutters and tell them they can run at 36 ipm [914 mm/min], they’re reluctant at first, until they get their first good machining results,” said Gharibians.

Walter’s Proto-max Inox solid carbide tool is specifically designed to address work hardening of titanium and other high-temperature metals. It features a sharp cutting edge and a tough substrate with a thin coating that is appropriate for the titanium. It has a lot of lubrication qualities which helps the material flow, and it can handle the heat that’s generated. The inserts have the same qualities as the Proto-max Inox, tough substrate, with a very sharp cutting edge and thin aluminum oxide coating applied with the PVD process. Applying the aluminum oxide with PVD process keeps the processing temperature lower which helps retain the integrity of the carbide and still retains the heat-resistant property and wear resistance of the aluminum oxide coating.

Which Non-Chipmaking Cutting Technology to Use?

Here’s the short answer from Hypertherm (New Brighton, MN). Certain processes will only cut certain types of materials. For example, oxyfuel can only cut carbon steel and plasma can only cut electrically conductive metals. This means if you need to cut more than just metal, you can immediately cross those two processes off your list and focus instead on laser or waterjet. Lasers can cut metal in addition to nonconductive material such as wood and plastic, while waterjet can cut just about anything. This includes metal, stone, plastics, rubber, foam, and food. Certain processes excel at different thicknesses so it is important to know which thickness of material you need to cut. Generally, laser is used to cut thinner materials, plasma mid-range to thicker materials, and oxyfuel very thick carbon steel. Waterjet can cut across the thickness range. The quality of a cut (or lack thereof) is based on the following properties: angularity, kerf or width, tolerance, size of heat affected zone (HAZ), dross, and edge quality.

The International Organization for Standardization (ISO) has developed a classification system for thermal cutting processes, such as oxyfuel, plasma, and laser. That standard—called ISO 9013—takes into account the above properties. Generally, waterjet produces the very best quality, followed by laser, plasma, and finally oxyfuel. Of note is that even the best process will provide poor cut quality if used on a CNC table without good motion control capabilities. It’s kind of like putting a high-performance engine in a budget car. When people think of productivity, they most often think of cut speed. While cut speed is important, you also need to consider the time it will take to get your system up and running (this includes preheating of the metal when discussing oxyfuel); the number of cutting heads the machine can accept; the efficiency of the nesting software and its ability to maximize cutting time; the ability to unload parts while the system is cutting; and finally the ability to eliminate secondary operations. The productivity of your cutting method will depend on material type and thickness. For example, laser is considered a highly productive process when cutting thinner material, but not as productive on thicker material. 

For more information, please go to http://tinyurl.com/smeforming.

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