Cutting Tools: Innovations Take the Heat
pose special challenges
By Jim Lorincz
Retooling for rough turning a forged Inconel hub for nuclear turbomachinery is credited with debunking a common myth about Inconel turning, according to Ramon Avery, product manager for turning tools, Ingersoll Cutting Tools (Rockford, IL). “When faced with insert rupture running Inconel, the knee-jerk reaction is to lighten the cut and take more passes to take load off the insert. This can make matters worse. Rupture often begins with an edge going dull, which in turn overloads the insert to the point of breaking down. It can happen in a few seconds, so it’s easy to miss wear as the root cause. Assuming that adequate machine horsepower is available, the better answer is to go to a stronger insert with a broader entry lead angle, and make fewer, deeper passes.”
In this case, a switch from square, flat-topped, zero-lead inserts to Ingersoll Hex-Turn inserts with a 45° lead angle and aggressive chipbreakers on the break face is credited with substantial savings in cycle time and tooling costs per piece. As forged, the hub measures 98" (2.5-m) diam × 7" (177.8-mm) long, and has a step down to a 93" (2.36-m) diam on one end. Previously, machining took 105 hr and 40 passes, required different inserts for turning and facing, and ate through 69 inserts per part. After optimizing the operation, the insert lasted the entire workpiece, despite more than doubling the cutting rate and increasing the cutting speed by 60%.
Difficult-to-machine materials like Inconel, in the example above, are driving cutting tool manufacturers to develop innovative solutions to some of the most challenging machining problems. Working in concert with their customers and with the most advanced machine tools, cutting-tool manufacturers are developing strategies to meet demanding applications in aerospace, medical, oil and gas, and, increasingly, in the automotive industries.
High temperature demands heat resistance, which is a major strength of materials like titanium, Ti-alloys, Inconel, and nickel-based and cobalt-based superalloys. For example, the heat resistance of Inconel 718 delivers important performance characteristics in the hot zone of turbine engines. This same heat-resistant characteristic can be detrimental during the machining process. If high heat is not effectively controlled in the cutting zone, it destroys tools, limits tool life, and reduces productivity. Strategies for removing heat through thermal management, and chip control through innovative insert design, figure prominently in solutions for machining these metals, and are additionally applied in the drilling and trimming of highly abrasive composite CFRP materials.
“The two classes of materials that are going to grow the fastest are titanium and CFRP,” says Francois Gau, Kennametal Inc. (Latrobe, PA). “Machining titanium [most recently the titanium alloy 5553] is easily 10× more difficult than machining aluminium. It requires a huge adjustment in learning how to machine the material, and the machine capacity to handle those materials, for example, rigid machines capable of high torque at low speed,” Gau says.
“Early in the process we decided that the key was thermal management, cooling the edge in the cut. We have introduced Beyond Blast technology for milling and turning that uses low-pressure coolant which is channelled through the interior of the insert to cool and enhance lubricity at the cutting edge,” says Gau. “ By channelling coolant through the insert at the interface of the milling cutter and insert, Beyond Blast delivers coolant directly where the tool cuts the material, ensuring more efficient coolant delivery, heat transfer, and lubricity. Tool life can be improved as much as 300%, depending on the application.”
“What happens with Beyond Blast technology is that the coolant is delivered to the hot side of the chip rather than flooding on the cold side,” explains Tilo Krieg, Kennametal’s director of global holemaking and solid end-milling products. “The coolant is delivered to the cutting zone where temperatures are hottest. With low-pressure coolant, you can run these new tools, even on existing legacy machines, at high feeds and speeds for higher metal removal rate, achieving longer tool life when machining Inconel and titanium,” says Krieg.
“When it comes to holemaking in titanium and nickel-based alloys, we introduced the Y-Tech drill, which features an asymmetric design to drill holes that are noted for roundness, straightness, and accuracy,” says Krieg. Available in sizes from 2.4 to 20-mm diam with specials to 25 mm, Y-Tech achieves its performance through a counterintuitive approach to drill design.
For drilling composites, Kennametal’s SPF drills feature a smooth CVD multilayer diamond coating that resists wear to provide longer tool life, reportedly reducing per hole costs by 68% in CFRP applications. The SPF drills are available in 3× D and 5× D lengths, and manufactured in diameters common to aerospace applications. “In the assembly stage, where a cowling must be joined with an engine, a composite skin to a wing, or a panel affixed to a stringer, drilling holes becomes a more complex challenge,” says Gau. “Now holes must be drilled through layered materials, CFRP on CFRP and CFRP on titanium, where the CFRP is like drilling through sandpaper, and drilling too fast in the titanium will create heat that will melt the CFRP and cause diamond to vaporize in the titanium. We have developed modular drills using KenTip KSEM replaceable tip drills with CBN-coated carbide for accuracy in finishing and speed without sacrificing tool life. Still another solution is using orbital drilling that depends on milling interpolation of the hole,” says Gau.
Developments in insert grades and product offerings for turning titanium and Inconel offered by Iscar Metals Inc. (Arlington, TX) recognize the challenge of removing heat from titanium, which is a poor conductor of heat, and chip control when turning Inconel to keep excessive heat from entering the tool. Matthew Schmitz, national product manager for GRIP Products, explains that many aerospace and other components often complicate turning applications. The reason: workpiece shapes are complex with thin walls and features that hinder chip evacuation.
“The solution for many of these components is found in unique, rigid tool designs using sharp inserts, resulting in better tool life,” says Schmitz, who points to a new carbide grade for machining Inconels. Iscar’s latest IC806 grade is a micro-grain, which means the grain size of the carbide is smaller than a micron. “Some of the newer carbide grades consist of a granular structure that is nearing one-half micron, producing even sharper tools and more consistency throughout the carbide,” he says.
“Chipbreaker design is important as is our groove and turn strategy. We use a grooving tool to side-turn, which changes the flow of chips across the tool itself while promoting chip control. Tang-Grip for parting and Sumo-Grip for groove-turn allow for better chip evacuation free from damaging chip wash, because the products’ design doesn’t have an upper jaw, and they feature tangential self-clamping in a rigid, secure pocket,” Schmitz explains. Another solution is Iscar’s Jet-Cut parting insert, which has a hole through the insert and delivers the coolant underneath the chip and directly to the cutting edge.
For milling titanium, Iscar’s strategy is to rough with its FTP FastFeed mill and then finish with other tools. “The FTP FF operates at lower DOC and higher feed with a small lead angle for chip thinning,” Hassan Narasimhan, national product manager milling products,
explains. “When the tool approaches the part, most of the force is directed at the spindle axially rather than radially, resulting in cubes [cubic inches per min] between 6 and 10 and the ability to be used on machines with less power and rigid setups than required for roughing with traditional end mills.” This is because the radial force (spindle bending force) with feed mill is only 15% compared to a traditional end mill.
At IMTS 2010, Seco Tools Inc. (Troy, MI) launched a microsite (www.secotools.com/us) and invited visitors to jump on board the virtual Seco Jet by clicking on industry applications. Once there, scrolling over various parts of the aircraft reveals the main components at the locale like landing gear, engine, or structural areas and tooling solutions available. The site also contains technical data on components and materials, according to Seco’s Don Graham. “Our intent with the data on the new web site is to help manufacturers choose productive tools as well as appropriate speeds and feed rates.”
“Reliability and predictability are high on our customer’s list of important considerations in machining.”
To meet the fast-moving material developments, Seco has introduced the new CBN 170 grade for finish turning nickel-based superalloys commonly found in aerospace, power generation, as well as automotive components. Designed for both toughness and wear resistance, CBN 170 works well in materials like Inconel 718, Udimet, Waspaloy, MAR M, and others, reportedly increasing tool life by 40% and cutting speed by 45% compared to whisker ceramics. For machining composite materials, Seco’s new Jabro family of solid carbide end mills includes five different geometries with cylindrical shank diameters ranging from 3 to 20 mm. “Many of the routers feature a DURA high-performance CVD diamond coating, which provides abrasion resistance to effectively manufacture composite materials,” says Graham.
“Reliability and predictability are high on our customer’s list of important considerations in machining,” says William Radtke, applications specialist, Walter USA (Waukesha, WI). “We’ve run across moditifed materials we haven't machined before. One such material was a modified stainless that would almost fall into the category of a high-temperature alloy.” Walter has introduced new PVD-coated inserts and grades, as well as a new uncoated grade for turning titanium. “In turning we’re constantly developing new chipbreakers for high-temperature alloys for chip evacuation.” For drilling, Walter is working with some applications drilling long, large holes in Inconel. “We’ve developed some new PVD coatings for drills and chipbreakers that improve tool life for these large-diam deep holes, and avoid blowing the drill out once you get deep into the Inconel.”
Sumitomo Electric Carbide Inc. (Mount Prospect, IL) has two newly developed insert grades for turning high-temperature alloys for aerospace, medical, and oil-industry components. The grades, AC510U and AC520U, specifically target Inconel 718, reportedly doubling tool life. “The TiAlNCrN coating was improved by adding a small amount of chromium, which hardens the coating layer by up to 40%, and increases cutting temperature capability up to 200° C,” explain Paul Ratzki and Rich Maton of Sumitomo. “Running speeds increased from 150–180 fpm to 180–225 fpm [45–55 m/min to 55–69 m/min], and finishing speeds from 200–225 fpm to 225–250 fpm [61–69 m/min to 69–76 m/min] for finishing Inconel 718,” Messrs. Ratzki and Maton report. “We prefer positive geometries for medical-component machining and sharpness, because of the cobalt-chrome materials used for bone replacement implants that are typically odd-shaped and small diam. Cobalt-chrome is abrasive, and you need something that is wear-resistant, yet tough enough to withstand shock.”
Sumitomo has introduced a new CBN grade, BN7500, for finishing powdered metals (P/M), which are being increasingly used in aerospace, oil and gas, and automotive industries. The BN7500 has a tougher substrate with a sharper edge to prevent generation of burrs on the machined edge of the component. A new grade, DA1000, is being used to machine carbon fiber and aluminum applications in aerospace and automotive.
Sean Holt, Aerospace applications manager at Sandvik Coromant’s Aerospace Application Center (Fair Lawn, NJ), believes that programming is underestimated as a way to improve results in processing difficult-to-machine materials. “Today, it’s very important that companies realize what the programmer does for them. He selects the cutting tools, the cutting data, the programming techniques, and ultimately determines the company’s profitability.”
Sandvik Coromant has held programming classes in which theory is discussed and programming strategies are demonstrated in machining titanium and Inconel. “One of the techniques we talk about is roll-in cut. When you are coming directly into contact with the part, whether milling or turning, you are always going to have a problem with notch wear in these difficult-to-machine materials. By changing the program by a simple technique called roll-in, a smoother toolpath is developed and tool life increases of 75–100% can result,” says Holt.
The seminars stress optimizing current technology and tools, but focus on techniques with an emphasis on high-pressure coolant for titanium and nickel-based alloys. “We always recommend high-pressure coolant whether it’s for milling or turning,” Holt says. Sandvik Coromant has developed a family of high-pressure coolant nozzles ranging from 1.4 to 0.6-mm diam to direct coolant to the depth of cut. “Selecting the right nozzle size is extremely important for chip breaking depending on the material, titanium or Inconel, being cut,” says Holt.
Sandvik Coromant has developed a method for machining pockets in solid by programming a technique that it calls spiral morphing. Predrilling a hole in the metal, the cutter spirals out rather than machining by turning 90° and running in straight lines. In spiral morphing, the cutter maintains the same width of cut and engagement as it walks out to the pocket dimension, eliminating vibration or recutting chips.
Frequently, the final process on a component is threading, and is carried out when the workpiece already has high-value content. This means that threading is one of the most critical processes. Failure at this point means expensive rework, which is costly, and possibly scrapping, which is totally unacceptable. Emuge Corp. (West Boylston, MA) offers a complete range of taps with the coatings and geometries required for difficult-to-machine materials. Increasingly, thread milling is becoming an important process, especially in aerospace parts, where full threads must be machined all the way to the bottom of the hole.
“We have taps designed specifically for difficult-to-machine materials for both through and blind-hole applications,” says Alan Shepherd, technical director. “Today we can easily tap 100– 150 holes in Inconel 718 using the latest advanced CNC machines with rigid tapping. When tapping heat-resistant materials, it’s important to use heavy-duty tapping fluids and high pressure to remove heat, so that taps don’t bind in the hole. Titaniums offer another challenge. They aren’t all created equal, and taps should be selected accordingly."
Thread milling is growing in importance in superalloys commonly used in the aerospace, oil and gas, medical, and defense industries, especially for high-value-added workpieces, according to Mark Hatch, manager, thread milling, for Emuge. “Tap breakage is a problem, and these materials require lubricity. Using tapping oil when tapping can be a negative with modern CNC machine operations, due to contamination of machine fluids.” Emuge thread mills are carbide tools available with different geometries in sizes up to 1" (25.4 mm). Above 1" diam, insertable carbide-tipped thread milling bodies are used. ME
This article was first published in the January 2011 edition of Manufacturing Engineering magazine. Click here for PDF.