Strategies require thinking outside of the conventional toolbox
By Jim Lorincz
Which came first the machine tool to cut the most advanced, difficult-to-machine materials, or the cutters, toolholders, fixturing, and chip evacuation coolant systems required to apply the power and precision required in chipmaking? The answer is as elusive, it seems, as that to the fabled query about the chicken and the egg. And, increasingly, machine tool builders, their customers, and their third-party suppliers of everything from tooling to software to coolant systems aren’t operating in a vacuum. They are collaborating to find solutions and strategies that can address applications in aerospace, medical, oil and gas, and, yes, in a resurgent global automotive industry. And what they are working with are innovations in substrates, geometries, and even concepts that are challenging long-accepted practices about how best to remove material.
The evolution of innovative cutting tool solutions has been driven by attention to industry applications that are truly game changers. In aerospace and land-based turbine applications, high temperatures demand heat resistance, which is the 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. These same heat-resistant characteristics can be detrimental during the machining process. High heat in the cutting zone, if not properly controlled, destroys tools, limits tool life, and reduces productivity.
Roughing Hardened 5553 Ti is Tougher
Strategies for more efficient chip formation along with innovative processing figure prominently in solutions for machining these difficult metals. What happens when the material being rough-machined is already problematic, like 10-2-3 or 5553 titanium, and then it’s hardened to Rc 50 ahead of roughing? Don Yordy, aerospace product manager, Ingersoll Cutting Tools (Rockford, IL) describes the challenge that this material represents. "It’s not uncommon to rough-macine parts that are pre-hardened. Tool steels and precipitating stainless steels are often pre-hardened at above Rc 40 before rough-machining, but we’ve only recently begun testing in hardened Ti. At this early stage, the tests are limited to functionality and edge-life tests applying our latest products for profiling, pocketing, contouring, face milling, and drilling this material."
Prior to testing in their Tech Center, Ingersoll felt optimistic based on a field test where their Evo-Tec shell mill was being used to profile hardened Ti. Yordy explains: "We were repeatedly getting two hours edge-life pulling over 5.0 cim with a 5" [127-mm] diameter shell mill and edge-wear as the failure-mode." But, once Ingersoll began general testing in the tech center with other cutter types and programming methods they began to appreciate the difficulty that roughing the harder Ti would pose, especially in deeper features. Yordy explains: "Much higher lateral forces were being generated during chip formation and this was resulting in edge chipping especially during entry and exit of the material. Plunge milling is typically a good method for combating this effect in annealed Ti but in the hardened Ti we anticipated less impact to the cutting edge using a high-feed mill."
The Need for Predictable Failure Modes
"With a high-feed mill we’re able to get the loads in compression as we enter the material. Most of the lateral loads that created vibration at the cutting edges and moments through the tooling are redirected axially and the effect allows the cutter to enter and exit the material gently." This method, says Yordy, eliminated the random edge chipping and fracture that we were recording when a plunge cutter and shell mill were tested in deeper features. "Especially when exiting the material in the deeper features with the shell mill, the relatively large decrease in torsion and compression would take the cutter out-of-load, bite the material and fracture the cutting edge," Yordy says.
"It’s a little more complicated when you uniquely design a chipbreaking function into a tap, but we’ve done it."
Ingersoll has several high-feed mills developed primarily for the demands of the die/mold industry. "It’s an industry where extended tooling is more common than not," says Yordy. "We tested two designs in the hardened Ti and had eliminated the random edge breakage, but we were not satisfied with the rate of edge wear. Still, we had changed the failure mode to repeatable and predictable, which allowed us to begin and since complete edge-life testing."
Currently, the high-feed mill performing best in the hardened Ti is a new design that can mount any of four insert styles into cutter bodies that are highly positive in both the axial and radial direction. Says Yordy, "We had anticipated that, given the aggressive rake in the cutter body, all of the insert styles would form a chip very efficiently, but there was uncertainty about how durable such an efficient design would be in a hardened material. Durability has not been a problem and we’ve recorded our best results using a flat-top version of the insert. Maximum DOC with this insert style is 0.140" [3.56 mm]. This series of high-feed mills is being introduced at IMTS as Hi-Quad F."
According to Kennametal Inc., the trend toward lightweight construction materials will continue to grow and not just in the aerospace environment. As far as lightweight construction is concerned, along with titanium, aluminum, and magnesium, carbon-fiber reinforced plastics (CFRP) and their composite materials in particular will dominate the field—for the moment that is. Thilo Mueller, Kennametal manager for solid carbide end milling in Fürth, Germany, sees the automotive industry as "the next technological driving force for fiber-reinforced plastics.
High-Volume Automotive is Different
Dealing with machining CFRP for high-volume automotive production would pose significant challenges. Anticipating this and to facilitate communication with the auto industry, Kennametal has classified CFRP materials into five categories ranging from pure carbon-fiber matrix to stacks of aluminum, CFRP, and titanium. They include CFRP in combination with nonferrous metals, high-temperature heat-resistant alloys, stainless, and nonferrous metals and high-temp alloys.
The problem of delamination and tool life are recognized as particularly difficult in drilling CFRP materials. Kennametal has developed diamond-coated carbide tools to machine CFRP and PCD tools to machine stacks, i.e. composite metal and CFRP materials. Kennametal’s diamond-tipped SPF drills are recommended for drilling pure CFRP materials in terms of service life and delamination where hundreds of holes must be drilled.
PCD tools, which can provide service lives that are up to 30–50% longer, have certain restrictions as to tool diameter and geometry. PCD tools with diamond crystals that grow directly on the carbide supporting shaft allow ground contoured cutting edges rather than brazed-tip cutting edges with positive front rake angles. Another recent development involves CNC-controlled orbital drilling. Here, in a similar way to circular milling, the drilling is done with a milling tool. The tool axis is slightly tilted along the Z axis to enable free cutting. This development work is being carried out in close cooperation with the Swedish machine manufacturer Novator (Stockholm, Sweden). Contrary to normal manual drill feed units, which are commonplace in the aircraft industry, the Novator machines feature a CNC-controlled machining unit, with a measurement procedure that can be used to compensate for tool wear by modifying the circular movement.
As well as drilling systems, Kennametal is also working on milling systems for CFRP with compression-style routers. Their V-shaped geometry, spiraled to the right at the tip and to the left on the shaft, provides the optimum setup for cutting grooves and trimming work. The V-shaped inverted spirals press the matrix layers together for fine surfacing work. For machining work on pure CFRP, Kennametal can provide the most universal tool, in the shape of the burr-style router. The down-cut style router has been specially designed for milling recesses and for surfacing work. The main advantage of the left-spiraled tool lies in the higher cutting speeds that can be reached. For this tool, the developers have chosen a substrate with a low cobalt content to enhance the adhesion of the diamond layer and therefore create a very easy-cutting tool.
The challenge of titanium machining is how to achieve high metal removal rates with low torque typical of the structurally stiff machining centers that are required. MAG IAS LLC (Erlanger, KY) has developed its Cyclo-Cut Max-Flute high-performance end mills that enable machining of internal structures in titanium, Inconel, and stainless steel components with removal rates up to 8 in.3/min with only 25 ft-lb (34 N•m) of torque. The innovative Max-Flute end mill features a 16-flute design. The high-density end mills run at 2037 rpm and a 5.8 m/min to achieve removal rates of up to 131 cm3/min with only 33.8 N•m of torque and 6.76 kW.
The secret of the Max-Flute tool’s design is that it uses shallow, radial widths of cut, which transfer less heat to the cutting tool and allow higher surface speeds for roughing titanium, Inconel, and other high-temperature alloys that have traditionally required high torque at low rpm to achieve desired removal rates. Typically, when cutting titanium, 60–70% of the heat generated is normally transferred to the tool, which dramatically reduces tool life.
Max-Flute end mills are designed with a differential pitch on the tool flutes to break up harmonics and reduce chatter. Titanium is very prone to chatter, which affects part quality and can cause unpredictable tool failure. By using extremely high feed rates and light radial cuts, a lot of material can be removed with very little risk of scrapping parts. The secret to Max-Flute’s ability to make heavier cuts at high speeds is using the tools in conjunction with TrueMill software from Surfcam (Camarillo, CA), which maintains a constant angle of engagement, making the radial cuts more consistent throughout the cutter path, increasing material removal rates and decreasing cycle times, while extending tool life. Max-Flute/TrueMill enables the machine to make heavier cuts at high speeds and by using the maximum flute density available, maximum feed rates are achievable.
Tools Better by Design
Industry-specific challenges often involve applications that generate solutions of a kind where one may not have existed before. Emuge Corp. (West Boylston, MA) manufactures taps, form taps, thread mills, and end mills, as well as the required toolholders for a wide variety of applications. Taps are effective for most common materials and have a longer reach that some workpieces may require. Thread mill products include inserts and toolholders to complete its line of threadmaking tools.
Emuge’s new Rekord DZBF series taps features an Emuge-designed chipbreaking technology that eliminates the formation of long continuous chips that commonly occur when tapping carbon steel, alloy steels, and austenitic stainless steel. "It’s a little more complicated when you uniquely design a chipbreaking function into a tap, but we’ve done it," explains Peter Matysiak, Emuge president. "Although this product is effective for many applications, it’s primarily aimed at problems manufacturers in the energy industry have in machining valves and other parts for the oil, power generation, and wind turbine industries." The Rekord DZBF series taps feature a newly developed cutting face geometry and spiral flute form, which combine to affect chip flow, chip curl, and chip length. The chipbreaking technology produces short, broken, controllable chip formations that eliminate flute clogging and potential failure of the machine tap due to chipped cutting teeth or breakage.
The Rekord DZBF Series taps are offered in inch sizes from ½ to 2" (12.7–51 mm) for use in both horizontal and vertical machining applications. Taps are ground with an increased pitch diameter, designated 2BX which optimizes the gage tolerance for a 2B class of fit. Taps are also made with long shanks, DIN length for extra reach, improved chip evacuation, cooling, and lubrication. Taps are surface treated with TiN to reduce friction between the tap and workpiece, which results in improved thread finishes and increased tool life in carbon and alloy steels, cast and forged steels, tool and die steels, and 300 stainless steel up to Rc 35. ME
This article was first published in the September 2012 edition of Manufacturing Engineering magazine. Click here for PDF.