Article From: December 2020 Manufacturing Engineering, Ed Sinkora, Contributing Editor
Making parts for making electricity
Need a little good news? America’s seemingly insatiable need for electricity is producing strong demand for the components that go into power generation equipment. That’s according to Phil Wiss, regional sales manager for the profile grinding division of United Grinding North America, Miamisburg, Ohio. He added that “‘work in process’ is still a dirty word, so manufacturers need to run that fine line between having enough parts in stock and meeting just-in-time delivery,” making this an excellent time to explore the technology that can tackle the challenges in this field.
The land-based gas turbines generating much of our electricity are not much different from aircraft turbines. “They’re basically a jet engine on steroids,” said Greg Bronson, sales director for the Americas, at toolmaker Greenleaf Corp., Saegertown, Pa. Land-based turbines aren’t designed with the same concern about weight, he explained, but they still rely on difficult-to-machine materials, especially for the hot sections. That means variations on the nickel-based, heat-resistant Inconels and René materials you’d find in aircraft engines, plus “some proprietary materials specific to OEMs like Siemens and GE,” he added.
As is the case with aircraft turbines, the effort to improve gas turbine efficiency by increasing pressures and temperatures has driven the development of ever more heat-resistant materials, which are correspondingly more difficult to machine. By way of illustration, according to Greenleaf’s machinability index, Inconel 718 is rated “100 percent machinable,” with a starting point of 700 sfm and 0.0075 ipr for its WG-300 whisker reinforced ceramic insert. But “some of the René materials and powdered alloys coming out have 35 to 40 percent machinability ratings, meaning you’d have to reduce feeds and speeds by 60 to 65 percent,” said Bronson.
The big difference between aircraft and land-based turbines is the size. For example, “a jet engine turbine wheel might be anywhere from 18 to 36″ [45.72 to 91.44 cm] in diameter, or a bit larger on some of the bigger engines like the GE 90,” said Bronson. “Whereas that same disc in land-based turbines would be 7 to 8′ [2.13 to 2.44 m] in diameter.”
Likewise, machining these parts requires a daunting amount of material removal. “You’re starting with a casting or a forging that may be in the realm of 8,000 lb [3,629 kg] and taking off 5,000 lb [2,268 kg] of chips,” said Bronson. For that job, Greenleaf recommends ceramic tooling, which offers “much higher speeds and similar feed rates to carbide,” said Bronson, resulting in material removal rates that are typically 4 to 8 times as high. With the proviso that metal removal rates are always a function of speed, feed, and depth of cut, plus material condition and hardness, he said Greenleaf has seen improvements of 10× or more.
He also observed that the surge in additive manufacturing, especially in the repair side of the business, brings the new challenge of laser-sintered materials. “These materials are very abrasive and have a scale condition that can reduce cutting speed and create a depth of cut notch that can cause tools to fail prematurely,” said Bronson. “In these cases you may need to go with a ceramic or diamond tool.”
Building larger parts on a stainless steel plate presents a further problem, he explained. After removing the laser-sintered part from the build plate, the remaining detritus must be cleaned up in order to reuse the plate. Thus the tool used to resurface the plate must be able to machine Inconel or another high-temperature alloy and stainless steel simultaneously. Bronson said Greenleaf’s latest ceramic, XSYTIN-1, “is capable of machining both the sintered nickel alloy and the softer stainless steel base plate with no adverse effect to the tool.”
Finish Machining High-Temp Giants
Despite their enormous size, land-based turbine components must be built to tight tolerances. And that size itself presents a challenge, explained Bronson, because in many cases the tool must last through the entire cut in order to avoid a mismatch. “I’ve run into cases where customers took seven, eight, or nine passes to remove the last few thou on the OD of a disc to bring it into tolerance, just because of taper issues and wear in the tools,” he said, adding that carbides are often the better solution in these situations. Because, although you can remove more material with a ceramic tool in the same amount of time, a carbide tool will have a longer time in the cut and can finish without the need to index.
Emuge Corp., West Boylston, Mass., has come up with a creative solution for cutting turbine blade root forms and the corresponding slots in the disc. They call the new tool Pagode because it looks like a pagoda. As Milling Application Specialist Evan Duncanson explained, unlike the fir-tree cutters typically used for this application, the Pagode “does not have one big gash all the way into the core of the end mill. With the latter approach, the tool’s gash must be as deep as the smallest diameter, which also means a massive amount of material must be removed at the largest diameter. This weakens the tool significantly. In the Pagode the gash follows the tool profile such that there is only a few millimeters of relief from the cutting edge.”
This leaves much more material at the core and allows Emuge to add more flutes, and hence more cutting edges, for a given tool diameter. “A typical fir-tree cutter has only three flutes,” Duncanson explained. “We can build a Pagode of the same diameter with six or eight flutes. And the more flutes you have, the faster you can go.”
Duncanson added that the additional flutes and larger core also enabled Emuge designers to use a larger helix angle than a typical fir-tree cutter. “That ensures there are always two cutting edges engaged in the part,” he said. This results in more consistent forces on the tool, which is better for tool life, surface finish, and the spindle. Plus the larger core diameter makes for “a much stronger tool overall that can take more of a beating. In testing at Emuge Germany and out in the field, we have seen anywhere from two to 10 times the life of a standard Emuge fir-tree cutter, and a cycle time reduction of two to 10 times versus a standard Emuge fir-tree cutter.” This also helps ensure the tool can last through the entire finishing cut on a large part.
Emuge can provide the tools in both carbide and HSS, and with multiple radial coolant holes in each flute, if desired. In fact, given the thicker core, Emuge is able to direct the coolant onto the cutting edge, whereas “in a standard fir-tree cutter, the flute is deeper and the coolant essentially just comes out straight,” said Duncanson. What’s more, Emuge is able to regrind each tool up to four times, and does so on the same machine that manufactured it. Thus it can guarantee 10 µm form accuracy on both new tools and regrinds.
Duncanson said the Pagode concept is also flexible enough to allow for the combination of several tools in one. “I’m currently working with a customer to create a tool that combines a bell profile at the end with a taper to a profile for cutting a straight wall, another profile for pressure surfaces, and corner radii on the outside. This would ordinarily be three or four different tools. The tool-change time on the huge machines used to cut these turbine parts can be 15 to 20 minutes, so combining tools is a significant improvement.” The Pagode has a higher initial cost than a fir-tree cutter, but Duncanson pointed out that the tool’s much higher throughput and the ability to regrind it multiple times, not to mention the possibility of combining operations, make it a much more cost-effective solution.
Greenleaf scored a victory in finishing multiple bearing surfaces on a gas turbine shafts, replacing a precision-ground braised carbide tool. “It was a tight tolerance part,” recounted Bronson, “and a nightmare to set up. They would run anywhere from seven to 13 or 14 of these tools, each of which were about $1,000 a piece, just to finish those bearing areas. And then they would hand polish them to get them to the required 0.4 µm Ra. We created an indexable carbide insert that not only greatly reduced their setup time, it also achieved an Ra of 0.34 µm—a near mirror finish. They flat out told us it was impossible, but we did it.”
Specialized Grinding Solutions
The predominant solution for achieving a fine finish on turbine components is, of course, grinding. And for compressor rotors, Danobat USA, Houston, offers some unique solutions. As Danel Epelde, business development director at Danobat’s Spanish headquarters, explained, the blades in many of the smaller (50-mW range) power-generation turbines are loose. But for efficient operation, it’s critical to control the clearance between the moving rotor and the stationary casing.
So Danobat developed a machine that spins the assembled rotor at up to 7,000 rpm and measures the blade tips in their working position while they are being ground. Since there was no commercially available inspection system that could do this with the required precision, Danobat created its own high-speed camera system. The new system compares the images to the desired tip radius and adjusts the grinding process accordingly. Danobat also supplies vertical grinding machines for the compressor casing, so it is responsible for creating a perfect match when the camera system and grinding machine are used together.
The problem of matching the blade profile to the compressor case exists for the giant 400+ mW gas turbines as well. But the cost of building a machine capable of spinning and grinding those giant rotors has seemed prohibitive. Epelde said OEMs know their turbines would be more efficient if they did this, but it’s not clear the improvement justifies the cost.
“They reach out to us every three or four years and I think sooner or later they will have to do it,” he said. In any case, Danobat boasts a broad portfolio that mixes and matches various technologies to suit the requirement. “If the parts are small or medium-sized, we use granite to make the structure because it’s a good way to achieve the required thermal stability and damping behavior,” explained Epelde. “Once you go over a certain size, granite becomes more expensive and we move to a stabilized pearlitic cast iron. We make machines that weigh 5 tons and machines that weigh 50 tons [4.54 or 45.4 metric tons]. We can grind parts that weigh 5 lb or 10,000 lb [2.27 or 4,536 kg]. If we’re spinning a 5-lb part, we can use a precision ball bearing workhead. With a 10,000-lb part, we’d use hydrostatic technology.”
Grinding the root form of the blades that go into these rotors takes a machine like the Blohm Profimat MT high-hp, heavy-duty, creep feed grinder, said Wiss. This machine uses continuous dressing of a wide conventional abrasive form wheel, but with little or no additional automation.
Conversely, for vanes United Grinding would recommend one of its Mägerle MFP 150, MFP 50, or MFP 51 platforms. “All three include automatic tool changing and overhead dressing,” said Wiss. “The overhead dressing allows us to do continuous dress creep feed, which combines a rapid metal removal rate with the ability to hold a tight tolerance. For some parts, like a radial slot, we’d use a plated CBN wheel. The toolchanger also gives us the ability to use a metal bond CBN, along with drilling and milling, in the same machine tool. We use the 80/20 rule.” In other words, this kind of multi-tasking grinder makes sense if 80 percent of the work calls for grinding while 20 percent calls for other machining. And everyone loves processing as many features as possible in one clamping.
Wiss also pointed out that the wheel/tool flanges include RFID chips, so the machine automatically checks to make sure the right tool goes to the spindle, even if the operator puts it in the wrong slot in the changer. Plus the operator has access to the changer during the machining process, so worn tools can be replaced at any time without interrupting production.
For an even more specialized application serving the industry, Mägerle and Blohm also have models for grinding the gear form in Hirth and Curvic couplings. “We have machines with a basement that swallows over a meter of shaft,” said Wiss, “while the workzone handles a coupling up to 1½ meters in diameter. You could fit a VW on the rotary table inside these machines, though we also make machines that are 80″ [203.2 cm] across the front for grinding small Hirth or Curvic couplings.”
United Grinding’s approach is to lay the coupling flat on the table, which is a “rigid arrangement and easy to set up,” Wiss added. This is aided by excellent software. “The part doesn’t have to be perfectly centered on the table. The machine probes the OD of the part to determine runout, and also probes both sides of each tooth and does a stock divide to calculate the required material removal,” he said. “Then it grinds automatically. On other grinders, you have to tap the coupling repeatedly on the OD with a rawhide or rubber mallet until it’s indicated perfectly before grinding the part.”
For its part, Danobat offers a double-column vertical grinder for turbine disks. “These parts are almost 10′ [3.048 m] in diameter and made out of Inconel,” explained Epelde, “so machining is quite challenging.” The part requires slots and holes, in addition to surfaces that call for grinding. At the same time, customers want to perform as many operations as possible in one clamping—operations that require very different torque and power curves. As a result, the machine has a grinding spindle and a separate boring and turning spindle.
Gages as Big as the User
The large size of land-based turbine components, and the many differences in design and manufacturing methods from OEM to OEM, call for customized gaging. Enter the team at the Special Gage Division for The L.S. Starrett Co. , Athol, Mass. Manager Andrew Morin said Starrett engineers, designs and manufactures gages for measuring all the machined diameters; various height gages; slot gages; and even area flow gages to measure the minimum area openings of turbine engine nozzles. That last example uses eight or more contacts that reach into the throat of the nozzle openings. Hydraulic cylinders take the measurements and transfer the data to a dial indicator, where they are displayed with 0.001 in2 (0.645 mm2) resolution. This allows the operator to locate and match the segment openings around the engine circumference to provide a balanced air flow.
For diameter gages that measure up to 90″ (228.6 cm), Starrett uses a lightweight honeycomb structure or carbon fiber and isolation handles so the operator’s body heat doesn’t influence the gage. “You can be checking a 5′ [1.52-m] diameter with a gage that weighs a couple of pounds,” said Morin. “They’re accurate down to tenths and they have a dial indicator so every operator can be sure they’re reading exactly the same. A lot of major customers have them because if you were to use a regular micrometer that large, it’s a two-person or a three-person job just to get the gage in place. With our honeycomb, it’s a one-person job and they can do it all day long.”
Starrett also make gages that reach into assemblies that are difficult or impossible to access bodily. “I’ve got a gage on my sample rack that’s 7′ [2.13 m] long and has to be lowered by a hoist into the part to be checked,” said Morin. “We also make things like double-turret gages that collapse to a small diameter, go through that small diameter, and then double their size to get the reading the operator needs without ever having to move or flip the part. We handle all sorts of awkward situations.”
Reinventing Machine Tool Construction
There’s a revolution in machine tool design and construction going on at the Department of Energy’s Oak Ridge National Laboratory in Tennessee. The team at the Manufacturing Demonstration Facility there has shown the viability of casting concrete machine tool bases and columns using 3D printed molds.
As Chief Manufacturing Officer Thomas R. Kurfess, Ph.D., P.E., explained, “The key thing is not so much the concrete base. It’s leveraging a large-scale additive manufacturing polymer printer to build the mold rapidly and at very low cost, such that we can pop out a base for a machine tool at very low cost.” That makes it easier, faster, and cheaper to both develop new machine tools and to create customized machines. And “if it’s suddenly easy to change the base, that really flips the rebuilding business around. You could keep any components that made sense and build a new base to suit the new purpose.”
While it’s possible to leave the mold on the finished base, Kurfess said the carbon fiber reinforced molds can also be reused several hundred times if designed to be removed from the casting. And if the builder doesn’t want to invest in large-scale printing, it can use a service bureau to produce and ship the molds. “It’s plastic, so it’s not particularly heavy,” observed Kurfess. Yet when filled with concrete, the base appears to offer an order of magnitude improvement in vibration damping relative to cast iron. Plus it’s thermally stable.
It’s also possible to embed cooling channels and sensors in the casting, including those that would be vaporized when casting molten metal. The Oak Ridge team is now testing various accelerometers, vibration sensors and temperature gages with an eye toward improved monitoring and real-time process control.