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Building Better Blisks

Kip Hanson
By Kip Hanson Contributing Editor, SME Media
The tolerances are tight, the metals difficult, the stakes exceedingly high. Simply put, machining gas turbine components is very challenging work.

It goes without saying that no commercial airline traveler wants to look out the window and see an engine burst into flames. Nor do they want to pay more for their ticket to Des Moines or Seattle due to poor fuel efficiency or excessive maintenance costs. Thankfully, the aerospace industry has practically eliminated such events by designing high-performance jet engines that are both robust and safe, with virtually no failures due to mechanical defects.

That’s high praise, because whether they’re used to power a jet airliner or a municipal power station, gas turbine engines are complex assemblies containing many thousands of individual parts. The list includes fuel injectors, bearings and seals, pumps, valves, gear trains and all manner of electronics to monitor operational performance.

Each is critical to a functioning engine. But it’s the rotating blades that bear the most abuse when a wayward flock of Canadian geese gets pulled in during flight, as with US Airways Flight 1549 that famously had to make an emergency landing on the Hudson River in 2009.

This is why turbine manufacturers make the blades and blisks (short for bladed disks) from titanium and various nickel-based, heat-resistant superalloys (HRSAs), such as Inconel, Waspaloy and the single-crystal CMSX-4. All are exceedingly tough and impact-resistant, never mind their ability to withstand operating temperatures that often approach or even exceed the metal’s melting point. They’re also quite difficult to machine, a task made even more challenging by complex, curved surfaces and unforgiving tolerances.

Pitch Perfect

While the engineers behind these modern wonders deserve kudos for their designs, it’s the programmers and machinists making the components who have to meet some of the most demanding requirements in the industry. And to do that, the processes must be sound, the tooling and machinery operating at top efficiency.

Renishaw’s MODUS point cloud sectioner generates “on-surface” sections on any plane through a cloud of tip centre data points that were previously captured by REVO. (Provided by Renishaw)

Dan Skulan, general manager for Industrial Metrology at Renishaw Inc., West Dundee, Illinois, has recommendations for the latter of these concerns. He points to the widely held belief that if you use modern design and simulation software tools, the finished product will be perfect.

“The fact of the matter is that no manufactured component is perfect, no matter how good the machine tool used to make it,” Skulan says. “And with five-axis machining centers, it’s especially crucial to fine-tune both the linear access travels and, more importantly, the rotary axis positioning.”

Skulan suggests that the best way to accomplish this is with routine calibration using a combination of laser calibration and subsequent diagnostic testing systems, such as the company’s XM60/XR20W and QC20W ballbar respectively. These procedures should be performed during initial machine installation, followed by regular checkups as needed to correct positioning errors.

Both are good advice no matter what’s being machined, but in the case of high-value, highly accurate components such as blades and blisks, it’s often done daily. “The forging alone might cost $50,000 or more, and by the time you’ve finished machining it, you could easily have several times that amount into the completed blisk,” Skulan says. “Unfortunately, just one or two thousandths of error in the table’s center of rotation can cause big problems when you’re down to the finishing pass.”

Skulan has plenty more to say about machine tool optimization, including the use of Renishaw’s AxiSet for quick checkups of a machine’s rotary axes, but instead shares his insights on another crucial aspect of blade and blisk machining: verifying their accuracy.

The REVO multi-sensor system supports five-axis measurement on a three-axis coordinate measuring machine, he explains. It is a probe-style head that can scan even extremely complex surfaces very quickly and generate the “high data density” needed in the aerospace and medical industries. And the latest iteration, the REVO-2, has several enhancements that might be of special interest to blade and blisk manufacturers.

“One of the challenges with these blade shapes is when the radius you’re measuring is smaller than that of the probe,” Skulan notes. “It’s a phenomenon in metrology that has plagued people forever. Renishaw has solved that using an algorithm in its Modus Metrology software called Cloud Point Sectioning. But we’ve also launched several other features, such as a fringe probe for detection of 3D topology and an ultrasonic thickness probe that allows the user to measure from one side of a workpiece to the other, up to half an inch thick. Each of these has applications in the blade and blisk market.”

Optimal Mass

Ryan Matz, applications engineer and five-axis machining specialist for DN Solutions America Co., Pine Brook, N.J. (formerly Doosan Machine Tools), agrees with Skulan’s earlier assessments on the need for extreme machine accuracy. “Proper maintenance and calibration is important for any CNC machine tool, but one of the biggest success factors in this sector is to start with an accurate machine,” he asserts.

“All it takes is one gouge from a broken tool and the part could easily be scrap,” Matz says. “That’s why DN Solutions equips most of its machines with glass scales on both the linear and rotary axes, thermal compensation, a dual-contact spindle interface, and the mass needed to take heavy cuts and minimize deflection.”

As with virtually everywhere else in manufacturing, automation is increasingly used in aerospace. Blade and blisk machining are no exception. Matz recommends that five-axis machine buyers look for a rotary coupling in the table, as this simplifies installation of the hydraulic or pneumatic clamping systems needed for robotic tending. Equally important is a spindle with coolant-through capabilities, preferably with a temperature-controlled and filtered, high-pressure coolant system.

“I also recommend that you balance your tools,” Matz adds. “We use the BIG-PLUS spindle interface, which is more rigid and more accurate than a single-contact steep taper, but you have to go a step further when looking at high-rpm spindles like those used in blade and blisk machining. I can tell you from first-hand experience that the difference in tool life and overall performance is amazing when using balanced tooling.”

Achieving Proper Balance

This is good advice in any machining application, but with blades, blisks and other flight-critical turbomachinery components, it’s practically mandatory. Brendt Holden, president of Haimer USA LLC, Villa Park, Illinois, states that any shop aiming for fine surface finishes or that wishes to eliminate vibration in high-speed spindles should balance toolholder assemblies to G2.5 or better.

The term “high-speed spindle” is admittedly ambiguous, but it’s a safe bet that most industry experts—Holden included—would set the bar at 15,000 rpm. “We have customers that balance to a G2.5 for speeds of just 8,000 rpm, but those are typically for single-point boring applications where the head is usually quite unbalanced. For everyone else, balancing of the complete toolholder assembly—the retention knob, collet and nut … everything—is the first step towards better tool life and surface finish, as well as increased spindle longevity,” Holden says.

As cutting forces are high when machining HRSA materials, and because titanium in particular is quite “grabby” and tends to pull end mills out of their holders (making for a very bad day), Holden recommends anti-pullout tools and toolholders.

“We’ve partnered with cutting tool manufacturers like Kennametal, Iscar and others on the Safe-Lock and the modular Duo-Lock systems, each of which eliminates the pullout that many have experienced when using mechanical, shrink-fit and hydraulic toolholders. Both products have since become a favorite of many aerospace shops.”

Circling Around

Cutting tool manufacturer Emuge-Franken USA, West Boylston, Mass., is one such Safe-Lock partner, although as Milling Application Specialist Evan Duncanson notes, his primary focus is on circle segment cutting, aka conical barrel milling.

The DVF-series five-axis machining centers are designed for high-precision machining of complex parts like this turbine component in a single operation. (Provided by DN Company)

These specialty end mills are almost as complexly designed as the workpieces they’re tasked with machining, so we won’t provide a deep dive into them here. What we will say is that, as their name describes, circle segment cutters utilize a small section of a much larger arc—in some cases, up to several feet in diameter—to do their work, thus allowing them to take bigger step-downs than their ball-nosed counterparts. This can reduce machining time significantly (up to 90% in some cases) while providing smoother surface finishes and extending tool life, making them an increasingly attractive option in die-mold applications, medical implant machining and, as Duncanson points out, the milling of blades and blisks.

This helps explain why Emuge-Franken has developed its Turbine Program, a lineup of circle segment and tapered ball-nose end mills with geometries “specially tailored to meet the requirements of materials and component designs in the aircraft and turbine industry.”

Greater productivity notwithstanding, circle segment cutters are a relative newcomer to the industry, and as Duncanson will tell you, retooling existing and approved processes in the aerospace (and medical) industry is rarely done. Because of this, the majority of all current blade and blisk machining is still performed with traditional ball-nose end mills, although this status quo will certainly change as new gas turbine engine programs come online.

Meet the Expert

Complicating matters further, the toolpaths needed to drive a circle segment cutter are significantly more challenging to generate. “Blade surfaces are very CAM-intensive, no matter what type of cutter you’re using. Circle segment brings that up a notch,” Duncanson says.

Karlo Apro is a strategic technical specialist at CNC Software LLC, Tolland, Conn., makers of Mastercam. He seconds Duncanson on the nature of blade and blisk toolpaths, and remembers the days when they “might take a week or more” to generate. Fortunately, those days are ancient history. “We introduced Blade Expert around 10 years ago to address this problem, and have since enhanced it to support circle segment cutting.”

Most of the toolpath routines in Mastercam are feature agnostic, or “just another tool in the toolbox,” and not designed for a specific type of workpiece, Apro explains. Not so with the Blade Expert, which makes various assumptions when the programmer initiates it. “It recognizes that you’re cutting turbine blade or propeller-like shapes on a five-axis machine, so it knows if it should tilt away when you’re getting too close to a wall, for example, and how to use different tools to blend the variable radii common in these parts.”

It also helps determine the best approach to roughing, semi-finishing, and finishing each blade. As Duncanson noted, this task must be completed in “just the right way,” lest the remaining material become too flimsy to support itself. As a result, the programmer generally finishes each blade profile in vertical sections, working deeper into the part and around the repeating sections until complete.

This has long made blade programming a trial-and-error affair, with a workpiece that—due to its high cost and even higher operational stakes—has virtually no room for error.

“The trick is to have a good handle on the material you’re cutting, how much stock to leave behind, how hard to push the end mill, what tool to use in each section and so on,” Apro says. “This is especially true with circle segment tools, which, thanks to their huge radius, are great for finishing, but also increase side pressure and can lead to deflection as the blade gets thinner. It’s definitely an art, but advanced programming software makes it a lot more scientific.”

Raising the Bar

There’s much more to the blade and blisk story than toolpaths and end mills. As any five-axis machining center programmer knows, workholding selection plays a critical role in the ease with which certain part features can be reached. It also helps determine how quickly an operator can set up one of these machine tools. And many shops are turning to automation as the cure for ongoing worker shortages.

It’s clear that shops of all kinds need modular, quick-change and strong yet compact workholding. San Diego-based 5th Axis Inc. is one of the many providers that specialize in this type of tooling, and has recently announced an automation offering to complement it.

The company’s RockLock zero-point system is now “robot compatible,” with a pneumatic clamping mechanism that eliminates the “sticking” that might cause a robotic arm to error out, according to Sales Manager Eric Nekich. 5th Axis also introduced a customizable shelving and storage solution that accepts industry-standard, zero-point pull studs with 52 mm or 96 mm spacing, along with specially tailored grippers for each.

Granted, none of this is specific to blades and blisks. Yet Nekich is quick to note that 5th Axis’ well-known dovetail-style clamping system is a common sight in many aerospace shops. “Turbine blades are often gripped on an integrated dovetail fixture. The entire workpiece sits above the clamping area, making it easy to get into all those deep, little contours without worrying about the spindle bouncing off the workholding,” he says.

“And for larger blades, they’ll use a pair of self-centering vises and a trunnion table to simultaneously grip both ends of the part. This provides clean access to the complex geometry without sacrificing rigidity.”

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