Smaller Holes Bigger Challenges
Holemaking in tough materials serves more demanding aircraft applications.
By Michael Tolinski
Given the dangers inherent to raising tons of metal off the ground and into the air, it’s not surprising that the aerospace industry favors tried-and-true manufacturing methods. At the same, its materials and high-performance designs are becoming more extreme, encouraging the use of less-traditional approaches for machining very detailed features.
For aerospace components, even the smallest machined features can have high importance. Turbine engine cooling holes, for example, improve the performance of aircraft engines when the holes are strategically placed, angled, and sized in engine components. Proper cooling requires multiple holes in high-heat nickel alloys at 0.25-mm diam or less.
Over the years, there has been an explosion in the requirements for these turbine cooling holes, says Mark Barry, vice president for Prima North America’s Convergent Lasers Division and Laserdyne Systems Division (Chicopee, MA). Modern turbine engines operate at hotter temperatures for higher efficiencies, requiring thin-film cooling, in which a layer of cooling air is passed through and around critical parts. Acting as passages for the air, cooling holes help shield components from temperatures that are often higher than the melting points of their materials.
“A novel idea for effusion cooling in a combustor twenty years ago involved less than 2000 holes,” says Barry. The company considered this a challenge to drill at the time. The challenge has since increased, with some engines now requiring hundreds of thousands of small holes.
One traditional approach for turbine holemaking is EDM. There are two reasons for this, according to Jeff Kiszonas, EDM product manager for Makino (Auburn Hills). “One, you can do it fairly quickly on anything conductive, and two, it’s a very well-known process in the aerospace industry, and aerospace is pretty conservative when it comes to change. Once they know something works and it doesn’t cause problems down the road, they don’t like to switch to other processes.”
As the number of turbine holes multiplies, EDM’s ability to drill many holes simultaneously becomes valuable. But machining rows of hundreds of holes along curving blade contours requires accurate hole placement. For smaller holes (0.2–1.0 mm), EDM offers the consistency and necessary finish quality, Kiszonas says. Fine-hole EDM machines, for example, “use varying voltages and current to achieve a very fine finish and a very precise hole.” These machines also can handle large length/diameter ratios up to 100:1.
Makino’s Edge2FH fine-hole EDM machine, based on a standard ram EDM platform, has an additional W-axis attachment and software to support fine holemaking. Kiszonas points to some extreme applications for this type of machine, such as 0.0035" (0.09-mm) diam holes in small tubes. Other aero applications include tight patterns of holes in aircraft fuel rails that deliver engine fuel, or small holes in hydraulic fluid lines.
The automation of the EDM process has added to its reputation in aerospace, says Gisbert Ledvon of Charmilles Technologies Corp. (Lincolnshire, IL). “If you compare it to other technologies, from an automation point of view EDM is probably the most secure and safe manufacturing method you could apply.” Operator inconsistency is generally a non-factor; EDM machines with interactive “expert system” programming only demand that the operator define the workpiece material, electrode material, hole depth, and surface finish. Based on this, the machine controls and optimizes the process.
An example of EDM automation is given by Robert Sorrell of GE Fanuc Automation (Charlottesville, VA). GE Fanuc supplies the brains and guts of the “Fast Hole” EDM from Beaumont Machine (Milford, OH). The system also reportedly can interface easily with a Fanuc Robot for more automated applications.
Hole sizes created with the EDM typically vary from 0.008 to 0.250" (0.2–6.4 mm) for typical aerospace parts, with unlimited hole density, configurations, and patterning, says Sorrell. Holemaking speed varies depending on material, depth, and part configuration, but the EDM “consistently drills three to four times faster than others.” For example, “With the GE Fanuc 160i-MB control, the cycle for diffuser holes was reduced from 6 min to 28 sec.”
But EDM’s limitations in aerospace prevent it from being the only choice for all holemaking applications. “With anything aerospace, there’s always a concern with the EDM process, because of what’s called the recast layer, or ‘white layer,’” says Makino’s Kiszonas. There’s virtually no recast layer in small, fine holes, but recast is still a worry for critical aircraft engine parts. “If there’s too much of it, there’s a fear that that recast can actually separate from the main material, and cause problems mechanically with the engine.”
Microcracking is also a concern with any high-heat process such as EDM and laser drilling. “If you develop too much heat, you can also develop microcracking on the surface of the material, which will develop into further cracking,” he says.
Moreover, EDM can only machine materials that are conductive. Thus, with more turbine blades and other high-heat components being coated with nonconductive thermal-barrier coatings, EDM steps out and laser drilling steps in. Some manufacturers are now investigating the use of ceramic turbine components and ceramic matrix composites, both of which are nonconductive, says Randy Gilmore, director of laser technology for The Ex One Company (Irwin, PA).
Laser drilling has other advantages over EDM. “Lasers can drill individual holes faster than EDM, and can produce individual holes with less thermal damage [recast],” says Gilmore. For certain applications, multiple holes with multiple entry angles must be individually created, favoring lasers, and making EDM’s talent for simultaneous hole-drilling less relevant.
Newer part designs typically don’t specify EDM or laser as a required process, adds PRIMA/Laserdyne’s Mark Barry. “If anything, the specifications are for metallurgical and dimensional requirements, leaving the decision to manufacturing.” A combustor, for example, is made of several sheetmetal formed parts with many thousands of holes with changing patterns, at multiple angles, and of different diam (typically 0.4–0.8 mm). “These part designs favor laser processing.” Barry argues that even a complex pattern and wide range of angles and hole sizes can be drilled in a single laser setup.
For small holes, the laser method of choice is percussion drilling, where the laser beam “pecks” at the material. This technique can be used for “drilling on the fly,” or turning a cylindrical part and machining it with specific laser pulse patterns to complete the holes, says Barry. Here, the time per hole is significantly reduced, requiring about one-third of a second to complete a 0.64-mm hole in 1.5-mm-thick material (compared to one second per hole with straight percussive drilling).
For larger holes (over 0.035" or 0.89 mm in diam), trepanning is used. This is the laser machining approach used by Ex One to trepan turbine blade holes with a laser beam 0.002" (50 µm) in diameter. The beam machines each hole by moving in a circular (planetary) motion. “The size of the hole drilled is determined by the beam spot diam plus the trepanning diam,” says Gilmore.
In Ex One’s SuperPulse Laser Machining System, trepanning at a small scale requires sophisticated control for attaining proper hole roundness, size, and angle. “The method of manipulating the trepanning mechanism relies upon very close synchronization between four motors.” These motors are synchronized to direct the laser beam at the correct angle and speed of rotation, via a five-axis (X, Y, Z, A, and C) motion-control system from Bosch Rexroth Corp. (Hoffman Estates, IL).
For the A and C rotary axes, the Ex One system uses synchronous kit motors or torque motors (sometimes referred to as direct-drive technology), says Peter Rochford, Bosch Rexroth application engineer. “With synchronous kit motors, either linear or rotary, the OEM integrates the stator and permanent-magnet rotor into the mechanical design of the axis. There are many advantages to direct-drive technology like greater positioning accuracy (i.e., no backlash), fewer components, faster speeds and accelerations, better regulation performance, [and] better static and dynamic stiffness.”
Concerns about laser drilling remain, despite its widespread use and solid control systems. For example, some people are concerned about potential damage to material on the other side of a through-hole by the laser beam coming through the hole.
“Whether you trepan or percussion drill [including ‘drill on the fly’] the beam will come through,” says Mark Barry. The potential for damage depends on the focal length and part size. If there is a risk, shields and material coatings can help protect delicate part surfaces. And Barry says Laserdyne has developed a detection sensor to help minimize the number of shots required to create a hole. “Laser drilling has been an art form, not a manufacturing process. That’s often cited as its largest drawback, and that’s the problem we’re attacking.”
Hard-tool drilling certainly is not out of the question in the aerospace game, despite the advantages of EDM and lasers for drilling superalloys. With the right drill material and design, drills are still needed for holemaking in aircraft alloys, composites, and “sandwich” structures that include layers of composites and metals.
Drilling in tough, high-nickel alloys is hard on the tool, but it can be done productively, says Francois Gau, aerospace and defense industry segment manager of Kennametal Inc. (Latrobe, PA). He points to an engine component made from Inconel 718 with multiple 4.5-mm holes. Here, switching to the right high-wear tool had dramatic effects. The company’s B284 uncoated carbide drill with a slightly convex cutting edge resulted in several times more holes per tool, and about 30% faster cutting rates.
Small threaded holes, used throughout aerospace assemblies, present their own problems. Screw threads can be hard to cut in typical aero materials, says Mark Hatch, product manager for Emuge Corp. (Northborough, MA). Inconel tends to “jam” when tapped, shrinking or “squeezing” material into the cutting tool, and taps have to be very application/material-specific, says Hatch.
There are also limitations in length/diameter ratio that shorten the usable thread length in a small hole. In some situations, the depth of the cut threads is too shallow to be useful, he says, so the company has focused on expanding thread depth to two times diam. And for difficult materials, carbide thread-milling tools are alternatives to thread-cutting or thread-forming tools, sidestepping the issue of tap breakage altogether.
Meanwhile, fiber composites are being used more for aircraft structures, motivating interest in new kinds of drills. In fact, it’s difficult for tooling specialists to keep up with composites technology, says Ken Williams, UK business development for aerospace for Sandvik Coromant (Fair Lawn, NJ). “I don’t think any of us know a huge amount about composites, because they’re changing all the time.”
In aircraft components made from composites, drilled holes are typically 3/32–3/8" (2.4–9.5 mm) pilot holes for inserting rivets or bolts in final aircraft assembly, says Williams. Some components call for holes that are chamfered, with countersinks in them, a real challenge in a composite material.
So composite drilling has been in constant development, given the problems presented by each new kind of composite. The fiber direction and weave affect the drilling process, whether they’re unidirectional or a cross or wave-weave with fibers in various directions, says Williams.
Delamination is the main problem encountered when drilling into composites. Fibers can separate from their matrix when the drill enters and exits, and you “can also have delamination half-way through the material,” adds Williams. Delamination tends to be reduced with multidirectional fiber orientations, but problems remain on drill entry and exit, when fibers are pulled out of the surface.
Sandvik has tried various approaches to reduce delamination. “We’ve played around with changes in point angle—increasing it, decreasing it.” Williams also refers to an early design, the hollow Phi drill, which is a kind of solid-carbide trepanning tool that cuts through the fibers to create a hole while minimizing axial pressure into the material. But he seeps this as an old solution that offers limited tool life. New drill designs may require PCD or new diamond coatings.
New tools and methods are also needed for drilling composite stacks of fiber-composite, aluminum, and titanium layers, says Williams. He points to a structural component on the Airbus A380 for attaching the wings to the fuselage. It’s composed of a stack of composite, aluminum, and titanium, with an overall thickness of 6" (150 mm).
One approach to drilling stacks is offered by Kennametal, in partnership with orbital-drilling OEM, Novator AB (Spanga, Sweden). Because traditional push-drilling can create burrs or delamination in the layers of the stack, the companies use orbital drilling to reduce burrs by combining the spinning of the drill on its own axis with a mechanical spindle that rotates eccentrically around a principal axis. The dual high-speed rotations are said to have the effect of feeding the tool through the material rather than pushing it, reducing axial forces and heat buildup.
The orbital motion also allows the tool diam to be smaller than the hole diam. This differential aids in chip and heat extraction, and allows one tool to be used to create holes of different diam, says Kennametal’s Francois Gau. (See “Holemaking With Precision” Manufacturing Engineering, November 2002, for more information on Novator’s orbital drilling technique.)
Robots in Step with Precision Holemaking
“Aerospace is pushing the envelope for the accuracy of robots,” says Virgil Wilson, senior engineer for material removal, Fanuc Robotics America Inc. (Rochester Hills, MI). He says the general trend in aircraft body assembly is from manual to robotic, with the hard machining done by the robots’ end-of-arm tooling (EOAT).
“There’s a lot of technology in an end-of-arm tool.” One EOAT can drill, rivet, and rout holes, changing drills as necessary to account for increasingly complicated combinations of aircraft structural materials. For example, a robot might be used for drilling out and inserting rivets around the door edge of a plane.
“The robots must be very precise in terms of positioning and maintaining normality,” Wilson adds. The positioning tolerance is especially tight, driving the need for robot accuracy.
This article was first published in the March 2006 edition of Manufacturing Engineering magazine.