Manufacturing operations frequently leave workpiece edges ragged and part surfaces rough. Metalcutting tools produce burrs, casting processes create unwanted parting line flash, and welding generates built-up areas and splatter.
In some cases, burrs and surface irregularities have no influence on part performance and can be ignored. More frequently, however, ragged edges and rough surfaces have one or more negative effects, from preventing clean mating of components to posing safety hazards to interfering with the function of a machine or even a human body. As a result, finding ways to remove burrs and improve part finishes is the subject of massive amounts of effort and expense.
The growing use of high-performance advanced workpiece materials multiplies that effort and expense. The list of those materials is long and growing, including high-temperature superalloys, titanium, high-silicon aluminums, compacted graphite iron, carbon-fiber-reinforced polymers, and even multilayered composites. The properties that enhance advanced material performance also make them more difficult to deburr and finish.
Suppliers of deburring and finishing tools and technologies work continually to match the proliferation of advanced materials with their own innovative tools and processes.
Some industries are hotbeds of advanced material application. T.J. Boudreau, category manager, high-volume production for Weiler Abrasives Group (Cresco, PA) said, “Automotive and aerospace are two places where new materials are forcing innovation in burr removal.” For example, the stronger, harder advanced aluminum alloys used in engine blocks and aircraft structural components usually are machined with expensive diamond tooling, and manufacturers run the tools as long as they can to minimize tooling costs. As the tools wear and lose sharpness, they create larger burrs.
A principal burr removal method uses abrasive-impregnated nylon brushes that conform to workpiece contours and scour the burrs away. Brush design incorporates a variety of factors. “You can’t just make a crazy aggressive deburring brush because the advantage of a brush is that it is conformable and can get into places and not change part geometry,” Boudreau said. Design elements include the mechanical structure of the brush, its rigidity and springback tendencies, and its approach angle to the burr. Boudreau said the goals are similar to those of machining: “How do I get the tool to that spot, gain the clearance to get to it, and do it as quickly as possible?”
Development of abrasive grain technology occurs in parallel with brush design. A few years ago most abrasive brushes were made with silicon carbide abrasive. “Silicon carbide is an excellent, sharp abrasive,” Boudreau said, “but it is very brittle and doesn’t hold up well to advanced materials because it tends to shatter instead of fracturing and creating new cutting surfaces.”
Most of Weiler’s design work centers on ceramic-impregnated brushes. Ceramic is very hard and durable. It is less brittle than silicon carbide, so it stays sharper longer. First used in high-pressure applications involving rigid grinding wheels, ceramic abrasive wasn’t well received when it entered the brush market about a decade ago. The abrasive was so durable that it generated excessive heat and brushes could melt and transfer to workpieces. Newer ceramics have lower specific cutting energy and are very effective in lower pressure applications such as brushes.
The newer ceramics paired with advances in brush filament technology have created a shift in the industry. “Today, 75% of what we manufacture uses ceramic grain,” Boudreau said. “Five years ago it was 80% silicon carbide.” Weiler’s abrasive-impregnated brushes are tradenamed Nylox.
Material properties, Boudreau said, “are one of several important factors in designing a deburring solution. For instance, we would never look at a SiC technology for compact graphite iron. That material generates a lot of pressure and heat, and from our experience the best choice is a ceramic.”
Regarding advanced materials such as carbon fiber reinforced composites (CFRP), Boudreau said, “When you cut carbon fiber it leaves a fuzz. Almost not even a burr. We have worked with different aerospace companies and have developed a deburring solution to defuzz carbon fiber parts.”
Deburring brushes provide maximum benefits in automated applications. “If you can automate a process using an existing piece of equipment, it is infinitely faster than taking it out of the machine and deburring it manually, Boudreau said. “The savings are earth-shattering.” Any time CNC machines can be used to optimize processes, it increases ROI. Many deburring brushes can be attached to a standard toolholder, loaded into a toolchanger, and applied automatically. “There is a huge opportunity for part variation when you do things by hand, and deburring on a CNC machining center will give you the most consistent parts in the world,” Boudreau said.
Deburring a perpendicular hole through a flat plate generally is a straightforward job. Deburring a hole that intersects a bore is more difficult, and internally deburring a 90˚ elbow made of aerospace alloy is an even larger challenge. In that case, said Stan Kroll, sales manager of J.W. Done Corp. (Hayward, CA), “You don’t have a nice flat circular exit, you have an edge on one corner of the elbow bore that blends out into the wall, and it is an interrupted cut. The back wall of the elbow is 180˚ from the burr that needs to be removed. It is very difficult, especially by hand, to use a carbide ball to gradually smooth and blend that corner without damaging the adjacent surfaces.”
J.W. Done’s deburring tool, the ORBITOOL, consists of a flexible shaft topped by a carbide sphere or hemisphere surrounded by a polished radial protective disk. The disk gives the tool the appearance of the planet Saturn.
J.W. Done Founder Michael Kapgan invented the ORBITOOL and patented it in 2001. It can be used manually with a hand grinder, but produces maximum benefits when employed with CNC control. Application is relatively simple. The tool is not rotating when it is moved into the cross hole on center. It is offset to one side of the cross hole so the protective disk contacts the wall of the hole and the flexible shaft bends, creating cutting pressure. The CNC program then rotates the tool and feeds it via helical interpolation past the intersection of the hole and the bore.
“As it feeds down it finds the edges of the intersection,” Kroll said, “The protective disk acts like a cam follower. As soon as it moves beyond the end of the wall of the hole, the carbide burr begins removing material at the intersection.” When the hole edge ends inside an elbow, the protective disk rides the wall and cutting doesn’t occur. A spherical version of the ORBITOOL performs a mirror-image process when deburring a hole that goes through both sides of a bore.
When a user desires a radius at an intersection to smooth fluid flow or relieve stress at the joint, changing parameters to increase metal-removal rate will create the radius. Controlling metal-removal rates involves manipulating four factors: Tool rpm, length of the tool shaft (clamping it shorter in the holder increases cutting pressure), feed rate (slowing feed increases the time the tool spends cutting), and thread pitch (a tighter helical interpolation spiral boosts material removal per distance of feed). Establishing the correct parameters involves “dialing it in until it gets the right amount of work done,” Kroll said. The tools come in seven nominal sizes, from 0.047 to 0.375″ (1.19–9.375 mm).
When machining advanced materials, Kroll said, tool life varies by workpiece. In a soft aluminum part that requires minimal deburring, an ORBITOOL might be able to process 50,000 holes. In deburring and creating a radius in a tough titanium fitting, tool life could be in the range of 5000 holes or less.
In aerospace manufacturing, welding, and fabrication operations, technicians employ right-angle or die grinders fitted with abrasive disks to deburr components and smooth machine marks, remove weld splatter, eliminate casting parting lines, and blend surfaces for coating. Mario Davila, product manager for nonwoven abrasives at Norton/Saint-Gobain (Worcester, MA) said many advanced materials possess high hardness or other characteristics that dictate careful choice of abrasives. “When working with special alloys and hard metals such as nickel alloys or titanium, conventional abrasive grains such as aluminum oxide or silicon carbide may not cut it. The tough and hard surfaces of these alloys tend to wear down conventional abrasives at a fast rate, reducing productivity and increasing costs. Ceramic abrasives, on the other hand, excel in high-pressure, heavy-duty applications. They possess superior hardness and grain microfracture capabilities that assure a smooth cut and controlled wear rate.”
Shops typically use a series of decreasingly-aggressive abrasives when finishing the surface of a component. Operations progress from heavy stock removal for eliminating parting lines and flattening welds, to less aggressive blending and finishing. A series of abrasives could begin with a coarse 36 grade coated abrasive, followed by a 60 grade flap disk utilizing overlapping strips of sandpaper, and finishing with a 120-grit sanding disk. Selecting and changing over between the different abrasive media add time and complication.
Davila said using nonwoven abrasives can help consolidate process steps. In these materials, resin is used to bond abrasive grains to flexible nylon fibers that in turn are attached to the backing of a disk. The 3D nature of the fiber/abrasive matrix is compressible and conforms to the contours of the workpiece, while also providing space to dissipate the debris removed.
Davila said Norton recently introduced a new line of nonwoven abrasives called Rapid Prep XHD. The heavy-duty nonwoven disks feature proprietary ceramic abrasives and span a performance range from aggressive metal removal to finishing. “Surface conditioning products are traditionally used for blending and finishing,” Davila said, “This product is more towards the grinding end of the spectrum. We are grinding metal, removing stock, and at the same time doing surface blending.” The resulting finish depends on several factors, including the operator’s technique, the pressure being exerted, and the rpm of the tool. The disks are available in coarse and medium versions. The medium grit is generally for use on softer metals, such as aluminum, copper and bronze, while the coarse grit is appropriate for steels.
For advanced materials, the blend of elements reduces smearing of resin even in elevated-temperature processing of high-nickel content alloys, while also resisting loading on aluminum and softer metals, according to Norton/Saint-Gobain. The roughing-to-finishing capability enables end users to save the time otherwise spent choosing different intermediate abrasives and changing them during processing.
Typical aerospace applications of the nonwoven abrasives in materials such as 7075 aluminum include removing parting lines from castings, smoothing welds, deburring and finishing machine marks. Norton Rapid Prep XHD MED disks could be applied in aerospace composites to remove graphite coatings used to heat-treat the aluminum skins of aerospace parts and turbine elements.
Many nickel-base alloys and titanium aerospace and medical parts are forged or cast to near-net shape, minimizing the occurrence of machining-generated burrs. However, according to Brett Eldridge, sales manger, mass finishing equipment at Rosler Metal Finishing USA (Battle Creek, MI), a significant proportion of the parts require high levels of finish to meet functional requirements.
For example, finishing efforts for aerospace components such as turbine engine blisks and segments focus on airflow characteristics. Generally, the components need a finish of 16 µin. Ra or better; some require lower than an 8 µin. Ra, “Where you are getting into a near-polish area,” Eldridge said. Rosler achieves such exacting results via vibratory mass finishing processes in which parts are immersed in polishing media in a rotating vibrating bowl. Depending on the parts being processed, the parts may either be fixtured or free-floating in the polishing media.
A Rosler process called Keramo-Finish combines special high-density media and an abrasive paste. The media is a mix of fully vitrified nonporous clays that contains no abrasive and weighs about 140 lb/ft3 (2242.58 kg/m3). It acts as carrier for the abrasive paste, which has the consistency of pancake batter. The mixture breaks down in use and provides progressively finer polishing action, similar to using a coarse 80-grit sandpaper and progressively using a finer grit paper to achieve a smooth or polished finish. “We are attacking the peaks of the cast material, cutting those peaks over and over again and smoothing it to a finer surface,” Eldridge said. “After 10–24 hours, depending on where we are starting and where we need to get, we can get mirror finishes that allow for proper airflow and good abrasion resistance.” The process can produce finishes as low as a 2 µin. Ra–3 µin. Ra. When the finish reaches 4 µin. Ra, the abrasive paste is washed out of the media and continuing processing produces the final mirror finish.
Finishing parts often requires multiple trips to the vibratory bowl; a forged turbine engine blade may have one session to remove forging scale, then another after a forming process, followed by further polishing after root-machining operations.
Different advanced materials require different media and operating parameters. “Inconel is hard, up in the RC 50-plus range, where titanium is softer, generally around RC 40,” Eldridge said. Because titanium is softer, Rosler often uses a polyester-based plastic abrasive media that, at 60–75 lb/ft3 (961.11–1201.38 kg/m3), is less dense than ceramic, making it less likely to roll over the ductile burr.
Finish specifications depend on customer requirements. The medical industry uses titanium for bone screws, bone plates, knee implants and other items. “Even though typical callouts on bone plates are somewhere between 4 µin. Ra and 8 µin. Ra, the customer may want a mirror finish for aesthetic quality,” Eldridge said.
Some simple, noncritical parts can be processed using established procedures, but many parts for critical applications, especially in the medical and aerospace industries, require development of custom finishing processes. “We are selling our customers a process and our equipment comes with that process. Obviously the machines are very critical to deburring and finishing, but without the right process, the customer is just guessing,” Eldridge said.
Rosler aims to develop processes that require minimal labor. A shop not using a vibratory method is most likely finishing parts by hand. Much training, skill and experience are required to achieve consistent results. Too much material removal on a medical part or blisk will create expensive scrap. Eldridge pointed out, “Our machines don’t change. Once the rpm and motors are set, and if the user maintains media level and so forth, as long as the raw parts are the same the machine will provide the same results every single time.”