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Helping Your Parts Shine On

Kip Hanson
By Kip Hanson Contributing Editor, SME Media

Avoiding the drudgery of deburring and finishing operations

The Diamond Burnish Face Mill Tool from Cogsdill Tool Products operates much like a face mill, but is designed to burnish flat surfaces down to single-digit Ra finishes.

The deburring and finishing of machined and fabricated parts is a necessary but often disregarded step in the manufacturing process. As a result, some shops still approach this task much as they did in the previous century, relying on manual deburring and polishing methods over more modern, automated, and predictable means. This is time-consuming and wasteful. It leads to inconsistent part quality and even increased machine downtime as operators attend to buffing wheels and drill presses, their CNCs idle. Worse, a simple mistake at this stage of the manufacturing process can mean scrapping out a workpiece that took hours or even days to produce.

This article has neither the time nor space to provide a detailed explanation of the dozens upon dozens of deburring and finishing technologies available today, but it will touch on some of the more common ones. For example, a Timesaver or equivalent rotary brush machine is a great way to deburr many flat workpieces. Belt-style versions of these machines perform similar tasks by straight-lining or graining parts, whereas rotary drum and vibratory bowl finishers filled with abrasive media can tackle even the toughest burrs on machined, stamped, or laser-cut parts.

There’s also thermal deburring, an improbable process that applies an explosive charge to blast away burrs. Electrochemical machining (ECM) uses an electrolytic fluid to dissolve burrs and does so in a surprisingly accurate manner. Abrasive flow machining (AFM) is commonly used in the automotive and hydraulics industries to deburr and polish complex internal part features, just as abrasive blasting smooths surfaces on everything from camshafts to cargo ships.

The Need for Non-Wovens

Then there are belts, discs, brushes, and other abrasive media, all of which make short work of burrs and rough surfaces. An expert on these is Michael Radaelli, senior product manager, non-woven abrasives at Norton | Saint-Gobain Abrasives, Worcester, Mass. He noted that the industry continues to develop ever more capable and productive abrasives, among them the company’s Rapid Prep XHD line of non-woven abrasive products. “By leveraging modern abrasive grains, bonding methods, and other technology improvements, shops can often reduce the number of steps needed to achieve the desired surface quality,” Radaelli said. “This helps cut labor time, abrasive inventory, and especially operator fatigue when right-angle grinding.”

He suggested that manufacturers make several common mistakes when selecting abrasives. During deburring operations, for example, operators must have the skill and experience needed to ensure they do not change part geometry and size, lest they inadvertently scrap the workpiece. This is often the result of using too coarse an abrasive, applying too much pressure, or simply choosing the wrong abrasive altogether. Similar problems can occur when removing parting lines on castings, or during blending and finishing operations on welded products.

Radaelli pointed to various field tests with the new Norton products. A manufacturer of emergency vehicles enjoyed a 26 percent faster cut rate and twice the abrasive life on its line of aluminum ladders after switching to XHD abrasives. A well-known locomotive builder eliminated two hand-grinding operations on welded carbon steel structures. A supplier of computer cabinets went from 480 to 1,040 in2 (3,097 to 6,710 cm2) per wheel using non-woven abrasives, with room to spare.

Norton Rapid Finish general purpose convolute wheels offer the flexibility to take on light deburring and blending work, remove deep scratches or surface irregularities, and produce a uniform surface.

Non-woven abrasive technology is becoming more popular in other finishing operations as well, notably cylindrical and centerless grinding. Radaelli said there are many examples where Norton Rapid Finish non-woven convolute wheels improved surface finish and increased time between dressing significantly over legacy resin-bond and vitrified wheels. “If someone wants to remove an 1/8" of material from a workpiece, this is not the wheel for them,” he said. “But in the far more common situation where a customer is taking a finishing pass and wants to improve productivity and part quality, we strongly encourage them to get out of their comfort zone and take a look at more modern, high-performance abrasives.”

Grinding Optional

Of course, it’s often possible to eliminate these types of grinding operations altogether. That’s according to Maury West, vice president of sales and marketing at Cogsdill Tool Products Inc., Camden, S.C., who noted that roller burnishing is a proven way to generate single-digit Ra surface finishes, consistent sizing, and work hardening on virtually any CNC lathe or machining center. What’s more, it just got a lot easier.

“We introduced our Universal Diamond Burnishing Tool (UDBT) around one year ago and they’ve been flying off the shelves ever since,” he said. “It’s typically a square shank tool with a radiused, replaceable diamond tip and indexable head, adjustable 90o in the left or right-hand direction in 15o increments. We also offer it with a standard Capto C6 shank. This allows burnishing faces, contours, and angled surfaces, and if the hole is large enough, internal features up to a certain depth.”

The UDBT can be used on any metal up to the mid-40s HRC, West explained. Like all burnishers, it works by “cold flowing” the peaks of machined surfaces, in essence turning mountain ranges into smooth prairies. Because it displaces material, there is a corresponding decrease in the workpiece diameter during the burnishing operation. For softer metals such as aluminum and mild steel, this comes out to 10 µin for every 10 Ra of roughness—in other words, burnishing a shaft with a 100 Ra surface finish to 10 Ra or less would knock around 0.001" (0.0254 mm) off the diameter. Harder metals like stainless and tool steels, Inconel, and other exotic metals see around half this amount.

The ORBITOOL’s flexible shaft design supplies radial pressure to the cutting tool below.

For flat surfaces, West recommends the Diamond Burnish Face Mill tool (DBFM). As its name implies, the DBFM resembles and acts like a face mill, and is designed to burnish flat surfaces using replaceable diamond inserts similar to the UDBT just described. “We’ve been able to get single-digit Ra finishes in plates 3' [0.914 m] long, and eliminate the residual tooling marks common with face and shell mills,” said West. “This has proven to be much more cost effective and less time consuming than polishing or lapping. Additionally, since there is no material removal with burnishing, it is an eco-friendly process.”

Tackling Hole Deburring

Cogsdill also offers various mechanical hole deburring tools, among them the single-piece “clothespin” style Burr-Off, the spring-loaded Burraway and Micro Burraway, and the Ellipti-Bur for curved or angled surfaces. All use a simple in-and-out action to deburr both sides of holes in a single pass, which can be performed on a CNC machine tool or manually. Carbide and HSS blades are available, with diameters from 0.040" (1 mm) up to 2.0" (50.8 mm).

J.W. Done Corp., based in Hayward, Calif., is another supplier of hole deburring tools, and specializes in removing burrs on intersections deep inside manifolds, valve bodies, and other challenging areas.

General Manager Stan Kroll described the company’s ORBITOOL Cross Hole deburring tool as a hemispherical cutter surrounded by a polished metal ring or disk and attached to a flexible shaft: the shaft applies constant pressure in manual or CNC operations; the fine-toothed cutter removes even very large, rigid burrs; and the disc protects the hole intersection’s adjacent surfaces.

“It’s both self-correcting and self-finding,” he said. “Anytime the tool is inside the hole, it rides on the disk until it encounters a burr. Because the shaft acts as a spring, it allows the disc to then slide past the burr, after which the cutter removes any excess material there. We also have a double-sided ORBITOOL that deburrs the top of the hole as well as any internal intersections.”

The ORBITOOL lineup has grown in recent months. What were once custom, 12" (30.48-cm) long tools have now become a standard offering. In order to maintain sufficient radial pressure, however, these have a shaft diameter 30-50 percent larger than that of their shorter, 6" (15.24-mm) long counterparts. Kroll explained that tool diameters from 0.055 to 0.375" (1.40-9.53 mm) in diameter are available in either length, and that the method of operation remains unchanged. “The heavier shaft also allows the user to be a bit more aggressive, which is particularly important for the removal of heavy burrs in large diameter holes.”

Bringing Plating Operations In-House

Birchwood Technology’s CNC black oxide finishing system uses an integrated programmable hoist and tank line to automatically process iron and steel components. It operates as a self-controlled unit that eliminates the labor costs associated with manual lines.

Though they don’t fall under the metal removal category, plating, painting, and coating operations can also be considered finishing processes. One of these is black oxide, a popular method of protecting part surfaces that some refer to as gun bluing. Unlike competing and generally more expensive processes such as nickel and chrome plating, powder coating, and most anodizing, black oxide produces virtually no change to part dimensions and can be performed with minimal equipment investment.

“Whether you’re talking about black oxide or an alternative process such as zinc phosphate, you’re looking at one to two microns of buildup.” That’s according to Jesse Vouk, general manager of Birchwood Technologies, Eden Prairie, Minn., a supplier of chemicals and equipment for these and other protective finishing technologies. “We also have a whole host of room-temperature products that are primarily cosmetic, but also provide various levels of lubricity and corrosion resistance. For those making hardware items, some oil and gas components, and anyone that needs a low-cost yet durable protectant, it’s an excellent option.”

Because these processes are considered low temperature and do not require electrodes, as with most plating operations, they are easy to bring in-house. “Typically, there will be a couple of heated tanks in a process line, starting with an alkaline cleaner to remove any machining fluids, oil and grease, and other contaminants,” Vouk said. “From there, it goes into a chemical solution, which for our mid-temperature black oxide operates at around 200 to 210°F [93.3 to 98.9°C]. It’s a direct replacement for the traditional hot black oxide process that operates at up to 290°F [143.3°C], so it is nowhere near as hazardous in terms of worker safety and waste treatment.”

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