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Deburring Processes and Challenges

 

Issues intermesh when choosing the right high-volume deburring method


By Michael Tolinski
Contributing Editor 

 

All manufacturers are sensitive to the edge conditions of their components, even though they may not regard deburring as a process. Automakers would like to claim they only use "burr-free" components, but LaRoux Gillespie, author of several publications about deburring over the last four decades, including the new Mass Finishing Handbook from SME and Industrial Press, says it doesn't require much effort to find excessively sharp edges on new car parts that can cut mechanics' hands or cause mechanical problems. In particular, automotive transmissions seem particularly prone to warranty problems. Their inner workings don't tolerate burrs on gears or other components, since burrs can interfere with fluid flow or cause noise, excess wear, overheating, or complete failure.

Given the importance of properly finished parts, it's fortunate that suppliers have several deburring methods to choose from. Common methods overlap somewhat in their burrremoval capabilities, but they vary in equipment cost, speed, and shop flexibility, meaning there's often no clearcut choice for a job.

For gear deburring, for example, "there are a variety of approaches that work well," says Gillespie. Common methods haven't changed much through the years, but the choice of method hasn't gotten any easier. Choices range from special rotary chamfering tools to electrochemical, thermal, brush, and other methods.

The most economical choice for gears and similar components hinges on answers to at least three major questions, he says. "The whole issue really depends on what's the volume, what's the edge condition you're trying to produce, and how big is the burr."

People often ignore the importance of knowing the complete dimensions of a burr. "Thickness is the biggest issue," says Gillespie. A long burr that's very thin is usually no trouble to remove, but one that's much thicker than it is long may have to be machined off. (Unfortunately, burr thickness is hard to measure; it may require using a measuring microscope or making a rubber mold of the part, sectioning it, and measuring the burrs with an optical comparator.)

Part size and shape are also issues. Certain areas of the part may be prone to large burrs—for example, at sharp, low-angle corners. For parts with different-size burrs at different locations, there's the danger of the deburring method removing too much stock overall just to remove the largest burr. Unfortunately, people often just focus on the worst edge or problem, and neglect the fact that all deburring methods remove more than just burrs, so critical dimensions can be lost in the process.

The selection of the best deburring method can be a trial-and-error approach. But for more informed, systematic decision-making, Gillespie recommends that people lay out all part requirements, volumes, and cost targets. Comparisons should be made between more than just a couple of reasonable choices from the 117 defined deburring methods he has identified.

Thermal energy deburring, for instance, is a straight-forward concept for bulk-deburring small parts. Here, parts in baskets or fixtures are placed in a sealed chamber. In a few milliseconds, fuel gas combusted with oxygen burns off the burrs without affecting the part's thick sections. Thermal deburring can eliminate loose, thin material, though it's limited by thicker burrs, says Gillespie. "It'll do a good job on grinding burrs, because they're very thin—in the neighborhood of 0.0005" [0.013 mm] thick—and it will put a little break on the edge." 

The Thermal Energy Method (TEM) of Extrude Hone Corp. (Irwin, PA) works with nearly all machined, forged, and cast metal workpieces. "TEM results are similar to standard vibratory tumble deburring, but with a lot more consistency and uniformity," the company's William Vande comments. Material removal can be controlled, though TEM is not necessarily the most precise process for transmission components, he says. For example, for producing a specific dimensioned radius of 1/8" (3.2 mm) where ±0.010" (0.25 mm) is the tolerance, other methods are recommended.

On the other hand, it's fast. "The real benefit of TEM is that we can produce the required results in a lot less time than tumbling or other abrasive methods." Theoretically, says Vande, a few hundred parts can be processed in less than 60 seconds, and "unlike vibratory tumbling, which only processes the exterior surfaces of a workpiece, TEM will also process internal surfaces."

Side issues with thermal deburring can include heat distortion and surface oxidation. Workpiece size can be another limitation, although the chamber of Extrude Hone's largest TEM unit, the P-400 XL, is 16" (406 mm) in diam by 20" (508-mm) high. Other issues with TEM such as safety and electronic controls have been solved over the last 15 years, says Vande.

TEM may be the low-cost solution (apart from the initial equipment investment), but Vande warns that different deburring methods shouldn't be compared apples-to-apples. "Each process is used to obtain different results, and is highly dependent on customer needs and objectives. It would be like comparing a drill press to a lathe.

"If a customer simply wants to remove burrs on the edge or teeth of a gear to prevent tooth chipping and improve appearance, then TEM is probably the best choice." In more critical applications like turbine engines, Vande recommends the abrasive and electrochemical methods the company offers.

A completely different approach for gears is to deburr at or near the hobbing machine. In large-scale gear production, deburring and chamfering with hard rotary tools on dedicated equipment is simply another step in the machining process. Rotary tools deburr and chamfer gear teeth along desired edges, eliminating later problems with handling and gear-face clamping. Such tools are supplied by the Gleason-Hurth division of Gleason Corp. (Rochester, NY).

According to Thomas Daniek of Gleason-Hurth project engineering in Munich, Germany, the tool setup consists of rotary deburring tools meshed with the workpiece gear in a parallel axis. The chamfering tool creates burrs on the gear face, which are then removed with secondary deburring disks. Burnishing wheels can also be used for additional finishing, or to push or roll secondary burrs back into the gear.

The various rotary tools are chosen based on workpiece requirements for chamfering and edge quality. This choice also depends on whether wet or dry machining is used, and on whatever shaving, honing, or other finishing processes are employed. The tools are used on dedicated, automated Gleason machines, some of which are connected to or integrated with the hobbing process.

Integrated hobbing and chamfering/deburring machines (CDM) seem like a total, space-saving solution. "The integrated CDM unit becomes part of the hobbing machine and its internal automation system," says Brian Cluff of Star-SU Inc. (Hoffman Estates, IL). Star-SU offers S-series hobbing/CDM machines from European machine-builder Samputensili. For example, the S-200 CDM accommodates workpieces as large as 250 mm in diam. 

Cluff emphasizes that this approach's good economics depends on high volumes. "It's cost-effective to invest in this type of combined hob/chamfer/deburr machine for mass-production gears, where the line is dedicated to a specific part over the life of the production-project cycle."

Certain gears resist neat, all-in-one deburring solutions. Bevel gears, for example, can't be deburred with rotary tools, says Hermann Stadtfeld, vice president in bevel gear technology for Gleason. "The chips [that] a face cutter for bevel gears produces are longer and thicker than the ones from cylindrical hobs; this creates a burr on the heel that is thick and difficult to remove by deburring."

In most cases, two different methods for chamfering and removing the secondary burr are used by bevel gear manufacturers. The chamfering might require a fly cutter, a chisel-shaped tool, or a small end mill, says Stadtfeld. Deburring might require shear disks, brushes, or files, mounted onto a machine's workhead.

Consequently, some gear and transmission component operations require flexible approaches to deburring. Brush deburring is one such approach, says Rick Sawyer, Engineered Solutions Manager, Weiler Corp. (Cresco, PA). It works well on gears, and can be particularly effective with transmission valve bodies.

Deburring with rotating brushes sidesteps some capital equipment investment, and fits more of a single-partflow type of production, Sawyer explains. Unlike batch deburring, it doesn't require a large backlog of work-in-process parts. "Parts can come off the hobbing, grinding, or turning operation and go right into the deburring system, so that you have a more balanced throughput system."

Brush-deburring machines are more easily integrated near existing primary equipment, he observes, rather than located in some other department. "Sometimes the operator who runs the hobbing machine can also run the deburring machine. Or a robot will take the part right from the last machining operation to the deburring machine." Robots can also take parts directly to the deburring media, depending on the end-user's floor plan and process flow.

The key to brush deburring is matching media with component. Nylon abrasive filament (NAF) radial brushes are commonly used on hardened parts such as transmission gears, and wire-filled radial wheel brushes are especially effective when used on gears prior to heat treating, says Sawyer. Aluminum transmission valve bodies use NAF disk brushes, where the filaments that contain abrasive particles are parallel to the rotating brush hub axis.

Wet methods can deburr areas the above dry methods can't reach. One flexible approach simply uses the kinetic energy of CNC-controlled high-pressure jets of water. "Its speed and effectiveness are substantially better due to the fact that you're cleaning the workpiece at the same time you're deburring it," says Rick Willard, an engineer with Sugino Corp. (Itasca, IL).

He points to high-pressure water's advantages for deburring transmission components. With gears, he says, water is very effective at removing burrs and contamination from cross-drilled oilgallery holes. With valve bodies, rotating water nozzles deburr the "worm trail" face, while Sugino's rotating lance nozzles "attack the spool valve bores right at the source of the burr."

At IMTS, Sugino introduced its improved high-pressure Jet-Clean system, which supplies water at pressures to 10,000 psi (68 MPa) and velocity to 1100 ft/sec (335 m/sec). Originally conceived as a small machining center, the unit includes a 600-rpm tooling spindle and optional CNC rotating indexing table.

High-pressure water has some limitations, however. "By itself, high-pressure water will not remove heavy root burrs caused by dull tooling or rough milling," Willard says. In these cases, hard tooling is used in conjunction with high-pressure water.

A different kind of wet method, electrochemical deburring (ECD), is essentially electroplating in reverse. ECD removes material from the workpiece using the directed flow of an electrolyte. At first glance, given its use of chemicals and electricity, the method may seem like an OSHA nightmare for a "dry" machining shop. But ECD is widely used by automotive components manufacturers in Europe, and process variations eliminate the hazardous chemicals, according to Phillip Miller of Faraday Technology Inc. (Dayton, OH).

As with other dedicated-machine methods, ECD is most easily justifiable when deburring high-volume, commodity-type parts, Miller says. By using properly designed tooling, the process can be controlled to remove metal along specific part edges.

He says the company's Faradayic process for edge and surface finishing makes ECD a better fit for more shops. The process uses a nonhazardous electrolyte. This is unlike most EC processes that use electrolytes like sulfuric and phosphoric acid, which focus electrical energy on a specific area of the workpiece, dissolving the metal. By contrast, "We use a pulsed or non-steady-state voltage-controlled electric field and a very conductive electrolyte, a simple water-based solution." This manipulated electric field controls metal removal.

 

Deburring Down Under

For deburring internal passages at cross-holes, candidates include thermal, abrasive flow, electrochemical, and high-pressure water methods. But an application at a firm located near Melbourne, Australia, shows how effective a relatively simple mechanical carbide tool can be.

Camshafts made at Ford Motor Company's Geelong engine plant contain eight 4-mm drilled radial holes that intersect axial holes and the central bore. "Ford was experiencing engine failures, and the root cause was traced back to the then-current cross-drilling process," says Leigh Milvain, national technical manager for Okuma Australia Pty. Ltd. (Rowville, Victoria, Australia). Burrs would break off and jam the variable cam timing gear, he explains.

Okuma, a supplier of the plant's capital equipment, helped investigate the root cause and solution. The company first considered adding a thermal deburring station, but its ideal solution was to integrate deburring within the existing robotic drilling cell—specifically, to follow each drilled-hole operation with a cross-hole deburring tool.

For the deburring tool, Okuma chose the Orbitool from J.W. Done Corp. (Hayward, CA). The spindle-mounted tool consists of a driveshaft linked to a semispherical carbide cutter via a flexible elastomeric coupling. The coupling allows the tool to flex when it's inserted into the drill hole. As the tool is lowered into the hole, a protective disk shields the hole's ID from the carbide cutter until it reaches the cross-hole intersection, where the cutter contacts the sharp burr-edge of the intersection, and the spinning tool is orbited to cut around it.

Experimentation determined the best drilling parameters to make the complete system work. "If drills are pushed too fast, either in speed or feed, the size of the burr produced affects the time taken to remove it," says Milvain.

 

This article was first published in the October 2006 edition of Manufacturing Engineering magazine. 


Published Date : 10/1/2006

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