Contributing Editor: Dave Davidson, Deburring/Finishing Technologist | 509.230.6821 | dryfinish@gmail.com | https://about.me/dave.davidson
If you have parts that need edge or surface finishing improvement and would like to have FREE sample part processing and a quotation developed for finishing the parts please contact Dave Davidson at dryfinish@gmail.com I can also be reached at 509.230.6821. Information about equipment for bringing Centrifugal Iso-Finishing capability to your facility is also available…
INDEX TERMS: Deburring, Edge Contour and Finish, Isotropic Surfacing, Mass Media Finishing, Nontraditional Finishing, Super-Polishing, Surface Finishing, Loose Abrasive, Edge Finishing, Super-Finishing, Burnishing, Abrasive Media, Polishing, Peening
Abstract
This material edited and annotated by SME (Society of Manufacturing Engineers) member and past-president LaRoux Gillespie, CMfgE, PE ( laroux1@earthlink.net ) will be of interest to anyone interested in the importance of deburring and surface conditioning to critical component functionality. Much of the discussion here centers around the risk involved in failing to adequately address edge finish and surface conditioning issues in aeronautical and aerospace manufacturing. The forum discussion was brought to the attention of SME’s Deburring and Surface Finishinggroup by Giovanni Cirani, an SME-Technical Group member and CBF specialist from Italy.
Annotated Forum discussion
The following paragraphs provide some insight into the issues caused by burrs and sharp
edges and surface roughness. This is the result of combining proven research with discussion and industrial experiences, particularly on aircraft parts. The book Hand Deburring: Increasing Shop Productivity (Gillespie, 2003) and De- burring and Edge Finishing Handbook (Gillespie, 1999) note that burrs and sharp edges create many problems. Understanding a customer’s specific concerns and terminology allows the manufacturing plant to adopt different approaches to edge finishing. Sharp edges on finished parts can result from neglecting a sharp edge or by producing a burr with many sharp facets. Burrs and sharp edges are problematic and raise many issues. Burrs on sheet metal parts cause premature tearing during formation. Plating over burrs and sharp edges allows early corrosion attack or a poor fit in assembly. Fine burrs on automotive cylinders (grinding micro-slivers) can cause engine failure. Burrs on life safety devices can provide unrealistic levels of perceived safety. Automotive mechanics regularly sustain cuts and bruises from burred and sharp edged components. Edge quality issues include performance, safety, cost, and appearance, as detailed in the following list of 25 issues:
• cut hands in assembly or disassembly • interference fits (from burrs) in assemblies • jammed mechanisms (from burrs) • scratched or scored mating surfaces (which allow seals to leak) • friction increases or changes (disallowed in some assemblies) • increased wear on moving or stressed parts • electrical short circuits (from loose burrs) • cut wires from sharp edges and sharp burrs · • unacceptable high voltage breakdown of dielectric · • irregular electrical and magnetic fields (from burrs) • detuning of microwave systems (from burrs) • metal contamination in unique aerospace assemblies • clogged filters and ports (from loose burr accumulation) • cut rubber seals and O-Rings • excessive stress concentrations • plating build up at edges • paint build up at edges (from electrostatic spray over burrs) • paint thin-out over sharp edges (from liquid paints) • edge craters, fractures, or crumbling (from initially irregular edges) • turbulence and nonlaminar flow • reduced sheet metal formability • inaccurate dimensional measurements • microwave heating at edges • reduced fatigue limits • reduced volumetric efficiency of air compressors • reduced cleaning ability in cleanroom applications • reduced photoresist adherence at edges, and to the list we would add: • less aesthetic appeal. —
Discussion:
In October 2002 Will K. Taylor in an Internet open forum stated, “It is standard aeronautical practice to: (a) attain 125- microinches Ra machined finish [or better] on cut and machined edges/surfaces; (2) deburr holes and chamfer/radius edges; and (3) round-off [radius] sharp [square-ish] exterior and interior corners.” He then asked, “What engineering and practical benefits are derived from following standard practices?” The many responses to this question generally reflect the above-bulleted issues – some in more depth and a few in questioning or less proven fashion. While the entire discussion is not recreated here, many of the ideas are. For complete details see http://www.eng-tips.com [aircraft engineering forum]. The editor of this paper does not authenticate the answers but believes the commentary is useful for further research.
Fatigue life, stresses, and strain
Fatigue life increases when decreasing surface roughness, and smoother surfaces have less preload loss when they are part of a mechanically fastened joint. Burrs increase stress concentration at hole edges, which already have three times the net section stress at the edge. Therefore, removing burrs decreases stress concentration, which increases fracture resistance and fatigue life. Lastly, burrs can interfere with proper seating of mechanical fasteners, so removing them reduces damage to fasteners and clamped components during assembly. Sharp corners increase stress concentration, so increasing radii decreases stress concentration, which increases fracture resistance and fatigue life. If water creeps under interfaces via higher surface roughness and fills up a cavity or interface, then freezes, it could create high stresses and/or accelerate material fracture, not to mention stress corrosion cracking at scores from the hidden, trapped water/chemicals. One author notes, “Sharp corners, burr holes etc. increase not only the stress but the strain as well. Looking at the strain we can have three different situations:
1. The strain can be inside the linear behavior. (Under the yield limit) 2. The strain can be between the ultimate and the yield limit. 3. The strain can reach the ultimate limit If the third situation is going to occur, the cracks can develop because of the material failure. In this case, the crack can also reach the material’s “critical value”. For this reason, round the corners, deburr the holes, and finishing the surfaces will help to pass from the third to the first situation.” Almost without exception fatigue cracks start at the surface of a part rather than internally. One possible reason may be that the highest stresses are usually found at the surface (e.g. bending and torsion) and the surface is vulnerable to stress raisers such as machining notches, scratches, and pits. Surface finish affects the strength of a part subjected to fatigue loading because most machining operations leave a notch pattern and fatigue cracks usually originate in a notch.”
See part examples below of parts that have been deburred (and in some cases – finished and polished) with various mass finishing processes…
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Quench cracking
If the part is to be heat treated, leaving any sharp external corners can lead to quench cracking because of the much greater local cooling rate.”
Corrosion and coating impact
Poor surface finish introduces millions of new points for crevice corrosion on the surface. Also, a rough surface can make it difficult to get good results with nondestructive testing methods like dye penetrants–especially when the roughness is in a pattern (such as produced by fly-cutting or milling). Rougher surfaces or sharper exterior edges can scratch coated or painted surfaces during assembly and might allow hidden corrosion to spread underneath what might temporarily appear as good finishes. The physics, electrochemistry, etc. are well documented about applying a coating to a sharp edge. When using any type of electrically catalyzed process (anodizing, electrocoating, electrostatic spray painting, etc.) current density fluctuations prevent the build-up of a uniform coating thickness. Variations in coating thickness have many negative aspects, such as variable friction at joint surfaces, areas for localized corrosion, pitting, galvanic cells, etc. Corrosion fatigue and stress corrosion cracking are obvious concerns.
Another author notes, “There is an actual field problem (which has been solved) where an Exacto knife had been used to trim away excess adhesive film in an adhesively bonded wing structure. The resulting superficial scratches on the wing skin (.002″ and under) eventually opened up as cracks leading to fuel leaks from the wet wing. The aircraft concerned is characteristic of an extraordinarily long-lived type and the field fix solves the problem, but the example demonstrates the extent of care required where surface imperfections are concerned. The problem, by the way, turned up 18 years after the aircraft was built.” Any sort of surface treatment (plating, chemical conversion coat, etc.) will require more material on a rough surface to achieve an equivalent film thickness. Enough to make a difference over a long production run. It will take less primer on aluminum parts if the surface is not rough that is a reduction in weight. A rough surface is harder to clean. For aircraft “Dirt is weight”
Good seating
A fastener hole with a good, sharp, burred corner will have obvious problems with seating when met with a fastener that has a radiused junction between head and shank. Poor bonding of structures, in lightning strikes, can cause catastrophic local structural failure.
Joint friction and preloads
Also, with riveted structure, friction (due to the clamping force of the fasteners) between faying surfaces in a joint serves a couple important functions. First, the friction provides a bit of ‘shear preload’–the joint can take a certain amount of shear without loading the fasteners or sheet in bearing. The greater the friction, the more resistant the joint will be to working loose and smoking rivets. This ties in nicely to the second function: high frequency (engine) vibrations throughout the structure are damped or dissipated through joint friction. The greater the friction, the greater the high-frequency fatigue resistance of a mechanically-fastened joint. If a burr is sitting between the fastened sheets preventing good contact of the faying surfaces, much of this friction is lost. A higher surface roughness will lead to higher friction forces to overcome when torquing a bolt. This means that less preload (Fi) will be developed, with a corresponding decrease in load at which gapping occurs (Fi/(1-C)), which increases chances for leaks (stuff coming out, or stuff going in), and also leads to worse fatigue performance (higher alternating tensile stresses). A higher surface roughness may also lead to preload relaxation – exacerbating all of the above. As one reader noted, “This is the classic “shanking and sheet gapping” syndrome, caused by burrs and “liberated burrs” [chips].” Rough surfaces provide less surface area of contact giving rise to higher and very localized contact stresses. If flavored with a little salt mixed in and throw in some corrosion this could be a disaster.
Static discharge
Sharp outside corners on structure act as electrical charge concentrators and can be a static discharge hazard. For the same reason, sharp corners can cause undesirable results in electroplating operations. One reader asks, “If an overly rough surface causes corrosion could this joint develop a static charge? If there are two conductive metal surfaces separated by a dielectric (oxide) and you add some movement or vibration – – presto –static charge because of rough surfaces (as opposed to burrs).
Ability to inspect
It will be easier on highly finished surfaces to find cracks with visual inspections, than in a rough face. Mating faces must be finely machined to:
• avoid friction • avoid heat due to friction. Excessive heat may change the properties of the material surface, with unpredictable consequences. • have better lubrication. The active film in a finely machined surface will be more efficient because there will be more surface in contact with the lubricant. This will permit better heat transfer from the part to the lubricant (there is a limit to how fine a finish a surface should have. The automotive industry intentionally adds some surface patterns to hold the oil in internal combustion engines. • Excessive roughness may develop high material wear, leading to high play, and high replace frequencies of the parts. • Roughness produces friction as stated above. Friction can lead to electricity (triboelectric effect). Electricity can lead to corrosion.”
Electrical issues
As noted above friction between rough surfaces will create electrical energy. That energy can create an accelerated galvanic- corrosion anode or cathode site, if all (most) other surfaces are coated or insulated. Burrs are sources of static discharge Burrs and surface roughness will both interfere with good, uniform surface contact between faying surfaces in a mechanical joint. This increases the electrical resistance of the joint and, if severe, can cause problems with the electrical bonding of structure; interfering with effective grounding of electrical equipment and/or antennae, and become a miniature plasma cutter in the event of a lightning strike.” Current density due to sharp edges and burrs can cut through protective coatings on mating surfaces and radii providing a minute area of “clean metal” electrical path to drive corrosion dramatically worse than if no protective coating were there to begin with because of the extremely high resultant current density. The hole punching force of high current density re- results in stress risers to enhance SCC and corrosion fatigue. For aircraft assemblies, sharp edges become spark over points whenever voltage is applied (static, lightning strikes…)
Zero timing (Aerospace application)
One reader asks “If the fastener holes that are deburred are inspected with the use of HFEC (High-Frequency Eddy Current) prior to installation into the aircraft, could those qualify as being zero timed? I was looking through an older Boeing Structural Repair Document (D6-81987) and there was a statement about increasing the inspection threshold of these fasteners (if the holes were zero timed) from 60,000 flight cycles to 100,000 flight cycles. That would be a significant reduction of cost for a maintenance shop.
Hydraulic and gas leaks
Higher values of surface roughness (and burrs) increase leakage rate under/around gaskets and seals. Nipping gaskets, seals, and o-rings on sharp edges during installation, or scouring them on rougher surfaces during operation of rotating equipment, can accelerate leakage. Sometimes a surface that is finished too well can hinder sealing. O-rings need something to hold on to – if your surface finish is too fine and the compression on the O-ring is too light- the O-ring is likely to fail. In one industry, engineers specify 63ra for most surfaces that will contact a secondary sealing element. (They do however require flatness and surface finish to an extreme on other parts – millionths of an inch for mechanical seal faces). There are times when a sharp edge is needed. Labyrinth seals in gas turbines spring to mind, as do squealer tips on compressor blades.
Increased air drag
Increased air drag or turbulence can occur over rougher, dirtier surfaces.
Ice build up
Rougher surfaces can create a better adhesive surface to build up (and hold on to) such that more ice stays on the aircraft, overstressing the structure or adversely affecting operation.
Peening issues
Excessive surface roughness can be an indication of over-peening, which negates the beneficial aspects of compressive residual stress. Aluminum and magnesium are especially prone to over-peening, which results in many localized areas of increased stress. Problems with fracture (stress intensity) and fatigue (crack nucleation sites) are then possible/probable. As others have already mentioned, joint problems can arise from excessive surface roughness, and over-peening is yet another method for creating surface roughness. Shot peening, surface polishing, deburring and rounding off adds a sustained compressive stress into the material. This stress will counteract the tensile stress caused by a crack and help to contain its propagation.
References
Gillespie, L. K. 1999. Deburring and Edge Finishing Handbook. Dearborn, MI: Society of Manufacturing Engineers (SME).
Gillespie, L.K. 2001. Deburring: a 70 year bibliography, Deburring Technology International, Kansas City, Missouri.
Gillespie, L. K. 2003. Hand deburring: increasing shop productivity. Dearborn, MI: Society of Manufacturing Engineers (SME).
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Centrifugal Iso-Finishing Technology for deburring and finishing parts and components, castings and AM 3D printed parts
Centrifugal isotropic Finishing
Centrifugal Isotropic Finishing (CIF) is a high-energy finishing method, which has come into widespread acceptance in the last few years. Although not nearly as universal in application as vibratory finishing, a long list of important CBF applications have been developed in the last few decades.
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Similar in some respects to barrel finishing, in that a drum-type container is partially filled with media and set in motion to create a sliding action of the contents, CBF is different from other finishing methods in some significant ways. Among these are the high pressures developed in terms of media contact with parts, the unique sliding action induced by rotational and centrifugal forces, and accelerated abrading or finishing action. As is true with other high energy processes, because time cycles are much abbreviated, surface finishes can be developed in minutes, which might tie up conventional equipment for many hours.
Centrifugal Barrel Finishing principles – high-intensity finishing is performed with barrels mounted on the periphery of a turret. The turret rotates providing the bulk of the centrifugal action, the barrels counter-rotate to provide the sliding abrasive action on parts.
The principle behind CBF is relatively straightforward. Opposing barrels or drums are positioned circumferentially on a turret. (Most systems have either two or four barrels mounted on the turret; some manufacturers favor a vertical and others a horizontal orientation for the turret.) As the turret rotates at high speed, the barrels are counter-rotated, creating very high G-forces or pressures, as well as considerable media sliding action within the drums. Pressures as high as 50 Gs have been claimed for some equipment. The more standard equipment types range in size from 1 ft3 (30 L) to 10 ft3, although much larger equipment has been built for some applications.
Media used in these types of processes tend to be a great deal smaller than the common sizes chosen for the barrel and vibratory processes. The smaller media, in such a high-pressure environment, are capable of performing much more work than would be the case in lower energy equipment. They also enhance access to all areas of the part and contribute to the ability of the equipment to develop very fine finishes. In addition to the ability to produce meaningful surface finish effects rapidly, and to produce fine finishes, CBF has the ability to impart compressive stress into critical parts that require extended metal fatigue resistance. Small and more delicate parts can also be processed with confidence, as the unique sliding action of the process seems to hold parts in position relative to each other, and there is generally little difficulty experienced with part impingement. Dry process media can be used in certain types of equipment and is useful for light deburring, polishing, and producing very refined isotropic super-finishes.
Below are some process video footage demonstrations of high-speed centrifugal isotropic finishing. These automated edge and surface finishing methods are capable of producing very refined low micro-inch surfaces that can improve functional part performance and service life.
Dave Davidson: Contributing Editor | dryfinish@gmail.com | 509.230.6821 https://about.me/dave.davidson | https://dryfinish.wordpress.com
CONTRIBUTING EDITOR BIOGRAPHY – David A. Davidson, [dryfinish@gmail.com]
Mr. Davidson is a deburring/surface finishing specialist and consultant. He has contributed technical articles to Metal Finishing and other technical and trade publications and is the author of the Mass Finishing section in the current Metal Finishing Guidebook and Directory. He has also written and lectured extensively for the Society of Manufacturing Engineers, Society of Plastics Engineers, American Electroplaters and Surface Finishers Association and the Mass Finishing Job Shops Association. Mr. Davidson’s specialty is finishing process and finishing product development.
More about Dave Davidson…
I am a deburring and surface finishing specialist, consultant and advisor to SME’s [Society of Manufacturing Engineers] Technical Community Network. The focus of my activity is assisting manufacturers and machine shops with reducing their dependence on hand or manual deburring and finishing methods, and helping them to upgrade the edge and surface finish quality of their parts. I currently work from Colville, WA but I assist clients nation-wide. I can arrange for free sample processing and process development for your challenging deburring and finishing needs and can provide you with either contract finishing services or the in-house capability to produce improved hands-free finishes on precision parts. I can be contacted at 509.230.6821 or dryfinish@gmail.com. Let me know if I can be helpful.
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