Feature Photo: Burrs are more than an industrial nuisance. They can be the source of significant problems when not properly removed, this is especially true within the aerospace industry. What follows is an annotated open-forum discussion of burrs common to aerospace manufacturing operations and the potential consequences when not ameliorated. (Photo credit: Extrude Hone)
This material edited and annotated by past SME President LaRoux Gillespie, CMfgE, PE 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 on 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 the DESC group by Giovanni Cirani, an SME-DESC Technical Group member and CBF specialist from Italy.
Abstract. 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.
• 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 buildup 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 clean room applications
• reduced photoresist adherence at edges, and to the list we would add:
• less aesthetic appeal. —
The sources describing the research on effects of burrs are listed in Deburring: A 70-Year Bibliography (Gillespie, 2001).
Above: Deburring and finishing can be accomplished in precision machining environments with Centrifugal Isotropic Finishing. This video was produced by the SME chapter in Spokane, WA on the factory floor of MacKay Manufacturing
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 [squareish] 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.
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 material failure. In this case, the crack can also reach the material’s “critical value”. For this reason, round the corners, deburring the holes, and finishing the surfaces will help to pass from the third to the first situation.”
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 non-destructive testing methods like die 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.
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”
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 torqueing 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.
Cast parts with sharp edges
Because of metal floor, gasses, and mold design sharp edges and corners make any sort of forging or casting of a part difficult if not impossible.
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.
Issues between moving parts
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 fine 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.
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 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 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…)
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.
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, mass finishing, 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.
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).
Further reading: Internet resources
(1) “Isotropic Mass Finishing for Surface Integrity and Part Performance”, Article From: Products Finishing, Jack Clark, from Surface Analytics, LLC and David Davidson, from SME Deburr/Finish Technical Group, Posted on: 1/1/2015, [Barrel, vibratory, centrifugal and spindle finish can improve part performance and service life.] http://www.pfonline.com/articles/isotropic-mass-finishing-for-surface-integrity-and-part-performance
(2) “Turbo-Charged Abrasive Machining Offers Uniformity, Consistency” Article From: Products Finishing, by: Dr. Michael Massarsky, President from Turbo-Finish Corporation, and David A. Davidson, from SME Deburr/Finish Technical Group. Posted on: 6/1/2012. [Method can deburr, produce edge contour effects rapidly] http://www.pfonline.com/articles/turbo-charged-abrasive-machining-offers-uniformity-consistency
(3) “Turbo-Abrasive Machining and Finishing”. MANUFACTURING ENGINEERING – Aerospace Supplement, by: Dr. Michael Massarsky, President from Turbo-Finish Corporation, and David A. Davidson, from SME Deburr/Finish Technical Group. [Method first developed for the aerospace industry can improve surface integrity and part performance] http://www.slideshare.net/dryfinish/turboabrasive-machining-me-aerospace-supplement-reprint
(4) “The Role of Surface Finish in Improving Part Performnce”, MANUFACTURING ENGINEERING, by Jack Clark, Surface Analytics.com and David A. Davidson, from SME Deburr/Finish Technical Group.
(5) “Free Abrasives Flow for Automated Finishing”, MANUFACTURING ENGINEERING, , by: Dr. Michael Massarsky, President from Turbo-Finish Corporation, and David A. Davidson, from SME Deburr/Finish Technical Group. [Exciting new methods of surface finishing that go beyond deburring to specific isotropic surface finishes that can increase service life] http://www.slideshare.net/dryfinish/october-2013-f2-deburring-1
(6) Turbo-Abrasive Machining Demonstration Video: https://www.youtube.com/watch?v=jYxqCxMIHNo
(7) SME Spokane, WA Factory Floor video, Centrifugal Finishing in the Precision Machine Shop: Demonstration) https://www.youtube.com/watch?v=dUdKjaysTYM
AUTHOR BIOGRAPHY – David A. Davidson, [email@example.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.