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.
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.
Gillespie, CMfgE, PE and past president of SME is shown here at the SME
Mfg4 Conference and Exposition in Hartford CT. Shown along with LaRoux
is Dr. Michael Massarsky, inventor of the Turbo-Abrasive Machining
method for deburring and finishing rotating aerospace hardware.
• 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 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).
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
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 www.eng-tips.com
[aircraft engineering forum]. The editor of this paper does not
authenticate the answers, but believes the commentary is useful for
Fatigue life, stresses and strain
life increases when decreasing surface roughness and smoother surfaces
have less preload loss when they are part of a mechanically fastened
- Typical exit burr, such as those developed from drilling operations. Photo courtesy of Extrude Hone
corners increase stress concentration, so increasing radii decreases
stress concentration, which increases fracture resistance and fatigue
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.
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
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
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.”
- Burr removal and edge contour developed with Abrasive Flow Machining. Photo courtesy of Extrude Hone Corporation.
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
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.
exit burr condition on this part of aerospace rotating hardware from
broaching operations. PHOTO courtesy of Turbo-Finish Corporation
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.”
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"
burrs removed, edge contour generated and isotropic surfaces developed
on disc surfaces blending in positively skewed machining or grinding
marks (notches) as well producing a more functionally useful surface
profile that is negatively skewed. Edge and surface effects were
produced by dry granular abrasive materials with the Turbo-Abrasive
Machining method. Note that the Turbo-Abrasive Machining method
involves rotating parts on a spindle through a fluidized bed of abrasive
granular material. Normally the part would be rotated and then
counter-rotated to achieve uniform edge effects. The part shown in the
above photo has been rotated in one direction only. A counter-rotation
cycle is required to produce the edge-contour on both sides of the tooth
geometry. Photo courtesy of Turbo-Finish Corporation.
Joint friction and preloads
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.
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.
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
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.
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
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 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.
high energy sequential processing (such as those used in centrifugal
barrel finishing) it is possible not only to to deburr and develop
edge-contour, but also to produce isotropic polished (or burnished)
surfaces that are very functioal in the aerodynamic sense. PHOTO by Dave
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
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.”
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...)
Zero timing (Aerospace application)
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.
Higher values of surface roughness (and burrs) increase leakage rate under/around gaskets and seals.
gaskets, seals, and O-rings on sharp edges during installation, or
scouring them on rougher surfaces during operation of rotating
equipment, can accelerate leakage.
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).
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 or turbulence can occur over rougher, dirtier surfaces.
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.
parts with extraordinary size and shape considerations are now
candidates for mass finishing processes, as this machined titanium
bulkhead for a fighter jet demonstrates. The advantages of utilizing
vibratory mass finishing processes on this type of component include
developing stress equilibriums, and a consistent feature-feature and
part-to-part consistency and uniformity of edge-contour and surface
finish not possible not possible with conventional hand or manual
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
others have already mentioned, joint problems can arise from excessive
surface roughness, and over-peening is yet another method for creating
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
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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