91406 f1 Deburring rev 1.qxp
11/27/2012
9:23 AM
Page 57
Deburring &
Finishing
EDGE FINISHING—
PRODUCT ENHANCEMENT
OR WASTED COST?
Image courtesy LaRoux Gillespie
Edge preparation is
critical to many parts;
in fact, edge prep
absolutely adds quality
to the product
LaRoux K. Gillespie
Secretary/Treasurer
SME Executive Committee
Dearborn, MI
E-mail: laroux1@myvine.com
Burrs on commercial miniature ground tap.
E
dge finishing is a relatively new term in manufacturing. It’s a new and deeper focus on
what many used to call deburring, edge honing, edge preparation, edge prepping, burring, chamfering, or edge blending. Edge finishing goes beyond any of those definitions.
Deburring, which is often considered wasted effort by managers, wrongly carries a negative connotation. In reality, deburring and edge-finishing processes add many benefits to
parts—they create highly desirable edge quality—the quality most products need.
June 2009 • www.sme.org/manufacturingengineering
57
91406 f1 Deburring rev 1.qxp
11/27/2012
9:23 AM
Page 58
Deburring & Finishing
Clearly, users do not want burrs,
but most really do not want perfectly
sharp “burr-free edges.” Most users
want smooth edges that assemble cor-
58
rectly and easily—automatically perhaps—and edges that don’t cause premature failure. Many deburring
processes provide exactly that—high-
www.sme.org/manufacturingengineering • June 2009
quality edge conditions that ensure
long life. But few users ask the question: “How can I double my product
life by adding quality to part edges?”
Consider parts that have absolutely
no burrs, but have perfectly square,
sharp edges. Such parts are simply not
acceptable for assemblies other than,
perhaps, welded assemblies. These
“burr-free,” perfectly sharp edges cut
wires, cause plating buildup, accentuate
RF disruptions, cut mating parts, gouge
parts stacked upon them or in contact
with them, create high stresses, reduce
fatigue life, etc. As a result, even if there
were no burrs, companies would have
to finish edges (improve edge quality)
to improve performance.
The underlying issue is not burrs,
but what edge quality companies
want, and what tradeoffs they are
willing to make to achieve both necessary performance and low part cost.
“Edge finishing” and “deburring” in
their fullest context are two different
views of the same need—delivering
edges that customers need or want.
What is edge quality? There are few
definitions of edge quality as a generic
field. “Edge quality” is a general term
expressing the needed conditions for
edges (i.e. the intersection of two surfaces) of parts. It’s different from surface properties and bulk characteristics. The elements that constitute edge
quality, however, have some of the
same characteristics as the rest of the
part—specifically geometry or topology and surface integrity. Surface
integrity actually includes a number of
subsurface issues, as can be seen from
the list in the sidebar titled Attributes
of Edge Quality.
The full list of issues is more extensive than that in the sidebar, and
depends upon the material being
machined. My book Mass Finishing
Handbook, available from SME, lists
42 different flaws that users want to
avoid, and all but three are found on
edges as well as other part surfaces.
91406 f1 Deburring rev 1.qxp
11/27/2012
Koya Takazawa was one of the
first individuals to discuss the term as
a result of his early work on Toyota
air-conditioning compressors. Exact
control of the edge configuration of
critical parts provides a significant
increase in compressor efficiency
(5–15%) and performance of other
products. Surface integrity issues
affect the static and dynamic life and
strength of parts.
Conventional metalcutting and
grinding all leave residual stresses
below the surface. Tensile stresses typically cause early part failure, while
compressive stresses provide restraining
stresses that fight tensile loading. In
other words, for most applications
compressive stresses below the surface
increase part life, and may somewhat
improve part strength. Readers can see
the effect of these residual stresses
induced by machining by gently polishing samples of parts, and looking at the
strain lines near a part edge. In almost
every case, the metal grains are
stretched out into bent lines. That distortion of the metal results in residual
stresses. It is most easily seen by
machining brass with a rounded or
worn cutting edge. The depth of the
disturbed layer can be approximated by
the formula y~1/3(F/K), where y is the
depth of the layer that has residual
stresses, F is the magnitude of the resultant cutting force per unit width, and K is
the static material yield shear stress.
The cutting tool industry is one of
the most advanced researchers of edge
finishing, and tens of millions of dollars are spent each year to finish the
edges of cutters. While toolmakers
have honed the edges of cutters in
some manner for thousands of years,
edge “honing” became a standard in
the 1970s with brush honing and polishing. Edge configurations went from
sharp (as-ground), honed (radiused),
chamfered, and chamfered and honed,
to those with a variety of lands adjacent to the edge. Correct finishing of
9:23 AM
Page 59
cutting-tool edges reduces edge chipping and flank wear, resulting in
longer tool life. The correct cutting
edge also reduces plowing in the work-
piece (which results in smaller forces),
improves surface finishes, and reduces
residual stresses in the machined product. Correct edge preparation depends
June 2009 • www.sme.org/manufacturingengineering
59
91406 f1 Deburring rev 1.qxp
11/27/2012
9:23 AM
Page 60
Deburring & Finishing
on the cutter material properties, workpiece properties,
machining parameters, and the many variations in coatings.
Each of the 109 different deburring and edge-finishing
processes now in use by industry produces a unique set of
edge conditions. Brushing with abrasive-filled fibers, for
example, is widely used to remove damaging, but minute,
sharp-edge variations on cutting-tool inserts. In short, the
sharp, ground insert edges must be radiused slightly. Most, if
not all, cutting-tool inserts undergo automated brushing to
provide uniform radii at low cost. In this industry, brushing
is noted for providing the needed edge radii—it is a process
that adds quality to the part—and more uniformity in the
edge translates into much longer and more uniform tool life.
Cutting-tool makers are also finding that high-energy
mass-finishing processes translate into beneficial higher
subsurface compressive stresses that result in longer tool
life. When end mills were submitted to drag finishing
their life doubled, according to Walther Trowal (Haan,
Germany). The longer life is the direct result of removing
sharp edges, as well as uniform radius generation and
60
www.sme.org/manufacturingengineering • June 2009
smoothing of the flutes to allow easier chip flow. In this
application, edge radii produced were controllable from
15 to 60 µm ±0.5 µm. The 3.5-min of run time resulted
in more than double the tool life.
The underlying issue is what edge
quality companies want.
Today other companies are investigating the centrifugal
barrel and turbo-abrasive processes, and even high-frequency vibratory finishing for improving fluted-tool life.
Some of the benefits of these processes are believed to be the
result of not only surface improvements, but subsurface
issues arising from better compressive residual stresses. Endmills, drills, spade drills, broaches, hobs, and even circular
saw blades are reportedly being finished by some loose-abrasive or mass-finishing process. Slurries of fine abrasives are
also used, as are abrasives impregnated in rubber wheels,
hand stoning, and both dry and wet blasting.
91406 f1 Deburring rev 1.qxp
11/27/2012
What is true for the carbide insert
industry is not necessarily true for
other cutting tools. Perfectly sharp
edges are not desired for most carbide
insert tools, as noted above, but sharp
edges on single-crystal diamond (SCD)
tools are highly desirable. They can
impart mirror surface finishes on aluminum. Why a difference between tool
materials? Carbides are made of many
small particles pressed and bound
together, while single-crystal tools are
made—as the name implies—from a
single crystal that shears or grinds
with far fewer edge nicks.
Polycrystalline diamond (PCD) tools
are often inspected at 50× magnification. Dave Novak of ST&F Precision
Technologies & Tools (Arden, NC)
notes: “The best finish a ground PCD
can produce is 15–50 µm Ra when the
50× magnification is the acceptance criteria, but tools inspected as free of nicks
at 150× magnification can produce
10–14 Ra surfaces. In contrast, SCD
tools produce finishes of 4 µm because
9:23 AM
Page 61
they are free of minute gaps and nicks
on the edge. Razor-sharp edges on SCD
tools are used for manufacturing special
metal mirrors and optical lenses.” Some
single-crystal tools show no discernable
edge roundness when viewed at
15,000× magnification.
Electropolishing with weak acids
enhances the surface finish of many surgical instruments. Hypodermic needles,
for example, are electropolished by the
millions to smooth surfaces, and to
remove any burrs or small slivers without generating large radii. Large radii
create more pain when the needle
enters your arm.
Other medical applications include
the use of electrochemical edge finishing with NaCl and NaNO3. Recent
work shows that this process can be
effective on titanium medical clips and
surgical-steel knives, as well as other
titanium and stainless products. Unlike
typical electrochemical deburring
(ECD), recent processes utilize ECD
with advanced programmable con-
Attributes of Edge Quality
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Acceptable edge geometry
Lack of burrs
Correct radius, chamfer, sharpness, or other shape
Uniformity along entire edge(s)
Lack of edge chip-out, fracture, or damage of any kind
Acceptable edge-surface finish
Acceptable surface texture
Freedom from foreign surface smeared material
Freedom from foreign imbedded material
Acceptable subsurface integrity
Freedom from cracks
Presence of appropriate residual stresses (usually compressive)
Freedom from molten metal or plastic
Freedom from heat affected zones
Freedom from “white metal”
Freedom from smeared metal
Correct morphology
Correct structure
Correct grain orientation
Freedom from crystallizing effects
Freedom from chemical changes
Freedom from chemical or physical absorption
Freedom from oxidation, hydration, or stains
Uniformity of all attributes across the entire edge
(unless otherwise defined)
June 2009 • www.sme.org/manufacturingengineering
61
91406 f1 Deburring rev 1.qxp
11/27/2012
9:23 AM
Page 62
Deburring & Finishing
trollers to provide tailored waveforms (an asymmetric, interrupted voltage waveform). The electrochemically based
processes all have one unique characteristic that is useful for
many applications—they do not introduce any residual
stresses. They often remove harmful tensile stresses while
eliminating sharp projections. This process reportedly finishes
knife edges in 2 sec.
Edge quality is an issue
not just for external edges,
but for many internal holes.
Magnetic abrasive finishing provides even finer surface
finishes, while removing EDM recast from parts without
damaging any surfaces. Magnetic abrasive finishing can
produce finishes of 0.4 µm Ra, while the mass finishing
processes cited above provide 4–8 µm finishes (Ra). The
magnetic approach also allows removal of as little as
0.000001" (0.000025 mm) from surfaces, if necessary.
62
www.sme.org/manufacturingengineering • June 2009
Edge quality is an issue not just for external edges, but for
many internal holes as well. Fuel-injection ports depend
upon nozzle configuration, as do many orifices. Orifices
affect the pressures and velocities downstream, as well as
spray patterns. Each of these orifices has well-defined edge
conditions. Edge radii or shape, hardness, resistance to erosion, and roughness are all issues for heavily used injection
ports or orifices. Diesel injection nozzles, for example, use
100–200 µm diam holes having 0.1–0.4 µm Ra surfaces. The
entrances also require such precision finishes. The wrong
edge configuration on these parts can influence not only fuel
efficiency, but emissions into the air. Edges affect the pressure
drop through holes as well as the actual size of the fluid
stream coming out of the hole. My Countersinking
Handbook (available from SME) describes 27 benefits of
careful edge control on holes.
Fatigue life specimens are commonly edge-smoothed to
remove the life-reducing impact of edge effects. Sharp edges
on the test sample can provide misleading endurance information on new projects. ASTM, for example, recommends
using a 0.006" (0.152-mm) radii
for these specimens.
Sheetmetal edges are sometimes
dimpled or coined to induce compressive stresses. Coining edges of
aircraft materials reportedly has
increased fatigue life by a factor of
four. A variety of edge configurations
can be produced by these two methods, but the benefit lies in increasing
the life of the product, rather than in
traditional edge shapes.
Data by R.E. Cohen, D.K. Matlock, and G. Krauss of the Colorado
School of Mines (Golden, CO) have
shown that rounded edges produce a
more uniform case-hardening depth
after carburizing than square corners. Rounding the edges allows uniform carbon diffusion that reduces
scatter in fatigue data. Their 1992
work provides several insights into
the metallurgical differences between
square and round edge samples.
Square-corner samples retained
almost 50% more austenite at edges
than did the rounded samples.
Hardness at the edge of square edges
was lower than for rounded edges,
and the endurance limit was 13%
lower than for rounded samples.
91406 f1 Deburring rev 1.qxp
11/27/2012
Edge finishing requires high precision for many parts, but a large portion of the processes used to finish
edges do not need high-precision
tooling to accomplish that. Brushing
and mass finishing are conformal
processes; they conform to the slight
variations in part location or tolerances. They do not need precision
control strategies to provide precision
results. Some remove tarnish, smooth
surfaces as well as edges, and provide
other part benefits. In short, edge finishing does more than just remove
burrs—it makes the product function
correctly, and last for its intended
lifetime. Some processes provide the
edge needed during a soft or green
state, while others will do it for only
very hard edges.
Many parts have more mundane
requirements than expressed above.
They simply have to assemble easily
into complex mechanisms, and a radius
or chamfer facilitates assembly. Thus,
edge preparation for these parts (as
opposed to just deburring) adds quality
to later operations. Appropriate edges
reduce RF emissions and cross talk,
prevent plating buildup, and reduce the
9:23 AM
Page 63
chance for corrosion (because sharp
edges act like antenna for electrical currents that hasten corrosion). Smoothed
edges significantly increase the forma-
bility of sheetmetal parts, and dimpled
countersink edges in thin aluminum
sheet can increase fatigue strength by as
much as 58%.■
WANT MORE INFORMATION?
LaRoux Gillespie has authored
many books including: Mass
Finishing Handbook, which provides how-to details of all mass
finishing/loose-abrasive finishing
processes, Deburring and Edge
Finishing Handbook, which provides an in-depth guide to deburring technologies, and Countersinking Handbook, which provides total coverage of issues
related to countersinking and
chamfering holes. For more information or to place an order,
contact SME Customer Service at
800-733-4763, 8 am–5 pm Eastern Time, Monday–Friday, or go
to www.sme.org/store and follow the prompts.
June 2009 • www.sme.org/manufacturingengineering
63