The document discusses how specialized mechanical finishing processes can improve the performance and lifespan of parts and tools subjected to wear, fatigue, or fracture. It explains that common machining leaves surfaces with undesirable properties like positive skewness and directional textures, while specialized processes can produce negatively skewed, isotropic surfaces with compressive residual stresses. One such process, PEGCO Super-Hone, uniformly treats all surfaces and edges to develop these beneficial properties, subtly modifying tools without compromising dimensions and significantly extending tool lifespan. Overall, optimizing factors like smoothness, bearing ratio, skewness, and texture through specialized finishing can dramatically improve parts in demanding applications.

1. MECHANICAL FINISHING ¥~r'
Microfinishing and Surface Textures
by David A. Davidson
PEGCO Process Laboratories, Bartlett, N.H., E-mail.pegco@worldpath.net
T
he role of mass finishing processes as a
method for removal of burrs, developing edge
contour, and smoothing and polishing parts
has been well established and documented for many
years. These processes have been used in a wide
variety of part applications to promote safer part
handling (by attenuation of sharp part edges);
improve the fit and function of parts when assem-
bled; and produce smooth, even microfinished sur-
faces to meet either functional or aesthetic criteria
or specifications. Processes for developing specific
edge and/or surface profile conditions on parts in
bulk are used in industries as diverse as the jewel-
ry, dental, and medical implant industries on up
through the automotive and aerospace industries
(see Figs. 1 and 2).
Figure 1. (Before) This micrograph was taken with an electron
miscroscope at 50OX magnification. It shows the surface of a
raw, unfinished "as cast" turbine blade. The rough initial surface
finish as measured by profilometer was in the 75-90 Ra (min.)
range. As is typical of most cast, ground, turned, milled, EDM,
and forged surfaces this surface shows a positive Rsk (Rsk -
skewness - the measure of surface symmetry about the mean
line of a profilometer graph. Unfinished parts usually display a
heavy concentration of surface peaks above this mean line,
generally considered to be an undesirable surface finish charac-
teristic from a functional viewpoint.)
Less well known and less clearly understood is
the role specialized variants of these types of
processes can play in extending the service life
and performance of critical support components
or tools in demanding manufacturing or opera-
tional applications.
Industry has always been looking to improve sur-
Figure 2. (AfterlThis SEM micrograph (500X magnification) was
taken after processing the same turbine blade in a multistep
procedure utilizing orbital pressure methods with both grinding
and polishing free abrasive materials in sequence. The surface
profile has been reduced from the original 75-90 Ra (min.) to a
5-9 Ra (min.) range. Additionally, there has been a plateauing of
the surface and the resultant smoother surface manifests a neg-
ative skew (Rsk) instead of a positive skew. This type of surface
is considered to be very "functional" in both the fluid and aero-
dynamic sense. The smooth, less turbulent flow created by this
type of surface is preferred in many aerodynamic applications.
Another important consideration is that surface and subsurface
fractures seem to have been removed. Observations with
backscatter emission gave no indication of residual fractures.
face condition to enhance part performance, and
this technology has become much better under-
stood in recent years. Processes are routinely uti-
lized to specifically improve life of parts and tools
subject to failure from fatigue and to improve their
performance. These improvements are mainly
achieved by enhancing part surface texture in a
number of different and sometimes complementary
ways.
To understand how microsurface topography
improvement can impact part performance, some
understanding ofhow part surfaces developed from
common machining, grinding, and other methods
can negatively influence part function over time. A
number of factors are involved.
POSITIVE VE SUS NEGATIVE SURFACE SKEWNESS
The skew of surface profile symmetry can be an
important surface attribute. Surfaces are typically
characterized as being either negatively or posi-
tively skewed. This surface characteristic is
referred to as Rsk (Rsk - skewness - the measure
10 Metal Finishing

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Figure 3.These profilometer graphs illustrate in 2-D the difference
between an as-cast positively skewed surface and the same typ-
ical surface processed with a sequence of high-energy loose
media operations to produce an improved surface topology that
is potentially very useful for mission critical components that
require improved wear, fracture, or fatigue resistance.
of surface symmetry about the mean line of a pro-
filometer graph). Unfinished parts usually display
a heavy concentration of surface peaks above this
mean line, (a positive skew) (see Fig. 3). It is
axiomatic that almost all surfaces produced by
common machining and fabrication methods are
positively skewed. These positively skewed sur-
faces have an undesirable effect on the bearing
ratio of surfaces, negatively impacting the per-
formance of parts involved in applications where
there is substantial surface-to-surface contact.
Specialized high-energy finishing procedures can
truncate these surface profile peaks and achieve
negatively skewed surfaces that are plateaued,
presenting a much higher surface bearing contact
area. Anecdotal evidence confirms that surface fin-
ishing procedures tailored to develop specific sur-
face conditions with this in mind can have a dra-
matic impact on part life. In one example the life
of tooling used in aluminum can stamping opera-
tions was extended 1,000% or more by improved
surface textures produced by mechanical surface
treatment.
DIRECTIONALIZED VERSUS RANDOM (ISOTROPIC)
SURFACE TEXTURE PATTERNS
Somewhat related to surface texture skewness in
importance is the directional nature of surface tex-
tures developed by typical machining and grinding
methods. These machined surfaces are character-
ized by tool marks or grinding patterns that are
aligned and directional in nature. It has been estab-
lished that tool or part life and performance can be
substantially enhanced if these types of surface tex-
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Figure 4. 3-~ microsurface topographical maps are coming into
increasing use to better quantify surfaces as they relate to part
service life and performance. This surface is one that has been
processed to blend-in parallel rows of surface peaks left behind
from fine grinding operations. The resultant surface is one that
is more isotropic or random in nature. This type of surface can
be an important surface attribute to parts that are subjected to
repeated stress or strain and parts that undergo high force load-
ing of opposing surfaces.
tures can be altered into one that is more random in
nature. Postmachining processes that utilize free or
loose abrasive materials in a high-energy context
can alter the machined surface texture substantial-
ly,not only reducing surface peaks, but generating a
surface in which the positioning of the peaks has
been altered appreciably (see Fig. 4). These "isotrop-
ic" surface effects have been demonstrated to
improve part wear and fracture resistance, bearing
ratio, and improve fatigue resistance.
RESIDUAL TENSILE STRESS VS. RESIDUAL
COMPRESSIVE STRESS
Many machining and grinding processes tend to
develop residual tensile stresses in the surface area
of parts. These residual tensile stresses make parts
susceptible to premature fracture and failure when
repeatedly stressed. Certain high-energy mass fin-
ishing processes can be implemented to modify this
surface stress condition and replace it with uniform
residual compressive stresses.
NEW PROCESSES
PEGCO Super-Hone is a group of proprietary
processes that improves edge condition and surface
physical properties on tooling. The processes involve
July 2002 11

3. Figure 5. The need for some edge preparation on cutting tools
in many applications has been well documented. Most of the
processes currently utilized (including manual ones) concen-
trate on modifying the edge to produce an edge land to
strengthen the edge. Although these types of processes
improve cutting edge geometries they do not typically address
surface profile topography issues at the cutting edge such as
profile skewness, isotropiciy, load bearing ratio, and residual
stress correction. Specialized high-energy methods can and do.
complete envelopment and processing of all edge
and surface areas of the tool and are nonselective in
nature. These processes develop surface compres-
sion and stress relief on all surfaces, as well as uni-
formly microhoning and polishing all edge areas,
regardless of tool or edge geometry (see Fig. 5).
Although the processes promote changes that are
significant and crucial to improved tool perform-
ance, the changes in the physical cutting edge are so
subtle that dimensional integrity ofthe tool is main-
tained. These alterations of the cutting edge surface
area and supporting structures contribute to an
improvement in mechanical strength and can avert
premature wear or fracture of the tool.
The processes also have a tendency to improve
overall symmetry of the cutting features. This is
especially contrasted with manual honing methods
in which abrasive filament wheels, abrasive sticks,
or even coated abrasive paper are utilized to pre-
condition edges by hand methods. Surface profile
improvement is isotropic in nature, minimizing the
fatigue and fracture problems associated with uni-
directional surface patterns in surfaces created by
production grinding methods. Smoother, more
isotropic surfaces provide greater lubricity for
improved chip flow, often facilitating higher feed
and speed rates with processed tools.
This edge conditioning procedure is an outgrowth
of research to improve the tribological functionality
of wear surfaces, with a view to extending useful
service life and preventing premature failure of crit-
ical components that are highly stressed or
strained. It has been found that surface treatment
processes that can develop some measure of com-
pressive stress and an isotropic microfinish to edges
and surfaces can improve load bearing ratios and
other wear-resistant factors far more than improve-
ment in one of these areas alone.
When utilized to develop tool surface and edge
improvements these processes can dramatically
improve tool performance and maximize productivi-
ty in a number of areas: greater tool life and dura-
bility, increased machine feed rates, increased tool
speed rates, improved surface finish on parts,
reduced tool change-over and downtime cycles, and
improved tool performance meeting high tolerance
requirements.
SUMMARY
Many parts or tools that are subject to fatigue,
fracture, or wear can gain substantial improve-
ments in life and performance from alterations to
their overall surface texture. Improvements in
overall smoothness, load bearing ratio, surface pro-
file skewness, and isotropicity can in many
instances improve life and performance and cut
operational costs dramatically. Manufacturers that
have not subjected their parts to an analysis to
determine the potential benefits ofthis kind of pro-
cessing may be making parts "that are not all that
they can be."
BIBLIOGRAPHY
Davidson, D.A., Metal Finishing 2002 Guidebook and
Directory, 100(lA):104-117; 2002
Massarsky, M.L. and D.A. Davidson, Abrasives, OctINov;
1999
Davidson, D.A., "High Energy Dry Process Polishing,"
SME Technical Paper MR90-389, Dearborn, Mich.,
Society of Manufacturing Engineers; 1990
Massarsky, M.L. and D.A. Davidson, "Turbo-Abrasive
Machining Theory and Application," SME Technical
Paper MR95-271, Dearborn, Mich., Society of
Manufacturing Engineers; 1995
"Surface Metrology Guide," http://www.predev.com/smg.
Milan Mich., Precision Devices Inc.; 2001
"PEGCO Finishing Links," http://www.nvo.comlkpegco/use-
fullinks, Bartlett, N.H., PEGCO Process Laboratories;
2001
MF
12 Metal Finishing