Developments in Mass Finishing

1. MECHANICAL FINISHING
Developments in Mass Finishing Technology
by David A. Davidson
Deburring/Surface Finishing Specialist; E-mail; ddavidson@mgnh.dyndns.org
MASS FINISHING FUNDAMENTALS
Mass finishing describes a group of industrial
processes by which large lots of manufactured
parts can be processed in bulk economically to
achieve a variety of surface effects. These eco-
nomical processes, in contrast with hand debur-
ring methods, develop these effects with a high
degree of part-to-part and lot-to-lot uniformity
and consistency. These effects might include edge
break, edge contour, surface smoothing and
improvement, tool mark blending, burnishing,
polishing, superfinishing, and microfinishing. In
these types of processes, energy is imparted to an
abrasive-embedded or abrasive-coated loose
material known as media that is contained with-
in the work chamber of a finishing machine.
Energy is then transferred from work chamber
motion to the media and to the work-pieces
placed in the media by way of a random rubbing
or scrubbing action. This achieves some sort of
edge or surface improvement and refinement.
The surface and edge effects produced are typi-
cally nonselective in nature, unless a part has
been partially masked or fixtured. While edge
geometries can be modified (contoured) to some
extent, it would be a mistake to consider these
processes for substantial material removal oper-
ations that are best left to traditional grinding
and machining methods.
Typical dry media used for dry finish and polish
applications in barrel, vibratory, and centrifugal
high speed equipment is shown in Figure 1. The
top row shows media shapes manufactured from
hardwood, the bottom row various granular
materials from agricultural sources. These media
are made effective by treating them with very
fine abrasive powders that can, on properly pre-
pared surfaces, produce very refined and highly
reflective surfaces on many metal and plastic
substrates. Because of their relatively light-
weight bulk density, these materials are not typ-
ically specified for use in every-day general
deburring application, where ceramic and plastic
media shapes are more commonly used. However,
in some circumstances, special blends of these
materials have been paired for use with high-
energy mass finishing equipment because of the
environmental advantages of processing parts in
a waterless system. Additionally, some manufac-
turers have developed specialized abrasive and
plasic resin shapes to be able to perform some
abrasive operations in a high energy dry environ-
ment.
Finishing plastic components to achieve high-gloss
surface finishes can be accomplished in barrel sys-
tems with dry media. Multistage processing with a
sequence of steps, each utilizing successively finer
abrasive materials, is crucial to developing low micro-
inch Ra finishes such as those shown in Figure 2. This
type of sequential step finishing has been adopted and
utilized with other mass finishing methods, to produce
similar surface effects, on metal parts as well.
PART FIXTURING AND SURFACE FINISHING
Included in these mass finishing methods are tradi-
tional barrel tumbling, vibratory, and centrifugal
July/August 2003 49
Figure 1. Photo courtesy of Tyha S. Davidson. Figure 2. Photo courtesy of PEGCO Process Laboratories.
Davidson.qxd 7/16/2003 1:54 PM Page 1

2. 50 Metal Finishing
finishing. A closely related set of processes would be
fixture-centric processing such as the spin-finish,
drag-finish, spindle-finish, and the turbo-finish
methods. The fixture methods produce results by
imparting motion to parts that are fixtured (by
either dragging, rotating, or developing a planetary
motion) and are immersed in loose abrasive or pol-
ishing media. The force with which part edges and
surfaces interact with loose media can be consider-
ably higher than that developed by mass media
processes, where parts are placed randomly within
the media mass, and are dependent on the loose
media motion to achieve the surfacing results.
Fixturing parts in more conventional barrel or
vibratory methods is also not uncommon. This is
done for a variety of reasons, including the need to
prevent any part-on-part contact but also to
increase the amount of force flow of media against
part surfaces, to accelerate cycle times, and produce
more pronounced surface finish effects.
Part applications for fixture finishing in conven-
tional equipment vary widely. For example, some
manufacturers of brass musical instruments (trum-
pets, French horns, trombones) fixture brass instru-
ment assemblies in barrel or vibratory chambers
and flow soft polishing granulate media through the
assemblies to replace multiple buffing operations.
Similarly, some manufacturers of medical and sur-
gical implant devices fixture the devices in high-
energy centrifugal barrels and produce very refined
surfaces on cobalt chrome and titanium substrates
by processing the devices through a sequence of suc-
cessively finer loose abrasive operations. Fixtured
processing in vibratory equipment can accommo-
date even very large structural parts. This is an
important application for large structural aerospace
parts. The method can be used to reduce the need for
costly manual deburring and finishing methods on
airframe components. More importantly, with the
proper sequence of abrasive and nonabrasive opera-
tions, it can be used to develop very significant com-
pressive stress and work hardening characteristics
to the parts, enhancing their wear and fatigue fail-
ure resistance dramatically.
OLD DOG — NEW TRICKS, SEQUENTIAL PROCESSING
One trait that many of today’s more sophisticated
mass finishing operations share is a reliance on
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3. July/August 2003 51
multiple-step sequential processing. In this type of
processing, very rough surfaces can be brought to a
highly polished or microfinished state. This is done
by initially processing the parts with coarse abra-
sive material, and then following up with a sequence
of finer abrasives. Each of the subsequent steps uses
an abrasive material that has been calculated to
clear and blend in the abrasive pattern left in the
surface by the preceding step. To use a common
everyday analogy, almost everyone understands
that to produce fine finishes in woodworking appli-
cations, it is necessary to use sanding operations
with successively finer abrasive grits to produce
cabinet or furniture quality surfaces. The same prin-
ciple holds true in mass finishing (or even hand-fin-
ishing) metal parts when very smooth or polished
surfaces are required.
One time-honored method for producing very
refined surfaces is dry barrel processing. This tech-
nology was originally developed and heavily utilized
in the northeastern U.S. as early as the 1930s; simi-
lar methods were developed concurrently in Europe.
The method was developed primarily to miti-
gate the high labor costs associated with hand
buffing large numbers of consumer-oriented arti-
cles such as eyewear and jewelry. This technique
was widely accepted as a standard method for
producing very refined consumer acceptable
product finishes that had previously been the
sole province of those buffing methods. It is still
utilized for these types of applications. This
sequential principle has been adapted for use in
other types of equipment for other part finishing
applications. Where reflective surfaces are
desired on parts being finished in vibratory
equipment, it is not unusual now to see second-
ary vibratory processes with burnishing media or
dry process polishing media develop those sur-
faces. Many processes have been developed for
centrifugal disk and centrifugal barrels where
three or more steps are utilized in order to bring
part surfaces to very low micro-inch surface pro-
files or to develop very reflective surfaces for cos-
metic reasons.
MASS FINISHING PROCESSES AND COMPRESSIVE
STRESS EFFECTS
Even simple tumbling can develop residual stresses
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8. 52 Metal Finishing
that can provide some functional improvements to
service life in certain components. High-energy
mass finishing methods can magnify this effect
many times. In the early 1990s some researchers
pioneered the use of scanning electron microscope
(SEM) analysis to evaluate surface finishes for
durability and functionality.
This early work showed that it was possible to
improve functionality and service life of many dif-
fering types of components by a two-fold improve-
ment in metal surface profile and integrity.
Processes, such as peening, are commonly used
for metal surface integrity improvement to miti-
gate crack propagation points and improve serv-
ice life by improving wear and metal fatigue
resistance.
It was found that high-energy loose media sequen-
tial finishing could develop not only compressive
stresses but very level or negatively skewed
plateaued surfaces. This provided a great deal more
bearing load surface to parts that interacted with
other part surfaces.
In one application, stamping dies used for forming
aluminum can tops were given a useful life of
approximately ten times the value of parts that had
not been surface finished with this method. Another
application cited by Richard Gilliam in a technical
paper describing centrifugal barrel processing noted
extensive cycling tests conducted by a spring manu-
facturer. “This ability to improve resistance to
fatigue failure is graphically demonstrated by the
results of some tests made by a manufacturer of
stainless steel coil springs. A group of springs was
taken from a standard production run. Half of the
sample was finished in the manufacturer’s usual
manner of barreling followed by shot peening, while
the other half was CBF-treated for 20 minutes. The
springs were then tested to failure by compressing
them to a stress change from 0 to approximately
50,000 psi. The results showed that all the springs
finished by the conventional method failed between
160,000 and 360,000 cycles. The springs that had
been processed by CBF failed at between 360,000
and 520,000 cycles, an average improvement of
60%.”
Substantial compressive stress effects can also be
generated in lower-energy types of equipment with
very dense metal media. In commenting on this,
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9. July/August 2003 53
John Rogers, Process Laboratory Manager for the
Abbott Ball Co., Inc., noted some points made in a
company publication: “Steel media is smooth and
heavy. It is not abrasive in action. Rather, the
media’s weight and strength increases the smooth-
ness and pressure of its finishing action. Workpieces
keep their tolerances intact, gain compressive
stress, and achieve the ultraclean, microscopically
smooth surface.
The wide selection of media shapes and sizes
allow full control over the type and amount of con-
tact obtained between the media and the part.
Steel media is heavy, weighing approximately 300
lb/ft3. The media mass forms a dense cushion that
produces rapid finishes yet does not harm fragile
parts.
As steel media impinges on a part, its surface is
work-hardened. The working action imparts com-
pressive stress as a beneficial byproduct of the fin-
ishing process. In many instances, the process can
replace steel shot peening as a work-hardening step.
Parts processed with steel media have longer cycle
lives and greater resistance to wear as a result of
this compressive stress action.
ELECTROCOAGULATION: AN EMERGING WASTEWATER
TREATMENT TECHNOLOGY
One of the most pressing problems faced by those
involved in surface finishing is compliance with an
increasingly stringent maze of regulations designed
to protect underground resources from industrial
contaminants. Penalties for failure to adhere to
waste effluent treatment and disposal can be dra-
conian. This can be a particularly vexing problem for
those involved in both the plating and mass finish-
ing industries because of the difficulty and complex-
ity involved in removing the heavy metals in sus-
pension or solution in their wastewater effluent
stream before discharging the water back to the
municipality. A variety of strategies involving chem-
ical treatment and the use of flocculent along with a
final dewatering process, such as filtration or evap-
oration, are commonly used. All of the strategies
employ a method for separating water from dis-
solved or suspended solids by coagulation, agglom-
erating solid particles together to precipitate them
or make them much more filterable. One emerging
technology for these kinds of applications is known
as “Electrocoagulation” — using electric current
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10. 54 Metal Finishing
instead of chemical flocculent consumables to pro-
duce this effect.
A leading proponent of the technology, Scott W.
Powell of Powell Water Systems, contrasted the dif-
ference between chemical and electrical coagulation
methods in a recent technical paper: “Coagulation
caused by altering the charge on metal ions, organ-
ics, and colloidal particles creates a large particle
that can be settled or filtered out. Chemical coagu-
lation typically uses a dissolved salt. Part of the salt
will attach to the material in the water to be coagu-
lated. The other part of the ion typically remains in
the solution. Chemical coagulation creates a hydrox-
ide sludge that attracts water. The hydrophilic
sludge holds water, which increases the volume of
sludge generated and increases the dewatering
time.
“Electrocoagulation adds electrons to the solution
by passing alternating current or direct current
through the solution from the power grid. The elec-
trons destabilize the material in the water creating
oxide sludge when sufficient activation energy is
present. … Heavy metal ions converted to metal
oxides will pass the leach tests making them non-
hazardous. Metal oxides can be smelted to recover
the metals in a usable form.”
The bottom-line for finishers and platers is the
possibility of deploying a much less complex and
less costly alternative to other wastewater treat-
ment methods.
TURBO-FINISH MACHINES AND TURBO-ABRASIVE
MACHINING
Dr. Michael Massarsky, the inventor of the Turbo-
Finish process, initially developed the process for
improving edge and surface finish methods for
rotating parts in the aircraft engine industry. The
process replaces much of the manual deburring for-
merly required on these types of parts. TAM
machines could be likened to free abrasive turning
centers. They utilize fluidized bed technology to sus-
pend abrasive materials in a specially designed
chamber. Parts interface with the abrasive material
on a continuous basis by having part surfaces
exposed and interacted with the abrasive bed by
high-speed rotational or oscillational movement.
This combination of abrasive envelopment and high-
speed rotational contact can produce important
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11. July/August 2003 55
functional surface conditioning effects. Deburring
and radius formation happen quickly. Unlike buff,
brush, belt, and polish methods, or even robotic
deburring, abrasive operations on rotating compo-
nents are performed on all features of the part
simultaneously. This produces a feature-to-feature
and part-to-part uniformity that is extraordinary.
TAM processes share characteristics common to
both machining and mechanical finishing processes.
A much higher degree of control is possible than is
the case with conventional finishing processes. TAM
processes can utilize very sophisticated computer
control technologies to create processes which are
custom tailored to the needs of specific parts. Like
machining processes, the energy to produce the cut-
ting or abrasive action that develops the desired
surface effect arise primarily from the rotational
energy of the part itself. Unlike both machining
processes and manual deburring processes with
their single point of contact, TAM processes perform
abrasive machining or grinding on all features of the
part by abrasive media envelopment.
TAM processes were developed originally to
address deburring and surface conditioning prob-
lems on complex rotating components within the
aerospace industry. Aerospace parts, such as turbine
and compressor disks, fan disks, and impellers, pose
serious edge finishing problems. Manual methods
used in edge finishing for these parts were costly
and time-consuming. Even more importantly,
human intervention, no matter how skillful at this
final stage of manufacturing, is bound to introduce
some measure of nonuniformity in both effects and
stresses in critical areas on the part. TAM provides
a method whereby final deburring, radius forma-
tion, and blending in of machining irregularities
could be performed in a single machining operation.
This machining operation can accomplish in a few
minutes what in many cases took hours to perform
manually. It soon became obvious that the technolo-
gy could address edge-finishing needs of other types
of rotationally oriented components such as gears,
turbo-charger rotors, bearing cages, pump impellers,
propellers, and many other rotational parts.
Nonrotational parts can also be processed by fixtur-
ing them to the periphery of disk-like fixtures.
Another important feature of the process is its use
of high-intensity small abrasive particle contact to
produce surface effects. This results in the ability to
process intricate or complex part shapes easily.
Although the abrasive material used for processing
is similar in some respects to grinding and blasting
materials, TAM produces an entirely different and
unique surface condition. One of the reasons for this
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12. 56 Metal Finishing
is the muitidirectional and rolling nature of abra-
sive particle contact with part surfaces. Unlike sur-
face effects created with pressure or impact meth-
ods, such as air or wheel blasting, TAM surfaces are
characterized by a homogeneous, finely blended,
abrasive pattern developed by the nonperpendicular
nature of abrasive attack. Unlike wheel or belt
grinding, surface finishes are generated without any
perceptible temperature shift at the area of contact
and the microtextured random abrasive pattern is a
much more attractive substrate for subsequent coat-
ing operations than linear wheel or belt grinding
patterns.
TAM processing can be especially useful when part
size, shape, or complexity preclude the use of other
mechanical finishing processes. TAM deburrs and
develops edge and surface finishing effects very rapid-
ly and has unique metal improvement and compres-
sive stress generation capabilities. Aqueous waste
treatment and disposal costs are avoided by a com-
pletely dry abrasive operation. The process is primari-
ly intended for external surface and edge preparation,
although some simpler interior areas and channels can
be processed as well. Complex geometric forms can be
easily accessed. Repeatability and uniformity can be
even further enhanced with PLC or computer-con-
trolled processing, and with all features of the part
receiving identical and simultaneous abrasive treat-
ment, feature-to-feature, part-to-part, and lot-to-lot
uniformity on parts can be extraordinary.
BIBLIOGRAPHY
Davidson, D.A., “Mass Finishing Processes,” 2002 Metal
Finishing Guidebook and Directory, New York,
Elsevier Science; 2002
Boitsov, V.B. et al., “Calculation of Residual Stress
Formation at Vibro-Strengthening,” Dynamics,
Strength Wear Resistance of Machines, (Electronic
edition), Cheliabinsk State University Press, Russia,
(abstract-English, Full text Russian, Vol. 5, pp. 69-72,
December 1998)
Massarsky, M.L. and D.A. Davidson, “Turbo-Abrasive
Machining-A New Technology for Metal and Non-
Metal Part Finishing,” The Finishing Line, Dearborn,
MI: Society of Manufacturing Engineers, Association
for Finishing Processes, October 30, 2002, pp. 1–18
(Electronic Edition, pdf file)
Massarsky, M.L. D.A. Davidson “Turbo-Abrasive
Machining and Finishing,” Metal Finishing,
95(7):29–31; 1997
Gilliam, R,. “The Future of Mass Finishing – A
Comparison Between Finishing Methods,” Chiron
International Corp., Huntington Beach, CA; 2000
Powell, S.W., “Water Reuse Eliminates Government
Required Treatments for Wastewater Discharges,”
Clean Tech 2003 – 10th Annual International
Cleaning Technology Exposition, Conference:
McCormick Place, Chicago, IL., Mar. 3-5, 2003,
Conference Proceedings, pp. 270-272
Rogers, J., “Steel Media and Its Finishing Applications”,
Abbott Ball Co., Hartford, CT: , pp. 1–5; 2000 MF
ABOUT THE AUTHOR
Mr. Davidson is a deburring and 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.
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