This document discusses developments in dry process mass finishing techniques. It begins by defining mass finishing as using abrasive media to improve the surface of large batches of parts. It contrasts dry processes, which do not use water, to wet processes. Dry processes are best for precision or decorative finishes while wet processes are better for material removal. The document provides examples of using dry media and compounds in barrel finishing and centrifugal barrel finishing to produce precision and reflective finishes on metals and plastics. It emphasizes that dry finishing techniques allow for near-buff quality finishes without hand polishing and help recover precious metals. The document aims to increase awareness of innovative dry finishing technologies.

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1989
MR89-147
Developments in Dry Process
-Mass Finishing
author
DAVID A. DAVIDSON
Vice President
PEGCO Process Laboratories Division
Bartlett, New Hampshire
abstract
Dry process mass finishing techniques are helping manufacturers meet difficult and
stringent surface finish cntena. These techmques are being used in an increasing
number of applications in which low RMS precision fintshes are required, as well as In
decorative fimshmg where highly refined, near-buff quality surfaces are needed. This
paper discusses several applications in which dry process fimshing materials and
techniques have been utilized in both conventional mass fimshlng equipment and high
energy centrifugal equipment. Also discussed are the practical considerahons regarding
each equipment type and the benefits which can be accrued as a result of implementing
these technologies.
conference
DEBURRING AND SURFACE CONDITIONING ‘89
February 13-16, 1989
San Diego, California
index terms
Barrel Finishing
Deburring
Finishing
Polishing
Vibratory Finishing
Society of Manufacturing Engineers l One SME Drive l P 0. Box 930
Dearborn, Michigan 48121 l Phone (313) 271-l 500
n

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SM E TECH NICA L PA PERS
This Technical Paper may not be reproduced in whole or in part in any
form including machine-readable abstract. without permission from the
Society of Manufacturing Engineers. By publishing this paper, SME does
not provide an endorsement of products or services which may be discussed in
the paper’s contents.

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MR89-147
What & Mass Finishing 3
This paper will deal with “mass finishing” processes that
utilize dry media or combinations of dry media and compounds.
At times there has been some confusion as to exactly what
surface finishing processes fall under the category of “mass
finishing”.
For the purposes of our discussion vithin this paper, ve
will require a definition that is somevhat more detailed.
“Mass Finishing’* is a term used to describe a group of
industrial procedures by which large lots of metal or other
parts can be processed to achieve a variety of surface
effects. In these types of processes, energy is imparted to
an abrasive embedded or coated material within the system
known as media. Energy is then transferred from the media to
the workpieces to achieve some sort of surface improvement
and refinement.
METHODS CONTRASTED
Although the role that “vet” finishing media (such as
ceramic and resin-bonded preforms) play in the metal
finishing industry is vell known and documented, many
engineers are not familiar vith dry process finishing methods
and materials. These media and processes have a very
different set of process capabilities and optimum uses that
the more familiar ceramic and plastic media utilized in vet
systems.
While vet process media and methods are used to best
advantage in situations requiring aggressive material removal
and abrasive smoothing, dry methods and material are best
used for developing highly reflective decorative surfaces, oz
in applications vhere micro-inch precision surfaces are a
requirement. The use of dry materials as a simple deburring
tool vould often be inappropriate, vith some exceptions.
Among these are some applications that make use of high-
energy finishing equipment, and other processes involving
plastic parts, where water can not be used because of its
potential for creating undesirable changes in the physical
characteristics of plastic substrates.
The most obvious and striking difference between "vet" and
‘*dry” processes is the use of, or lack of water as an
integral element of the process. Water is an important
element of *‘vet” process finishing because it is used suspend
soils and abrasive and metallic fines until they can be
removed from the system, keeping both parts and media
relatively clean. One disadvantage to this type of system is
that it has an unvelcome by-product: effluent.
In contrast with this, dry processes do not make use of any
water. Additionally, they do not make use of synthetic media

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which has had abrasive grains or particles embedded within
the media. Dry media are coated with powdered abrasive
materials which are bound to the media by specially
formulated bonding agents. As is the case with vet process
media, dry media are available in various random (sawdust,
corn cob, walnut shell) and preformed (hardwood shapes)
types.
NEED APPLY INNOVATIVE FINISHING TECHNOLOGIES
Nearly all consumer-oriented products require substantial
surface refinement to assure end-product acceptance. High
quality surface finish quality is becoming of increasing
importance for even non-consumer oriented items. Surface
finish criteria have become more stringent as many
manufacturers find themselves competing vith offshore
products with high surface finish quality.
For many years it vas felt that the only mechanical
finishing method capable of producing very high quality
surface finishes vas buffing. From the manufacturer's
viewpoint hand buffing or manual polishing has some serious
drawbacks. Quite often in these types of operations, each
vorkpiece must be individually handled and Worked, leading to
non-uniformity problems and unacceptable reject/rework rates.
A more economically viable alternative to which many
manufacturers are turning is "mass finishing".
Despite this self-evident need to improve surface finishing
capabilities, it seems that much of American industry lacks
an awareness of the variety of finishing technologies nov
available. In studies conducted on behalf of the Society of
Manufacturing Engineers (SME) and others it is apparent many
American manufacturers have failed to keep abreast vith
developing finishing technologies. As a result, many American
companies are much less sophisticated in their utilization of
mass finishing techniques than many of their off-shore
competitors. Despite the undeniable correlation betveen
surface finishing.quality and end-user or consumer
acceptance, many American manufacturers fail to see the need
to exploit nevly developed finishing technology aggressively.
As is true with other areas of manufacturing operations,
sufficient capital and human resources are not being
allocated to allow for future competitiveness. This is
unfortunate, long-term investments made now in applying
innovative technologies to common manufacturing problems are
going to have a payback far out of proportion to the size of
the original investment.
Nev developments in process and abrasive technology point
to substantial economies and surface quality improvements
available to manufacturers vho adopt mass finishing methods
that employ dry finishing media and dry abrasive or polishing
compounds.

5. DRY PROCESSES AND DECORATIVE FINISHING
Almost any type of part fabricated by common manufacturing
methods (casting, blanking, stamping, molding, machining,
extruding etc.) requires some type of abrasive smoothing
and/or deburring in order to be marketable. Mass finishing
methods, utilizing common “vet” process media such as plastic
or ceramic, have long been recognized as the most cost-
effective and uniform vay to clear away surface
irregularities, blend in machining marks and imperfections,
and improve surface smoothness.
Although the resulting surface condition is a decided
improvement over that left by fabricating operations, it
often has an unacceptable dull and unattractive appearance.
This type of surface condition occurs because of a random
pattern of scratches and score-marks imparted to the surface
by the action of the abrasive finishing media. The dull
appearance ensues because light is being diffracted or
diffused by the sharp “peaks and valleys” visible in a
magnified cross-section of the part surfaces.
Dry mass, finishing OK polishing processes that can lessen
the size of these “peaks and valleys” or lessen the acuteness
of their singularity, vi11 improve surface reflectivity
appreciably. There are tvo different explanations given for .
the theoretical mechanism behind the reduction of these peaks
and valleys. The first is that polishing operations are an
extension of grinding or cutting operations, the difference
being that the abrasive materials being employed are much
finer. As a result, the surface scratches, (or peaks and
valleys) are much finer and represent a mucih smaller
deviation from a perfectly flat surface that would reflect -
light. The second of the two explanations ‘holds that
reflective surfaces are the result of physical changes vhich
occur in a micro-thin layer on the surface of the part during
polishing operations. During these operatio,ns it is thought
that. this layer becomes “plasticized” due. to friction induced
heat in localized areas, causing “peaks” to flow into
adjacent “valleys”, and improving surface reflectivity. Some
credence might well be given to both explanations, the first
could veil explain hov finishing methods with comparatively
lover energy levels (ie. horizontal barrel, vibratory) could
develop highly reflective surfaces vith dry media. Thesecond
explanation may shed some light on what happens when abrasive
particles are interacted vith part surfaces at high rates of s
speed lie. conventional buffing, centrifugal barrel finishing
[CBFI, centrifugal disc [CDMI 1.
PRACTICAL DRY PROCESS APPLICATIONS
Much work has been done in recent years on developing and
improving dry process medias and compounds. These type of
materials have found a growing list of applications in all of

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the major types of mass finishing equipment. (including
conventional rotary barrels, bowl and tub vibrators, and high
energy finishing equipment such as centrifugal barrel (CBF),
centrifugal disc (CDM) as veil as spin/spindle finish and
drag finishing equipment]. This material has proved to be
useful for producing both precision industrial finishes and
very highly refined reflective finishes for cosmetic or
decorative purposes. Besides finding wide use among both
fashion and precious jewelry manufacturers, these materials
are the key component for finishing operations on precision
parts within the aerospace, medical, electronic, ball and
roller bearing, fineblanking, plating and coating industries.
PRACTICAL CONSIDERATIONS
Some have made the mistake of thinking of centrifugal barrel
processes as simply accelerated barrel finishing. This is a
mistaken assumption, despite the obvious parallels in the two
processes, the forces at work in each are actually quite
different. Barrel finishing is a positive displacement
process where most of the work is accomplished within a
rotational/gravity induced slide zone. In centrifugal barrel
equipment the opposing rotational forces induced by the
counter-rotating barrels and turret can produce pressure
of media against parts that are many times the force of
gravity.
The media volume requirements of each type of machine
differs greatly as well, especially where dry media and
compounds are concerned. Ideally barrel equipment vould
filled to 50% of volume capacity in order to achieve the
longest possible slide zone and optimum process efficiency.
Dry media loading requirements of CBF equipment can take
advantage of the fact that slide zone requirements are much
more modest than the conventional barrel as most of the
rubbing or wearing action within this system is caused by
centrifugally induced pressures. This is a distinct advantage
for CBF equipment, as fill levels of 90% or more can be
utilized to protect especially delicate parts. [Filling CBF
equipment to 50% volumes vith dry materials will in many
cases cause part-on-part impingement.1
One advantage CBF equipment brings to the finishing
engineer's arsenal, is its ability to utilize dry media for
cutting, smoothing and polishing processes within restrictive
time cycle constraints. The precious metal jewelry industry
has found this to be especially useful. Some larger
manufacturers had been experiencing significant problems
associated vith their vet process cutting and smoothing
operations. Although they would expend considerable effort in
attempting to reclaim precious metal.fines from their mass
finishing equipment effluent through the use of settling
tanks, filters etc., significant amounts of gold vere not
being recovered. This problem was alleviated when a dry

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smoothing or grinding step in CBF equipment was instituted
utilizing hardwood or granular dry media used in conjunction
with an abrasive paste compound. When worn out these dry
materials could be incinerated, with a close to 100% metal
recovery,
FINISHING PRECIOUS METALS
Hardwood media vith accompanying granular and/or paste
compounds are finding increasing use on precious and fine
jewelry finishing. These materials can be used in both
conventional or high-energy equipment to create near-buff and
buff-equivalent finishes, obviating the need for labor-
intensive hand polishing operations and easing both precious
metal recovery and effluent problems as mentioned above. To
achieve jewelry quality finishes it is often necessary to run
the parts through a three step abrasive finishing process.
(1) Some manufacturers vi11 take the castings or as-
fabricated parts and run them through an initial
smoothing process vhich may utilize plastic or
ceramic media in conventional finishing equipment.
Many have had success in using high energy CBF
equipment and a hard granular dry media treated with
a specially formulated abrasive paste.
(2) The second step usually involves finer cutting or
grinding abrasives which have been applied to in the
case of CBF equipment a mixture, of dry media granules
and flakes. In conventional barrel equipment, this
maiy take the shape of a hardwood media/granular
compound mixture. [A general rule to keep in mind is
that the grade of media particles used in CBF
processes is considerably finer than vhat vould be
considered acceptable for running a similar process
in conventional equipment. The reason for this is
that CBF processes generate much heavier compressive
forces in the area of contact betveen the media and
part, coarser or larger sized media pieces may in
fact impinge the parts. 1
(3) The third and final step utilizing micro-fine
abrasive materials is used to develop highly
reflective near-buff or buff equivalent surfaces.
Care must be exercised in selecting the correct size,
grade and type of media particle to achieve high
quality surfaces. In the case of softer metal
substrates in particular, the harder granular
materials need to be avoided.
FINISHING NON-PRECIOUS BETATaS
Dry materials are being used to produce high quality
precision finishes on both ferrous and non-ferrous metal

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stampings, castings, forgings, die-castings and the like.
In many cases, dry processes are used as a secondary
operation, after heavy abrasive runs to bring part surfaces
down to low micro-inch finish levels. Considerable vork of
this nature is nov performed in high-energy CBF machines. In
most cases a single dry polish or cut/color cycle in CBF is
adequate to produce reflective surfaces of sufficient quality
to meet industrial or commercial needs. This type of process
is gaining increasing industry acceptance as a replacement
for buffing in situations vhere low RMS matte surfaces vi11
not produce optimum post-plate reflectivity standards
required by consumer oriented products.
HARDWOODMEDIA IN CONVENTIONAL EQUIPMENT
Many of the more successful dry finishing applications
utilize hardwood media shapes. Hardwood media have a number
of unique features that make them important industrially. One
especially important characteristic is a limited and
controlled adsorption vhich allovs these media to pick up,
carry, and apply various special dry process compounds for
smoothing, grinding and polishing.
This type of media is made primarily from various species
of birch and maple, and occasionally beech. There are some
qualities inherent to this material not to be found in any
type of synthetic media. For example vood has grain and this
factor necessitates some slight dimensional irregularities
not found in p'reforms made from synthetic materials. It's
bulk density is also quite different from that of synthetic
media, having half the density of plastic media, and one-
fourth that of most ceramics, making the number of pounds
required to fill a finishing machine the lovest of any
popular media type. Like other common preformed media, adroit
size and shape selection can promote access to complex part
shapes, and still provide for easy mechanical separation at
the end of the finishing cycle.
FINISHING PLASTIC PARTS
The most common application for dry finishing media and
compounds is the abrasive smoothing and final polishing of
plastic materials in barrel finishing operations. Among the
many types of parts being finished in this manner are:
eyeglass frames, sunglasses, buttons, toys, novelties,
acrylic castings and embeddments, costume jevelry, phenolic
luggage handles, bakelite pull components, composition tool
handles and others. Thermosetting plastics such as acrylic,
acetate, polyester and others seem to lend themselves more to
barrel finishing operations than do members of the
thermosplastic family such as nylon, ABS, delrin and others.
These thermoplastic materials seem to develop softer almost
rubber-like surfaces in barrel finishing operations that
preclude them from developing highly reflective polished

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MR89-147-7
surfaces.
Most plastics finishing operations appear to have lengthy
time cycles when compared with what is common in the metal
finishing industry. It is not uncommon to see processes vhere
a series of steps ranging from coarse grinding and smoothing
through final polish vith micro-fine abrasives may take
several days to complete. Some efforts have been attempted
to reduce the length of these cycles by adapting "vet process
metal finishing technology". The success of these efforts has
been limited.
One of the limiting factors seems to be that many plastic
substrates have their physical properties changed in
finishing environments that have water. Sometimes the
deleterious effects on the surface integrity of the parts do
not become apparent until they are in the hands of the
consumer. This has been especially noticeable in high-end
products such as eyeglass and sunglasses frames.
Some experimental vork has been done vith plastic
materials in high energy finishing equipment [both CBF and
CDMI vith mostly dissapointing results. One,limiting factor
seems to be the heat generated in these types of processes,
which has an adverse effect on plastic part surfaces.
Some experimental work has also been performed in vibratory
equipment. Besides the problems regarding "vet" environments
noted above, the micro-amplitude motion in vibratory
equipment.seems less able to develop highly refined plastic
surfaces free of "orange peel" than the roll and flow type
motion associated with barrel finishing.
FINISHING CERAMIC m COMPOSITE MATERIALS
One of the more interesting areas vhere dry materials have
found use is in the surface conditioning of ceramic and
composition parts. Hardvood media has been found to be an
effective vay of removing and cleaning firing sand from
parts, and in some applications dry materials can be used to
mass polish these type of materials, and impart highly
reflective surfaces.
MASS FINISHING WOODPARTS
Because of its lighter density, hardwood media can be a
useful tool for creating a variety of surface effects on
components manufactured from hardvood (softvood parts do not
lend themselves to this type of process). Simple barrel or
vibratory finishing applications using hardvood media and
powdered abrasives can replace and automate many hand-sanding
operations which are nov performed manually. Additionally,
hardwood media and barrel finishing equipment can be used in
mass transfer coating operations. Large numbers of parts can

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be stained, oiled or coated vith similar materials, without
the costs and headaches commonly associated with spraying
processes.
SUMMARY
There is a vide variety of surface finish problems to vhich
never and innovate dry finishing technologies can be applied.
The necessity of having an in-house ability to meet exacting
surface finish requirements is self-evident to those
manufacturers vho vish to remain competitive in a globally
integrated marketplace. Many of these manufacturers find
themselves turning to dry process mass finishing methods and
materials. In doing so, they are reducing the cost of day-to-
day operations by ameliorating both vet waste effluent
problems and reducing direct labor costs. Moreover, as
dependence on manual finishing techniques lessen, there is a
quantum leap in part-to-part and lot-to-lot surface finish
uniformity and quality control.

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BIBLIOGRAPHY
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MI. 1980, lM7-80
.Hiqnett, B., Mechanical Finish inq. - The Future for this
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Manufacturing Engineers, Dearborn MI. 1981
Landau, R., w Economic Growth, SCIENTIFIC AMERICAN, June,
1988
Davidson, D. A., Current ADDlications & Hardvood Media
in J&y Process Mass Finishina, Conference Proceedings --
Deburrinq and Surface Conditioning '85, Burr, Edge, and
Surface Technology Division, Society of Manufacturing
Enqineers (SME), Dearborn, MI., 1985
Davidson, D. A., Develonmentq b L>lr Process Pre-Plate. .
lshina. 74th AESF Annual Technical Conference
Proceedings, Chicago, IL, American Electroplaters and
Surface Finishers Society, Inc, Orlando, FL., 1987
Burkart, Dr. W., Grinding and Polishina. Theory and
Practice. Portcullis Press, London, UK, English
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TECHNOLOGY, Spring, 1987
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Engineers, Dearborn MI., 1981
Thompson, S., Selection & Vibratorv Finishina EcIuiDment,
SME Technical Paper MR81-381, Society of Manufacturing
Engineers, Dearborn MI., 1981
10. Davidson, D. A., Almost Buffed, PRODUCTS FINISHING,
December 1986
11. Davidson, D. A., Refinina Plastic Surfaces b Mass. . . a Methods. PLASTICS ENGINEERING, April 1986
12. Davidson, D. A., Common Abrolications for Hardvood Media
b m ~shinns, FINAL FINISH TECHNOLOGY, Vol. II, No.
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Finishins, FINISHERS MANAGEMENT, Vol. 33, No. 7,
September, 1988, p. 43-46
.

12. MR89-147-10
Figure 1. Vertical, Four com~:artmnt
CEF r:achine, 1.0 cu. ft., 26 liter cal;acit:l

13. MR89-147-11
Figure 2. Parts dry process
finished in conventional equipment; hardwood media in fcreground
Figure 3. Parts dry process finished.
larger Farts dry vibratory Einishcd, sn,allcr parts CCL dry finis;leG