Green finishng with high energy finishing equipment and dry media

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Green Mass Finishing with Dry
Abrasive and Polishing Media
Novel finishing process allows refined surface edge effects
while avoiding conventional wet-waste discharge.
All mass finishing processes utilize
a loose or free abrasive material
referred to as media within a contain-
er or chamber of some sort. Energy is
imparted to the abrasive media mass
by a variety of means to impart
motion to it and to cause it to rub
against or wear away at part surfaces.
Although by definition, the term
mass finishing is used generally to
describe processes in which parts
move in a random manner through-
out the abrasive media mass, equip-
ment and processes that utilize loose
abrasive media to process parts that
are fixtured all fall under this heading.
Most current mass finishing appli-
cations—such as barrel, vibratory,
centrifugal and spindle-finish
processes—produce edge and surface
conditioning on parts by processing
them in machinery work-chambers
with abrasive media and treated
water. The water, treated with spe-
cial chemical compounds, helps pro-
mote smooth roll and flow of media
inside the work-chamber; rinse and
clean the parts; and prevent sludge
build-up and redeposition of abra-
sive or metallic particulate on part
surfaces during the process.
The type of compound used to
treat the water can have a material
effect on the resultant surface fin-
ish. Some compounds are formulat-
ed to improve the abrasive cutting
action of media. Other compounds
have been formulated to comple-
ment the action of burnishing
media, such as metallic media,
porcelain or vitreous aluminum-
oxide nuggets in developing reflec-
tive surfaces by reducing surface
roughness with the compressive
rolling action of the media.
One drawback this type of
mechanical finishing entails is the
creation of a wet waste effluent or
discharge that must be treated prior
to disposal. Depending on the meth-
ods used and the materials being fin-
ished, treatment can be complex,
problematical and expensive.
In a very well-rounded discussion
of the potential problems that may
be involved, LaRoux Gillespie, in his
Mass Finishing Handbook1
, noted that
the treatment requirements can be
formidable. Waste from common
mass finishing operations that use
water and compounds can include:
1) Fine, abrasive particles in a
sludge
2) Metal fines or dissolved metal
fines
3) Fine, abrasive particles sus-
pended in water
4) Oils—dissolved, dispersed, etc.
5) Plastic resins
6) Vitrified clay.
The amount of effluent created by
batch-oriented mass finishing sys-
tems, such as barrel and centrifugal
barrel equipment, can be modest
when compared to industrial-sized
vibratory systems. These kinds of sys-
tems are draining effluent and
adding make-up water and com-
pound on a continuous basis. A 30-
cubic-ft. capacity vibratory finishing
machine can develop a 30-gallon-per-
hour waste stream that may include
two to six pounds of dissolved or sus-
pended solids per every hour of oper-
ation. The treatment of the waste-
stream is often made more compli-
cated by the use of chelating com-
pounds in the compound solution,
which makes separation of dissolved
or suspended metals difficult.
In recent years, one alternative has
been developed is combining dry
media with high-energy methods to
avoid the need for utilizing water-
based processes. Although many
mass finishing applications might
not lend themselves to an all “dry”
approach, there are some that do.
Plus, if a dry approach can be uti-
lized, waste disposal considerations
can be greatly simplified by this
“green” approach, thereby obviating
the need for effluent treatment and
secondary part drying operations.
One characteristic that has placed
dry media at a disadvantage relative
to conventional wet process media is
its relatively light weight or bulk
density. Plastic or ceramic media can
weigh two to three times more per a
given unit of volume. Also, the abra-
sive is an integral part of the media
mass, so fresh abrasive particles are
continuously being exposed as the
media surface is being worn away. A
number of approaches are being
used to overcome this handicap:
1) Newer dry media formulations
utilizing nylon impregnated with
abrasive.
TECHNICALLY
speakingBY DAVID A. DAVIDSON,
SOCIETY OF MANUFACTURING ENGINEERS
Figure 1: Some dry media is made up of natural materi-
als, such as the wooden shapes (top row), and the agri-
cultural by-products (bottom row). These kinds of
materials are treated with abrasives, such as pumice, or
polishing materials, such as jeweler's rouge or other
abrasive materials similar to those found in buffing
compound formulations. Photo by Tyha Davidson.

2. May 2007 I metalfinishing I 46 www.metalfinishing.com
ing. A closely related set of processes
would be fixture-centric processing,
such as the spin-, drag-, spindle-, and
turbo-finish methods (see Figure 2).
The fixture methods produce results
by imparting motion to parts directly
thatarefixturedonspindle fixtures(by
either dragging, rotating or developing
a planetary motion) and are immersed
in loose abrasive or polishing media.
The force with which part edges and
surfaces are interacted with loose
media can be considerably higher than
that developed by mass-media process-
es, whereby parts are placed randomly
2) The use of high energy cen-
trifugal equipment.
3) Fixturing parts inside finishing
machine work chambers to increase
the force and flow of media contact
against part surfaces.
ACCELERATING CYCLE TIMES
WITH FIXTURING.
Included in these mass-finishing
methods are traditional barrel tum-
bling, vibratory and centrifugal finish-
within the media mass, and are
dependent on the loose media motion
to achieve the surfacing results.
Fixturing parts in more convention-
al barrel or vibratory methods is also
common. 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 accel-
erate cycle times; and produce more
pronounced surface finish effects.
Part applications for fixture finish-
ing in conventional equipment vary
widely. To cite a few examples: Some
manufacturers of brass musical instru-
ments (trumpets, french horns, trom-
bones) fixture brass instrument assem-
blies 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 surgical implant devices
fixture the devices in high-energy cen-
trifugal barrels, and produce very
refined surfaces on cobalt chrome and
titanium substrates by processing the
devices through a sequence of succes-
sively finer loose abrasive operations.
SEQUENTIAL PROCESSING
One trait that many of today’s more
sophisticated mass finishing opera-
tions share is a reliance on multiple-
step sequential processing. In this
type of processing, very rough sur-
faces can be brought to a highly pol-
ished or micro-finished state. This is
done by initially processing the parts
with coarse abrasive material, and then
TECHNICALLY
speaking
Figure 2: The Turbo-Finish dry process (spindle-finish
method) is a unique surface- and edge-preparation
method designed to produce vibratory or barrel-finish-
style isotropic surface finishes on parts with media sim-
ilar to that used in dry blasting operations. The high
spindle speeds used in the process can develop edge
contour and refined surface conditions very rapidly,
resulting in smooth surface conditions. Photo courtesy
of Turbo-Finish Corporation.
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3. www.metalfinishing.com May 2007 I metalfinishing I 47
methods were developed concurrent-
ly in Europe.2
The method was developed prima-
rily to mitigate 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
following up with a sequence of finer
abrasives(seeFigure1).Eachofthesub-
sequent 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 borrow an analogy from the
wood finishing industry, almost
everyone understands that to pro-
duce fine finishes in woodworking
applications that it is necessary to use
sanding operations with successively
finer abrasive grits to produce cabinet
or furniture quality surfaces. The
same principle holds true in mass
finishing (or even hand-finishing)
metal parts, when very smooth or
polished surfaces are required.
One time-honored method for pro-
ducing very refined surfaces is dry
barrel processing. This technology
was originally developed and heavily
utilized in the northeastern United
States as early as the 1920s; similar
been the sole province of those buff-
ing methods, and 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 sur-
faces are desired on parts being fin-
ished in vibratory equipment, it is
not unusual now to see secondary
vibratory processes with burnishing
media or dry process polishing
media develop those surfaces (see
Figure 3).
Many processes have been devel-
oped for centrifugal disk and centrifu-
gal barrels whereby three or more
steps are utilized in order to bring part
surfaces to very low micro-inch sur-
face profiles, or to develop very reflec-
tive surfaces for cosmetic reasons.
One recent development in using
these techniques reflects an under-
standing this equipment is capable of
processing much larger work-pieces
than had previously been thought.
Larger, single-piece parts are being
TECHNICALLY
speaking
Figure 3: Turbo-Finish decorative and functional surface
finish effects. The parts in this group have been fixture
processed via dry spindle finishing to prepare surfaces
for further decorative or functional effects. Photo cour-
tesy of Turbo-Finish Corporation.
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4. May 2007 I metalfinishing I 48 www.metalfinishing.com
nylon with various abrasives impreg-
nated within the media shape.3
These
media types are generally used in
high energy finishing equipment,
which has been modified from stan-
dard designs to accommodate and
evacuate the dust developed as a by-
product of the process. Some studies
made at the university level in Japan,
where the media was developed, indi-
cate that this kind of process can also
be used to develop beneficial com-
pressive stress and promote surface
integrity on critical parts.
German manufacturers of “drag-
finish” equipment are utilizing dry
finishing and polishing materials to
develop edges and surfaces on cut-
ting tools that both improve the sur-
face finish the tools produce as well
as substantially increase the service
or operational life of the tools them-
selves. The driving force behind the
development of these materials has
been specifically the need to develop
waterless technologies that do not
have associated effluent stream treat-
ment costs.
DRY PROCESSING
OF MINIATURE PARTS
One area where dry processes have
had particular success is in the high-
energy processing of a variety of
small, even miniature machined parts
(such as Swiss turning or screw
machine applications). In one
extreme example, one large manufac-
turer had been machining large vol-
umes of precision speedometer shafts
using several dozen tumbling barrels
to surface finish the shafts in a 48-
hour cycle to a 10Ra finish. This wet
process was creating an effluent
waste stream in excess of 10,000 gal-
lons per day, which was highly objec-
tionable to the municipal authorities.
This entire operation was
replaced by two 120 liter capacity
centrifugal barrel finishing
machines utilizing a walnut shell-
abrasive mixture. The end result:
cycle times were reduced from 48
hours to two 2-hour cycles; effluent
discharge was reduced from 10,000
gallons per day to zero; and ending
surface roughness on the shafts was
reduced from 10 Ra to 4-5 Ra.
run in barrel compartments by being
fixtured within the barrel compart-
ment, or in some cases are being run
in barrel compartments that have
been over-filled with media deliber-
ately to cushion the part during the
rotational movement.
NEWER DRY MEDIA
FORMULATIONS
In recent years, waterless processes
have been developed by using special
dry media made primarily from
SUMMARY
Dry media mass finishing processes
can be used to produce refined edge
and surface effects. These processes
are not as well known as conventional
processes that make use of aqueous
solutions in concert with abrasive
media. Although not universal in
potential applications, these materials
and methods should be investigated
as a potential method for green fin-
ishing by companies seeking to avoid
rather than treat wet waste discharge
from mass finishing operations.
REFERENCES
1. Gillespie, LaRoux, Mass Finishing
Handbook, (New York, NY:
Industrial Press) 2007, pp. 200-
203.
2. Koichi Kitajima, Atsushi
Yamamoto, M. Sasai, Finishing
Characteristics and Residual
Stress on Workpiece Surface in
Dry Barrel Finishing,
Department of Mechanical
Engineering, University of
Kansai, 3-3-35 Yamate-Cho,
Suita-Shi, Osaka Prefecture,
Japan, 2007, www.scientific.net
3. Massarsky, M. L., Davidson, D.
A., “Turbo-Abrasive Machining
and Turbo-Polishing in the
Continuous Flow Manufacturing
Environment”, SME Technical
Paper MR99-264, CONFER-
ENCE PROCEEDINGS: 3rd
International Machining and
Grinding Conference, Cincinnati,
OH, Oct 4-7, 1999, Dearborn,
MI: Society of Manufacturing
Engineers, 1999
4. Davidson, D. A., “Micro-
Finishing and Surface Textures”,
Metal Finishing”, (White Plains,
NY: Elsevier Science) July, 2002
BIO
David A. Davidson is a member of the
Society of Manufacturing Engineers,
Deburring, Edge-Finish and Surface
Conditioning Technical Group. He can be
reached via e-mail: ddavidson@debur-
ringsolutions.com
TECHNICALLY
speaking
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TECHNICALLY
speaking
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