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INTRODUCTION
TO MASS FINISHING
 Mass
finishing is the general description for vibrating or flowing an
abrasive media around usually a number of non fixtured parts, moving
randomly within the mass of the abrasive media.
Various types of equipment generate energy which
is transferred thru the media to the part being processed. The transfer
of energy and randomly moving parts automates the finishing process,
with part loading and unloading to be addressed.
There are a number of variables that affect mass
finishing which includes:
- The
equipment
- Media
- Soap
compounds
- The
process
The
effects mass finishing has on parts include:
- Deburring
- Refining
RMS finishes
- Burnishing
(Brighting)
- Improving
strength
- Pre
plate
- Pre
paint adhesion
- Improving
oil retention on surfaces
- Cleaning
Mass
finishing processes are repeatable and one of the least expensive
ways to automate finishes on parts.
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TUB
VIBRATORY MACHINES
 The
tub vibratory machine is a horizontal urethane lined container
with a u-shaped bottom and an open top. It can be manufactured
with a variety of channel diameters and lengths making it an excellent
choice for long or large parts. The tub can also be divided to
run large parts preventing them from touching one another. The
tub, built with extended lengths, lends itself to straight line
automation of vibrated parts.
The tub vibratory machine generates quite a bit more energy than
rotating barrels and a minor amount of energy faster than bowl
vibratory machines. The system is commonly powered or vibrated
by a shaft with an eccentric attached to the tub. The shaft is
rotated by a v belt, sheave and motor drive. The eccentric weights
can be added or subtracted and adjusted around the shaft to increase
or decrease vibration. There is another uncommon tub vibratory
drive system that is driven with magnets. The tub vibratory machines
were developed in the mid 1920's and was a welcome improvement
over the rotating barrels that were in production prior to the
tubs inception.
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BOWL
VIBRATORY MACHINE
 The
bowl vibratory machine is a horizontal urethane lined donut shaped
container with an open top and vertical eccentrics shaft drive
built thru the center collum. The media mass rotates around the
center column in a rolling forward feed motion. The machine can
be built with a variety of channels and overall diameters, it
is now being built with multiple channels feeding parts from one
channel to another, allowing a variety of processes and drying
with one machine.
The bowl vibratory machine tends to be a better finishing machine
than the tub, it does a better job of keeping parts relatively
orientated to minimize damage than a tub. The bowl can also be
divided for large parts.
The most significant advantage of a bowl over a tub is that ramps,
gates and screens can be added to the bowl allowing self unload
of the parts away from the media and out of the machine.
The self unload and improved finishing process of the bowl makes
it a more popular choice over the tub.
The bowl vibratory machines were developed in the 1950's with
self unloading versions being pattend in the 60's. The bowl vibratory
machines were again a welcomed improvement over the tub and barrel
systems previous to it.
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CENTRIFUGAL
DISC
 The
centrifugal disc is a stationary urethane lined bowl with a rotating
bottom urethane lined disc that accelerates the media outward
to the sidewall which then decelerates and moves inward to the
center of the disc for reacceleration. This change in acceleration
and deceleration of the media creates a flowing action over the
part, generating energy up to 15 times faster than vibratory with
the increased energy. The disc machine will deburr mall tough
parts and achieve very low RMS finishes on parts for pre plate
or pre electro polished finishes.
The disc was developed in the late 60's and is being utilized
today because of its ease of load and unload. It's decrease process
time makes it highly favorable for cellurer manufacturing.
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CENTRIFUGAL
BARREL
 The
centrifugal barrel machine has enclosed urethane lined barrels
mounted on a turret. As the turret rotates one direction the barrels
are belt or gear driven to rotate to a given ratio in the opposite
direction. The individual barrels are loaded with the media, parts
to be processed, water and compounds. The action within the barrel
is a compressive sliding action creating the maximum energy available
today in mass finishing.
The process is excellent for small parts with large burrs. This
system has the energy to drive small media into hard to get to
areas. The system can also utilize one media to cut and brighten
with one step. The bowl can also be divided for large parts.
The centrifugal barrel is also being used to brighten parts with
dry medias to get as close to an as buffed finish as possible.
The centrifugal barrel machine was developed in the late 60's
and is becoming more and more popular as processes are being developed.
One of the disadvantages of the centrifugal barrel finishing is
the load and unload of parts is a bit labor intensive.
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SPINDLE
MACHINES
 The
spindle machine is a variable speed rotating bowl of random shaped
or preformed media. The spinning of the bowl slings the media
to the outside wall. The part is rotated by a spindle that holds
it into the spinning mass. The collision of the media hitting
the part as it rotates by is what creates the energy.
The individual part holding spindle can be programmed to rotate
at various speeds, and can orientate the part to be worked in
certain areas or dwelled to concen trate in an area. The flexibility
of orientation, rotation and dwell of the spindle allows parts
such as gears to be worked to achieve different radious in var
ious areas of the gear teeth.
The energy of the spindle machines are higher than vibratory.
Spindles were deve loped in the 70's and have been popular for
gear deburing. The American built spindle machine generally use
2 to 4 spindles per machine. Other manufacturers tend to utilize
many more spindles in a given machine.
Dry process spindle machines are used to obtain an as buffed finish
on large complex parts such as band instruments.
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DRAG
MACHINES
Drag
machines are bowl machines utilize light vibration with fixtures
holding and dragging the part thru a heavy mass.
The drag machine produces more work than conventional vibratory
systems resulting in reduced time cycles on complex parts.
These systems have been a recent development and are used on a
variety of parts like boat propellers, and air tool handles.
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BARREL
FINISHING
 A
hexagon or octagon barrel rotating horizontal will process parts
slower than vibratory finishing, but produces amazing finishing
results.
The barrel is rubber or urethane lined to protect parts. As the
barrel rotates it lifts the media which eventually slides. The
slide of the media containing the parts is the working action
of the barrel. A faster RPM results in a thinner slide area, a
slower RPM produces a thicker slid, 18-22 RPMS tends to be the
optional slide for most applications.
The process involves water and compound, the more water and compound
used the more cushioning action is achieved. Normal water levels
are just above the media & parts. Media level is very important,
50% full with parts will produce the longest slide and therefore
is a good starting point. Higher media levels will shorten the
slide and provide more part protection.
Time cycles range from 1 to 24 hours, 1 for burnishing, 2 for
deburring and longer cycles when deburring and burnishing in on
operation.
The load and unload of barrels is labor intensive, therefore vibratory
finishing has replaced many barrel applications. Internal areas
of parts are not finished well with barrel processes, but when
it comes to keeping flat parts from sticking together or super
luster finishes without part damage, barrels have always been
an excellent choice.
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OVERVIEW
MEDIA SELECTION INFORMATION
Vibratory
or mass finishing media transmits the energy from the machine to
the work piece. It carries the compound for cleaning or brightening
and it suspends and protects the parts.
The weight and size of the media has a lot to do with the amount
of work it is capable of doing. Heavier and larger media cut faster
and do more work because of the heavier hammer effect. The larger
medias also create more voids within the mass allowing a more aggressive
action. A smaller media will create a tighter mass capable of holding
more fluid, and with less voids within the mass it will work to
protect the part.
The shape of the media allows it to get into an area to get something
done or stay out of an area so it will not lodge. A round or cylindrical
shape will finish better because of single point contact with the
part, like a ball pean hammer effect. A flat shape, such as a triangle,
has a wide surface to rub over an edge allowing quicker deburring.
Media hardness has a lot to do with the finish depending on the
type of material and hardness of the part. This and other factors
will be discussed under media selection.
MEDIA
TYPE |
COLOR |
APPROX.
WT./CU.FT. |
CUTTING
ABILITY |
SURFACE
FINISH |
MEDIA
HARDNESS |
GENERAL
COMMENTS |
| CERAMIC |
|
|
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All
ceramics are cleaner running and in general, less expensive
to operate than Plastic Medias. They give a scratchy Matte.
Finish Best RMS Finish in vibratory is app. 12. |
| High
Density |
Brown |
120
lbs. |
Fastest |
Dull
Rough Matte Finish |
60-65
Brinell |
| Super
Fast |
Grey |
70-90
lbs. |
Very
Fast |
Scratchy
Matte Finish |
60-65
Brinell |
| Fast
Cut |
Grey |
70-90
lbs. |
Fast |
Scratchy
Matte Finish |
60-65
Brinell |
| Medium
Cut |
Grey |
70-90
lbs. |
Medium
Fast |
Matte
Finish |
60-65
Brinell |
| Light
Weight |
White |
40
lbs. |
Very
Fast |
Matte
Finish |
50
Brinell |
| Polish |
White |
70-90
lbs. |
Very
Little |
Bright
Reflect |
70-90
Brinell |
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| PLASTICS
Synthetic: Plastic |
|
|
|
*Very
clean running media. *All medias are biodegradable. *All synthetics
are capable of a preplate finish depending on metal type. *This
media will cut a burr when a ceramic media will roll it over. |
| Fast |
Pink |
45
lbs. |
Fastest
of Synthetics |
Good
(Matte) |
0
on Brinell |
| Medium |
Tangerine |
45
lbs. |
|
Good
(Matte) |
0
on Brinell |
| Preplate |
Light
Green |
45
lbs. |
|
Bright
Matte |
0
on Brinell |
| SP |
Tan |
45
lbs. |
Slowest |
Bright
Polish |
0
on Brinell |
| |
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| POLYESTER:
Plastic |
|
|
|
*The
media hardness is 45 Brinell. *Longer wearing than the synthetic
medias. |
| High
Density |
Grey |
55
lbs. |
Fastest
of Polyester |
Good(Dull
Matte) |
42-45
Brinell |
| Fast&Medium |
Green/Red |
55
lbs. |
|
Very
Good(Dull) |
42-45
Brinell |
| Preplate |
Light
Green |
55
lbs. |
Slowest |
Dull
Matte |
42-45
Brinell |
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This
media is identified by its bonding agent which is ceramic. The cutting
abrasive bonded by the ceramic is most commonly aluminum oxide between
80 to 220 grit size. Silicon carbide grit is also used in ceramic
media.
Ceramic
media is a very popular choice in finishing processes because of
its long lasting and clean running characteristics. There are different
grades of cut within the ceramic media family:
| *
Polish |
|
*
Super Fast Cut |
| *
Medium Cut |
|
*
High Density Fast Cut |
| *
Fast Cut |
|
*
Light Weight(low density) |
Below are the factors affecting media cutting performance:
*Abrasive Size And Percentage - Polish medias have no abrasives
where faster cut medias have courser grits and more of it.
*Bonding Agent Strength - Faster cutting medias are bonded
softer allowing the bond to wear away exposing new abrasives which
increases cut rates. Polish medias are very hard.
*The Ceramic Itself - Different clays will assist cut and
affect weights of medias. Heavier and larger media will cut faster.
The manufacturing process determines the shapes ceramic medias are
available in. The ceramic and abrasive are mixed wet or green and
then extruded thru dyes. This limits the shapes available which
are equal sides. As the media is extruded it is cut for thickness
and then baked in a continuous or batch klin to dry and harden.
The ceramic cone and ball are a couple exceptions to the extruded
equal side shapes.
Ceramic is a good all around media and is one of the first choices
to make for all metals. The 45 to 65 rockwell hardness of ceramic
can sometimes damage softer metal, testing prior to processing is
always advised.
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Plastic media is known by its bonding agent. The
two primary bonding agents of plastic are:
- Polyester
Synthetic
The polyester is the original formulation of plastic media. It
is produced from an oil derivative, therefore, non biodegradable.
The polyester breakdown is a white chalky sediment that's difficult
to clean off of parts.
The synthic plastic is a recent development and runs cleaner than
polyester. The synthetic is softer and lighter than polyester
achieving superior finishes. Synthetic is also biodegradable.
The cutting element in plastic medias is usually quartz. The cutting
element is limited to a given percentage because the manufacturers
process starts with a liquid and like adding sugar to water and
mixing it, if too much sugar is added it will drop out. After
the quartz is added a catalyst is added to harden the mix that's
poured in a mold. The molds provide a variety of interesting shapes:
cones, pyramid, tetrahedron and wedges.
Plastic medias are proccesed in a variety of different cuts:
Fine Cut (Pre Plate)
Medium Cut
Fast Cut
- Extra Fast Cut ( High Density Zirconium, 100 LBS Cu. Ft.)
Plastic medias in most cases do not cut as fast as ceramic but
do give lower R.M.S. finishes and eliminates the nicking or damaging
of edges on softer metals.
Below are a number of advantages of plastic medias:
- Light weight and soft bonding agents allow it to wear a burr
off with out rolling the burr onto the part. Once a burr is rolled
it's very difficult to remove.
Certain plastic shapes, particularly a wedge, will sharpen as
it wears allowing points to get into inside radious of parts.
In large mass finishing systems with large parts, hard medias
compressed between the machines sidewall and the rotating part
can gouge and damage the part. Softer plastic medias eliminate
this type of damage.
- Low RMS finishes.
In summary plastic media has many advantages. Your parts should
be run in a lab prior to media selection to determine its future
success.
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 RANDOM
MEDIAS
Random
medias are comprised of a variety of natural shapes and types
of materials, one of the oldest recorded applications of random
media used in mass finishing were by the Romans. They gathered
certain types of rock, put them in pouches attached to their belts
and polished their uniform ornaments as they marched.
Today, some popular random medias are micro-crystalline aluminum
oxide, silicon carbide and sintered ceramic. These medias are
tough, chemically inert, economical, non toxic and uniformally
sized.
Random medias can cause more lodging problems than the more commonly
used preformed medias.
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DRY
PROCESS MEDIA
 The
most common dry process media are wood pegs, corn cobs and walnut
shells. The are produced in a variety of shapes and sizes.
The dry process medias are most often impregnated with dry polishing
powders or creams. The dry polishing powders are chrome rouge,
iron oxide or calcide aluminum that are normally impregnated by
the media manufacture. The polishing cream also contain polishing
powder combined with other elements that are added to the dry
process media by the user and recharged between part batches.
Dry medias are used in most mass finishing machines, including:
vibratory, rotating barrels, centrifugal barrels, centrifugal
disc and spindle machines.
The most common applications of dry finishing media is surface
brightening (an as buffed finish) on final finish refinement.
The dry media process can be a one step process, however, it's
usually the last step of a two step finishing process. Cleaning
(especially with the creams) may also be required.
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STEEL
MEDIA
 Steel
media is a cold headed, heat treated, ground and finished carbon
or stainless steel shape. It is highly finished. Steel media is
used in vibratory or barrel mass finishing to burnish, clean,
improve compressive strength, dull edges, smooth finishes and
reduces porosity on plated parts.
Steel media can last 10 years, runs very clean and eliminates
lodging of media in parts. The media is available in a variety
shapes and sizes.
The weight is approximately 300 lbs per cu ft. Specially built
heavy duty machines are usually required to run steel because
of it's weight.
Process times are relatively short and waste treatment is simplified
because of non sediment waste stream.
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SOAP
COMPOUND
Soap compounds are critical and in many cases have more importance
in obtaining processes and finishes in mass finishing than the media
itself.
Soap compounds accomplish many things:
- Lubricates
the mass resulting in extended media life.
- Cushions
the part eliminating part damage.
- Suspends
the dirt, oils and media residues and flushes them from the system
keeping the parts and media clean.
- Inhibits
rust or corrosion of parts in the finishing process. Extended
shelf life inhibiting should be done as a post dip or spray with
water soluble inhibitors.
- Burnishing
(Brighting)
- Descaling
- Accelerates
media cut
Soap compounds are identified in the following categories lthough
many compounds will do multiple functions:
- Deburring
compounds (General Purpose)
- Burnishing
compounds (Brighting)
- Rust
inhibitors (Post operation sprays or dips)
- Chemical
accelerations (Metal oxidizes)
- Descalers
Soap
should be mixed with water and then flowed thru the system. The soap
and water in many cases can pass straight to sewer and now more commonly
recirculated back thru the system after being filtered.
The general soap concentration and water flows are 1 to 2 oz of soap
per gallon of water and 1 to 2 gallons of water per cubic ft per hour.
A good starting point for the following processes are as follows:
Ceramic Media - 1 oz of soap per gallon of
water and 1 gallon of water per cubic foot per hourPlastic Media - 1 oz of soap per gallon of
water and 1 1/2 gallons of water per cubic foot per hour
Dense Media - 1 oz of soap per gallon of
water(small shapes) and 1/2 gallon of water per cubic foot
You
will need to increase these minimums as parts get dirtier or oiler.
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WORK
LOAD
The work load or amount of parts that can be processed in a given
batch will largely determine the finishing cost of the part.
Factors that affect the work load are:
- Part
size and configuration
- Part
weight
- Alloy
of material part is make of
- Media
to be used
General
comments about work load:
- Larger
parts and or sharp cornered parts can be more susceptible to part
on part damage.
- Heavy
parts may require larger media to support and suspend the part
within the mass.
- Smaller
media (a dense mass) provides more protection, therefore allowing
more parts to be run without damage.
- Parts
that require very fine finish refinements or bright finishes cannot
be loaded as heavy as a deburring application.
The work load is determined by volume and is expressed by media to
part ratio.
Media to part ratio guidelines are as follows:
- Part
on part processing using no media
|
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0:1 |
- Very
small stamping, forging, castings
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1:1 |
- Very
small machined parts
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2:1 |
- Minimum
amount required for self unloading machines
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3:1 |
|
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4:1 |
- Softer
non ferrous metals
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5:1 |
- Very
fine finishes and bright burnished parts
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7:1 |
|
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10:1 |
- Parts
that cannot touch one another
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compartmentized |
Using media to parts ratio to calculate parts
per cubic foot:
Once you've estimated the media to part ratio one can calculate the
parts per cubic foot by multiplying part L" x W" x H"
x media to part ratio (on ratio 4:1 use 5) % 1728 = parts per cubic
foot.
Calculation of machine size required:
Example: use the following parameters
- If
you've estimated 12 parts per cubic foot
- One
hour total cycle (50 Min. process, 10 Min. unload)
- Daily
production 1000 parts 1 shift, on 7 one hour cycles. Divide 1000
by 7 = 143 parts per cycle
- Machine
size required 143 divided by 12 (Parts per cu. ft.) it will require
a 12 cu. ft. machine to process the 1000 parts per day.
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WORK
LOAD COST CALCULATION
Calculation of media attrition (wear out) rate and media weights:
| Media |
Attrition
Rate |
Weight |
| Ceramic
med cut |
.005 |
80
LBS cu. ft. |
| Ceramic
fast cut |
.01 |
80
LBS cu. ft. |
| Polyester
plastic |
.0125 |
50
LBS cu. ft. |
| Synthetic
plastic |
.015 |
45
LBS cu. ft. |
| Cost
calculation: |
| Once
you've determined the, |
| Use
the following finishing cost justification formula: |
Cost per part: |
| 50
minute cycle time |
| 10
minute unload ($10.00 labor rate) |
| Medium
cut ceramic .005 attrition rate,80 LBS cu.ft.) |
| 143
Parts per cycle |
| 1
oz of soap per gallon of water, 1 gallon of water per cu.
ft. 5 cents per oz for soap compound |
| Finishing
Cost Justification: |
| Compound
used per hour 12 oz. x $.005 per oz........... |
$
.60 |
| Media
Cost per load: |
| 960
LBS per load x $.80 per lb. x .005% attrition...... |
$3.84 |
| (80
lbs cu. ft. x 12 cu. ft. = 960 lbs) |
| Total
consumables..... |
$4.44 |
| Total
Cost per load: |
| Total
hourly rate $4.44 x .83 hours per load........... |
$3.69 |
| (50
min. is .83% of hour) |
| Direct
labor rate (+fringes) $10.00 per hours x .16 |
| hours
per load......................................... |
$1.60 |
| Total..................................... |
$5.29 |
| Total
cost per part: |
| Cost
per load $5.29 = $.074 per piece. |
| piece
per load 143 |
| The
part above costs less than 7.5 cents per part to process. |
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DEBURRING
Deburring is one of the most common applications of mass finishing.
Below is general information on deburring applications:
- External
burrs are easier removed than internal burrs.
- Burrs
can be created too large to remove.
- Small
parts with large burrs can be difficult requiring longer time
cycles or higher energy machines.
- Burrs
can be rolled by heavier medias therefore requiring light weight
medias to cut them instead of rolling them. Once a burr is rolled
and flatted to the part it becomes impossible to remove by mass
finishing.
- Stainless
burrs will work harden themselves off the part instead of being
worn off like other types of metals.
- Larger
media, faster cutting and heavier media will remove larger burrs.
- Increased
time cycles will assist in removing larger burrs.
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SURFACE
FINISHING
Surface finishing in mass finishing refers to refining or smoothing
a surface to lower the RMS finish. Pre plate finishes are often
refered to in the industry as surface refinement.
Various medias will affect surfaces in different ways. Medias
that are sharp cornered or harder than the work pieces can nick
and damage parts.
Ceramic medias may do an excellent job of bringing an RMS from
64 to 16 but a secondary plastic media cycle may be required to
bring the surface from 16 to 6. High energy systems that flow
media and or special medias will be required to bring surfaces
in the 2 to 3 RMS range.
Time cycles can become extended in surface refinement ranging
between 2 to 6 hours in vibratory or one hour in high energy systems.
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