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Abrasive Control Factors
For Mass Finishing Systems
By A. F. Kenton
There is a growing concern in the
surface finishing industry of the aging of those in the business. It
is not a glamorous trade but absolutely necessary to all industrial
and commercial products and unlike CNC equipment, there are subtle
processing control factors instead of software programs. Maybe for
that reason, fewer and fewer young people are seeking the trade
knowledge or careers in the business. That also means that as the
trade ages, more and more is left up to the people performing the
surface finishing work. It is for that reason that certain basic
fundamentals of material removal and surface finishing should be
passed along, because the graying trend is especially noticeable in
the mass finishing industry.
As a general
statement, what you put into a mass finishing system determines what
you get out of it. All mass finishing systems are designed to
perform uniform edge or surface modification on machined or cast
parts. The major difference between the various finishing systems is
the application of energy forces, which relates to the speed or time
necessary to produce the desired surface modifications
Next to equipment,
the specific gravity and weight of the abrasive is the most
important element effecting speed or time, as well as actual surface
texture and smoothness. Just as all equipment will eventually do the
job of material removal, so too will all abrasives. However, there
are literally hundreds of media compositions, sizes, and shapes
available. As a general rule, the greater the pressure that can be
exerted onto a part, either by equipment or the weight of the media,
the greater the material removal. Therefore, here are some
guidelines can be helpful for selecting media supplies for specific
applications and working particular parts.
Random and Preformed Shapes
The most typical application of
mass finishing equipment is to deburr parts en mass in the cheapest
and fastest way possible without getting stuck or creating other
finishing problems. To do this, you want to select the largest
abrasive possible that is still small enough to reach all the part
areas that have to be worked without getting stuck.
Unfortunately, finding the perfect
abrasive size is not always easy. One size or shape does not fit
all. Media choice depends upon the surface finishing requirements
that are effected by different part configurations which limits
access and the raw material of the part which determines the
hardness and size of the abrasive. An abrasive that works well on
one part may not be able to achieve the desired end result on
another. That is because there is a relationship to the abrasive
size, its hardness, and the part to be worked. For that reason,
nonferrous or soft metals are not normally finished with the
coarsest abrasive, because the surface of the metal will have a
rougher finish after processing than it did before being worked.
For cost purposes, usually the
cheapest form of any abrasive is loose random shaped, naturally
occurring mineral compounds classified by screen sizes. Although
this material is commonly used with abrasive blast finishing
equipment, it is rarely used in mass finishing systems because it
varies in size and shape making it more likely to jam and get stuck.
Therefore, it is more common to use manmade shapes in mass finishing
systems, because it is more controllable and predictable.
Unlike loose random natural
abrasives, which can be more than 1 inch in size, the largest
abrasive particle in a preformed shape rarely exceeds .060 in size.
Manmade media is made with uniform size particles which
predetermines the surface finish of the part in being worked.
Technically, you can not have a surface finish finer than the
largest abrasive particle used in the make up of the media. However,
over all media size is determined or usually referred to by its
physical size in L x W x H and abrasive composition first followed
by its preformed shape.
Ceramic and Plastic Preformed Shapes
Most preformed deburring shaped
media used in mass finishing systems is made with either a ceramic
or plastic bonding agent to hold the uniform abrasive particles
together. Ceramic media is made like cement and extruded and cut to
size to make a shape; whereas, plastic media is made like an epoxy
and then injected into small molded shapes. Both are then baked to
achieve an overall hardness and to create a very hard abrasive
shape.
Ceramic shapes are made with
inorganic materials and primarily aluminum oxide abrasive grits, but
silicon carbide, silicon, zirconia, and porcelain compositions are
also available. The binders used to hold the abrasive shape together
are formulated to decompose at a predetermined rate to expose new
sharp abrasive particles. The harder the bond, the longer the shape
lasts and the finer the surface finish of the part being worked.
The faster the bond breaks down, the faster the media removes
material and the coarser the finish on the end product. Ceramic
preformed shaped media is relatively hard and very rigid, similar to
a grinding wheel, so it is typically used on hard carbon steel parts
or those parts requiring a lot of material removal.
Plastic media is softer and more
flexible than ceramic media so it is usually used on nonferrous
parts. Even though the abrasives used to make the shapes are the
same as ceramics the media behaves differently. Plastic media will
produce equal surface finishing results on nonferrous parts as
ceramic does on steel, but plastics will take a long time to do
anything on steel parts and therefore is not recommended for that
application.
While ceramic media comes in a
number of inorganic bonds, which determines their rate of
decomposition, plastic comes in either a polyester or urea variation
of the bonding agent. The urea formulations seem to be a little
softer and cheaper than the polyester bonds. Ceramic media
compositions are basically shades of gray or brown and plastic is
multi colored resin formulations that also comes in many more shapes
than ceramic, because they are molded. Generally speaking, the
darker the color of ceramic media the faster the material removal
rate. However, plastic media has no industrial standards or uniform
color code that relates to the speed of cut or decomposition;
therefore, be carefully and go by the verbal description of the
composition and not the color.
Earlier I mentioned that after
equipment, specific gravity and weight of the abrasive was the most
important factor effecting the speed of processing parts. So, the
more media and parts you can get into a machine system, the faster
it works, with a couple of exceptions. A barrel system needs a
definite air gap for the media to properly slide during processing.
For normal deburring in vibratory and high energy systems, the
process requires about 60% media to parts by weight, and up to 80 or
90 % for burnishing and good looking finishes. Also, remember
normally the faster the media breaks down or decomposes the faster
it works.
As a guide for
deburring, abrasive media averages 100 lbs/ cu.ft. but can weight
between 90 to 120 lbs. per cu.ft., some fine polishing porcelains
can go as high as 150 lbs. per cu.ft., and on the opposite side are
some hybrid light weight ceramics that are used on both ferrous and
nonferrous materials that come in around 60 lbs. per cu.ft. Plastic
media normally weights anywhere from 55 to 80 lbs.per cu.ft. and
some zirconia abrasive plastic shapes approach 100 lbs. per cu.ft.
range.
Because both ceramic and plastic
media are formulated solids, they have a porosity factor that
effects moisture content and the weight of the abrasive media. That
means with age and depending upon the storage environment, this
moisture content can and will vary. Even though this problem does
not significantly effect processing, it does effect comparative test
results. When one tries to compare one media against another, the
media is weighted before and after testing to determine performance
and attrition. The weight factor due to moisture is the real wild
card that can play havoc with testing and there is almost no way to
compare apples and apples accurately.
Shape Effectiveness
As a general statement, preformed
media comes in two basic shapes or the shapes have two different
behavior characteristics, plus one. Both work and there is no
conclusive studies to indicate one shape is better than the other.
In slang terms, what you have is basically a steamroller versus a
bulldozer.
Spherical shapes,
such as cylinders, cones, or spheres/balls have a lot of rounded
surface features and are therefore very mobile like a steamroller.
As they move, they rotate en mass and by themselves. These shapes
work extremely well on parts with holes, because the media tends to
poke itself slightly into holes and rotate before moving on. They
also work flat areas very well without marking it.
Because these shapes are so mobile
en mass, they do not hold, support, or restrict parts from reaching
the bottom of the work chamber. That means the full weight of the
media and mass is used to put pressure on the parts. These shapes,
especially the balls, are not necessarily effective for finishing
inside corners and angles resulting in a shadow appearance in these
areas.
Angular
geometric shapes, such as triangles and tri-stars move more
slowly than curved shapes because their alignment and edges resist
movement creating a pushing effect like a bulldozer. This
resistance seems to transmit more energy or pressure to the parts on
a more constant basis make this shape very good for working edges
and inside angles on parts. This resistance characteristic also does
not allow parts to sink en mass and it also creates a slightly
louder sound while processing than rollers.
As mentioned, there is a plus one,
hybrid shape that needs special clarification. There is one
shape called a cylinder wedge that is probably the best general
purpose shape media of all applications because it uses both a round
and angular configuration. There is also something else interesting
about this shape that makes it unique. Unlike all the other shapes
that have their center of gravity directly in or near the center of
its form, this shape has its center of gravity on its outside edge.
That fact means that this media shape is the most unstable and most
mobile of all the shapes and it also means that it is less likely to
get stuck in most part recesses.
Dry finishing options
All of the media shapes discussed
up to this point, even the random shape materials, is typically used
in wet processing systems because dry inorganic materials do not
leave parts clean and they may also produce surface finishes rougher
than before processing. Also, when used dry, inorganic materials
become easily contaminated and can not be easily removed or
separated from the media. Therefore, water and a chemical compound
are used in most deburring processes.
A better dry alternative media is
wood, wood sawdust, corncob, and shell products. When used by
themselves these organic materials don’t have much capability to
deburr, but they can clean well. Because their small random particle
size, which rarely exceeds 1/8 of an inch, they are light weight
making their processing times to be very long and almost
prohibitive. When inorganic pumice is added to this organic media,
deburring drastically improves and it works well on jewelry detail
and flat parts, but time cycles are still long. Processing times can
be improved more by adding larger precut wood shapes or porcelain,
but other preformed abrasives are not recommended as an addition
because of porosity.
Of the organic materials, walnut
shell media is the heaviest at 35 to 45 lbs./cu.ft., next is corncob
at about 23 to 33 lbs./cu.ft., and then wood at between 21 to 27
lbs./cu.ft. The low number is the media in its untreated natural
condition and the high side represents treated materials. Pumice
additives add very little weight to these lower numbers but
polishing additives push the upper weight limit.
In the last five years, a new media
bonding resin process has been developed that produces these small
loose random organic and inorganic materials to create a media shape
and are run only dry. It looks, feels, and behaves like ceramic or
plastic media, but it is used without water or compound, yet it
produces a cleaner and smoother surface finish than traditional
media.
The new resin bonded media comes in
four formulations or grades and can have more inorganic than organic
composition, therefore, it is hard to classify. The key to this
media seems to be the resin bond that also gives it the longest life
of any media on the market today. The attrition or break down rate
is anywhere from 5 to 20 times that of the ceramic or plastic
formulations. Therefore, despite its high cost of around $12.00 per
pound, it is over all more cost efficient than other media over the
long run.
Another advantage of using this
media dry is that there is no need to treat wastes. Chemicals,
rising, inhibitors, and drying processes are eliminated, and the
maintenance to parts and equipment is removed. Parts come out clean
and can go directly into another operation without waiting. Some
precautions may be necessary to adapt existing equipment systems to
this dry process such as closing and sealing drains and putting a
dust cover over open machines or get an adequate air collection
system nearby.
This new resin bonded media is only
distributed in the USA by Finishing Associates Inc., a Sinto America
group company located in Huntingdon Valley, PA. which also makes
special equipment with air collection systems built in.
Conclusion
Although surface finishing industry
personnel are retiring and/or going out of business there is a need
for the technology and that technology is still changing.
Unfortunately, the decreasing number of people in the trade are not
as knowledgeable as they once where. The knowledge of equipment is
separate from processing. Machines will only produce mechanical
energy forces to the media that is in the work chamber. Processing
of parts can be summed up with an earlier statement of, what you put
into a machine determines what you get out of the machine. If you
don’t know what to put into it, along with parts, you are not going
to get the results you are looking for. Hopefully, this information
will help you with some of those decisions.
For more information, contact A.F. Kenton,
president of Nova Finishing Systems Inc., Huntingdon Valley,PA.
19006 or call 215-942-4474 e-mail novafinish@aol.com
Mr Kenton has been president and owner of Nova
Finishing Systems Inc. for over 13 years. His company makes small
bench top deburring equipment. He is a soon to be published author
of a book entitled “ Understanding Deburring and Mass Finishing
Systems”, which establishes a classification system for all material
removal systems.
Credits
Mike Cantwell
Finishing Associates Inc
1610 Republic Rd.
Huntingdon Valley, PA. 19006
David Davidson
Pegco
P.O Box 248
Bartlett, NH. 03812
George Bull
Illinois Electro- Deburring
9393 Seymour
Schiller Park, IL. 60176
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Nova Finishing Systems Inc., manufactures small, heavy-duty bowl
finishers that stack up to most of the big equipment on the market,
but cost much less. Nova series vibratory equipment also comes with
the same warranties of the larger machines. Form more information
on this equipment line, contact:
Nova
Finishing
PO
Box 185, Hatboro, PA 19040 * 1610 Republic Rd. Huntingdon Valley,
PA. 19006
215-942-4474
* 800-444-4159 * Fax 215-953-1342
novasales@novafinishing.com
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