The
mechanical finishing equipment focused on in this article is the
type that relies on some type of motion and some form of force to
apply an abrasive material to a part. Motion is acceleration, velocity
and deceleration. Of these, velocity is the most significant factor
in processing, although rates of acceleration and deceleration can
affect a finish. Force for finishing is gravitational, mechanical
or centrifugal. Force is mass multiplied by acceleration, so size
and speed matter. Kinetic energy also affects the process as media
and parts sizes vary, as does the density of different media and
part material.
For
material removal you need to move the part and abrasive under pressure
in a uniform contact fashion against one another at a certain velocity.
In simple terms, each machine uses the energy it creates to push,
shake or slide the abrasive material against the surface of the
part at some rate of speed.
Mass
finishing equipment is defined as any machine that can run parts
and media together in mass to achieve a certain finishing result.
Mass finishing is the equipment and application that will be discussed
in more detail. Some of these technologies have been adapted to
run a single part by fixture or some other means, but we will focus
on equipment capable of running in mass.
Tumbling
Barrels
The oldest machine for mass finishing is the tumbling barrel.
This comes in either the horizontal, closed barrel or the open-end
barrel. There are still some applications that only the barrel effectively
processes, but for the most part, these units are for small shops
with no critical finishing applications. Tumbling is a slow process
at best and does not match up well to material handling. Open-end
barrels can be automated with loaders and separation, but process
times are still lengthy. There is also the possibility of an explosion
in closed barrels from gases generated by the process.
In
tumbling barrels, the friction is created by gravity as the media
slides across the diameter of the barrel. The motion is created
by rotating the barrel by motor at an RPM that is fast enough to
finish effectively but does not throw the media. The only work being
done in a barrel is as parts and media slide together across the
top of the mass as the barrel rotates slowly (relative to other
technologies). Although pressure is greater at the bottom of the
barrel, lack of motion equals lack of work. To optimize the amount
of work being done, the barrel should be filled about 60% so the
part of the mass doing the work crosses the barrel at it widest
point.
Vibratory
Finishing
By far the most popular form of finishing in the U.S., vibratory
finishing can be a reliable, cost-effective solution. This technology
has taken many forms, but spinning a shaft with out-of-balance weights
produces the motion, with few exceptions. This is true for both
vibratory bowls and tubs with the position of the shaft relative
to the work chamber varying. Some machines use direct-mounted motors
with the weights fastened to their shaft. The alternative is to
drive the shaft with the weights via pulley and belt.
In
the vibratory bowl machines, the shaft spinning with the out-of-balance
weights creates a back-and-forth motion that “scrubs”
the media over the parts. The mass itself moves from the outside
of the bowl to the inside at top and inside to outside at the bottom
of the mass. It also rotates around the chamber. The path followed
would be similar to following the wire of a slinky wrapped in a
circle with the two ends held together.
The
amount of weight set off-center of the shaft affects the degree
of amplitude, with the greater weight creating the greater amplitude.
An amplitude range of 2-8 mm is common in vibratory finishing. (Lower
amplitudes are used for polishing and more delicate operations;
larger amplitude for quicker metal removal.) The number of times
that the machine shakes is relative to the motor RPM. Most vibratory
equipment runs between 1,200 – 1,800 RPM. A machine set with
amplitude of 5mm and running at 1,800 RPM would exact approximately
9,000mm of work across a part in a minute.
Out-of-balance
weights on a round bowl are found at the top and bottom with the
bottom leading the top weight in the direction the mass is moving.
Changing the angle of the top and bottom weights to each other can
change the tightness of the spiral the mass travels in the bowl.
The bottom leading the top weight by 90 degrees is considered common.
In
vibratory tubs, the amplitude and rolling motion are created the
same way. The only difference would be the lateral rotation that
lends itself to unloading in the round bowls is not a factor. Motion
in the tub from end to end, for unloading or continuous tub operation,
can be created by gravity and raising the tub at one end to create
a slope in the direction travel is needed.
Ratios
depend on the process needed. General deburring may be at a 3:1
ratio (media to parts), while pre-plate finishing may be at a 6:1
ratio. Pushing more parts into a defined media-to-parts ratio is
a common problem in mass finishing as production tries to get more
from the process. Media levels and load size (parts) should be clearly
defined and checked routinely.
Another
reason for the popularity of vibratory machines is their versatility.
They can accommodate a range of part sizes, making them easy to
adapt from part to part. Vibratory machines allow for easy and fast
changeover of media, compound and screens.
Vibratory
bowls and tubs allow for peripheral equipment to be added easily.
This includes loaders in batch or piece, automatic compounding,
separation and media classification. Multiple machines for multiple
processes can be set up to load one into another for different applications.
For example, a vibratory bowl could be set to debur and then unload
into another bowl equipped with heaters and cob for drying. Another
common example would be a cleaning and surface improvement process
going straight to a polishing or steel ball burnishing process.
Vibratory
equipment, despite continually trying to shake apart, is relatively
low maintenance equipment if designed and built correctly. Besides
lubrication of the main drive and dam blade (if applicable), there
are not many items to concern you on these workhorses. If you are
going to be using an aggressive media, your unit should have a good
quality hot-poured lining of urethane.
Batch
bowls and tubs require the entire load to be emptied from the machine
for separation, unless magnetism can be used and is effective, and
then reloaded. Round bowls with internal separation are very good
at separating and unloading the parts with the media never leaving
the machine. To further control separation, make sure the length
of the screen is adequate. Variable speed controls will also improve
your ability to thoroughly separate your parts from the media. Curved
wall chambers with spiral bottoms can achieve 100% separation of
parts and media in relatively short time.
Although
the round bowl and tub are the most common examples of vibratory
machines, some others include oval or racetrack; multiple channel
(multipass); long radius; continuous (flow through); divided tubs
and bowls.
High
Energy Centrifugal Disk
Centrifugal disk finishing is considered 4-15 times faster
than vibratory finishing processes. It is also very fast from floor
to floor, including loading, processing, unloading, 100% separation
and can incorporate rinsing, drying and multi-batch processing.
If you are doing a high volume of parts within a certain size range
(6 inches or less) the centrifugal disk is an excellent option.
The
energy in the disk is created by a spinning bottom disk, which pushes
the media outward, where it climbs the stationary wall of the chamber.
Against the chamber wall a braking action occurs and gravity brings
the media back down to the center bottom of the chamber. This torroidal
action constantly repeats itself, creating a finishing environment
where the parts are finished quickly. A sliding motion, where the
energy created in the bowl is enhanced by centrifugal forces working
against the physical barrier of the process chamber, produces a
uniformed and aggressive action. High-energy equipment will allow
the smaller media needed for certain applications to work the same
as more aggressive large media. Despite the aggression of the process
in regards to metal removal or polishing, certain parts are less
likely to impinge than in vibratory machines.
Machines
ranging in size from ¼-12 cu ft run at different RPM. The
RPM is tied to the diameter of the spinner and calculated to produce
a certain peripheral velocity that works best within the process
chamber
Centrifugal
disk finishing is the best for assuring complete, automated unloading
and separation. Not only does it 100% unload and separate, it can
be done in as few as seven minutes. The disk machines also have
a relatively small footprint.
The
critical area of this technology is the gap between the moving bottom
disk and the stationary bowl. Different approaches have been taken
to reduce wear and damage to this area. The technology has been
around for more than 30 years, is extremely popular in countries
like Japan and the gap is manageable. It would benefit you to check
the history of your vendors design and talk to people who use this
technology as it has been clouded with many different designs from
different vendors offering different solutions.
High-Energy
Centrifugal Barrel
Not a new technology but one that has seen resurgence in
past years is centrifugal barrel finishing. Although there are different
configurations for the machines, the most common have become the
units with four equal-size barrels set up opposite each other on
a turret. These machines range from a few liters up to 800 liters
or larger. Most common applications fit in the 30-240 liter range.
The motion of these units can create forces up to 30 times gravity
to be used in the finishing process. This allows for process times
that are 8-30 times that of a vibratory process.
The
machine creates its energy by turning the main turret at up to 250
RPM. Each barrel is set to counter rotate so that they basically
stay in the same position (like you are when seated on a ferries
wheel), or rotate backwards at some ratio. This ratio could be from
1:1 to 4:1 depending on the make of machine. Some machines also
support the barrels on both sides while others just on a single
side.
In
regards to versatility, centrifugal barrels offer the widest range
of processing options. The machines can perform the most aggressive
cutting or produce the highest polishes by simply changing the media
and compound. Processes include both wet cutting and dry polishing
with organic (cob, shell) media. Process times rarely exceed 60
minutes, usually for polishes. Deburring can happen in minutes with
the right process parameters.
These
units create both heat and pressure that allow the exceptional finishing
times. Thirty-minute cycles can generate enough heat to cause burns.
In the wet processes this can produce hot slurry, so caution must
be taken in opening the barrels after processing. Longer time cycles
often require a delay time and external cooling before the chambers
can be unloaded. It is this heat and pressure that allows the dry
organic media to work like a buffing wheel and produce high polishes.
The
down side to this technology has always been floor-to-floor time,
especially loading and unloading. With several closed barrels filled
with a mix of parts, media, water and compound, each must be loaded
and unloaded individually. Recent designs incorporate loaders and
separators into the machines but operators must still control the
function and assure the units are loaded correctly. Certain critical
applications often require layering of the parts as they are loaded,
making up a large percentage of the floor-to-floor time. The finishes
produced, however, often justify the process, since handwork is
the sole alternative.
Early
on, these machines were known to be a maintenance problem. Many
design upgrades and a better understanding of the variables that
affect the machine have all but eliminated this concern. With balanced
loads and good maintenance, these units will run trouble free.
Two
other types of high-energy equipment are drag finishers and spindle
finishers. Both require the fixturing of parts on multiple arms
that are immersed and rotated in the media mass. In Spindle finishing,
the tub spins the mass at a high speed and the parts are rotated
around as the media pass over it. Drag finishers spin the fixtured
parts through the mass at high speed while rotating them. Both are
very effective processes for parts that cannot be subject to any
impingement. Both are also capable of dry organic processing to
high polishes.
All
of the above machines are capable of running parts in mass by providing
an energy that allows the abrasive media to do its job. Each has
its advantages and disadvantages. By knowing what technologies are
available for consideration, and studying the parameters discussed
above that affect your overall process, you should be able to determine
what technology is best for your application.
To
get an idea of the hourly cost it takes to run each machine the
following can be used:
- Tumbling
Barrel: $35.00/hr.
- Vibratory
Tub: $30.50/hr.
- Vibratory
Bowl: $24.75/hr.
- Centrifugal
Disk: $25.25/hr.
- Centrifugal
Barrel: $29.75/hr.
The
variables that help calculate this cost change from plant to plant
and area-to-area, so you should spend the time to see what your
own cost are. The key to accurately defining your cost is to accurately
define the parameters of your process.
The
secret to the processing ability of any technology is how it creates
and balances velocity and pressure. This, combined with an understanding
of abrasives (media) and compounds, is how an effective process
is determined. Understanding these factors will help you see your
mass finishing as a process that can be defined and managed and,
therefore, improved as needed.
Keep
in mind, however, an effective mechanical finishing operation must
also make sure your parts move smoothly through the process, minimizing
handling time and cost. This floor-to-floor movement will have a
great influence on the type of equipment that will best suit your
production environment. Defining all the variables that apply to
your particular process will allow the efficiency and economy you
need from it.
Serving
the Finishing Industries. Since 1936.
PF Onine and all contents are properties of Gardner Publications,
Inc.
All Rights Reserved.