Vibratoru Finishing

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Vibratory finishing is now the most popular type of mass finishing and, next to
hand deburring, the most common surface conditioning method used by industry. This
versatile process is used for cleaning, deburring, deflashing, descaling, edge and corner
radiusing, surface finishing, and stress relieving. Workpieces in a wide variety of sizes
and shapes are processed, as are all metals and many nonmetallic materials. Large
quantities of parts can be run in batch or continuous process setups without handling or
fixturing, thus minimizing costs.

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Page 1
VIBRATORY FINISHING
Vibratory finishing is now the most popular type of mass finishing and, next to
hand deburring, the most common surface conditioning method used by industry. This
versatile process is used for cleaning, deburring, deflashing, descaling, edge and corner
radiusing, surface finishing, and stress relieving. Workpieces in a wide variety of sizes
and shapes are processed, as are all metals and many nonmetallic materials. Large
quantities of parts can be run in batch or continuous process setups without handling or
fixturing, thus minimizing costs.
1. EQUIPMENT BASICS
Vibratory equipment is made in two basic configurations: rectangular tub and round
bowl types. A third type, tubular, is made by at least one company, but is not widely used.
The first tub-type vibratory finishing machine was introduced commercially in 1957, and
the bowl-type about five years later. Both types use an open-top work chamber
containing an aggregate of media, compound, water, and the workpieces. While the
chamber is vibrated, work is performed by the media and compound’s scrubbing or
peening action on the workpieces.
1.1 Tub-Type Vibratory Equipment
With tub-type machines workpieces are loaded into the open top of a container
holding the media, compound, and water. The rectangular tub (container) generally has a
U-shaped cross-section with flat parallel ends, and is usually mounted on coil or rubber
springs. On some machines the containers are suspended on air bags, while certain small
units employ composite or metal strips for suspension. Processing containers other than
U-shaped, such as keyhole shapes and enclosed tubular units, are sometimes used to
improve unrestricted mass movement. Tub liners, generally polyurethane, are used to
prevent wear, reduce noise, and resist chemical attack. They also transmit energy to the
media. Removable separators can be installed to divide the rectangular tub into a number
of compartments. This permits processing workpieces individually or the use of different
media to finish various parts at the same time.
Tub replacement liners are about 15-25% of the cost of a new machine. Repair kits
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are also available for some machines. These will add up to a year of additional life to the
liner.
Vibration of the container is accomplished through several means, including the
following:
- A vibratory motor having counterweights on its shaft and attached to the bottom
of the container. This method is used on some bowl-type machines.
- A single shaft or twin shafts with eccentric weights driven by a standard motor.
- A system of electromagnetic vibration generators.
Machines equipped with magnetic vibrators are usually limited to less than 10 cu ft
of capacity. For this type of system the vibration occurs under the machine at a constant
3600 cycles per second (cps). Increasing the voltage from the rectifier console
strengthens the magnetic field, resulting in a longer pull to the armature, thus increasing
the stroke. The operating principle here is that the variable stroke acts like the variable
speed and amplitude of a mechanical vibrator without having to change weights.
Action of the media against the workpieces takes place throughout the entire load.
The media rubs against the workpieces while the entire load is turning over within the
container. As a result cycle times are substantially shorter for this kind of vibratory
finishing than for barrel finishing. Other advantages of tub-type vibratory finishing
include the capability for in-process inspection, unloading and reloading without
stopping the machine, and the possibility of automating for either batch or in-line
processing. Stated differently, vibratory finishing lends itself readily to continuous
processing, and hence to integration into automatic process lines.
Vibratory equipment can provide more finishing action in the recesses of
workpieces than barrels, and it can process larger parts. Tub vibrators have been built to
process components such as aircraft wing spars 30 ft (9 m) or more in length. Modular-
type machines mounted in series can handle parts over 100 ft (30.5 m) long. Modular tub
units are used to make very long tub vibrators from a series of shorter ones. Each unit
must be separately driven and the units must be joined in such a way that maintains as
good a vibratory action in the connection area as elsewhere.
With long tub-type vibrators, fully automated continuous processing of small parts
is possible. Workpieces are loaded at one end of the tub and unloaded at the opposite end.
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At the unloading end the media is separated from the workpieces and is generally
returned to the tub by a conveyor, often through reclassification systems. Post processing
operations, such as rinsing and drying, can be incorporated into such continuous systems.
The first 40 ft (12 m) long machine had a capacity of 200 cu ft (5.66 m3) and was used to
finish die cast parts up to 36 in. (1 m) in diameter. Some tub-type machines are designed
to allow both forward and reverse rotation of tub contents.
1.2 Round Bowl Type Equipment
Round bowl or toroidal vibratory finishing machines have a doughnut-shaped
chamber that permits a continuous circular flow of media and workpieces. The bowls
may have either flat or spiral bottoms. As with tub-type machines the chambers are
provided with liners.
Vibration of the bowl is accomplished either by a vibratory motor or by an
eccentric weight system mounted vertically in the center tube of the bowl. With either
method the amount of weight placed on the top and bottom of the eccentric system and
the angular displacement between the two weights control the following:
- the finishing action (the amount of media vibration against the workpieces)
- the speed at which the mass rolls over within the bowl
- the speed at which the mass rotates around the bowl.
1.3 Tubular Machines
Tubular machines come in at least two basic designs, but their overall shape looks
like a large-diameter, almost horizontal tube. One version is a continuous spiral tube
similar to popular amusement park water slides. Such machines are totally enclosed
except for the ends, which are open to allow automatic loading and unloading. The tube
is completely lined 360° with a rubber-like liner, which usually drops the noise level by
20 dB or more compared to open designs. A single motor drives the several tubes, saving
energy. The location of the motor equalizes forces within each tube, resulting in
consistent media action from one end to the other. The machine drive is versatile enough
to run all types of media that vary in density such as ceramic, plastic, dry polishing
media, and even steel media. Typically, the space requirement for an in line flow-through
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system having 10 ft of linear tub length is 7 ft wide
х
20 ft long. The new multiple-tube
machine (called “MULTI-TUBE” by the manufacturer) allows over 70 ft of linear tube
length to fit in this area.
1.4 Stacks of Tub Machines
One manufacturer provides a series of tubs in a single machine much like the seats
in a Ferris wheel. This allows long parts to be placed in one tub for roughing, into
another for finishing and into yet another for cleaning, all in the same basic piece of
equipment. Up to five different tubs, each handling parts up to 30 in. (76 cm) long, can
be placed in the machine.
1.5 Vibratory Mixer Deburring
Laboratory mixers have been used for deburring small parts. These tabletop
machines generally produce a vibrating, figure-8 motion in a closed container to mix
powders. Small parts including jewelry can be finished in them, but they are not in
common usage.
2 MACHINE BASICS
There are probably more machines equipped with the under-tub vibrator than any
other design. The introduction of tub-type vibrators in the 1960s featured this method and
it continues to be used today. The vibrator is attached beneath the tub itself, having pie-
shaped flyweights fitted to the shaft ends. Variable speed vibrator is a desirable feature
that aids in controlling any variables in mass density.
Machines equipped with magnetic vibrators are usually limited to less than 10 cu ft
(0.28 m3) in capacity. The vibration input is from the underside of the tub and operates at
a constant 3600 cycles per minute. Voltage is controlled via a rectifier console.
Increasing the voltage strengthens the magnetic field resulting in a longer pull to the
armature, thus increasing the stroke. The theory is that variable strokes simulate the
variable speed and amplitude of a mechanical vibrator without changing weights.
Round bowl vibrators are available in a number of “diameter ratios”—the ratio of
the center section diameter to the channel cross-sectional diameter. A 1:1 ratio means the
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processing channel has the same diameter as the center section diameter. A 4:1 ratio
means the center section diameter is four times that of the channel diameter. Generally,
the closer this ratio is to unity (1:1), the lower the cost of the machine.
Ratios of 1:1 are common for “non-elevating” chambers where no internal
separation is expected. Higher ratios make internal separation easier—2:1 to 4:1 ratios
are not uncommon for these machines when conventional ceramic and plastic media are
used. Ratios up to 7:1 can be found when difficult-to-move, hardened steel media is used.
When the diameter ratio is about 1:1, part retention times are quite small, generally on
the order of 1/5 to 1/3 min per cu ft (0.064 to 0.1 min per m3) of machine capacity. When
the diameter ratio is increased to 5:1, retention times of 1/2 to 1 min per cu ft (0.17 to
0.34 min per m3).
Special designs satisfy a great number of additional industrial requirements such as
ball burnishing machines, which are designed to handle much heavier steel ball media.
Some machines allow predetermined speed alternatives to maximize the type of work
that is possible in a single cycle.
3 MACHINES VARIABLES
There are at least six types of batch and continuous tub vibrators: U cross-section,
inverted keyhole section, half-U cross-section, single, dual, or multi drives. Batch
equipment comes with or without dividers. Continuous machines may or may not have a
means to control continuous process cycle in time.
Round vibrators are of at least four types: with dividers, without dividers, curved
outer wall, or straight outer wall. Round vibrators are available with or without a
separation system.
Oval vibrators possess the variables associated with round vibrators plus the
potential additional variable of ratio of length to width.
Portable tub machines can be used at the point of machining. Some of these allow
recirculation of the compounds, reconstitution of the compound, filtering, and chemically
recharging the effluent. While most machines of this design are small, one manufacturer
provides 3 and 5 cu ft capacity portable units. The settling tank, with pump and hose, is
in the base of the unit for self-contained recirculation of solution. These machines can be
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converted to flow-through use for optimum cleanliness.
3.1 Continuous Machines
Continuous machines are popular for the deburring of parts in relatively short
cycles. In these systems the separation deflector is fixed in the separation position so that
media and parts are always deflected up onto the separation deck. Media drops through
the screen where new parts are loaded continuously. Cycles are generally in the 3- to 30-
minute range depending on media, parts, amplitude, machine design, and size. These
machines hold 10 to 20% more media than batch machines because the area under the
screen where parts are loaded is filled.
Compartmented machines, like tub vibrators, are designed to make a number of
small machines out of one big one. The small compartments insure no part-on-part
contact, but are loaded and unloaded by hand. Compartments can be fixed in place in the
channel or they can be mounted on a fixture that allows them to rotate with the mass.
Some compartmented machines separate different media or different part numbers.
These may contain more than one part per compartment.
Oval-shaped vibratory machines are said to offer better separation of parts from
media, better control of retention times, and other advantages. They are driven by a
central eccentric weight system, like the round versions. Therefore they do not offer a
uniform action (i.e., media and parts close to the center do not act the same as those that
are farther away). Single shaft oval machines are no longer made, but dual shaft
machines, which provide better action are still produced. Dual shaft design produces a
true orbital motion at the center of the rotation—a centrifugal force that optimizes input
energy. This gives the fastest possible speed of mass rotation. This machine design can
be up to five times faster than single shaft machines.
3.2 Multiple Pass Designs
Multiple pass machines are newer. They permit longer retention times by making
the media and parts mass travel around the machine two or more times. Alternatively,
several operations can be performed on the parts, such as deburring followed by drying in
one continuous operation. Two or more different types of media could be used in the
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same machine at the same time without mixing. The narrow paths essentially prevent
parts from hitting each other. Parts merely follow one another around the bowl. Part-on-
part processing is also possible if desired, but the prevention of contact is of particular
benefit for many parts. Uniform parts spacing occurs automatically.
3.3 Bench Top Units
Most (but not all) of the small bench top machines are marginally designed devices
that cannot compare favorably with their grown-up fellows. In order to hold down costs
manufacturers of these machines have removed several important items, such as the
adjustable weight system and a good solution system. Leaving these two items alone out
of the design can turn a good machine into a bad one. (There are good machines on the
market, but they cost more.)
3.4 General Considerations
Heavy, bulky, or long parts need a tub style machine, while parts of more
geometrically uniform surfaces process better in bowls. If a machine is for metal media
processing the manufacturer will supply the machine set specifically to the application.
Continuous equipment is more directed in its use and usually applied to high volume
production.
The choice a user makes between tub and bowl is not based on personal preference
but on a review of part size and anticipated production. Part size will dictate the channels
requirements, while production and process time will determine the machine size, style,
and number of units required. However, it is not unusual for manufacturers to purchase
large bowl machines to process bulky parts that would finish quicker and more efficiently
in a tub unit 1/4 the capacity of the bowl. On the other hand, the use of tubs to process
larger quantities of small parts with no provision for automatic unloading or separation is
just as inefficient.
Some machines are not well designed: they will shake themselves apart after a few
months of operation. The vibration can also be transmitted through the floor to other
parts of the plant. One manufacturer provides an impressive display of vibration control.
He does so by standing a U.S. nickel on edge on the base of his machine while it is
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running. The nickel does not move, fall over, or roll away. Vibrations can be controlled!
3.5 Channel Cross-Sections
Like the tub units, bowl channels have a curved, hemispherical bottom, and,
normally, straight and parallel sides. When small parts and small media are involved, a
curve to the outer wall can be used to prevent media “back-roll.” Seldom is the center
section wall curved.
Regardless of the type of machine (tub, bowl, or continuous), the processing
channel shape is important. Scratches, impingement, and tangling of parts occur when the
channel cavity restricts freedom of mass motion. Deep channels develop forces at the
bottom area that can cause distortion to delicate pieces; in shallow channels large pieces
hit the bottom and one another. It would seem that a circular channel would be the better
design, but, once again, this depends upon the application.
A deep narrow channel has containment. If the part size is such that the mass
motion is unrestricted excellent results can be expected when utilized for deburring
applications. Shallow channels will accept long objects well, while bulky items will be in
constant contact with the channel lining and other workpieces.
Other popular channel configurations are manufactured. The above examples
illustrate that the part size, material, volume, and process are the key factors upon which
to base decisions. Operator work height and reach requirement should be carefully
reviewed. Certain units may require a platform for operator convenience and ergonomic
considerations.

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