Ever since the first Roman soldiers tossed their armor into a barrel
full of water and river stones to shine it for battle, mass finishing
has played a major role in efficient post-production surface improvement.
That sort of approach is great—so long as the parts in question
are, well, armor-like. But, what about today’s world of advanced
materials, miniaturized components, electrical connectors and micro
devices? Finishing challenges have followed suit, giving metal finishing
professionals increasing numbers of small, delicate parts that present
special challenges in surface preparation, processing and handling.
Processing delicate parts is one of the greatest ongoing challenges
in mass finishing. And, here’s the great irony: the smallest parts
are often the ones produced in the greatest quantities. So, they
are begging for a mass-finishing solution. But, due to their gage,
shape and construction, they are often the ones least likely to
succeed in a mass-finishing process.
Defining “Delicate”
While the term “delicate” is strictly in the eye of the beholder
(producers of two-gram steel stampings or two-pound jet engine turbine
blades may both legitimately see their parts as being delicate),
we’ll limit our discussion in this article to particular types of
parts that can present the most “tricks” to metal finishers:
- Thin parts. Small, flat parts, such as stampings thinner
than 0.025-inch present several challenges. First, they can easily
bend or twist in-process. Second, their profile introduces the
issue of capillary attraction; the tendency for thin, flat parts
to stick together and “stack,” effectively negating consistent
surface finishing. This is especially true in wet finishing processes
that, despite advances in dry processing methods, represent more
than 90% of all mass-finishing applications.
- Small parts. Die casting is an inexpensive way to produce
high quantities of detailed, decorative “near net shape” parts.
Unfortunately, the typically thin walls and soft material often
cannot withstand the weight, contact or force of the parts/media/water
finishing mass without distorting. Other smaller parts such as
powdered metal components may court damage due to their softer
metallurgical structures.
- Protrusions, tabs or slots. Often, a delicate parts-finishing
challenge comes not from size or weight but from shape. Parts
with long “arms,” protruding tabs, large slots or openings present
challenges of part-part entanglement, media lodging or damage/distortion.
- Fine finish requirements. The final category of delicate
parts is that in which the quality of the finish itself is the
issue. For the customer who insists on a “mirror finish” or finished
parts that “look like jewelry,” this may require zero-impingement
processes or other gentle handling requirements that produce the
kind of low Ra results needed.
What can you do if you have parts that fall into one of these categories?
As with most issues in mass finishing, start by learning your options.
The easiest way to break them down is by examining each of the four
main mass-finishing variables: machinery, media, chemistry and parts/media
ratios.
The Machinery Connection
Tumblers and vibratory systems. This is where delicate
parts finishing hits its first troubling paradox. While it may seem
logical that delicate parts require delicate processes, “kinder,
gentler” machines such as tumblers and vibratory tubs are not necessarily
the best options. Small, flat parts tend to stick together (and
to the sides of the processing chamber) and these machines can have
a harder time breaking the capillary attraction that binds them.
Changing to a processing chemistry with more “slip” can help, but
doing so takes more energy away from the deburring, descaling, edge-radiusing
or other jobs that the process is meant to accomplish. Parts handling
can also be an issue, as flat parts are more likely to hang up on
vibratory parts discharge ramps and chutes, making 100% parts accountability
difficult at best.
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Parts with thin profiles,
tabs, slots or unusual shapes present special challenges to
metal finishers. Parts shown range in thickness from 0.187-0.012
inches, penny shown for scale.
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That said, vibratory tubs and bowls are often the only mass-finishing
systems that can do the job on parts with odd-shaped protrusions
or tabs that can easily bend in higher-energy processes. Likewise,
some super-smooth/mirror-finish processes (such as those used to
burnish hand tools) are designed and optimized only for vibratory
or tumbling processes. Also, vibratory tubs and bowls lend themselves
to the use of dividers that section the processing chamber into
discrete sub-chambers. This allows process development in low- or
zero-impingement environments (e.g. just a few parts or even one
part per “section”). One tip: if vibratory proves the way to go,
choose a machine with fully-adjustable amplitude and frequency so
that processes can be fine-tuned. A small change in either parameter
can have immediate and visible results.
High-energy systems. Counter-intuitive as it may seem, high-energy
finishing machinery like centrifugal disc and centrifugal barrel
systems can provide excellent solutions for delicate parts finishing.
They have the raw power to overcome capillary attraction problems,
and many offer parts loading/unloading/handling systems that can
be very gentle on parts. Properly designed centrifugal barrel finishing
(CBF) systems can actually offer the greatest process flexibility
for delicate parts. In systems where barrel-to-turret rotation ratio
is adjustable, processes can be designed from very aggressive to
incredibly gentle—all while imparting up to 10 G of energy to the
finishing mass. Even without adjustable barrel or turret speeds,
CBF processes can be “cushioned” by simply increasing the amount
of media or water/compound solution in the barrel. The small “mini”
CBF machines are often the only high-energy choice for tiny parts.
Probably the biggest downside of CBF is the heat generated in a
closed, high-energy barrel. While gentler cycles are possible, these
cycle times are generally longer and can generate significant heat
in the processing mass. This can reach the point of requiring intermittent
pauses to pressure-relieve the barrels—thereby losing some of the
advantage of shorter cycles.
Centrifugal disc (CD) systems present no such concerns, since the
top of the processing chamber is open. And, at two to three G of
energy, they provide plenty of processing power. So, many of CBF’s
advantages translate to CD. However, there is one unique feature
of CD designs that can limit its usefulness in small parts processing—the
gap between the stationary processing chamber and rotating disc
or “spinner.” Typically, this gap begins its life (in a new machine
or spinner/chamber pair) around 0.01-0.015 inches. Over time and
with use, this gap eventually wears and widens, with obvious results—thin
parts or protruding tabs getting caught and jamming the machine
or damaging the part. Also, due to its aggressive “slide” action,
parts bending, tangling or part-on-part impingement can become issues.
Still, in the hands of a skilled process developer, centrifugal
disc can be a powerful processing tool that also offers perhaps
the best opportunities for pre- and post-process parts handling
and accountability. If centrifugal disc is an option, choose one
with both an adjustable gap (preferably one that can be set and
held at or below 0.01 inch) and infinitely variable spinner speed.
The Media is Part of the Message
Metal finishing media is a subject for a full-day seminar.
The sheer array of materials, cuts, weights, shapes, abrasive materials
and binders is dizzying. But, when processing delicate parts, the
options are a bit more limited. Steel media, for instance, is often
rejected pretty much out of hand because of its weight (with the
exception of some burnishing operations). Likewise, many ceramic
pre-forms can be too heavy to produce material removal without misshaping
delicate parts. However, smaller ceramics with lower cut rates can
offer good solutions. So can plastic media, which can be one-third
the weight of ceramic, offer long wear life and cut rates that range
from smooth burnishing to medium material removal. Porcelain media
can also offer interesting possibilities. While they typically are
not the choice for material removal, porcelain can be effective
for burnishing or achieving low Ra finishes (the downsides: long
cycle times and extremely higher per-pound cost). There are also
some new media being developed right now with porcelain-like weight
but with cut rates approaching aggressive plastics. The key is to
select a medium that will do the job without imparting too much
stress on the part—especially in high-energy machinery, which applies
a physical force that reduces the aggression needed from the media.
Generally speaking, when dealing with delicate parts, you’ll have
to live with trading off cycle time for the ability to mass finish
them at all. But, as media technologies improve, this gap will close.
In some machines, including centrifugal disc, delicate parts can
be processed in a “closed-loop” process. While most processes allow
water and process chemistry to flow through the processing chamber,
some call for closing the drain valve and allowing the water to
fill the chamber. By using porcelain or other low-abrasive media
and adding a fine abrasive powder (such as silicone carbide, aluminum
oxide, etc.) very small or delicate parts can be gently processed.
In these cases, the media is used mostly to support the parts while
the powder and water “slurry” does the work. The down-side is that
without a continuous flow of water to cool the chamber, friction
from the process causes heat to build up in the mixture, especially
in high-energy machinery. When not properly controlled, this heat
can reduce the effectiveness of the process, damage delicate or
sensitive parts and even damage the process chambers of some machines—especially
in centrifugal disc machines with urethane liners.
This could also be a time to look into so-called “dry processing.”
Using special media (and, usually specially designed machinery that
goes with them), a small percentage of parts are candidates for
being finished with no water at all. The upsides: no capillary attraction,
no wastewater stream and media that are exceptionally light and
therefore gentle on parts. The downsides: dry processes can be dusty
or dirty, usually require specialized equipment (which can be very
expensive compared with similarly sized conventional machinery)
and often require that the part be cleaned and completely dried
before and after deburring. Several “dustless” media are becoming
available. They are produced primarily in the Orient where high-energy
dry processing seems to be catching on. These media may offer a
solution in some applications, but the media is not commonly available
and is often expensive.
Developing the Right Chemistry
The overriding theme of chemistry in any process is simply
to keep parts and media clean, while minimizing water use. There
may be other special considerations such as the need to descale,
degrease, rust-inhibit or lubricate parts. But, the chemistry you
select depends primarily on the media you plan to run and the type
of part being processed.
Generally speaking (and, especially when dealing with delicate
parts) finishers should avoid high-foaming agents—or the common
temptation to run unnecessarily high concentrations of soap. In
fact, according to U. S. Chemical Company’s Dave Govoni, “Excessive
foam only dampens the machine’s processing capabilities. It can
also retain fluid in the machine, increasing the potential for re-deposit
of soils onto the part. In most mass- finishing processes, we look
for chemistry concentrations of around one percent. If you can’t
do the job at that rate, you’ve probably picked the wrong compound.”
That said, many delicate parts will benefit from chemistries that
add lubricity or “slip” to the process. This can help finishers
overcome challenges of capillary attraction, part-on-part impingement
and in-process parts cleaning. Ask your metal finishing chemical
supplier about compounds that provide low-foaming lubrication. Combined
with a “flow-through” process, such as those that can be run in
vibratory bowls or centrifugal disc systems (one of the upsides
of the CD’s spinner-chamber “gap”) a good cleaning compound can
go a long way to creating a clean, gentle processing cycle for delicate
parts.
Parts/Media Ratios
The standard rule-of-thumb for a typical parts/media ratio
in mass finishing is 3:1 by volume—thus, in a four-cu-ft working
capacity machine, you would load three cu ft of media along with
one cu ft of parts. This is always a good place to start. But, with
very delicate parts, you may have to stop thinking in terms of a
load of parts lying in a box. Instead, picture them tumbling and
turning in a mass-finishing machine. To create this new rule-of-thumb
ratio, determine the “orbital volume” of the part—a spherical shape.
Calculate it by first measuring one-half of the longest dimension
of the part. This becomes the radius of the sphere. Plug that dimension
into this standard mathematical formula for calculating the volume
of a sphere: 4Pr3/3. This more accurately represents the shape and
“volume” of the part in-process. Now, convert that into the 3:1
media:parts ratio and go from there. You may be able to get more
aggressive as you develop your process, but it’s a good, safe starting
point for parts that are easily damaged or cannot tolerate part/part
or part/machine contact.
A Gentle Conclusion
Delicate parts can represent a challenge for even experienced
mass-finishing professionals. As always, the only real answer is
to continually try new processes and be willing to experiment. Work
with your local metal finishing rep or job shop. They’ll help you
approach process development with a logical, scientific and experienced
approach. Take advantage of the free parts testing and process development
services that most media and equipment manufacturers offer. Remember
—they want your business and are usually willing to help you develop
processes in order to get it. Above all, don’t listen to those who
say it “can’t be done.” Sometimes, the most impossible-looking combination
of parts, media, machinery and chemistry turns out to be the breakthrough
process you were looking for.
