Tips on finishing problem parts Essay

Many manufacturing engineers and managers are ignoring the great
potential for mechanical finishing. Until recently, finishing was
solely cosmetic: A smooth and shiny part simply was more salable. Today,
however, there are compelling reasons for finishing parts to tight
tolerances–everything from fuel economy in aircraft and autos to more
life for highly stressed, precision computer parts.



In many cases, improved functional finishing is mandatory. A
component with smooth edges and surfaces can operate better and longer.
Safety considerations are important as well; burrs and sharp edges must
be removed from parts intended for consumers.



While mechanical finishing for general applications is widely
understood, it’s harder to determine proper procedure for very
large, very small, and unusually shaped parts, as well as those with
unusual edge and surface requirements. A broad base of technical data
and personal intuition is needed. Factors to consider include:



Material properties–hardness, toughness, density, abrasion resistance, melting point, and chemical composition.



Burr properties–width, lenght, mass, toughness, relative
repeatability, and accessibility.



Part definition–size, tolerance, edge requirements, function, and
surface finish.



Once such data is obtained, a review of production requirements
(i.e., volume of parts to be processed per batch, hour, shift, etc) is
necessary. A specific type of finishing equipment then is selected,
e.g., a high-energy centrifugal-disc or centrifugal-barrel machine.


In one case, we received a request to provide a 0.028″ radius
on a part made of a tough alloy. The workpiece, when processed in a
vibratory tub, achieved a 0.016″ radius after 24 hr of processing.
We suggested centrifugal-barrel finishing (CBF). Now the specified
radius, along with superior color and surface finish, are achieved in
less than 2 hr.



Unusually large problems



Tub-vibrators have been built for finishing parts as large as
complete aircraft wing spars. Such systems reduce finishing costs by as
much as 85 percent and inspection costs by 25 percent. The quality of
the finish and consistency of edges and surfaces also are improved.



For these large parts, manual deburring and finishing with files
and scrapers cause sporadic defects (e.g., excessive edge radius).
Previously, this dictated reworking, or even scrapping, the spars. Less
apparent flaws, if not detected, could precipitate field failure.



Vibratory finishing can generate uniform edge and corner radii while improving a spar’s surface from 50 to 16 microinches AA.
(Manual finishing produces an 8 to 30 microinch finish with no
consistency.)



Further, complete wings of the largest aircraft can be finished
automatically via a sanding machine. One, in fact, is installed at a
major west-coast aircraft builder.



Prior to installing the machine, the wings, some as long as 105 ft,
were placed on trestles and manually sanded. The automatic machine is
capable of finishing both sides of a wing (up to 9 ft-6″ wide) in
one pass. In operation, the wing remains staionary while the
trolley-mounted sanding machine moves along the wing’s length,
removing material from both sides to within 0.0005″.



Capital equipment justification was made originally on savings
compared to manual finishing; however, the main benefit is improved fuel
efficiency for the aircraft because of lower air resistance. Many
aircraft manufacturers now finish wing and body skins this way. The cost
of the equipment is justified on fuel savings alone during the first six
months of a plane’s operation.



The complete armature of a steam turbine is another large part
being automatically finished to deburr, radius edges, and improve
surface finish. The equipment is justified by reduced manual labor.
Another benefit is reduced rework.



Like the wing-sanding machine, finishing equipment for steam
turbine parts is custom disigned and built using standard components.
Because they are programmable, the machines are adaptable to design
changes–programs can be set to handle variations in edge quality, or to
generate different radii on different component areas.


Refer to Figure 1 for a final example of unusually large parts that
challenge finishing operations.



Not so small problems



The watch industry was first to use precision mass finishing for
miniature parts. Development of centrifugal-barrel processing was most
effective for deburring because it generated precise edge and corner
radii and fine finishes–all economically.



Precision miniature deburring and surface finishing applications in
the aerospace industry also are done with CBF, Figure 2. This is a
mass-finishing process like conventional tumbling and vibratory
finishing, but has advantages of high-speed processing under forces as
high as 100 g’s. Fine media is used for handling precision parts,
and very fine finishes can be achieved, even on fragile workpieces.



Dealing with complex shapes



There are proven processes for finishing complex parts when
relevant edges and surfaces are accessible to wheels and belts, buffs,
brushes, abrasive media, and abrasive or nonabraise shot. But, ability
to remove burrs and improve finishes of areas inaccessible to such
processes is less understood.



With the advent of more complex mechanisms, particularly hydraulic
and pneumatic components, specifications for edge and surface
conditioning are increasingly stringent. Development of processes to
finish internal holes and recesses is critical. Currently, there are
three effective techniques: Thermal-energy beburring (TED), abrasive
flow, and electrochemical deburring (ECD).



TED, Figure 3, romoves burrs from all edges and corners of a
component. If the burrs are uniformly thick, deburring will be complete
and the edge condition uniform, while no action is taken on other
surfaces.



The process is ideal for metal parts through which fluids of any
type must flow. Processing costs are low.



On the downside, an oxide coating is deposited on parts as a result
of the rapid oxidation, so expect expenses for subsequent cleaning.
Successful applications include carburetor bodies, lock bodies, and
pneumatic and hydraulic pump bodies.



Abrasive flow is a means of deburring, andedge and surface
conditioning, by extruding an abrasive-laden semisolid medium across
edges and surfaces. The machine has two directly opposed media
cylinders; theworkpiece is fixtured between them.



Media are forced from one cylinder, through the workpiece, into the
other cylinder, and back again. As media passes through the part, edges
and surfaces are smoothed.



The process only affects areas in contact with the flow. It can
finish several surfaces, or even several parts, at one time. Very fine
finishes are possible.



After processing, the media must be removed and the parts cleaned.
Abrasive flow is more expensive than mass finishing, but is capable of
handling intricate parts. Typical applications include extrusion dies,
compacting dies, coldheadling dies, and complex aircraft components.



ECD, Figure 4, is esentially the reverse of electroplating. By
using it, you can selectively remove burrs while having no effect on a
workpiece, except in the immediate vicinity of the burr. Like TED, ECD
isn’t a surface-finishing process, and like abrasive flow,
it’s selective.



The difference between ECD and electrochemical machining (ECM) is
that in the former, the cathode is located in a fixed position
immediately adjacent to the burr, while in the latter the cathode is
driven into the part as metal is removed. In the case of ECD, when the
gap increases to more than about 0.025″ metal removal ceases.
That’s when deburring is completed andafine edge radius is
genearated.



ECD tooling is made to suit the particular workpiece and bur. The
tooling must approach the work so it is parallel to the burr. Also, the
tooling must be insulated everywhere except immediately adjacent to the
burr.



Electrolyte is pumped either around or through the tool, exiting at
the burr zone. Seccess, by the way, hinges on proper cathode design.



ECD costs sinificantly less than TED, and is usually lower than
abrasive flow. Applications include valves and precision components in
the auto, aerospace, business machine, and defense fields.



Typically the process produces stray machining effects on areas
immediately adjacent to the edge being deburred (evidenced by a slight
darkening). Immediately after processing, parts prone to corroding must
be washed.



For semiprecision or precision parts, where burrs are consistent in
size and selective deburring is needed, ECD is the first process to
consider.



Complex conditions



A combination of several conditions determines the quality of a
surface. These include surface smoothness, edge radius, radius
smoothness, surface scratch pattern, scratch shape (and any the
condition, contamination by the finishing medium, and stresses imparted
to edges, corners and surfaces.



Some finishing situations must be accomplished under especially
difficult conditions. One example is finishing carbide inserts. A
radius of as little as 0.001″, or as great as 0.007″, may be
required, and a variation of 10 percent can impact tool performance.
Tolerances must be kep to less than 5 percent for optimum machining
results.



Spindle finishing and rubber-wheel pressure finishing were used,
but required considerble skill. Automated buffing now is preferred
because it can hone insert edges even to the largest radius with fully
automatic monitoring and control; production rates are up to 2000
parts/hr.



Automotive drive chains are another example of finishing under
complex conditions, Figure 5.



In summary, deburring, and edge and surface conditioning, are far
more important today than they were 10 years ago, and will be
increasingly so. Some computer components, for example, have tighter
tolerances than aerospace parts. Mechanical finishing must be better
understood to get finishing costs under control and to specify the most
appropriate technique for a job.



For more information about centifugal-barrel finishing, circle E20.

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