Tips on finishing problem 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.
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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|Author:||Hignett, J. Bernard|
|Publication:||Tooling & Production|
|Date:||Nov 1, 1984|
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