Tips on finishing problem parts Essay

Many manufacturing engineers and managers are ignoring the greatpotential for mechanical finishing.

Until recently, finishing wassolely cosmetic: A smooth and shiny part simply was more salable. Today,however, there are compelling reasons for finishing parts to tighttolerances–everything from fuel economy in aircraft and autos to morelife for highly stressed, precision computer parts. In many cases, improved functional finishing is mandatory. Acomponent with smooth edges and surfaces can operate better and longer.Safety considerations are important as well; burrs and sharp edges mustbe removed from parts intended for consumers.

While mechanical finishing for general applications is widelyunderstood, it’s harder to determine proper procedure for verylarge, very small, and unusually shaped parts, as well as those withunusual edge and surface requirements. A broad base of technical dataand 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, relativerepeatability, and accessibility. Part definition–size, tolerance, edge requirements, function, andsurface finish. Once such data is obtained, a review of production requirements(i.

e., volume of parts to be processed per batch, hour, shift, etc) isnecessary. 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″ radiuson a part made of a tough alloy.

The workpiece, when processed in avibratory tub, achieved a 0.016″ radius after 24 hr of processing.We suggested centrifugal-barrel finishing (CBF). Now the specifiedradius, along with superior color and surface finish, are achieved inless than 2 hr. Unusually large problems Tub-vibrators have been built for finishing parts as large ascomplete aircraft wing spars. Such systems reduce finishing costs by asmuch as 85 percent and inspection costs by 25 percent. The quality ofthe finish and consistency of edges and surfaces also are improved.

For these large parts, manual deburring and finishing with filesand scrapers cause sporadic defects (e.g., excessive edge radius).

Previously, this dictated reworking, or even scrapping, the spars. Lessapparent 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 noconsistency.) Further, complete wings of the largest aircraft can be finishedautomatically via a sanding machine. One, in fact, is installed at amajor 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 iscapable of finishing both sides of a wing (up to 9 ft-6″ wide) inone pass.

In operation, the wing remains staionary while thetrolley-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 savingscompared to manual finishing; however, the main benefit is improved fuelefficiency for the aircraft because of lower air resistance. Manyaircraft manufacturers now finish wing and body skins this way. The costof the equipment is justified on fuel savings alone during the first sixmonths of a plane’s operation.

The complete armature of a steam turbine is another large partbeing automatically finished to deburr, radius edges, and improvesurface finish. The equipment is justified by reduced manual labor.Another benefit is reduced rework. Like the wing-sanding machine, finishing equipment for steamturbine parts is custom disigned and built using standard components.Because they are programmable, the machines are adaptable to designchanges–programs can be set to handle variations in edge quality, or togenerate different radii on different component areas. Refer to Figure 1 for a final example of unusually large parts thatchallenge finishing operations. Not so small problems The watch industry was first to use precision mass finishing forminiature parts.

Development of centrifugal-barrel processing was mosteffective for deburring because it generated precise edge and cornerradii and fine finishes–all economically. Precision miniature deburring and surface finishing applications inthe aerospace industry also are done with CBF, Figure 2. This is amass-finishing process like conventional tumbling and vibratoryfinishing, but has advantages of high-speed processing under forces ashigh 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 whenrelevant edges and surfaces are accessible to wheels and belts, buffs,brushes, abrasive media, and abrasive or nonabraise shot. But, abilityto remove burrs and improve finishes of areas inaccessible to suchprocesses is less understood.

With the advent of more complex mechanisms, particularly hydraulicand pneumatic components, specifications for edge and surfaceconditioning are increasingly stringent. Development of processes tofinish internal holes and recesses is critical. Currently, there arethree effective techniques: Thermal-energy beburring (TED), abrasiveflow, and electrochemical deburring (ECD). TED, Figure 3, romoves burrs from all edges and corners of acomponent. If the burrs are uniformly thick, deburring will be completeand the edge condition uniform, while no action is taken on othersurfaces. The process is ideal for metal parts through which fluids of anytype must flow. Processing costs are low. On the downside, an oxide coating is deposited on parts as a resultof the rapid oxidation, so expect expenses for subsequent cleaning.

Successful applications include carburetor bodies, lock bodies, andpneumatic and hydraulic pump bodies. Abrasive flow is a means of deburring, andedge and surfaceconditioning, by extruding an abrasive-laden semisolid medium acrossedges and surfaces. The machine has two directly opposed mediacylinders; theworkpiece is fixtured between them. Media are forced from one cylinder, through the workpiece, into theother cylinder, and back again. As media passes through the part, edgesand surfaces are smoothed. The process only affects areas in contact with the flow. It canfinish several surfaces, or even several parts, at one time. Very finefinishes are possible.

After processing, the media must be removed and the parts cleaned.Abrasive flow is more expensive than mass finishing, but is capable ofhandling intricate parts. Typical applications include extrusion dies,compacting dies, coldheadling dies, and complex aircraft components. ECD, Figure 4, is esentially the reverse of electroplating. Byusing it, you can selectively remove burrs while having no effect on aworkpiece, except in the immediate vicinity of the burr. Like TED, ECDisn’t a surface-finishing process, and like abrasive flow,it’s selective. The difference between ECD and electrochemical machining (ECM) isthat in the former, the cathode is located in a fixed positionimmediately adjacent to the burr, while in the latter the cathode isdriven into the part as metal is removed. In the case of ECD, when thegap increases to more than about 0.

025″ metal removal ceases.That’s when deburring is completed andafine edge radius isgenearated. ECD tooling is made to suit the particular workpiece and bur. Thetooling must approach the work so it is parallel to the burr. Also, thetooling must be insulated everywhere except immediately adjacent to theburr. Electrolyte is pumped either around or through the tool, exiting atthe burr zone. Seccess, by the way, hinges on proper cathode design.

ECD costs sinificantly less than TED, and is usually lower thanabrasive flow. Applications include valves and precision components inthe auto, aerospace, business machine, and defense fields. Typically the process produces stray machining effects on areasimmediately adjacent to the edge being deburred (evidenced by a slightdarkening). Immediately after processing, parts prone to corroding mustbe washed.

For semiprecision or precision parts, where burrs are consistent insize and selective deburring is needed, ECD is the first process toconsider. Complex conditions A combination of several conditions determines the quality of asurface. These include surface smoothness, edge radius, radiussmoothness, surface scratch pattern, scratch shape (and any thecondition, contamination by the finishing medium, and stresses impartedto edges, corners and surfaces. Some finishing situations must be accomplished under especiallydifficult conditions. One example is finishing carbide inserts.

Aradius of as little as 0.001″, or as great as 0.007″, may berequired, and a variation of 10 percent can impact tool performance.

Tolerances must be kep to less than 5 percent for optimum machiningresults. Spindle finishing and rubber-wheel pressure finishing were used,but required considerble skill. Automated buffing now is preferredbecause it can hone insert edges even to the largest radius with fullyautomatic monitoring and control; production rates are up to 2000parts/hr. Automotive drive chains are another example of finishing undercomplex conditions, Figure 5. In summary, deburring, and edge and surface conditioning, are farmore important today than they were 10 years ago, and will beincreasingly so.

Some computer components, for example, have tightertolerances than aerospace parts. Mechanical finishing must be betterunderstood to get finishing costs under control and to specify the mostappropriate technique for a job. For more information about centifugal-barrel finishing, circle E20.

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