隔振系统实验台总体方案设计(论文+DWG图纸)
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南通大学毕业设计附录（英文资料翻译）SOUD FREEFORM FABRICATION ISFF) AND RAPID PROTOTYPING11 SOUD FREEFORM FABRICATION ISFF)METHODSSeveral manufacturing processes are available to make the important transition from computer aided design(CAD)to a prototype partSeveral new technologies began to make their appearance after 1987In that year，stereolithography (SLA) was first introduced bv 3D Systems Inc，and over the next five years several rival methods also appearedTllS created the family of processes known as solid freeform fabrication fSFFlAs with most new technologies at the beginning of the“market adoption Sshaped curves”in Chapter 2the SFF domain iS accompanied by a relative amount of advertising“hype”SFF processes are sometimes described as： Parts on demand From arttopart Desktop manufacturing Rapid prototyping At the time of this writing，stereolithography (SLA)，selective laser sintering (SLS)，fused deposition modeling(FDM)，and layered object modeling(LOM) are being used on a day-to-day basis by commercial prototyping companiesThe three-dimensional(3一D)printing process in cornstarch，plastic，and ceramics is also being used commerciallyThe methods lower on the list show promise but do not seem to be in great use by third party prototyping houses to make their daily incomeCasting is a special caseIt is still used to make oneof-akind prototypesFurthermore，for batch runs in the 10 to 500 category it is a very cost effective method to use once an original mold has been made by a process such as stereolithographyMachining isalso used to make oneof a kind or several prototypes111 Summary of SFF and Rapid Prototyping ProcessesIn daily commercial use：Stereolithography (SLA)Selective laser sintering (SI S)Laminated object modeling (LOM)Fused deposition modeling (FDM)More at the research and development(R&D)stage：3-D printing in cornstarch，plastic，or ceramic3-D printing with plastics followed by planarization using machiningSolid ground curing(similar to SLA)Shape deposition modeling ra combination of addition and subtraction) Non-SFF(traditional)：MachiningCasting Comparisons done in the early 1990s by the Chrysler Corporation revealed that the SLA process was ahead of its rival nontraditional prototyping methods in terms of cost and accuracy(these studies excluded an evaluation of machining and casting)Following the technical descriptions in this chapteradditional figures and tables are thus included to compare these costs and accuraciesOver the last decade， SLA has further emerged as the most used SFF process,especially for the generation of the master patterns for casting and injection moldingAt the time of this writing，SLS，FDMand LOM have the most visibility after SLA112 The History of SFF MethodsDuring the late 1970s,Mead and Conway(1980)created the groundwork for the fast prototyping of very large scale integrated(VLSll circuitsDesigners were encouraged to think in terms of five two-dimensional(2D)patternsThese patterns defined three stacked interconnection layers on a metaloxidesemiconductor fMOSl wafer and their mutual connections through holesThe patterns descry bed the actual geometrv of the connection runs and via holes that one would see when looking down onto the circuit chip，regardless of the exact process and number of masking steps that were used to implement the chip(see MOSIS,2000) Inspired by this success，beginning in the 1970s,several companies tried to create layered manufacturing for mechanical partsAISO by the mid1980s,severa U.Sgovernment studies analyzed the possibilities of a“mechanical MOSIS”(Manufacturing Studies Board，1990；Bouldin，1994；NSF workshop I，1994，and II，1995) The prospects for a mechanical MOSIS were thus frequently linked to the fabrication processes in the lists mentioned(Ashley,1991，1998；Heller，1991；Kruth，1991；Woo，1992，1993；Au and Wright，1993；Kochan，1993；Kai，1994；UCLA，1994；Wleiss and Prinz，1995；Cohen et a1，1995；Dutta，1995；Jacobs，1992，1996；Beaman et a1，1997；Kumar et a1，1998；Sachs et a1，2000) The introduction of the first commercial SFF technology-stereolithography-was accompanied by the advent of the STereoLithography (STL)representation of a CAD obiect“STUis a modified CAD format that suits a subsequent slicing operaction and the“downstream”laserscanning paths on a physical SLAFDMor SLS machine Is a soccer ball round? The answer depends on how carefully the balIis measuredNominally,it is a perfect sphereHoweveron closer inspectionthe leather is sewn together from about 20 little hexagonal patches and a few pentagonal patches to create the curvatureIn reality it is an approximation to a sphere Likewisethe“STLformat approximates the boundary surfaces of a CAD model by breaking it down into interconnected small triangles-a process called tessellationEach triangle is represented by the xyz coordinates of each of its three vertices,enumerated by the righthand rulethat isCounterclockwise (ccw) order as viewed from the outside of the bodv The vector normal to the surfaee of each triangle is also specified1his tessellated surface is stored as an“STL file”this filePerhaps containing up to 200，000 triangles，is sent over the Internet to a prototyping shop As shown in Figure 41this tessellated CAD model is then sliced like a stack of playing cardsFor 3D Systemsmachines this is known as the SLI or sliced fileOther rapid prototyping machines use the slicing technique but have their own file creation details and namesEach slice for the imaginary soccer ball will thus be a circleHoweverbecause of the tessellation procedure it will not be a perfect circleThe slicing action cuts through the triangles on the boundaryThus，each circular slice (or disc will actually be a multisided polygon running inside the“bounding circle”The number of sides on this inner polygon is of course related to how finely divided the original tessellation was made Inside the SLA machinethe laser first creates the outer boundary of each slice and then“weaves”across each slice in a hatching pattern to create the layerThe number of slices and the style of the weaving pattern are chosen by each rapid proTotyping shopEspecially for SLA and SLS a certain amount of trial and errorOr craftspersonship，begins to play a role at this stageThis is reviewed in more detailover the next few pages “STL，is now the standard exchange format for SFF processesHoweverit is inadequate for many reasonsFirst，the files are large due to the tessellation methodSecondthere are redundancies in the“STLformat0ne example of redundancy is as follows：the triangles are represented by the“counterclockwise rule”so that it is clear in which direction the outer-surface normal actsHoweverit has also become customary to specify the surface vector as wellInconsistency can be introduced as a result of this redundancy, and no rules exist for resolving it McMains (1996)describes how“STLdoes not capture topology or connectivity, making it difficult to fix some of the common errors found in filessuch as cracks，penetrating or extraneous faces，and inconsistent surface normals without resorting to guessing the designers original intentMore general digital interchange formats have also been used with SFlF These include ACIS(1993)and IGES (Heller，1991)However，as described in NSF(1995)，problems arise with these formats，too One aspect of ongoing research is thus to improve this representation language (McMains et a1，1998)12 CASTING METHODS FOR RAPID PROTOTYPING121 IntroductionThe classic manufacturing texts by DeGarmo and associates(1997)，Kalpakjian (1997)，Schey (1999)，and Groover (1999)are remarkably comprehensive in their coverage of the casting processThe several methods of casting include：Lost-wax investment castingCeramic-mold investment castingShell molding Conventional sand molding Die casting Rather than duplicate the material found in other books, this section focuses on casting as it is done by rapid prototyping companiesBatch sizes from 50 to 500 are typicalThe key market strategy is that casting is cheap and fastHowever，it may not be the choice for the final product because of its tolerancesDepending on the type of casting chosen，the tolerances vary from+/一75 microns(0003 inch)for lost-wax processes to+一375 microns(0015 inch)for standard sand castings(also see Chapter 2)122 LostWax Investment CastingAs mentioned in Chapter 1the fundamentals of casting were invented by Korean and Egyptian artists many centuries agoThe following steps are known as the lost-wax investment cas“ng process(Figure 416)：(ac)a master pattern of an engineering or art object is first carved from wax；(d-f)it is surrounded by a ceramic slurry that soon sets into solid around the wax；(g)the wax is melted out through a hole in the bottom，leaving a hollow cavity；(h)this hole is plugged，and liquid metal is poured into the open cavity from the top；(i)after a while，the metal solidifies and the ceramic shell can be broken away to get the part；(j)some cleaning，deburring ，and polishing are needed before the object is finished The process was greatly improved and made more accurate during World Warn II for aeroengine componentsToday it is used for products such as jet engine turbine blades and golf club headsOn the top line of Figure 416wax patterns are formed from injection molds，assembled on treelike forms，and then treated with the slurry Alternate layers of fine refractory slip (zircon flour at 250 sieve or mesh size) are applied，followed by a thicker stucco layer( sillimanite at 30 sieve or mesh size)The coated components are dipped in fluidized beds that contain isopropyl silicate and liquid acid hardenerDrying takes place in ammonia gasThe next step is to eliminate the wax in a steam autoclave at 150。C，fire the mold for 2 hours at 950。C，then pour in the liquid steel or aluminum In summary, the modern lost-wax method has one of the best tolerances in the casting family because the original wax patterns are made in nicely machined molds Today, tolerances of+-75 microns r0003 inch)are readily obtainableAlso the ascast surface is relatively smooth and usable for the same reason0ther advantages include：Figure 416 The lostWax investment casting processUpper diagrams (a) through(C) lead to the tree of Wax master patterns . Middle diagrams show the slurry and stucco being appliedLower diagram shows the casting (adapted from literature of the Steel Founders Society ofAmerica) No parting lines if the wax original is hand finished Waxes with surface texture can give direct features such as the dimples on a Golf club Automation of the slurry dipping is possible using robots，thereby reducing costs Products such as turbine blades can be unidirectionally solidified，giving good mechanical properties in the growing direction123 Ceramic-Mold Investment Casting ProceduresThe snag about the previous method is that the wax pattern is destroyedThe ceramic-mold investment casting technique therefore employs reusable submaster patterns in place of the expendable wax patternsThis version of investment casting ideally involves five steps to make it efficient and to retain，as much as possible，the fine care and expense that go into creating the original master positive in Step 1The steps are as follows：Step 1Positive：make an original master pattern with stereolithography or machining Step 2Negative：create a shell around the master with highly stable resinA negative space is created around the original positive master patternThis shell can be pulled apart to give a parting lineStep 3Positive：create reusable submaster rubbery molds from the shellsStep 4Negative：create the destroyable slurryceramic moldsStep 5Positive：pour metal into the ceramic molds，which are then broken apart to get the components ,which must then be degated and deburred SLA can be used to make the original master pattern，or a CNC machine can be used to mill the master from brass，bronze，or steelOf course，the process can start at Step 3，but this might damage the original master，especially if it is SLAAlsoto get high productivity in the factory ,it is preferable to have many molds at Step 3All of which can be made from the stable resin negative in Step 2 Prototyping companies like to use the hard resin to fabricate the negative in Step 2，because the resin has good dimensional stabilityNote that it is typical to have two resin molds，one for each side of the casting，separable by a parting line Once the hard resin shells have set，they can be filled with a slurry gel that solidifies to a hard“rubbery positive”for Step 3111is intermediate submaster mold can be stripped away from the resin shells while it is still“rubbery”the material is ideal for the rather rough handling environments of a foundry and the rubbery properties mean that no draft angles are needed for stripping these submasters off the resin shells The Step 4 negative mold is made from a graded aluminosilicate with a liquid binder(ethyl silicate)and isopropyl alcoh01This is poured around the submasters from Step 3Once the slurry has set，the two ceramic halves are joined to create the inner cavity ,the slurry is fired at 950。C to give it strength，and the casting processsay with molten aluminum，can begin After solidification，the component is broken out of the ceramic，cleaned up，and deburred The parting line can cause problems，but in general，good accuracy is obtained：+-125 to 375 microns(+-0005 to 0015 inch)124 Shell MoldingAn alternative form of highaccuracy casting is shell moldingMetal pattern plates are first heated to 200。C to 240。CA thin wall of sand，5 to 15 millimeters f025 to 075 inch)thick，is then sprayed over the platesThe sand is resincoated to ensure adhesion to the metal platePhenolic resins, with hexamethylene -tetramine additives，are combined with the silica to ensure rigid thermosetting of the sprayed sandthe next steps are to cure，strip，and dry the sand molds ,which are comparably very accurate for castingOnes the excess sand is removed and casting is finished，accuracies can be as low as+一75 microns f0003 inch)125 Conventional Sand MoldingThe cruder，cheaper version of casting called sand castingA sand impression starting with wooden or plaster patterns is made around the pattern with gates and risers for the poured metalThis gives tolerances of+一375 microns(0015 inch) Newer developments include：1A highpressure ioltand-squeeze method：Here mechanical plungers push the sand against the mold at a jolt of 400 psi This gives a tighter fit of the sand against the pattern and hence better tolerances after casting2Carbon dioxide block molding：Here the interfacing between the sand and the pattern is made up of a special material about 12 millimeters(05 inch)thickIt is a refractory mix of zircon or very fine silicabonded with 6sodium silicate，which is then hardened by the passage of carbon dioxide126 Die CastingDie casting is predominantly done by the highpressure injection of hot zinc into a permanent steel dieToday ,the die or mold for this type of casting is almost certain to be milled on a threeor fiveaxis machine t001 Die costs are relatively highbut smooth components are produced with accuracies in the range of+-75 microns(0003 inch)However，these high costs for the permanent molds mean that die casting does not really fit into the rapid prototyping familyIt is mostly used for large-batch runs of small parts for automobiles or consumer productsSince low melting point materials such as zinc alloys are used in the process，component strengths are relatively modest Today , the injection molding of plastics(Chapter 8)is often preferred over zinc die casting13 MACHINING METHODS FOR RAPID PROTOTYPlNG131 OverviewChapter 7 deals with the generalized machining operation including the mechanics of the processThis chapter focuses on advances in CAD，CAM software that allow CNC machining to be more of a“turnkey rapid prototyping”processOne goal is to fully automate the links between CAD and fabricationAnother goal is to minimize the intensely hands-on craft operations(eg，process planning and fixturing )that demand the services of a skilled machinist CyberCut0M is an Internetbased experimental fabrication test bed for CNC machiningThe service allows client designers on the Internet to create mechanical components and submit appropriate files to a remote server for process planning and fabrication on an openarchitecture CNC machine t001Rapid tool-path planningNovel fixturing devices ,and sensor-based precision machining techniques allow the original designer to quickly obtain a high-strength，goodtolerance component (Smith and Wright，1996)132 WebCAD ：Design for Machining on the Internet”On the Client SideA key idea is to use a“process aware”CAD tool during the design of the partThis prototype system is called WebCAD (Kim et a1，1999)Sun MicrosystemsJava a portable，objectoriented，robust programming language similar to C+is being used as a framework for serving miniapplicationsThe GUI is a 25D featurebased。design system that uses the destructive solid geometry(DSG)idea introduced in the last chapter ( Cutkosky and Tenenbaum ，1990；Sarma and Wright，1996)Recall that the user starts out with a prismatic stock and removes primitives or “chunks” of materialBy contrast，conventional constructive solid geometry(CSG)means building up a part incrementally from“nothingnessIn the “destructive” paradigminstead of allowing arbitrary removal，the user is also constrained to removing certain shapes of material，referred to as featuresThese features take the form of pockets，blind holes，and throughholes WebCAD also contains an expert system capturing rules for machinabilityAt the top of Figure 417the designer is shown being guided by these rulesFor exampleA forbidden zone” is imposed around a through-hole feature to prevent it from being designed too close to an edgeIn the event that the designer violates a rule，a “popUp” window advises on an appropriate remedy by moving the hole further into the block-typically bv its radius dimensionWebCAD also uses ar、，SI、VYG what you see is what you get”)environment，with explicit cutting tool selection and visible comer radii on pocketsAt the time of this writing，further improvements also include freeform is face editing and selection of different cutting tool sets depending on final fabrication location(Kim，2000) The rationale for imposing destructive features upon the designer is that each of these features can readily be mapped to a standard CNC milling processThe scheme thus resembles the interaction between a word processor and a printer regarding the“printability”of the documentIt is easy to criticize that the restriction to DSG limits the set of parts that can be designedHoweverthe key advantage of this design environment is that the design-to-manufacture process is more deterministic than conventional methods，which rely on unconstrained design and on looser links between design，planning，and fabricationExperience shows that designers are somewhat concerned at first that they are constrained；however，the opportunity to be provided with the correct part very quickly proves to be attractive133 Planning on the Server SideWizen the clients design is finished，theinternet to a process planner residing on resulting geometry can be sent over the a remote serverAn automated software pipeline takes the geometry and determines in which order the features should be cut，the exact tool paths to traverse，cutting feeds，and spindle speeds for a machine toolMacroplanning orders the individual features and creates the specific machining setups in fixturesCyberCuts current macroplanner is a feature recognition module that can reliably extract the volumetric features
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