1271-隔振系統(tǒng)實(shí)驗(yàn)臺(tái)總體方案設(shè)計(jì)

1271-隔振系統(tǒng)實(shí)驗(yàn)臺(tái)總體方案設(shè)計(jì),系統(tǒng),實(shí)驗(yàn),試驗(yàn),總體,整體,方案設(shè)計(jì) 南通大學(xué)畢業(yè)設(shè)計(jì)附錄（英文資料翻譯）- 1 -SOUD FREEFORM FABRICATION ISFF) AND RAPID PROTOTYPING1．1 SOUD FREEFORM FABRICATION ISFF)METHODSSeveral manufacturing processes are available to make the important transition from computer aided design(CAD)to a prototype part．Several new technologies began to make their appearance after 1987．In that year，stereolithography (SLA) was first introduced bv 3D Systems Inc． ，and over the next five years several rival methods also appeared．TllS created the family of processes known as solid freeform fabrication fSFFl．As with most new technologies at the beginning of the“market adoption S．shaped curves”in Chapter 2．the SFF domain iS accompanied by a rela．tive amount of advertising“hype． ”SFF processes are sometimes described as：·Parts on demand·From arttopart·Desktop manufacturing·Rapid prototypingAt 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 companies．The three-dimensional(3 一 D)printing process in cornstarch，plastic，and ceramics is also being used commercially．The 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 income．Casting is a special case．It is still used to make one—of-a—kind prototypes．Furthermore，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 stereolithography．Machining isalso used to make one…of a kind or several prototypes．1．1．1 Summary of SFF and Rapid Prototyping ProcessesIn daily commercial use：·Stereolithography (SLA)·Selective laser sintering (SI S)·Laminated object modeling (LOM)南通大學(xué)畢業(yè)設(shè)計(jì)附錄（英文資料翻譯）- 2 -·Fused deposition modeling (FDM)More at the research and development(R&D)stage：·3-D printing in cornstarch，plastic，or ceramic·3-D printing with plastics followed by planarization using machining·Solid ground curing(similar to SLA)·Shape deposition modeling ra combination of addition and subtraction) Non-SFF(traditional)：·Machining·CastingComparisons 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 chapter．a(chǎn)dditional figures and tables are thus included to compare these costs and accuracies．Over 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 molding．At the time of this writing，SLS ，F(xiàn)DM．a(chǎn)nd LOM have the most visibility after SLA．1．1．2 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 circuits．Designers were encour．a(chǎn)ged to think in terms of five two-dimensional(2．D)patterns ．These patterns defined three stacked interconnection layers on a metal．oxide ． semiconductor fMOSl wafer and their mutual connections through holes．The 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 parts．AISO by the mid．1980s,severa U.S．government studies analyzed the possibilities of a“mechanical MOSIS”(Man．ufacturing 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 南通大學(xué)畢業(yè)設(shè)計(jì)附錄（英文資料翻譯）- 3 -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． “．STU’is a modified CAD format that suits a subsequent slicing oper．a(chǎn)ction and the“downstream”laser．scanning paths on a physical SLA ．FDM．or SLS machine．Is a soccer ball round? The answer depends on how carefully the balIis meas．ured ．Nominally,it is a perfect sphere．However．on closer inspection．the leather is sewn together from about 20 little hexagonal patches and a few pentagonal patches to create the curvature．In reality it is an approximation to a sphere．Likewise．the“．STL’’format approximates the boundary surfaces of a CAD model by breaking it down into interconnected small triangles--a process called tes．sellation．Each triangle is represented by the x／y／z coordinates of each of its three ver．tices,enumerated by the right．hand rule——that is．Counterclockwise (ccw) order as viewed from the outside of the bodv The vector normal to the surfaee of each triangle is also specified．1his tessellated surface is stored as an“．STL file． ”this file．Perhaps containing up to 200，000 triangles，is sent over the Internet to a prototyping shop．As shown in Figure 4．1．this tessellated CAD model is then sliced like a stack of playing cards．For 3D Systems’machines this is known as the SLI or sliced file．Other rapid prototyping machines use the slicing technique but have their own file creation details and names．Each slice for the imaginary soccer ball will thus be a circle．However．because of the tessellation procedure it will not be a perfect circle．The slicing action cuts through the triangles on the boundary．Thus，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 machine．the laser first creates the outer boundary of each slice and then“weaves”across each slice in a hatching pattern to create the layer．The number of slices and the style of the weaving pattern are chosen by each rapid pro．Totyping shop．Especially for SLA and SLS a certain amount of trial and error．Or craftspersonship，begins to play a role 南通大學(xué)畢業(yè)設(shè)計(jì)附錄（英文資料翻譯）- 4 -at this stage．This is reviewed in more detailover the next few pages．“．STL， ’is now the standard exchange format for SFF processes．However．it is inadequate for many reasons．First，the files are large due to the tessellation method．Second．there are redundancies in the“．STL'’format．0ne 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 acts．However．it has also become customary to specify the surface vector as well．Inconsistency can be introduced as a result of this redundancy, and no rules exist for resolving it．McMains (1996)describes how“．STL'’does not capture topology or connectivity, making it difficult to fix some of the common errors found in files—such as cracks，penetrating or extraneous faces，and inconsistent surface normals —without resorting to guessing the designer’s original intent． More 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)．南通大學(xué)畢業(yè)設(shè)計(jì)附錄（英文資料翻譯）- 5 -1．2 CASTING METHODS FOR RAPID PROTOTYPING1．2．1 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 process．The several methods of casting include：·Lost-wax investment casting·Ceramic-mold investment casting·Shell molding·Conventional sand molding·Die castingRather than duplicate the material found in other books, this section focuses on casting as it is done by rapid prototyping companies．Batch sizes from 50 to 500 are typical．The key market strategy is that casting is cheap and fast．However，it may not be the choice for the final product because of its tolerances．Depending on the type of casting chosen，the tolerances vary from+/一 75 microns(0．003 inch)for lost-wax processes to+／一 375 microns(0．015 inch)for standard sand castings(also see Chapter 2)．1．2．2 Lost·Wax Investment CastingAs mentioned in Chapter 1．the fundamentals of casting were invented by Korean and Egyptian artists many centuries ago．The following steps are known as the lost-wax investment cas“ng process(Figure 4．16) ：(a—c)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 components．Today it is used for products such as jet engine turbine blades and golf club heads．On the top line of Figure 4．16．wax patterns are formed from injection molds， assembled on treelike forms，and then treated with the slurry ．南通大學(xué)畢業(yè)設(shè)計(jì)附錄（英文資料翻譯）- 6 -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 hardener．Drying takes place in ammonia gas．The 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 r0．003 inch)are readily obtainable．Also the as—cast surface is relatively smooth and usable for the same reason．0ther advantages include ：Figure 4．16 The lost—Wax investment casting process．Upper diagrams (a) through(C) lead to the tree of Wax master patterns . Middle diagrams show the slurry and stucco being applied．Lower 南通大學(xué)畢業(yè)設(shè)計(jì)附錄（英文資料翻譯）- 7 -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 aGolf club． ‘·Automation of the slurry dipping is possible using robots，thereby reducingcosts．·Products such as turbine blades can be unidirectionally solidified，giving goodmechanical properties in the growing direction．1．2．3 Ceramic-Mold Investment Casting ProceduresThe snag about the previous method is that the wax pattern is destroyed．The ceramic-mold investment casting technique therefore employs reusable submaster patterns in place of the expendable wax patterns．This 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 1．The steps are as follows：·Step 1．Positive：make an original master pattern with stereolithography or machining． ·Step 2．Negative：create a shell around the master with highly stable resin．A negative space is created around the original positive master pattern．This shell can be pulled apart to give a parting line．·Step 3．Positive：create reusable submaster rubbery molds from the shells ．·Step 4．Negative：create the destroyable slurry／ceramic molds．·Step 5．Positive：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 steel．Of course，the process can start at Step 3，but this might damage the original master，especially if it is SLA．Also ．to get high productivity in the factory ,it is preferable to have many molds at Step 3．All 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 stability．Note that it is typical to have two resin molds， one for each side of the casting，separable by a parting line．南通大學(xué)畢業(yè)設(shè)計(jì)附錄（英文資料翻譯）- 8 -Once the hard resin shells have set，they can be filled with a slurry gel that solidifies to a hard“rubbery positive”for Step 3．111is 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 alcoh01．This is poured around the submasters from Step 3．Once 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 process． say 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(+／-0．005 to 0．015 inch)．1．2．4 Shell MoldingAn alternative form of high—accuracy casting is shell molding．Metal pattern plates are first heated to 200。C to 240。C．A thin wall of sand，5 to 15 millimeters f0．25 to 0．75 inch)thick，is then sprayed over the plates．The sand is resin．coated to ensure adhesion to the metal plate．Phenolic resins, with hexamethylene -tetramine additives，are combined with the silica to ensure rigid thermosetting of the sprayed sand．"the next steps are to cure，strip，and dry the sand molds ,which are comparably very accurate for casting．Ones the excess sand is removed and casting is finished，accuracies can be as low as+／一 75 microns f0．003 inch)．1．2．5 Conventional Sand MoldingThe cruder，cheaper version of casting called sand casting．A sand impression starting with wooden or plaster patterns is made around the pattern with gates and risers for the poured metal．This gives tolerances of+／一 375 microns(0．015 inch) Newer developments include：1．A high．pressure iolt．a(chǎn)nd-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 casting．2．Carbon dioxide block molding：Here the interfacing between the sand and the pattern is made up of a special material about 12 millimeters(0．5 inch)thick．It is a refractory mix of 南通大學(xué)畢業(yè)設(shè)計(jì)附錄（英文資料翻譯）- 9 -zircon or very fine silica．bonded with 6％sodium silicate，which is then hardened by the passage of carbon dioxide．1．2．6 Die CastingDie casting is predominantly done by the high—pressure injection of hot zinc into a permanent steel die．Today ,the die or mold for this type of casting is almost certain to be milled on a three． or five．a(chǎn)xis machine t001．Die costs are relatively high．but smooth components are produced with accuracies in the range of+／-75 microns(0．003 inch)．However ，these high costs for the permanent molds mean that die casting does not really fit into the rapid prototyping family．It is mostly used for large-batch runs of small parts for automobiles or consumer products．Since 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 casting．1．3 MACHINING METHODS FOR RAPID PROTOTYPlNG1．3．1 OverviewChapter 7 deals with the generalized machining operation including the mechanics of the process．This chapter focuses on advances in CAD，CAM software that allow CNC machining to be more of a“turnkey rapid prototyping”process．One goal is to fully automate the links between CAD and fabrication．Another goal is to minimize the intensely hands-on craft operations(e．g． ，process planning and fixturing )that demand the services of a skilled machinist．CyberCut0M is an Internet．based experimental fabrication test bed for CNC machining．The 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 open．a(chǎn)rchitecture CNC machine t001．Rapid tool-path planning．Novel fixturing devices ,and sensor-based precision machining techniques allow the original designer to quickly obtain a high-strength， good—tolerance component (Smith and Wright，1996)．1．3．2 WebCAD ：Design for Machining on the Internet”O(jiān)n the Client 南通大學(xué)畢業(yè)設(shè)計(jì)附錄（英文資料翻譯）- 10 -SideA key idea is to use a“process aware”CAD tool during the design of the part．This prototype system is called WebCAD (Kim et a1． ，1999)．Sun Microsystems’Java a portable，object—oriented，robust programming language similar to C++—is being used as a framework for serving mini．a(chǎn)pplications ．The GUI is a 2．5D feature．based。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 material．By contrast，conventional constructive solid geometry(CSG)means building up a part incrementally from“nothingness~’’In the “destructive” paradigm．instead of allowing arbitrary removal，the user is also constrained to removing certain shapes of material，referred to as features．These features take the form of pockets，blind holes，and through．holes．WebCAD also contains an expert system capturing rules for machinability．At the top of Figure 4．17 ．the designer is shown being guided by these rules．For example．A ‘‘forbidden zone” is imposed around a through-hole feature to prevent it from being designed too close to an edge．In the event that the designer violates a rule，a “pop．Up” window advises on an appropriate remedy by moving the hole further into the block--typically bv its radius dimension．WebCAD also uses aⅥr、 ， SI、VYG what you see is what you get”)environment，with explicit cutting tool selection and visible comer radii on pockets．At 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 process．The scheme thus resembles the interaction between a word processor and a printer regarding the“printability”of the document．It is easy to criticize that the restriction to DSG limits the set of parts that can be designed．However．the 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 fabrication．Experience shows that designers are somewhat concerned at first that they are 南通大學(xué)畢業(yè)設(shè)計(jì)附錄（英文資料翻譯）- 11 -constrained；however ，the opportunity to be provided with the correct part very quickly proves to be attractive．1．3．3 Planning on the Server SideWizen the client’s design is finished，theinternet to a process planner residing on resulting geometry can be sent over the a remote server．An automated software pipeline takes the geometry a