外文翻譯--實體自由成型制造與快速原型制造 (2)

1、<p><b>　　中文6627字</b></p><p><b>　　4100單詞</b></p><p>　　SOUD FREEFORM FABRICATION ISFF) AND RAPID PROTOTYPING</p><p>　　1．1 SOUD FREEFORM FABRICATION ISFF)M

2、ETHODS</p><p>　　Several 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

3、 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

4、 fSFFl．As with most new technologies at the beginning of the“market adoption S．shaped cur</p><p>　　·Parts on demand</p><p>　　·From arttopart</p><p>　　·Desktop manufac

5、turing</p><p>　　·Rapid prototyping</p><p>　　At the time of this writing，stereolithography (SLA)，selective laser sintering (SLS)，fused deposition modeling(FDM)，and layered object modeling(LO

6、M) 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

7、 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 i</p><p>　　also used to make one…of a kind or several prototypes．</p&g

8、t;<p>　　1．1．1 Summary of SFF and Rapid Prototyping Processes</p><p>　　In daily commercial use：</p><p>　　·Stereolithography (SLA)</p><p>　　·Selective laser sintering

9、 (SI S)</p><p>　　·Laminated object modeling (LOM)</p><p>　　·Fused deposition modeling (FDM)</p><p>　　More at the research and development(R&D)stage：</p><p&g

10、t;　　·3-D printing in cornstarch，plastic，or ceramic</p><p>　　·3-D printing with plastics followed by planarization using machining</p><p>　　·Solid ground curing(similar to SLA)<

11、/p><p>　　·Shape deposition modeling ra combination of addition and subtraction) Non-SFF(traditional)：</p><p>　　·Machining</p><p><b>　　·Casting</b></p>

12、<p>　　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

13、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 emerge

14、d as the most used SFF process,especially for the generation of the mast</p><p>　　1．1．2 The History of SFF Methods</p><p>　　During the late 1970s,Mead and Conway(1980)created the groundwork for

15、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．

16、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 </p>

17、<p>　　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

18、a“mechanical MOSIS”(Man．ufacturing Studies Board，1990；Bouldin，1994；NSF workshop I，1994，and II，1995)．</p><p>　　The prospects for a mechanical MOSIS were thus frequently linked to the fabrication processes in

19、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

20、a1．，1998；Sachs et a1．，2000)．</p><p>　　The introduction of the first commercial SFF technology--stereolithography--was accompanied by the advent of the STereoLithography (．STL)representation of a CAD obiect．“

21、．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．</p><p>　　Is a soccer ball round? The answer depends on

22、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

23、 reality it is an approximation to a sphere．</p><p>　　Likewise．the“．STL’’format approximates the boundary surfaces of a CAD model by breaking it down into interconnected small triangles--a process called tes

24、．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 no

25、rmal to the surfaee of each triangle is also specified．1his tessellated surface is stored as an“．STL file．”this file．Perhaps co</p><p>　　As shown in Figure 4．1．this tessellated CAD model is then sliced like

26、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 im

27、aginary 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</p>

28、<p>　　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 p

29、attern 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 at this stage．This is reviewed in more detailover the next few p

30、ages．</p><p>　　“．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

31、 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 cus

32、tomary to specify the surface vector as well．Inconsistency can be introduced as a result of </p><p>　　McMains (1996)describes how“．STL'’does not capture topology or connectivity, making it difficult to f

33、ix 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 f

34、ormats 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 r</p><p>　　1．2 CASTING METHOD

35、S FOR RAPID PROTOTYPING</p><p>　　1．2．1 Introduction</p><p>　　The classic manufacturing texts by DeGarmo and associates(1997)，Kalpakjian (1997)，Schey (1999)，and Groover (1999)are remarkably compr

36、ehensive in their coverage of the casting process．The several methods of casting include：</p><p>　　·Lost-wax investment casting</p><p>　　·Ceramic-mold investment casting</p><

37、;p>　　·Shell molding</p><p>　　·Conventional sand molding</p><p>　　·Die casting</p><p>　　Rather than duplicate the material found in other books, this section focuse

38、s 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

39、 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)．</p><p

40、>　　1．2．2 Lost·Wax Investment Casting</p><p>　　As mentioned in Chapter 1．the fundamentals of casting were invented by Korean and Egyptian artists many centuries ago．The following steps are known as th

41、e 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 w

42、ax is melted out through a hole in the bottom，leaving a hollow cavity；(h)this hole is plugged，and liquid metal is poured into t</p><p>　　The process was greatly improved and made more accurate during World W

43、arn 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

44、，and then treated with the slurry．</p><p>　　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 s

45、ize)．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

46、the mold for 2 hours at 950。C，then pour in the liquid steel or aluminum．</p><p>　　In summary, the modern lost-wax method has one of the best tolerances in the casting family because the original wax patterns

47、 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：</p>&l

48、t;p>　　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 diagram shows the casti

49、ng (adapted from literature of the Steel Founders ’Society ofAmerica)．</p><p>　　·No parting lines if the wax original is hand finished．</p><p>　　·Waxes with surface texture can give di

50、rect features such as the dimples on a</p><p>　　Golf club． ‘</p><p>　　·Automation of the slurry dipping is possible using robots，thereby reducing</p><p><b>　　costs．<

51、;/b></p><p>　　·Products such as turbine blades can be unidirectionally solidified，giving good</p><p>　　mechanical properties in the growing direction．</p><p>　　1．2．3 Cerami

52、c-Mold Investment Casting Procedures</p><p>　　The snag about the previous method is that the wax pattern is destroyed．The ceramic-mold investment casting technique therefore employs reusable submaster patter

53、ns 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 origina

54、l master positive in Step 1．The steps are as follows：</p><p>　　·Step 1．Positive：make an original master pattern with stereolithography or machining． </p><p>　　·Step 2．Negative：create a

55、 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．</p><p>　　·Step 3．Positive

56、：create reusable submaster rubbery molds from the shells．</p><p>　　·Step 4．Negative：create the destroyable slurry／ceramic molds．</p><p>　　·Step 5．Positive：pour metal into the ceramic m

57、olds，which are then broken apart to get the components ,which must then be degated and deburred ．</p><p>　　SLA can be used to make the original master pattern，or a CNC machine can be used to mill the master

58、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

59、Step 3．All of which can be made from the stable resin negative in Step 2．</p><p>　　Prototyping companies like to use the hard resin to fabricate the negative in Step 2，because the resin has good dimensional

60、stability．Note that it is typical to have two resin molds，one for each side of the casting，separable by a parting line．</p><p>　　Once the hard resin shells have set，they can be filled with a slurry gel that

61、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

62、 a foundry and the rubbery properties mean that no draft angles are needed for stripping these submasters off the resin shells．</p><p>　　The Step 4 negative mold is made from a graded aluminosilicate with a

63、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 gi

64、ve it strength，and the casting process．say with molten aluminum，can begin． </p><p>　　After solidification，the component is broken out of the ceramic，cleaned up，and deburred ．The parting line can cause pro

65、blems，but in general，good accuracy is obtained：+／-125 to 375 microns(+／-0．005 to 0．015 inch)．</p><p>　　1．2．4 Shell Molding</p><p>　　An alternative form of high—accuracy casting is shell molding．

66、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．Phen

67、olic 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

68、 ac</p><p>　　1．2．5 Conventional Sand Molding</p><p>　　The cruder，cheaper version of casting called sand casting．A sand impression starting with wooden or plaster patterns is made around the patt

69、ern with gates and risers for the poured metal．This gives tolerances of+／一375 microns(0．015 inch) Newer developments include：</p><p>　　1．A high．pressure iolt．a(chǎn)nd-squeeze method：Here mechanical plungers push

70、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．</p><p>　　2．Carbon dioxide block molding：Here the interfacin

71、g 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 zircon or very fine silica．bonded with 6％sodium silicate，which is then hardened by the pas

72、sage of carbon dioxide．</p><p>　　1．2．6 Die Casting</p><p>　　Die casting is predominantly done by the high—pressure injection of hot zinc into a permanent steel die．Today ,the die or mold for thi

73、s type of casting is almost certain to be milled on a three．or five．a(chǎn)xis machine t001．</p><p>　　Die costs are relatively high．but smooth components are produced with accuracies in the range of+／-75 microns(0

74、．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 pr

75、oducts．Since low melting point materials such as zinc alloys are used in the process，component strengths are relatively modest．</p><p>　　Today , the injection molding of plastics(Chapter 8)is often preferred

76、 over zinc die casting．</p><p>　　1．3 MACHINING METHODS FOR RAPID PROTOTYPlNG</p><p>　　1．3．1 Overview</p><p>　　Chapter 7 deals with the generalized machining operation including the

77、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

78、．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．</p><p>　　CyberCut0M is an Internet．based experimenta

79、l 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

80、．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 (Sm

81、ith and Wright，19</p><p>　　1．3．2 WebCAD ：Design for Machining on the Internet”O(jiān)n the Client Side</p><p>　　A key idea is to use a“process aware”CAD tool during the design of the part．This protot

82、ype 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 featu

83、re．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 wit</p><p>　　WebC

84、AD 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

85、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 dim

86、ension．WebCAD also uses a</p><p>　?、鰎、，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 improvement

87、s also include freeform is face editing and selection of different cutting tool sets depending on final fabrication location(Kim，2000)．</p><p>　　The rationale for imposing destructive features upon the desig

88、ner 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

89、 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 conv&l

90、t;/p><p>　　1．3．3 Planning on the Server Side</p><p>　　Wizen the client’s design is finished，theinternet to a process planner residing on resulting geometry can be sent over the a remote server．An a

91、utomated 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 tool．</p><p>　　Macroplan

92、ning orders the individual features and creates the specific machining setups in fixtures．CyberCut’s current macroplanner is a feature recognition module that can reliably extract the volumetric features from 2．5D parts．

93、The output of the module is not just a machining feature set but a rich data structure that also gives important connectivity information that relates one feature to another．A recent advance in the macroplanner is its ab

94、ility to recognize and process features containing </p><p>　　Microplanning and tool-path planning decompose the DSG volumes into specific tool motions．Colloquially speaking，this is the step that is like lawn

95、 mowing：each volume has to be carved out with a specific tool diameter，and the overlap between each strip has to be considered in relation to part tolerance and surface roughness．The corners of pockets(just like lawns)mi

96、ght require special methods．Freeform surfaces must be divided into flat and steep regions．The flat regions are machined with a project</p><p>　　1．3．4 Fabrication by Milling on the Server Side</p><

97、p>　　Finally ,a stream of NC commands performs the machining on an open-architecture milling machine．(By contrast，if it had been determined along the way that the client would have been better served by SFF technology,

98、 CyberCut can connect to a fused deposition modeling FDMl machine．)The particular milling machine being used is an open．a(chǎn)rchitecture machine that can execute advanced tool—path trajectories．One example is a machine path

99、interpolator that can traverse complicated freeform paths represent</p><p>　　實體自由成型制造與快速原型制造</p><p>　　1．1實體自由成形制造方法</p><p>　　可以用幾種制造方法將CAD信息轉(zhuǎn)換為原型物體。自1987年以來，又出現(xiàn)了幾種新的技術(shù)來完成這一轉(zhuǎn)換。就在1987