kalpakjian 11

Rapid-Prototypin g Processes and Operations °

°

U

°

This chapter describes the technologies associated with rapid prototyping, sharing the characteristics of computer integration, production without the use of traditional tools and dies, and the ability to rapidly produce a single part on demand; they all have the basic characteristics of producing individual parts layer by layer. The chapter discusses the (nonmetallic and metallic) materials used in rapid prototyping and describes the commercially important rapid-prototyping technologies. These processes include fused-deposition modeling, stereolithography, multijet modeling, polyjet modeling, three-dimensional printing, and selective laser sintering. The chapter ends with a description of the revolutionary practice of applying rapid-prototyping techniques to the production of tooling (rapid tooling) that can be used in other manufacturing processes.

20.l 20.2 20.3

20.4 20.5

Introduction 525 Subtractive Processes 528 Additive Processes 530 Virtual Prototyping 54| Direct Manufacturing and Rapid Tooling 542

EXAMPLES: 20.1

Functional Rapid

Prototyping 526 20.2 20.3

20.4 20.5

Coffeemaker Design 534 Production of Second Life® Avatars 537 Fuselage Fitting for Helicopters 538 Casting of Plumbing Fixtures 547

CASE STUDY:

Typical parts made: A wide variety of metallic and nonmetallic parts for product design analysis, evaluation and finished products. Alternative processes: Machining, casting, molding, and fabricating.

20.l

20.l

lnvisa|ign® Orthodontic Aligners 543

Introduction

In the development of a new product, there is invariably a need to produce a single example, or prototype, of a designed part or system before allocating large amounts of capital to new production facilities or assembly lines. The main reasons for this need are that the capital cost is very high and production tooling takes considerable time to prepare. Consequently, a working prototype is needed for design evaluation and troubleshooting before a complex product or system is ready to be produced

and marketed. A typical product development process was outlined in Fig. 1.3 in the General Introduction. An iterative process naturally occurs when (a) errors are discovered or (b) more efficient or better design solutions are gleaned from the study of an earlier generation prototype. The main problem with this approach, however, is that the

525

2

Chapter 20

Rapid-Prototyping Processes and Operations

(D)

(H)

(C)

Examples of parts made by rapid-prototyping processes: (a) selection of parts from fused-deposition modeling; (b) stereolithography model of cellular phone; and (c) selection of parts from three-dimensional printing. Source: (a) Courtesy of Stratasys, Inc., (b) and (c) Courtesy of 3D Systems, Inc. FIGURE

20.|

production of a prototype can be extremely time consuming. Tooling can take several months to prepare, and the production of a single complicated part by conventional manufacturing operations can be very difficult. Furthermore, during the time that a prototype is being prepared, facilities and staff still generate costs. An even more important concern is the speed with which a product flows from concept to a marketable item. In a competitive marketplace, it is well known that products that are introduced before those of their competitors generally are more profitable and enjoy a larger share of the market. At the same time, there are important concerns regarding the production of high-quality products. For these reasons, there is a concerted effort to bring high-quality products to market quickly.

technology that speeds up the iterative product-development process considerably is the concept and practice of rapid prototyping (RP)-also called desktop manufacturing, digital manufacturing, or solid free-form fabrication. Examples of rapid-prototyped parts are shown in Fig. 20.1. A

EXAMPLE 20.l

Functional Rapid Prototyping

Toys are examples of mass-produced products that have universal appeal. However, some toys are actually quite complex, and the function of a computeraided design (CAD) cannot be ensured until prototypes are produced. Figure 20.2 shows a CAD model and a rapid-prototyped version of a water squirt gun (Super Soaker Power Pack Back Packm

water gun), which was produced on a fused-deposition modeling machine. Each component was produced separately and assembled into the squirt gun, and the prototype could actually hold and squirt water. The alternative would be to produce components on CNC

milling machines or fabricate them in another fashion, but this can be done only at much higher cost. By producing a prototype, interference issues and assembly problems can be assessed and corrected if necessary. Furthe; from an aesthetic standpoint, the elaborate decorations on such a toy can be more effectively evaluated from a prototype than on a CAD file and can be adjusted to improve the appeal of the toy. Each component, having its design verified, then has its associated tooling produced, with better certainty that the tooling as ordered will produce the parts desired.

Section 20.1

(H)

Introduction

(D)

FIGURE 20.2

Rapid prototyping of a Super Soakerm squirt gun. (a) Fully functional toy produced through fuseddeposition modeling; (b) original CAD description. Source: Courtesy of Rapid Models and Prototypes, Inc., and Stratasys, Inc.

Developments in rapid prototyping began in the mid-1980s. The advantages of this technology include the following: °

°

°

Physical models of parts produced from CAD data files can be manufactured in a matter of hours and allow the rapid evaluation of manufacturability and design effectiveness. In this way, rapid prototyping serves as an important tool for visualization and for concept verification. With suitable materials, the prototype can be used in subsequent manufacturing operations to produce the final parts. Sometimes called direct prototyping, this approach can serve as an important manufacturing technology. Rapid-prototyping operations can be used in some applications to produce actual tooling for manufacturing operations (rapid tooling, see Section 20.5 .1). Thus, one can obtain tooling in a matter of a few days.

Rapid-prototyping processes can be classified into three major groups: subtractive, additive, and virtual. As the names imply, subtractive processes involve material removal from a workpiece that is larger than the final part. Additive processes build up a part by adding material incrementally to produce the part. Virtual processes use advanced computer-based visualization technologies. Almost all materials can be used through one or more rapid-prototyping operations, as outlined in Table 20.1. However, because their properties are more suitable for these operations, polymers are the workpiece material most commonly used today, followed by metals and ceramics. Still, new processes are being introduced continually. The more common materials used in rapid-prototyping operations are summarized in Table 20.2. This chapter is intended to serve as a general introduction to the most common rapid-prototyping operations, describe their advantages and limitations, and explore the present and future applications of these processes.

52

528 TABLE

Chapter 20

Rapid-Prototyping Processes and Operations

20.l

Characteristics of Additive Rapid-prototyping Technologies Type of

Layer creation technique

Stereolithography

Liquid

Liquid layer curing

Photopolymerization

Materials Photopolymers (acrylates, epoxies, colorable

Multijet/polyjet modeling Fused-deposition modeling

Liquid

Liquid layer curing

Photopolymerization

Photopolymers

Liquid

Extrusion of melted polymer

Solidification by cooling

Ballistic-particle manufacturing Three-dimensional printing

Liquid

Droplet deposition

Powder

Selective laser sintering

Powder

Binder-droplet deposition onto powder layer Layer of powder

Solidification by cooling No phase change

Polymers (such as ABS, polycarbonate, and polysulfone) Polymers and wax

Electron-beam melting

Powder

Layer of powder

Melting

Laminated-object manufacturing

Solid

Deposition of sheet material

No phase change

20.2

Subtractive Processes

Process

Supply phase

phase change

resins, and filled resins)

Sintering or melting

Ceramic, polymer, metal powder, and sand Polymers, metals with binder, metals, ceramics and sand with binder Titanium and titanium alloys, cobalt chrome Paper and polymers

Making a prototype traditionally has involved a series of processes using a variety of tooling and machines, and it usually takes anywhere from weeks to months, depending on part complexity and size. This approach requires skilled operators using material removal by machining and Hnis/cling operations (as described in detail in Part IV)-one by one-until the prototype is completed. To speed the process, subtractive processes increasingly use computer-based technologies such as the following: ° ° ° °

Computer-based drafting packages, which can produce three-dimensional representations of parts. Interpretation software, which can translate the CAD file into a format usable by manufacturing software. Manufacturing software, which is capable of planning the operations required to produce the desired shape. Computer-numerical-control (CNC) machinery with the capabilities necessary to produce the parts.

When a prototype is required only for the purpose of shape verification, a soft material (usually a polymer or a wax) is used as the workpiece in order to reduce or avoid any machining difficulties. The material intended for use in the actual application also can be machined, but this operation may be more time consuming, depending on the machinability of the material. Depending on part complexity and

Section 20.2

Subtractive Processes

529

TABLE 20.2

Mechanical Properties of Selected Materials for Rapid Prototyping Tensile strength (MPa)

Elastic modulus (GPa)

Elongation in 50 mm (%)

Somos 7120a

63

2.59

2.3-4.1

Somos 9120a

32

1.14-1.55

15-25

WaterClear Ultra

56

2.9

6-9

WaterShed 11122

47.1-53.6

2.65-2.88

3.3-3.5

32

2.2-2.6

12-28

FC720

60.3

2.87

20

FC830

49.8

2.49

20

FC 930

1.4

0.185

218

Polycarbonate

52

2.0

3

ABS-M30i

36

2.4

4

PC

68

2.28

4.8

Duraform PA

43

1.6

14

Duraform GF

27

4.0

1.4

SOMOS 201

-

0.015

1 10

137 120

12-16

Process

Material

Stereo-

lithography

DMX-SL 100 Polyjet

Fused-

deposition modeling

Selective laser

sintering

Electronbeam melting

ST-100c Ti-6Al-4V

305

970-1030

10

Notes

Transparent amber; good generalpurpose material for rapid prototyping Transparent amber; good chemical resistance; good fatigue properties; used for producing patterns in rubber molding Optically clear resin with ABS-like properties Optically clear with a slight green tinge; mechanical properties similar to those of ABS; used for rapid tooling Opaque beige; good general-purpose material for rapid prototyping Transparent amber; good impact strength, good paint adsorption and machinability White, blue, or black; good humidity resistance; suitable for general-

purpose applications Semiopaque, gray, or black; highly flexible material used for prototyping of soft polymers or rubber White; high-strength polymer suitable for rapid prototyping and general use Available in multiple colors, most commonly white; a strong and durable material suitable for general use; biocompatible White; good combination of mechanical properties and heat resistance White; produces durable heat- and chemical-resistant parts; suitable for snap-fit assemblies and sandcasting or silicone tooling White; glass-filled form of Duraform PA has increased stiffness and is suitable for higher temperature applications Multiple colors available; mimics mechanical properties of rubber Bronze-infiltrated steel powder Can be heat treated by HIP to obtain up to 600-MPa fatigue strength

machining capabilities, prototypes can be produced in a few days to a few weeks. Subtractive systems can take many forms; they are similar in approach to the manufacturing cells described in Section 39.2. Operators may or may not be involved, although the handling of parts is usually a human task.

0

Chapter 20

Rapid-Prototyping Processes and Operations

20.3

Additive Processes

Additive rapid-prototyping operations all build parts in layers, and as summarized in Table 20.1, they consist of stereolitliograpliy, Multiiet/polyiet modeling, fuseddeposition modeling, ballistic-particle manufacturing, three-dimensional printing, selective laser sintering, electron-beam and laminated-object manufacturing. In order to visualize the methodology used, it is beneficial to think of constructing a loaf of bread by stacking and bonding individual slices on top of each other. All of the processes described in this section build parts slice by slice. The main difference between the various additive processes lies in the method of producing the individual slices, which are typically 0.1 to 0.5 mm thick and can be thicker for some systems. All additive operations require elaborate software. As an example, note the solid part shown in Fig. 20.3a. The first step is to obtain a CAD file description of the part. The computer then constructs slices of the three-dimensional part (Fig. 20.3b). Each slice is analyzed separately, and a set of instructions is compiled in order to provide the rapid-prototyping machine with detailed information regarding the manufacture of the part. Fig. 20.3d shows the paths of the extruder in one slice, using the fused-deposition-modeling operation described in Section 20.3.1. This approach requires operator input in the setup of the proper computer files and in the initiation of the production process. Following that stage, the machines generally operate unattended and provide a rough part after a few hours. The part is then subjected to a series of manual finishing operations (such as sanding and painting) in order to complete the rapid-prototyping process. It should be recognized that the setup and finishing operations are very labor intensive and that the production time is only a portion of the time required to obtain a prototype. In general, however, additive processes are much faster than subtractive processes, taking as little as a few minutes to a few hours to produce a part.

20.3.l Fused-deposition Modeling In the fused-deposition-modeling (FDM) process (Fig. 20.4), a gantry robotcontrolled extruder head moves in two principal directions over a table, which can be raised and lowered as needed. A thermoplastic filament is extruded through the small orifice of a heated die. The initial layer is placed on a foam foundation by extruding the filament at a constant rate while the extruder head follows a predetermined path (see Fig. 20.3d). When the first layer is completed, the table is lowered so that subsequent layers can be superimposed. Occasionally, complicated parts are required, such as the one shown in Fig. 20.5a. This part is difficult to manufacture directly, because once the part has been constructed up to height a, the next slice would require the filament to be placed in a location where no material exists beneath to support it. The solution is to extrude a support material separately from the modeling material, as shown in Fig. 20.5 b. Note that the use of such support structures allows all of the layers to be supported by the material directly beneath them. The support material is produced with a less dense filament spacing on a layer, so it is weaker than the model material and can be broken off easily after the part is completed. The layers in an FDM model are determined by the extrusion-die diameter, which typically ranges from 0.050 to 0.12 mm. This thickness represents the best achievable tolerance in the vertical direction. In the x-y plane, dimensional accuracy can be as fine as 0.025 mm-as long as a filament can be extruded into the feature. A variety of polymers are available for different applications. Flat wire metal

Section 20.3

fié

(H)

(D)

Side View Model

yii

Model

'/2: Su ort pp 'Q

Z ,&» /2 ,~%v §§§!fé”'

\

\

ea

A

/

(C)

/‘

Support

(Cl)

FIGURE 20.3 The computational steps in producing a stereolithography (STL) file. (a) Three-dimensional description of part. (b) The part is divided into slices. (Only 1 in 10 is shown.) (c) Support material is p l anne d. (d) A set of tool directions is determined to manufacture each slice. Also shown is the extruder path at section A-A from (c) for a fused-

deposition-modeling operation.

deposition uses a metal Wire instead of a polymer filament, but also needs a laser to heat and bond the deposited Wire to build parts. Close examination of an FDM-produced part will indicate that a stepped surface exists on oblique exterior planes. If this surface roughness is objectionable, a heated tool can be used to smooth the surface, the surface can be hand sanded, or a coating can be applied (often in the form of a polis h'mg Wax ). However, the overall tolerances are then compromise d unless care is taken in these finishing operations.

Additive Processes

32

Chapter 20

Rapid-Prototyping Processes and Operations

Thermoplastic filament

z ,V

X

Heated build head moves in x-y plane

Table moves in z-direction

Efggfgdffde'

gi

minutes

--~___f~e;» "-.,,_A_

V'

Fixtureless

Filament supply

2 (D)

(H)

FIGURE 20.4 FDM 900mc,

a

(a) Schematic illustration of the fused-deposition-modeling process. (b) The fused-deposition-modeling machine. Source: Courtesy of Stratasys, Inc.

Ceiling within

Desired part

Gussets

Island

an arch

Ceiling

(H)

(D)

(C)

(Ol)

(G)

protruding section that requires support material. support structures used in rapid-prototyping machines. Source: After P.F. (b)-(e) Common Jacobs, Rapid Prototyping ca” Manufacturing: Fundamentals of Stereolithography. Society of Manufacturing Engineers, 1992. FIGURE 20.5

(a) A part with a

Although some FDM machines can be obtained for around $20,000, others can cost as much as $300,000. The main differences between them are the maximum size of the parts that can be produced and the numbers and types of materials that can be used.

20.3.2 Stereolithography common rapid-prototyping process-one that actually was developed prior to fused-deposition modeling-is stereolit/aography (STL). This process (Fig. 20.6) is based on the principle of curing (hardening) a liquid photopolymer into a specific shape. A vat containing a mechanism whereby a platform can be lowered and raised is filled with a photocurable liquid-acrylate polymer. The liquid is a mixture of acrylic monomers, oligomers (polymer intermediates), and a photoinitiator (a compound that undergoes a reaction upon absorbing light). A

Section 20.3

Additive Processes

533

At its highest position (depth a in Fig. 20.6), a Platform motion shallow layer of liquid exists above the platform. A UV light source laser generating an ultraviolet (UV) beam is focused UV curable upon a selected surface area of the photopolymer liquid and then moved around in the x-y plane. The beam \ I’ Liquid cures that portion of the photopolymer (say, a ringx ; surface shaped portion) and thereby produces a solid body. V c The platform is then lowered sufficiently to cover Formed part the cured polymer with another layer of liquid polyVat mer, and the sequence is repeated. The process is re| | peated until level b in Fig. 20.6 is reached. Thus far, Platform we have generated a cylindrical part with a constant wall thickness. Note that the platform is now lowered by a vertical distance ab. FIGURE 20.6 Schematic illustration of the stereolithography At level b, the x-y movements of the beam process. define a wider geometry, so we now have a flangeshaped portion that is being produced over the previously formed part. After the proper thickness of the liquid has been cured, the process is repeated, producing another cylindrical section between levels I9 and c. Note that the surrounding liquid polymer is still fluid (because it has not been exposed to the ultraviolet beam) and that the part has been produced from the bottom up in individual “slices.” The unused portion of the liquid polymer can be used again to make another part or another prototype. Note that the term “stereolithography,” as used to describe this process, comes from the facts that the movements are three-dimensional and the process is similar to lithography (see Section 28.7), in which the image to be printed on a flat surface is ink receptive and the blank areas are ink repellent. Note also that, like FDM, stereolithography can utilize a weaker support material. In stereolithography, this support takes the form of perforated structures. After its completion, the part is removed from the platform, blotted, and cleaned ultrasonically and with an alcohol bath. Then the support structure is removed, and the part is subjected to a final curing cycle in an oven. The smallest tolerance that can be achieved in stereolithography depends on the sharpness of the focus of the laser; typically, it is around 0.0125 mm. Oblique surfaces also can be of very high quality. Solid parts can be produced by applying special laser-scanning patterns to speed up production. For example, by spacing scan lines in stereolithography, volumes or pockets of uncured polymer can be formed within cured shells. When the part is later placed in a postprocessing oven, the pockets cure and a solid part forms. Similarly, parts that are to be investment cast will have a drainable honeycomb structure which permits a significant fraction of the part to remain uncured. Total cycle times in stereolithography range from a few hours to a daywithout postprocessing such as sanding and painting. Depending on their capacity, the cost of the machines is in the range from $100,000 to $400,000. The cost ofthe liquid polymer is on the order from $80 per litre. The maximum part size that can be produced is 0.5 >< 0.5 >< 0.6 m.. Stereolithography has been used with highly focused lasers to produce parts with micrometer-sized features. The use of optics required to produce such features necessitates thinner layers and lower volumetric cure rates. When stereolithography is used to fabricate micromechanical systems (see Chapter 29), it is called microstereolithography.

3

"b

Chapter 20

Rapid-Prototyping Processes and Operations

20.3.3 Multijet/Polyjet Modeling The Multijet Modeling (MJM) or Polyjet process is similar to inkjet printing, where print heads deposit the photopolymer on the build tray. Ultraviolet bulbs, alongside the jets, immediately cure and harden each layer, thus eliminating the need for any postmodeling curing that is needed in stereolithography. The result is a smooth surface of thin layers as small as 16 ,um that can be handled immediately after the process is completed. Two different materials are used: One material is used for the actual model, while a second, gel-like resin is used for support, such as these shown in Fig. 20.5. Each material is simultaneously jetted and cured, layer by layer. When the model is completed, the support material is removed with an aqueous solution. Build sizes are fairly large, with an envelope of up to 500 >< 400 >< 200 mm. These processes have capabilities similar to those of stereolithography and use similar resins (Table 20.2). The main advantages are the capabilities of avoiding part cleanup and lengthy postprocess curing operations, and the much thinner layers produced, thus allowing for better resolution.

EXAMPLE 20 2 Coffeemaker Design

Alessi Corporation is well known for its high-end kitchen products. Although it makes products out of a wide range of materials, it is best known for its highly polished stainless~steel designs. An example is the Cupola coffeemaker, a market favorite that was to be redesigned from the bottom up while preserving the general characteristics of the established design. Alessi engineers used Multijet modeling to produce prototypes of components of the coffeemaker, as shown in Fig. 20.7. The prototypes allowed engineers to evaluate the ease and security of mechanical assembly, but a significant effort was expended on the design of the coffeemaker’s lip in order to optimize the pouring of coffee. A large number of lip prototypes were constructed and evaluated to obtain the most robust and aesthetically pleasing design. The ability to compare physical prototypes to the existing product was deemed essential to evaluating the designs. After a final design was selected from the numerous prototypes produced, it was found that a 5-6-week time savings was achieved in product development. The time savings

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Coffeemaker prototypes produced through Multijet modeling and final product. Source: Courtesy Alessi Corporation and 3D Systems, Inc. FIGURE 20.7

translated into cost savings, as well as assuring timely market launch of the redesigned product. Source: Courtesy Alessi Corporation and 3D Systems, Inc.

20.3.4 Selective Laser Sintering Selective laser sintering (SLS) is a process based on the sintering of nonmetallic or (less commonly) metallic powders selectively into an individual object. The basic elements in this process are shown in Fig. 20.8. The bottom of the processing

Section 20.3

Galvanometers I

E

I

Sintering laser

Optics

Laser

gp

§` Process-control computer

EnvironmentalCOHUOI Unit

+

' Wh C

'n~~

*

I

-W

Schematic illustration of the selective-laser-sintering process. Source: After C. Deckard and P.F. McClure. FIGURE 20.8

chamber

is

equipped with two cylinders:

powder-feed cylinder, which is raised incrementally to supply powder to the part-build cylinder through a roller mechanism. 2. A part-build cylinder, which is lowered incrementally as the part is being formed. I. A

First, a thin layer of powder is deposited in the part-build cylinder. Then a laser beam guided by a process-control computer using instructions generated by the three-dimensional CAD program of the desired part is focused on that layer, tracing and sintering a particular cross section into a solid mass. The powder in other areas remains loose, yet it supports the sintered portion. Another layer of powder is then deposited; this cycle is repeated again and again until the entire three-dimensional part has been produced. The loose particles are shaken off, and the part is recovered. The part does not require further curing-unless it is a ceramic, which has to be fired to develop strength. A variety of materials can be used in this process, including polymers (such as ABS, PVC, nylon, polyester, polystyrene, and epoxy), wax, metals, and ceramics with appropriate binders. It is most common to use polymers because of the smaller, less expensive, and less complicated lasers required for sintering. With ceramics and metals, it is common to sinter only a polymer binder that has been blended with the ceramic or metal powders. The resultant part can be carefully sintered in a furnace and infiltrated with another metal if desired.

20.3.5 Electron-beam Melting process similar to selective laser sintering and electron-beam welding (see Section 30,6), electron-beam melting uses the energy source associated with an electron beam to melt titanium or cobalt-chrome powder to make metal prototypes. The workpiece is produced in a vacuum; the part build size is limited to around 200 >< 200 >< 180 mm. Electron-beam melting (EBM) is up to 95% efficient A

Addntive Processes

Chapter 20

Rapid-Prototyping Processes and Operations

from an energy standpoint (compared with 10-20% efficiency for selective laser sintering), so that the titanium powder is actually melted and fully dense parts can be produced. A volume build rate of up to 60 cm3/hr can be obtained, with individual layer thicknesses of 0.05 0-0.200 mm. Hot isostatic pressing (Section 17.3.2) also can be performed on parts to improve their fatigue strength. Although applied mainly to titanium and cobalt-chrome to date, the process is being developed for stainless steels, aluminum, and copper alloys.

20.3.6 Three-dimensional Printing In the three-dimensional-printing (3DP) process, a printhead deposits an inorganic binder material onto a layer of polymer, ceramic, or metallic powder, as shown in Fig. 20.9. A piston supporting the powder bed is lowered incrementally, and with each step, a layer is deposited and then fused by the binder. Multijet modeling and polyjet processes (described in Section 2O.3.3) are sometimes referred to as three-dimensional printing approaches, because they operate in a similar fashion to ink-jet printers but incorporate a third (thickness) direction. However, three-dimensional printing is most commonly associated with

printing a binder onto powder. Three-dimensional printing allows considerable flexibility in the materials and binders used. Commonly used powder materials are blends of polymers and fibers, foundry sand, and metals. Furthermore, since multiple binder printheads can be incorporated into one machine, it is possible to produce full-color prototypes by having different-color binders (see Example 20.3). The effect is a three-dimensional analog to printing photographs using three ink colors on an ink-jet printer. A common part produced by 3DP from ceramic powder is a ceramic-casting shell (see Section 11.2.4), in which an aluminum-oxide or aluminum-silica powder is fused with a silica binder. The molds have to be postprocessed in two steps: (1) curing at around 150°C and (2) firing at 1000° to 1500°C. The parts produced through the 3DP process are somewhat porous and therefore may lack strength. Three-dimensional printing of metal powders can also be

|1>

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I!! 1.

Holler mechanism

,/-Powder Eli* _;;fi¢==

Spread powder

Binder

ET” Elle

~;;2;;;,,.

if

2. Print layer

3. Piston

movement

A..

4.

Intermediate stage

FIGURE 20.9

Source: After

E.

5. Last layer printed

6.

Finished part

Schematic illustration of the three-dimensional-printing process. Sachs and M. Cima.

,

Section 20.3

Additive Processes

537

infiltrating metal, permeates into P/M

Binder deposition

\

\\

li/licrostructure detail

is powder Bidder Metal

powder ‘

(21)

Q

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lower-melting-point metal

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infiltrated by

20.|0 Three-dimensional printing using (a) part-build, (b) sinter, and (c) infiltration steps to produce metal parts. (d) An example of a bronze-infiltrated stainless-steel part produced through three-dimensional printing. Source: Courtesy of Kennametal Extrude FIGURE

Hone.

combined with sintering and metal infiltration (see Section 17.5 to produce fully dense parts, using the sequence shown in Fig. 20.10. Here, the part is produced as before by directing the binder onto powders. However, the build sequence is then followed by sintering to burn off the binder and partially fuse the metal powders, just as in powder injection molding described in Section 17.33. Common metals used in SDP are stainless steels, aluminum, and titanium. Infiltrating materials typically are copper and bronze, which provide good heat-transfer capabilities as well as wear resistance. This approach represents an efficient strategy for rapid tooling (see below). In a related ballistic-particle manufacturing process, a stream of a material (such as plastic, ceramic, metal, or wax) is ejected through a small orifice at a surface (target) using an ink-jet type mechanism. A powder is not involved; the material deposited by the ink-jet mechanism is used to build the prototype. The ink-jet head is guided by a three-axis robot to produce three-dimensional prototypes. )

EXAMPLE 20.3

Production of Second Life® Avatars

Second Life® and World of Warcraft® are examples of virtual worlds accessed through a website and are enjoyed by millions of people worldwide. To participate, users create an “avatar” that depicts their

alter ego in the fictional world. Many modern computer games (such as Rock Band 2) also allow users to produce very detailed avatars, with a unique appearance and unique personalities. Avatars contain

38

Chapter 20

Rapid-Prototyping Processes and Operations

(H)

FIGURE 20.l

(D)

Rapid-prototyped versions of user-defined characters, or avatars, produced from geometric descriptions within popular websites or games. (a) Second Life® avatar, as appears on a computer screen (left) and after printing (right); (b) an avatar known as “Wreker” from World of \X/arcraft®. Source: Courtesy Z Corporation, Figure Prints and Fabjectory, Inc. I

three-dimensional geometry data that describes their appearance, which can be translated to a file format suitable for rapid prototyping. Avatars can be printed in full color to a 150-mm high figurine with Z-Corp Spectrum Z51O or ZPrinter

450 three-dimensional printers (Fig 2O.11). Users can order their avatar prototypes on the web, which are then printed and shipped to the user within days.

EXAMPLE 20.4 Fuselage Fitting for Helicopters

Sikorsky Aircraft Company needed to produce a limited number of the fuselage fittings shown in Fig. 20.12a. Sikorsky wanted to produce the forging dies by means of three-dimensional-printing technologies. A die was designed using the CAD part description. Forging allowances were incorporated and flashing accommodated by the die design. The dies were printed using a three~ dimensional printer produced by ProMetal and are shown in Fig. 20.12b. The dies were made by producing 0.178-mm layers with stainless-steel powder as the workpiece media. The total time spent in the 3DP machine was just under 45 hours. This

was followed by curing of the binder (10 hours, plus 5 hours for cooldown), sintering (40 hours, plus 17 hours for cooldown), and infiltration (27 hours, plus 15 hours for cooldown). The dies then were finished and positioned in a die holder, and the part was forged in an 800-ton hydraulic press with a die temperature of around 300°C. An as-forged part is shown in Fig. 20.12c and requires trimming of the flash before it can be used. The dies were produced in just over six days-compared with the many months required for conventional die production. Source: Courtesy of Kennametal Extrude Hone.

Section 20.3

Additive Processes

45 mm

Flange thickness = 3 mm internal radii = 5 mm External radii = 10 mm (6) |-|,_»_

(D)

FIGURE

20.l2

A

r\r_s.___|,___.;

(C)

fitting required for a helicopter fuselage. (al CAD representation with added dimensions. three-dimensional printing. (c) Final forged workpiece. Source: Courtesy of Kennametal

(b) Dies produced by

Extrude Hone.

20.3.7 Laminated-object Manufacturing Lamination implies a laying dovvn of layers that are bonded adhesively to one another. Several variations of laminated-object manufacturing (LOM) are available. The simplest and least expensive versions of LOM involve using control software and vinyl cutters to produce the prototype. Vinyl cutters are simple CNC machines that cut shapes from vinyl or paper sheets. Each sheet then has a number of layers and registration holes, which allow proper alignment and placement onto a build fixture. Figure 20.13 illustrates the manufacture of a prototype by laminated-object manufacturing with manual assembly. Such LOM systems are highly economical and are popular in schools and universities because of the hands-on demonstration of additive manufacturing and production of parts by layers. LOM systems can be elaborate; the more advanced systems use layers of paper or plastic with a heat-activated glue on one side to produce parts. The desired shapes are burned into the sheet with a laser, and the parts are built layer by layer (Fig. 20.14). On some systems, the excess material must be removed manually once the part is completed. Removal is simplified by programming the laser to burn perforations in crisscrossed patterns. The resulting grid lines make the part appear as if it had been constructed from gridded paper (With squares printed on it, similar to graph paper).

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Because the chip thickness is always greater than the depth of cut, the value of than unity. The reciprocal of r is known as the chip-compression ratio or chip-compression factor and is a measure of how thick the chip has become compared with the depth of cut; hence, the chip-compression ratio always is greater than unity. The depth of cut also is referred to as the undeformed chip thickness, as may be visualized by reviewing Fig. 21.3. The cutting ratio is an important and useful parameter for evaluating cutting conditions. Since the undeformed chip thickness, to, is a machine setting and is therefore known, the cutting ratio can be calculated easily by measuring the chip thickness with a micrometer. With the rake angle also known for a particular cutting operation (it is a function of the tool and workpiece geometry in use), Eq. (21.1) allows calculation of the shear angle. Although we have referred to to as the depth of cut, note that in a machining process such as turning, as shown in Fig. 21.2, this quantity is the feed. To visualize the situation, assume, for instance, that the workpiece in Fig. 21.2 is a thin-walled tube and the width of the cut is the same as the thickness of the tube. Then, by rotating Fig. 21.3 clockwise by 90°, note that it is now similar to the view in Fig. 21.2. r is always less

Section 2 1.2

Shear Strain. Referring now to Fig. 21.4a, we can see that the shear strain, y, that the material undergoes can be expressed as

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Note that large shear strains are associated with low shear angles or with low or negative rake angles. Shear strains of 5 or higher have been observed in actual cutting operations. Compared to forming and shaping processes, the workpiece material undergoes greater deformation during cutting, as is also seen in Table 2.4. Furthermore, deformation in cutting generally takes place within a very narrow zone. In other words, the dimension d = OC in Fig. 21.4a is very small. Thus, the rate at which shearing takes place is high. (We discuss the nature and size of the deformation zone further in Section 21.3.) The shear angle has great significance in the mechanics of cutting operations. It influences force and power requirements, chip thickness, and temperature. Consequently, much attention has been focused on determining the relationships among shear angle, cutting process variables, and workpiece material properties. One of the earliest analyses was based on the assumption that the shear angle adjusts itself to minimize the cutting force or that the shear plane is a plane of maximum shear stress. This analysis yielded the expression

¢=4y+§-Q,

(mm

,B is the friction angle and is related to the coefncient of friction, /.L, at the tool-chip interface by the expression /.L = tan B. Among the many shear-angle relationships developed, another useful formula that generally is applicable is

where

¢=4v+a-5.

(mm

The coefficient of friction in metal cutting generally ranges from about 0.5 to 2, indicating that the chip encounters considerable frictional resistance while moving up the tool’s rake face. Experiments have shown that /.L varies considerably along the tool-chip interface because of large variations in contact pressure and temperature. Consequently, /.L is also called the apparent mean coefhcient of friction. Equation (21.3) indicates that (a) as the rake angle decreases or the friction at the tool-chip interface (rake face) increases, the shear angle decreases and the chip becomes thicker; (b) thicker chips mean more energy dissipation because the shear strain is higher [see Eq. (21.2)]; and (c) because work done during cutting is converted into heat, the temperature rise is also higher. The effects of these phenomena are described throughout the rest of this chapter.

Velocities in the Cutting Zone. Note in Fig. 21.3 that (since the chip thickness is greater than the depth of cut) the velocity of the chip, VC, has to be lower than the cutting speed, V. Since mass continuity has to be maintained, Vto = Veta,

Hence, VC

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. VC

V sind>

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Mechanics of Cutting

Chapter 21

Fundamentals of Machining

velocity diagram also can be constructed, as shown in Fig. 21.4b, in which, from trigonometric relationships, we obtain the equation A

V

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A

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2l.2.I Types of Chips Produced

in Metal Cutting

The types of metal chips commonly observed in practice and their photomicrographs are shown in Fig. 21.5. The four main types are as follows:

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2|.5 Basic types of chips produced in orthogonal metal cutting, their schematic representation, and photomicrographs of the cutting zone: (al continuous chip with narrow, straight, and primary shear zone; (b) continuous chip with secondary shear zone at the chip-tool interface; (c) built-up edge; (d) segmented or nonhomogeneous chip; and (e) discontinuous chip. Source: After M.C. Shaw, P.K. Wright, and S. Kalpakjian. FIGURE

Section 21.2 ° ° ° °

Continuous Built-up edge Serrated or segmented Discontinuous.

Let’s first note that a chip has two surfaces. One surface has been in contact with the rake face of the tool and has a shiny and burnished appearance caused by rubbing as the chip moves up the tool face. The other surface is from the original surface of the workpiece; it has a jagged, rough appearance (as can be seen on the chips in Figs. 21.3 and 21.5) caused by the shearing mechanism shown in Fig. 21.4a. This surface is exposed to the environment and has not come into any contact with any solid body.

Continuous Chips. Continuous chips usually are formed with ductile materials that are machined at high cutting speeds and/or high rake angles (Fig. 21.5a). The deformation of the material takes place along a narrow shear zone called the primary shear zone. Continuous chips may develop a secondary shear zone (Fig. 21.5b) because of high friction at the tool-chip interface; this zone becomes thicker as friction increases. Deformation in continuous chips also may take place along a wide primary shear zone with curved boundaries (see Fig. 21.3b), unlike that shown in Fig. 21.5 a. Note that the lower boundary of the deformation zone in Fig. 21.3b projects helou/ the machined surface, subjecting it to distortion, as depicted by the distorted vertical lines in the machined subsurface. This situation occurs generally in machining soft metals at low speeds and low rake angles. It usually results in a poor surface finish and induces surface residual stresses, which may be detrimental to the properties of the machined part in their service life. Although they generally produce a good surface finish, continuous chips are not necessarily desirable, particularly with computer-controlled machine tools in wide use, as they tend to become tangled around the toolholder, the fixturing, and the workpiece, as well as around the chip-disposal systems (see Section 23.3.7). The operation may have to be stopped to clear away the chips. This problem can be alleviated with chip breakers (discussed shortly), by changing parameters such as cutting speed, feed, and depth of Cut, or by using cutting fluids. Built-up Edge Chips. A huilt-up edge (BUE) consists of layers of material from the workpiece that gradually are deposited on the tool tip-hence the term built-up (Fig. 21.5c). As it grows larger, the BUE becomes unstable and eventually breaks apart. Part of the BUE material is carried away by the tool side of the chip; the rest is deposited randomly on the workpiece surface. The cycle of BUE formation and destruction is repeated continuously during the cutting operation until corrective measures are taken. In effect, a built-up edge changes the geometry of the cutting edge and dulls it (Fig. 21.6a). Built-up edge commonly is observed in practice. It is a major factor that adversely affects surface finish, as can be seen in Figs. 21.5c and 21.6b and c. However, a thin, stable BUE usually is regarded as desirable because it reduces tool wear by protecting its rake face. Cold-worked metals generally have less of a tendency to form BUE than when in their annealed condition. Because of work hardening and deposition of successive layers of material, the BUE hardness increases significantly (Fig. 21.6a). As the cutting speed increases, the size of the BUE decreases; in fact it may not form at all. The tendency for BUE formation can be reduced by one or more of the following means: ° ° ° ° °

'

Increase the cutting speeds Decrease the depth of cut Increase the rake angle Use a sharp tool Use an effective cutting fluid Use a cutting tool that has lower chemical affinity for the workpiece material.

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Allowable Wear Land. We realize that we have to sharpen a knife or a pair of scissors when the quality of the cut deteriorates or the forces required are too high. Similarly, cutting tools need to be replaced (or resharpened) when (a) the surface finish of the machined workpiece begins to deteriorate, (b) cutting forces increase significantly, or (c) the temperature rises significantly. The allowable u/ear land (VB in Fig. 21.15 a) for various machining conditions is given in Table 21.4. For improved dimensional accuracy, tolerances, and surface finish, the allowable wear land may be smaller than the values given in the table. The recommended cutting speed for a high-speed steel tool is generally the one that yields a tool life of 60 to 120 min, and for a carbide tool, it is 30 to 60 min. However, depending on the particular workpiece, the operation, and the highproductivity considerations due to the use of modern, computer-controlled machine tools, the cutting speeds selected can vary significantly from these values. Optimum Cutting Speed. We have noted that as cutting speed increases, tool life is reduced rapidly. On the other hand, if the cutting speed is low, tool life is long, but the rate at which material is removed is also low. Thus, there is an optimum cutting speed. Because it involves several other parameters, we will describe this topic further in Section 25.8.

|.4

Allowable Average Wear Land (see VB in Fig. 2I.l5a) for Cutting Tools in Various Machining Operations Operation Turning Face milling End milling Drilling Reaming

Allowable wear land (mm) High-speed steel tools Carbide tools 1.5 1.5

0.3 0.4 0.15

1~4 __

==

4. This

_ 1-3,

or that tool life is increased by 300%. Thus, a reduction in cutting speed has resulted in a major increase in tool life. Note also that, for this problem, the magni~ tude of C is not relevant.

0.5v,\/ir; = V1\/Tl.

TABLE 2

577

0.4 0.4 0.3 0.4 0.15

Note: Allowable wear for ceramic tools is about 50% higher. Allowable notch Wear, VBWX, is about twice that for VB.

578

Chapter 21

EXAMPLE 21.3

Fundamentals of Machining

Effect of Cutting Speed on Material Removal

The effect of cutting speed on the volume of metal removed between tool changes (or resharpenings) can be appreciated by analyzing Fig. 21.16. Assume that a material is being machined in the “one” condition (that is, as cast with a hardness of 265 HB). We note that when the cutting speed is 60 m/min, tool life is about 40 min. Thus, the tool travels a distance of 60 m/min >< 40 min = 2400 m before it has to be re~ placed. However, when the cutting speed is increased to 120 m./min, the tool life is reduced to about 5 min

and the tool travels 120 m/min >< 5 min = 600 m before it has to be replaced. Since the volume of material removed is directly proportional to the distance the tool has traveled, it can be seen that by decreasing the cutting speed, more material is removed between tool changes. It is important to note, however, that the lower the cutting speed, the longer is the time required to machine a part, which has a significant economic impact on the operation (see Section 2S.8).

2l.5.2 Crater Wear Crater u/ear occurs on the rake face of the tool, as shown in Figs. 21.15 a, and c, and Fig. 21.18, which illustrates various types of tool wear and failures. lt readily can be seen that crater wear changes the tool-chip interface contact geometry. The most significant factors influencing crater wear are (a) the temperature at the tool-chip interface and (bl the chemical affinity between the tool and workpiece materials. Additionally, the factors influencing flank wear may affect crater wear.

Thermal cracks in interrupted cutting

® Flank wear (wear land) ® Crater wear

Primary groove or depth of cut line Secondary groove @ (oxidation wear) © Outer-metal chip notch @ Inner chip notch

Chamfer

C9

Carbide

High-speed steel

Ceramic

(3)

G) Flank wear

® Crater wear © Failure face

Chamfer

QD

Primary groove or depth-of-cut line

® Outer-metal chip notch © Plastic flow around failure High-speed steel tool, thermal softening, and plastic flow

face

Ceramic tool, chipping, and fracture

(bl

(a) Schematic illustrations of types of wear observed on various cutting tools. illustrations of catastrophic tool failures. A wide range of parameters influence these wear and failure patterns. Source: Courtesy of VC. Venkatesh.

FIGURE 2

l.l8

(b) Schematic

Crater wear generally is attributed to a diffusion mechanismthat is, the movement of atoms across the tool-chip interface. Since diffusion rate increases with increasing temperature, crater wear increases as temperature increases. Note in Fig. 21.19, for example, how rapidly crater wear increases within a narrow range of temperatures. Applying protective coatings to tools is an effective means of slowing the diffusion process and thus reducing crater wear. Typical coatings are titanium nitride, titanium carbide, titanium carbonitride, and aluminum oxide and are described in greater detail in Section 22.6. In comparing Figs. 21.12 and 21.15a, it can be seen that the location of the maximum depth of crater wear, KT, coincides with the location of the maximum temperature at the tool-chip interface. An actual cross section of this interface, for steel cut at high speeds, is shown in Fig. 21.20. Note that the crater-wear pattern on the tool coincides with its discoloration pattern, which is an indication of the presence of high temperatures. 2

0.30 co

E

0.15

FIGURE 2l.l9 Relationship between craterwear rate and average tool-chip interface temperature: (1) high-speed steel, (2) C1 carbide, and (3) C5 carbide (see Table 22.4). Note how rapidly crater-wear rate increases with an incremental increase in temperature. Source: After B.T. Chao and K.]. Trigger.

|.5.3 Other Types of Wear, Chipping, and Fracture

----l

l

now describe the factors involved in other types of cuttingtool wear and fracture. Rake face Nose u/ear (Fig. 21.15a) is the rounding of a sharp tool due to mechanical and thermal effects. It dulls the tool, affects chip formation, and causes rubbing of the tool over the workpiece, raising its temperature and possibly inducing residual stresses on the machined surface. A related phenomCrater wear enon is edge rounding, as shown in Fig. 21.15a. An increase in temperature is particularly important for high-speed steel tools, as can be appreciated from Fig. 22.1. Tools also may undergo plastic deformation because of temperature rises in the cutting zone, where temperatures can easily reach 1000°C in machining steels and can be higher in stronger materials. Notc/Jes or grooz/es observed on cutting tools, as shown in Figs. 21.15a and 21.18, have been attributed to the fact Chip Flank face that the region they occupy is the boundary where the chip is no longer in contact with the tool. Known as the depth-of-cut FIGURE 2l.20 Interface of a cutting tool (right) and line (DOC) with a depth VN, this boundary oscillates because chip (left) in machining plain-carbon steel. The of inherent variations in the cutting operation. Furthermore, discoloration of the tool indicates the presence of the region is in contact with the machined surface generated high temperatures. Compare this figure with the tempeduring the previous cut; the thin work-hardened layer that rature profiles shown in Fig. 21.12. Source: Courtesy can develop will contribute to the formation of the wear of P.K. Wright. groove. If sufficiently deep, the notch can lead to gross chipping of the tool tip because of its reduced cross section, as well as the notch sensitivity of the tool material. Scale and oxide layers on a workpiece surface also contribute to notch wear, because these layers are hard and abrasive. Thus, light cuts should not be taken on rusted workpieces, and the depth of cut should be greater than the thickness of the oxide film or the work-hardened layer. In Fig. 21.3, for example, the depth of cut, to, should be greater than the thickness of the scale on the workpiece. In addition to being subject to wear, tools may undergo chipping, in which a small fragment from the cutting edge of the tool breaks away. This phenomenon, We

80

Chapter 21

Fundamentals of Machining

which typically occurs in brittle tool materials such as ceramics, is similar to chipping the tip of a pencil if it is too sharp. The chipped fragments from the cutting tool may be very small (microchipping or macrochipping), or they may be relatively large, in which case they are variously called gross chipping, gross fracture, and catastrophic failure (Fig. 21.18). Chipping also may occur in a region of the tool where a small crack or defect already exists. Unlike wear, which is a gradual process, chipping is a sudden loss of tool material and a corresponding change in its shape. As can be expected, chipping has a major detrimental effect on surface finish, surface integrity, and the dimensional accuracy of the workpiece. Two main causes of chipping are the following: ° °

Mechanical shock (i.e., impact due to interrupted cutting, as in turning a splined shaft on a lathe). Thermal fatigue (i.e., cyclic variations in the temperature of the tool in interrupted cutting).

Thermal cracks usually are perpendicular to the cutting edge of the tool, as shown on the rake face of the carbide tool in Figs. 21.15 d and 21.18a. Major variations in the composition or structure of the workpiece material also may cause chipping. Chipping can be reduced by selecting tool materials with high impact and thermal-shock resistance, as described in Chapter 22. High positive rake angles can contribute to chipping because of the small included angle of the tool tip, as can be visualized from Fig. 21.3. Also, it is possible for the crater-wear region to progress toward the tool tip, thus weakening the tip because of reduced material volume and causing chipping.

2I.5.4 Tool-condition Monitoring With computer-controlled machine tools and automated manufacturing, the reliable and repeatable performance of cutting tools is a critical consideration. As described in Chapters 23 through 25, modern machine tools operate with little direct supervision by a machine operator and generally are enclosed, making it impossible or difficult to monitor the machining operation and the condition of the tool. It is therefore essential to continuously and indirectly monitor the condition of the cutting tool so as to note, for example, wear, chipping, or gross tool failure. In modern machine tools, tool-condition monitoring systems are integrated into computer numerical control and programmable logic controllers. Techniques for toolcondition monitoring typically fall into two general categories: direct and indirect. The direct method for observing the condition of a cutting tool involves optical measurements of wear, such as the periodic observation of changes in the tool profile. This is a common and reliable technique and is done with a microscope (toolmakers’ microscope). However, this requires that the cutting operation be stopped for tool observation. Another direct method involves programming the tool to contact a sensor after every machining cycle; this approach allows the detection of broken tools. Usually, the sensor has the appearance of a pin that must be depressed by the tool tip. Indirect methods of observing tool conditions involve the correlation of the tool condition with parameters such as cutting forces, power, temperature rise, workpiece surface finish, vibration, and chatter. A powerful technique is acoustic emission (AE), which utilizes a piezoelectric transducer mounted on a toolholder. The transducer picks up acoustic emissions (typically above 100 kHz), which result from the stress waves generated during cutting. By analyzing the signals, tool wear and chipping can be monitored. This technique is effective particularly in precision-machining

Section 21.6

Surface

operations, where cutting forces are low because of the small amounts of material removed. Another effective use of AE is in detecting the fracture of small carbide tools at high cutting speeds. A similar indirect tool-condition monitoring system consists of transducers that are installed in original machine tools or are retrofitted on existing machines. They continually monitor torque and forces during cutting. The signals are preamplified, and a microprocessor analyzes and interprets their content. The system is capable of differentiating the signals that come from different sources, such as tool breakage, tool wear, a missing tool, overloading of the machine tool, or colliding with machine components. The system also can compensate automatically for tool wear and thus improve the dimensional accuracy of the part being machined. The design of transducers must be such that they are (a) nonintrusive to the machining operation, (b) accurate and repeatable in signal detection, (c) resistant to abuse and the shop-floor environment, and (d) cost effective. Continued progress is being made in the development of sensors, including the use of infrared and #beroptic techniques for temperature measurement during machining. In lower cost computer numerical-control machine tools, monitoring is done by tool-cycle time. In a production environment, once the life expectancy of a cutting tool or insert has been determined, it can be entered into the machine control unit, so that the operator is prompted to make a tool or cutter change when that time is reached. This process is inexpensive and fairly reliable, although not totally so, because of the inherent statistical variation in tool life.

Surface Finish and Integrity

2|.6

Surface finish influences not only the dimensional accuracy of machined parts but also their properties and their performance in service. The term surface finish describes the geometric features of a surface (see Chapter 33), and surface integrity pertains to material properties, such as fatigue life and corrosion resistance, that are strongly influenced by the nature of the surface produced. With its significant effect on the tool-tip profile, the built-up edge has the greatest influence on surface finish. Figure 21.21 shows the surfaces obtained in two

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