reference free part encapsulation

Journal of Manufacturing Systems Vol. 16/No. 1

1997

Reference Free Part Encapsulation: A New Universal Fixturing Concept Sanjay E. Sarma, Massachusetts Institute of Technology, Cambridge, Massachusetts Paul K. Wright, University of California-Berkeley, Berkeley, California

Abstract Fixturing is an essential prerequisite to machining a

three-dimensional component. With traditional techniques, the range of fixturable shapes is limited, and the identifica- tion of suitable fixtures in a given setup involves complex reasoning. As a result, automated fixture planning systems tend to be slow and conservative, and they remain a major stumbling block in the development of computer-aided process planning systems. This paper presents a new universal fixturing technique called Reference Free Part Encapsulation (RFPE), which solves the problems of immobilization, location, and support of the component independently of its shape. At first glance, RFPE closely resembles other phase-change techniques in that it first relies on encapsulating the workpiece in a low melting point filler matrix for immobilization and support. The subtle difference arises, however, in the manner in which RFPE solves the traditional problems of loss of Iocational information when a setup change is effected. In RFPE, the filler block is machined to a known shape like a cube. After machining features in any setup, the block is refilled and restored to its original shape. The faces of the filler block are now used as Iocational cues in refixturing the block in a new setup. The fact that RFPE is independent of the shape of the component makes planning easier, expands the range of machin- able parts, and makes it an ideal component for rapid prototyping by machining. Some technological aspects of RFPE and their implications on design and process planning are discussed. Finally, schemes for integrating RFPE-based fixturing into the functionality of an open architecture machine tool are presented.

Introduction To manufacture a mechanical component on a

milling machine, it is necessary to immobilize, support, and locate it in each setup. This is referred to as workholding or, alternatively, fixturing. This paper presents the preliminary concepts of a new universal workholding technique called Reference Free Part Encapsulation (RFPE).

The current practice in workholding includes the use of a number of workholding elements, such as vises, parallels, clamps, toe-clamps, V-blocks, modular plates, and so on. For each operation, an appro- priate workholding configuration must be designed

and assembled from these elements depending on the shape of the component and the tool paths. For oddly shaped components, it may be necessary to construct special dedicated fixturing elements when the standard equipment does not suffice. This practice has a number of drawbacks, as follows:

The range of shapes fxturable with these standard devices is limited. In fact, fixturing considerations place serious constraints on the creativ- ity of the designer.

The determination of a fixturing plan for an arbitrarily shaped component is a craft that does not lend itself to efficient automation. Today, the development of fixture planning systems remains a major stumbling block in general automated process planning.

The actual physical construction of the fxture assembly is a human-intensive, and consequently expensive, process.

In recent years, the problems associated with conventional technology have invited research into novel, universal workholding techniques. Phase change based fxturing is a universal workholding technique that is commonly used for fxturing arbitrarily shaped components. As the name suggests, the workpiece is immersed in a bath of a molten fixturing alloy. The alloy is then permitted to freeze, thereby trapping the workpiece irrespective of its shape. Typically, this technique is used in conjunction with locating dies in a pallet, as shown in Figure 1.

Fixturing has three important functions: immobilization, support, and part location. Unfortunately, typical phase change based fixturing does not aid in the location/orientation of the workpiece. Dies are necessary to determine or ensure the correct orientation of the workpiece within the bath of molten filler. These dies are component-specific. Different dies are necessary for different setups, unless the component is symmetric. No robust or economically viable

35

Journal of Manufacturing Systems Vol. 16/No. 1 1997

Low melting point material ~=ece Cutting Tool

~omponent

Mold Locating die Filler material

Figure 1 Phase Change Based Fixturing

way has yet been devised to immobilize and locate components independently of their shape (that is, without using locating dies). Each time a setup change is necessary during machining, all locational information is lost and the component has to be relo- cated using dies or probes. This has made the use of phase change based techniques cumbersome. In general, they are only resorted to when absolutely necessary, for example, during the manufacture of turbine blades. ~ Understandably, the aerospace industry has been a leader in the use of phase change based fixturing in the manner explained above.

Reference Free Part Encapsulation is a phase change based universal fixturing technique that pre- serves the location and orientation of the workpiece across setups. RFPE has been developed as a fixturing technique for the rapid prototyping of arbitrarily shaped components on a milling machine. The basic concepts of RFPE, practical issues, and some appli- cations advantages are presented in this paper. The next section reviews some related work in universal fixturing.

Background: Previous Research in Phase Change Based Fixturing

There are two broad classes of phase change based fixturing techniques: temperature-induced and pseudo-phase change. In temperature-induced phase-change techniques, the filler material is formulated to have a low melting point and can be melted for pouring and removal. Pseudo-phase change based fixturing refers to devices that mimic the effect of molten alloys by conforming to the surface of the component.

Temperature-induced phase-change materials have been used for fixturing in the aerospace industry for many years. Common phase-change materials include thermoplastic materials like Rigidax

and fusible alloys such as those produced under the Cerro brand. Fusible alloys have the advantage that they can be formulated to have negligible expansion coefficients, although they tend to be more expensive. An extensive description of the use of these materials is given in Boyes) Stangrom 3 also describes the use of electrorheological fluids as an alternative to conventional temperature-induced phase-change materials. Application of voltage changes the phase of the material from liquid to solid; however, although electrorheological fluids may be more amenable to automation, the high volt- ages required (2-4 kV/mm) and the very low yield strengths of such materials rule them out for application to machining.

Pseudo-phase change fixtures are attractive because they are easier to dispense and do not require the cooling time required of phase-change materials. The fluidized bed technique uses a con- tainer filled with spherical particles rather than a molten material. 4 The bed is fluidized by permitting controlled amounts of compressed air to pass between the spheres. While in a fluid state, a workpiece is inserted into the fixture. Once embedded, the workpiece is "fastened" by applying pressure on the bed. In a different patented system, 5 ferromag- netic particles are used in conjunction with an elec- tromagnet to fluidize the bed.

Phase and pseudo-phase change based fixturing techniques work by conforming to the surface of the workpiece. This effect can also be approximated by an array of fingers or plungers. A conformable turbine blade clamp is described in Cutkosky, Kurokawa, and Wright? Researchers at the Massachusetts Institute of Technology designed a set of clamps that use shape memory alloy actua- tors. 6 An extensive review of these and other tech- nologies is presented in Hazen and Wright. 7

Basic Concept Consider the imaginary scenario in which a stock

of metal could be suspended midair using, say, magnetic forces. All faces of the stock would then be exposed for machining. Any component could be machined entirely in a single "setup" by an appro- priate machine that can access all the sides of the component. Unfortunately, such a magnetic device does not exist. As a result, physical forces of contact must be relied on to immobilize objects.

36


1997

Mechanisms that deliver forces by physical contact also prevent machine access in the regions where the contact occurs. For example, a vise prevents access to the clamped faces and usually to the bottom faces during milling. Thus there is the concept of setups-- the object must be refixtured in a different configuration to provide access to previously hidden faces. The action of changing setups, unfortunately, entails loss of locational information. For example, when an object is released from a vise, all locational references are lost. Upon immobilization in a new configuration, these references must be re-established using further locational cues. This is a complex and time-consuming task that has traditionally required human expertise and effort.

RFPE is a mechanism that conceptually "freezes the component in space." Instead of space, the workpiece is embedded in a solid block of low melting point filler material. During change of setups, the solid block of filler material is relied on to preserve locational information. The steps taken to make a three-dimensional object using RFPE are described below and shown in Figure 2.

1. The stock is initially embedded within a cube of the filler material as shown in Figure 2.

2. The filler cube is then held in a vise as any square component would be.

3. Machining operations are carried out on the stock and the filler block. For example, to make a hole on the top surface, a drilling operation is carried out on the top surface through the filler.*

4. Once all the features on the top surface are completed, the block is refilled with molten filler and, after solidification, resurfaced to restore it to its original cubical dimensions. Areas of the stock that have been machined out are now occupied by frozen filler material. This step is referred to as restoration. Restoration may be performed either within the same setup or exter- nally in a different setup.t

5. A change of setup can now be carried out by flipping the filler block over. Because the stock remains immobilized in the filler block, and

* For deeper features, it may be necessary to mill access pockets to provide access to the toolholder.

t The significance of internal and external setups in manufacturing systems is described in Black?

B

Rller

Stock

I I Initial block Machine Refill with

features filler

Machining

=: i

A'n" A

B Face-mill Perfect cube

top restored

Restoration

I i Change Machine Refill with Face-mill Perfect cube setup features filler top restored

After ~ I1~ melting

Figure 2 Steps in RFPE

because the orientation of the filler block in its old and new setups is exactly known, the location of the stock in the new setup is exactly known.

6. Machining operations are carried out in this new setup as discussed in Step 3. Once completed, the block is refilled and resurfaced as discussed in Step 4. Every setup is handled in this manner.

7. Once all the features have been completed, the filler material is melted and recycled. The completed component remains.

8. The machined filler material (either thermoplastic or alloy) can be recovered for reuse.

Practical Issues in Implementing RFPE

Since the original conception of the underlying principle of RFPE in 1994, an effort has been made to develop it to a stage where it can be practiced on the shop floor. This section presents a preliminary and qualitative description of some practical issues in this emerging technology. The development of quantitative models is in its incipience and is a topic of ongoing research.

The following sections describe the making of the initial RFPE blank, the problems related to the material properties of the filler material, and some vari- ant forms of RFPE that mitigate the disadvantages related to the properties of the filler material.

37


Initial Blank The initial blank is cast in an open cubical mold.

The mold needs to have high finish and accuracy and must also facilitate easy retrieval of the blank. The accuracy of the box directly impacts the accuracy of the machined component in simple RFPE. "Supported RFPE," described later, makes the accuracy of the RFPE technique independent of the box tolerances.

Figure 3a shows the creation of the initial blank. If the machining is to start from an initial rough stock, where all critical features will be machined using RFPE, then the exact location of the stock in the filler cube is not critical. As long as the volume occupied by the finished component is within the volume occupied by the rough stock, it does not matter where the rough stock is exactly. This is shown in Figure 3b. Once machining is commenced using RFPE, all following features will be properly aligned with respect to each other, and because there are no pre-existing features to reference off, the component will be accurate.

When the filler block is removed from the mold, all faces except the top face are nominally accurate (problems of shrinkage are discussed later). The top face needs to be machined to make a perfect cube. If the side faces are not reliably cast, it may be necessary to square the entire filler block before machining is begtm. If the initial workpiece already has some features on it before RFPE is used, then it is

(a) Filler (b) Eventual ~ l l '~ /Mo ld ~===~omponent

Rough I ! i l I i Stock ~ I I

envelope: ~,#,l,,,,,,,,,,=,,J I ock

~ S t a n d As long as the volume of the eventual component is within

the rough stock, location of the Making the initial mold. The stock is not critical. top face is machined after

solidification. (C)

If the component has pre-existing features, an initial locating die is

necessary.

Figure 3 Making the Blank

necessary use a locating die (or some other locating device) before RFPE is begun. This is shown in Figure 3c. Later features need to be properly oriented with respect to earlier features. The location requirement in such cases is no worse than that of conventional fixturing, with the advantage that all subsequent setups can be performed without the need for any further referencing. A question that comes up at this point is how this locating mold can be manufactured. One solution is to manufacture it using a rapid prototyping technique such as stereo- lithography or part-printing. 9 Another alternative is to machine the locating die in a single 2-1/2D setup.

In the long run, standard filler blocks (with stocks of specified material and dimensions within) of different sizes may be available for use in rapid prototyping. This is analogous to standard paper sizes available for printing and photocopying (letter size, legal size, and so on).

A third issue that needs to be considered in the creation of the initial blank is mold-release agents. Initial experiments have shown that temporary coat- ings of chalk powder and soap solution work well for manual operation. However, there is a need to develop more permanent alternatives, such as non- stick materials, that can be permanently coated onto the inside surface of the mold. This will be especially important if RFPE is to be automated, as discussed later.

Problems Related to Material Properties of Filler

The process step that distinguishes RFPE from classical encapsulation techniques is restoration, which involves repouring and refreezing filler material. Because the accuracy of the casting is very important, the behavior of the filler material during the restoration step is critical to the accuracy of RFPE. Some potential problems related to the behavior of the filler material, especially during restoration, are discussed below.

Strength~Stiffness The most obvious problem related to the use of

filler materials such as Cerro alloys and Rigidax polymers is their lower strength and stiffness when compared to the tool steel used in conventional fixtures. As shown in Figure 4, the strength of Rigidax WI Green is more than two orders of magnitude less

38

Journal of Manufacturing Systems Vol. 16/No. [

1997

E

==

10

9

8

7

6

5

4

3

2

1

0 i I I I I 0.02 0.04 0.06 0.08 0.1 0.12

Strain

It should be noted that, although to a lesser extent than other polymers, Rigidax ~ is in fact viscoelastic. The data are therefore strain-rate dependent. The representative data above were collected at a strain rate of 0.05 ips. Ongoing work is modeling the dynamic behavior of Rigidax ~ with the goal of using it to damp machining vibration.

Figure 4 Stress-Strain Response of Rigidax WI Green

than that of tool steel. Fortunately, this problem is greatly mitigated by the effect of elastic averaging; because there is much greater area of contact between the workpiece and the fixture in encapsulation techniques, actual average stresses are much lower than in conventional fixtures. As a result, the ill effects of using a weaker material are much less than expected. Nevertheless, the extra plastic deformation that results from encapsulation techniques cannot be ignored. Preliminary experiments have shown that in extreme cases the loss of tolerance from these factors may be up to 25 microns. One such experiment is summarized in Figure 5. It is noted that the availability of the open architecture machine tool facilitated the force measurements and probing routines. 1'"

Measured data also show that the strength/stiffness properties of an encapsulated fixture scale increase favorably with increase in size. As the size

Objective: Determine the loss of accuracy in machining from the use of RFPE

Rigk encap:

DOC)

ONOC)

Machining specimen: A15052 embedded in Rigidax Wl Green

Experimental procedure

Probe End mill End m ill ~ .4l

Dynamometer Embedded i Dynamometer i '- - - - J length . . . . . . . . . . . . . . . . .

Workpiece is fixturad in vise. Shoulder feature is machined, and achieved dimension is probed.

Cutting force is measured.

Probe

Process is repeated with component embedded in Rigidax ~. Difference in deflections equals

loss of accuracy attributable to the deformation of Rigidax ~.

Results

Conclusions For relatively severe cuts (low embedded length, significant cutting forces), Rigidax e may contribute upward of 25 microns (0.001") of machining error. However, these errors can be reduced by using

conservative cutting parameters. Metallic fillers are expected to reduce errors further.

Figure 5 Loss of Machining Accuracy with Rigidax WI Green

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Journal of Manufacturing Systems Vol. 16/5Io. 1 1997

increases, the cross-sectional area increases as the square of the length scale. Even if the length of the filler section increases proportionally to the charac- teristic length, the effective deflection for a given force decreases as the size of the component increases. In other words, for components larger than the one shown in Figure 5, the deflection error from Rigidax may be expected to be less than 25 microns for the same cutting forces.

Shrinkage Shrinkage is a primary cause of loss in accuracy

in RFPE. Thermoplastics like Rigidax tend to shrink more (5%) than fusible alloys like CerroTru (< 0.05%). Shrinkage, and the resulting warpage of the initial filler block, may make the reference faces of the block unreliable.

Thermoplastics and non-eutectic alloys tend to have higher warpage as a result of the gradual phase change as the temperature changes. Eutectic alloys like CerroTru and CerroBend, which have sharp melting points, are likely to warp less. Furthermore, because fusible alloys tend to have negligible shrinkage, they are preferable when shrinkage and warpage are serious problems. In preliminary experiments, the problems of warpage have been avoided by using supported RFPE, described in later sections.

Drift is the movement of the embedded stock in the filler block during repouring. It is caused by locally molten zones at the interface between the stock and the frozen filler, as shown in Figure 7. Initial experiments with Rigidax WS have shown that drift can be

Temperature differential in non-eutectic alloy

[ure

nperature)

Filler alloy: Phase diagram

Eutectic alloys require a smaller temperature differential between pouring temperature and yield temperature; therefore, the melt zone is smaller,

and problems of ddft are less significant.

Figure 6 Drift in Eutectic Alloys

as high as 25 microns in severely undersupported sections. Drift was found to be insignificant with Rigidax WI Green (less than 10 microns).

The accurate prediction of drift is a difficult problem, and research is currently under way to develop a reasonable model. In general, however, drift can be controlled by one or more of the following practices:

Use of higher melting point filler material. This increases the cooling rate in ambient circumstances and reduces the chance of drift. Unfortunately, higher temperatures increase thermal errors as well as exacerbate problems of shrinkage.

Acceleration of cooling rate with air circulation. A thin "cold pour"; a layer of filler material

poured in at just above the lower freezing point. This layer insulates the previously frozen layer and reduces the melt zone.

The use of a eutectic alloy. If the pour is con- ducted just above the eutectic temperature, the amount of local melting is greatly reduced, as shown in Figure 6. With non-eutectic alloys, the pour temperature needs to be much higher.

Supported RFPE An important concept in RFPE is the transference

of coordinates between setups through the mainte- nance of the cubical shape of the filler block. The accuracy of the dimensions of the cube is critical to the accuracy of the technique. If the filler material is not sufficiently hard, it is possible to compromise the finish of the block during clamping. Furthermore, the natural problems of shrinkage and warpage during pouring and freezing tend to make the face finish of the block unreliable for some materials, like Rigidax . To combat these problems, a family of RFPE techniques called supported RFPE have been

Component is Component machined in a poured to refill previously drifts into the given setup the block frozen filler melt zone

melts at the under gravity interfaces

Figure 7 Drift

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1997

developed. Supported RFPE techniques make the accuracy largely independent of problems like shrinkage. The basic idea of these techniques is that the reference and/or strength are provided by metallic surfaces embedded on the boundary of the filler block.

Cage RFPE In cage RFPE, an external cage, or skeleton, is

embedded on the outside of the filler block, as shown in Figure 8. The mold is used merely to con- tain the molten filler material and not to provide dimensional accuracy. As a result, the precision of the mold does not impact the accuracy of the technique. Furthermore, a certain amount of shrinkage actually helps because the filler material does not tend to "bulge" out, thereby not interfering with the reference surfaces.

An obvious drawback of cage-supported RFPE is the reduced access to the stock, especially for angled features, due to the support frame. A solution to this problem is to use disposable, off-the-shelf stands to assemble the solid cage. If the strut obstructs access to a certain feature, it can be machined away. This introduces the need, during the process planning stage, to ensure that such operations are carried out later so that the absence of the struts affects few features. An alternative is to use a partial cage, that is, a single comer of the original cubical framework. This reference comer floats in the filler block in a comer that is not accessed for any feature. This comer can be used as a reliable reference surface during clamping.

Stock-Enclosed RFPE A second way to provide support is to use the

stock itself as the exoskeleton of the filler blank. This is referred to as stock-enclosed RFPE (SE-RFPE). The first step in SE-RFPE is to square a stock of the workpiece material to a size that contains the required component. Features are then machined in the first setup, and the cavities are filled with filler material. The stock is switched to a new setup and the process repeated. Because the stock is larger than the required component, the outer comers of the stock remain untouched. These comers act as the support- ing structures as in cage RFPE. Figure 9 shows an SE-RFPE component in its fifth setup.

It should be noted that because SE-RFPE involves much more material removal than other forms of RFPE, it is more convenient for softer materials like

I

Figure 8 Solid-Cage RFPE

Figure 9 Stock-Enclosed RFPE

plastic and aluminum. The essential advantage of SE-RFPE is that it can be used to machine components of odd dimensions without separately machining a cage.

2-1/2D RFPE In practice, a large fraction of the components

machined in job shops are "2-1/2D"; they can be machined entirely in two setups. Stock-enclosed RFPE can be modified to handle 2-1/2D components conveniently. This is referred to as 2-1/2D RFPE, which more closely resembles conventional phase change based fixturing.*

Figure 10 describes the steps in 2-1/2D RFPE. Initially, a stock is squared and a deep pocket is machined to create a tray. Next, the features on the lower side of the 2-1/2D component are machined at the bottom of the tray. The tray is then filled with filler material. This step is merely for support rather than for transference of locational coordinates. The workpiece can now be flipped around and features on the opposite

* This idea, along with the gripper design, is due to Mark MacKenzie. 12

41


Fill tray with molten filler

Machine bay; Machine dorsal

~Machined lower Retrieve component 1), features

~'~ ~ Machine boundary

Figure 10 2-1/2D RFPE

side can be machined. Once all the features have been machined, the finished workpiece can be retrieved.

Advantages of RFPE RFPE was developed to address certain immedi-

ate needs in the field of machining. This section describes the advantages of RFPE. Specifically discussed is how, by expanding the range of parts that can be fixtured, RFPE dramatically simplifies design and process planning. Finally, it is shown how RFPE is amenable to automation.

Process Planning Process planning may be defined as the determi-

nation of a complete set of machining instructions to manufacture a given component. Lower-level planning tasks include tool selection, path generation, and cutting parameter selection. Important planning tasks at the higher level include operation sequenc- ing, fixture design, and setup selection.

The legal sequences in which the machining operations can be performed are constrained by so-called feature interactions--ordering constraints that ensure that the requirements of each machining operation are met. Most interactions can be repre- sented as pairwise ordering constraints and can therefore be handled by straightforward graph-theo- retic means, la These are referred to as local interactions. However, there are two classes of interactions-namely those arising from fixturing concerns and those arising from component rigidity concerns--that do not lend themselves to such a simple

analysis. In fact, fixturing and rigidity concerns remain important stumbling blocks in the development of automated process planning systems today. These are referred to as global interactions. Simply put, if process planning is done in an environment that only uses conventional vises and toe-clamps, the generation of a fixture plan that adequately immobilizes, locates, and supports a component is a very difficult task with many ambiguities.

Reference Free Part Encapsulation greatly simplifies process planning because it eliminates the two important sources of global interactions, namely fixturing considerations and part rigidity concerns.

Fixturing Constraints With RFPE, the shape of the component is imma-

terial. There are no requirements on the order in which operations must be performed because fixturability can be ensured in any orientation. Furthermore, the need to locate the component in every new setup is completely eliminated, thus obvi- ating the need for reference surfaces and datums.

Part Rigidity Constraints After all the features in a given setup in RFPE are

machined, the RFPE filler block is restored to its original cubical state by repouring frozen filler and permitting it to freeze. The volume of metal removed during machining is now replaced by frozen filler material, and all cavities in the component are filled and supported. Figure 11 shows this in greater detail. As a result, the possibility of deformation during cutting or clamping in the next setup is eliminated. This is an important advantage of RFPE because it not only expands the range ofman- ufacturable components but also relieves planning of an important and difficult responsibility.

The elimination of global interactions means that planning can be carried out entirely within the realm of a single directed-graph representation of the

Rller material supports slender

sections

Figure 11 Eliminating Rigidity Concerns Through RFPE

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Journal ofManufactunngSystems Vol. 16/No. 1

1997

remaining manufacturing constraints. As a result, more efficient algorithms are possible, and greater plan optimality is guaranteed.

Implications of RFPE on Design The previous section discussed that conventional

fixturing methods make process planning difficult. However, a perhaps more important disadvantage of conventional fixturing techniques is that they greatly limit the range of components that can be machined. By eliminating global interactions, RFPE vastly expands the range of designs that can be manufactured. In this section, some interesting new possibilities intro- duced by the capabilities of RFPE are discussed.

Arbitrary Component Shape Ensuring that the part shape lends itself to fixtur-

ing in conventional circumstances restricts the cre- ativity of the designer. For example, to be fixtured in a vise, the part must yield two parallel and accessi- ble faces in every setup. However, by nature of being a phase change based fixturing technique, RFPE is independent of the shape of the component. As a result, almost any shape can be machined as long as the capabilities of the machine tool permit it.

RFPE is especially pertinent in light of recent interest in complex three-dimensional surface modeling and multiaxis milling. In most discussions on this topic, the essential problem of fixturing is ignored, perhaps with the tacit assumption that dedicated fixtures will be used or that machining can be performed in a single setup. With RFPE, it is actually possible to sculpt general three-dimensional shapes across many setups.

Delicate Components and Built-In Flexures As indicated earlier, the workpiece in RFPE is

always embedded in a supportive filler matrix. Consequently, it is possible to realize a wide variety of small and delicate component designs with little or no extra effort. Furthermore, it is possible to design flexures and springs into a component as shown in Figure 12. This permits the designer t con- solidate many functionalities into a single component and potentially simplifies mechanism design.

Reference Free Parts In conventional fixtufing, it is necessary to use

datums in the workpiece to locate and orient it in each new setup. Locational information is trans-

ferred across setups through the datum surfaces defined by the designer in dimensioning the component. These surfaces must be machined to reasonable accuracy even if they play no part in achieving the intended functionality of the component. Orientational information between setups is usually transferred through "squared" surfaces on the workpiece. Squaring, which is the process of reducing a rough stock to a nominally rectangular block, is usually performed by the machinist to ensure orthogo- nality between features in different setups. Most experienced designers and machinists are cognizant of these factors and will often create a number of "pseudo-features" or datum surfaces to transfer locational/orientational information across setups.

In RFPE, the enclosing filler block provides all the necessary cues to locate the workpiece in its new setup. Consequently, operations like squaring and finishing of nonessential faces become redundant, and individual features can be accurately oriented with respect to each other independently of such cues. This new design paradigm is referred to as reference free component design. An example is shown in Figure 13. The intent of the design is to create an elbow for a robotic arm. The two bearing holes are required to be perpendicular and offset by a certain distance. To specify this part for conventional fixturing, the user must rely on datums 1, 2, and 3 as shown. Although these datum surfaces are not critical to the functionality of the part, they must be machined for transfer of coordinates between setups. Furthermore, to machine datums 1, 2, and 3 (which are the back, top, and right faces of the block as shown) it is also necessary to machine the three faces opposite to them (that is, the front, bottom, and left faces). Effectively, therefore, the part must be completely squared and will take at least six setups. With RFPE, however, the two holes can be drilled in

directly onto gripper chassis

6ripper

This gripper can be machined in one setup with a two-dimensional version of RFPE

Figure 12 Delicate Gripper with Built-In Flexures

43


Yl

F ront~ ~'~R~ght

Robot elbow

Datum 1

Top

Front

I

(

I I Datum 2

~ Datum 3

Figure 13 Datums in Mechanical Design

exactly two setups. The six faces of the block need not be machined. In other words, there is no need to introduce intermediary faces or cuts for information transfer across setups. Squaring becomes an option- al, cosmetic step.

Design for Manufacture In conventional fixturing, each fixturing arrange-

ment is a solution to a unique problem. It is difficult to generalize a fixturing strategy to a range of problems. The ad hoc nature of conventional fixturing makes design for manufacture very difficult because an assessment can only be made by actually finding a reasonable fixturing arrangement (such as by process simulation). With RFPE, on the other hand, it is easy to generalize and quantify the "envelope of fixturability." An assessment of whether a part can be manufactured merely involves simple checks related to the dimensions of the part (in comparison to the RFPE equipment). Moreover, a minor modifi- cation to a component in RFPE is less likely to ren- der it unfixturable than in conventional fixturing. Consequently, RFPE can be thought of as a more predictable and "forgiving" technique than conventional techniques, and it lends itself more easily to design for manufacture.

Rapid Prototyping with RFPE Fixture manipulation is the one aspect of milling

operations that has resisted automation. RFPE offers

a means for automating the CNC machine tool completely to the extent where general three-dimensional parts can be prototyped rapidly "at the touch of a button?' This section describes a speculative scenario for integrating the operation of RFPE into a milling system.

The first step in automating RFPE would be the development of a mechanism that can simultaneous- ly act as a mold and a vise. Such mechanisms are referred to as mold-vises. A mold-vise concept is shown in Figure 14. Essentially, a mold-vise makes it possible to perform the restoration within the vise holding the blank; by completely closing around the blank, the mold-vise acts as a mold and prevents molten filler material from leaking.

Filler block Embedded stock

Bottom plate

(a) Top view: open (b) Top view: closed (c) Side view

Figure 14 A Mold-Vise Mechanism

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1997

The mold-vise mechanism can be incorporated into the working zone of a milling machine (Figure 15) as follows:

At the beginning of each setup, the filler block resides in the mold-vise. The lifting plunger is up.

Features are machined out of the filler block and the embedded stock.

The table of the (milling) machine is moved such that the mold-vise is under the dispenser. The lifting plunger is retracted.

Molten filler material is poured into the mold- vise. The pour is controlled to approximate the amount of material removed.

The molten material is permitted to freeze. Air circulation may be initiated by the dispensing device to accelerate freezing.

After the material freezes, the lifting plunger is pushed up. The top face is faced off to restore a reference surface.

This setup is now completed. A new setup may be assumed. The change of setup can be performed automatically by a robot. Manipulation is simplified by the fact that the block is a cube.

Results and Discussion Since its conception in 1994, RFPE has been used

to manufacture a number of components that would be very hard to machine by conventional means. Some examples are shown in Figure 16. This section describes these test parts and discusses the present and future capabilities of RFPE.

Machining Tests Components 1-3 in Figure 16 are robot grippers

that were machined for a separate project at the

/

CNC Milling Machine

robot to manipulate block between setups

(now shown)

Figure 15 The automated machine tool

University of California-Berkeley. Gripper 1 was machined from 5052 aluminum, while grippers 2 and 3 were machined from ABS. These components demonstrate the capability of machining flexures integrally into the component (as also shown in Figure 12). The flexure thickness in gripper 1 is 0.5 mm, while in 2 and 3 it is 0.8 mm. All three components were machined in two setups each using 2-1/2D RFPE. These components would have been very expensive to machine by conventional means because they would have required at least four setups and a sacrificial baseplate.

Figure 16, component 4, shows a virtual reality headgear assembly consisting of seven separate sub- components that were machined using 2-1/2D RFPE. The longer struts in this assembly are made of ABS, 100 mm long, and have square cross sections that are 3 mm (1/8") on the side. Multiple sup- porting fixtures would have been required to machine comparably fragile components if using conventional fixturing techniques.

Components 5-7 in Figure 16 were machined using the general RFPE in six setups including squaring. Components 5 and 6 were machined from

Figure 16 Components Manufactured with RFPE

45


aluminum, while component 7 was machined from ABS. Component 7, which is described topologically as a trefoil knot, is especially remarkable because the struts are only 1.5 X 1.5 mm (1/16 X 1/16") in cross section. This component would have required at least 12 setups and multiple dedicated fixtures conventionally. In fact, the range of topologies that can be machined using RFPE is quite surprising. It is possible to machine multiple functional components, even those topologically linked, simultane- ously from a single stock. In one such example, not shown here, two intertwined links of a chain were machined from one stock.

Finally, Figure 16, component 8, shows a pair of ABS computer casings (top and bottom, shown here in assembled form) machined using 2-1/2D RFPE. These components were approximately 300 X 275 mm (11 x 12") in area. The web at the bottom of the casings was less than 3 mm (1/8") in thickness. Attempts to machine these components by conventional means were unsuccessful because of the vibration of the unsupported web section. However, this component was successfully (and easily) machined using RFPE.

Table 1 summarizes the fixturing and machining times for each component.

Current and Future Directions Currently, accuracies in the range of 250 to 500

microns (5 to 10 x 10 -3 in.) are being achieved rou- tinely in components machined by RFPE. Theoretically, machining accuracies better than 25 microns should be achievable. This discrepancy can be explained by the (current) reliance on supported RFPE techniques such as cage RFPE and SE-RFPE. In such techniques, the support structures reduce access to the workpiece and therefore mandate the use of longer tools. Tool deflection in deep pocket machining is a well-known reason for loss of machining tolerances and finish. Future plans are to ameliorate these problems by using partial cages and high-speed cutting (~15,000 rpm as opposed to the 2000 to 3000 rpm currently used). Furthermore, all experiments to date have been with the polymer Rigidax . In the future, test materials like CerroTru will be used; in particular, the possibility of eliminating the need for support altogether will be explored.

Another future goal is to automate RFPE. In the cases shown in Figure 16, RFPE was performed entirely by manual operation. However, the methodology has been developed to the extent that it can be performed by an untrained operator. In other words,

Table 1 RFPE Manufacturing Times

Par t Component Number Fixmring Machining Total # Name of Time T ime Manufacturing

Setups (offline) Time

1 Simple 2 35 a 10 45 A1 gripper

2 ABS long 2 50 5 55 gripper

3 ABS swivel 2 35 15 50 gripper

4 VR headgear 14 b 300 200 500 (7 pieces)

5 Bracket 6" 150 100 250 6 Escher 3 75 75 150

part 7 Trefoil 6 150 80 230

knot 8 Computer 4 100 110 140

casings (2 pieces)

a. For 2-1/2D RFPE, this time includes the creation of the tray. b. Setups reflect total for seven separate components. c. Components 5-7 were machined entirely from unsquared stock. This is reflected in the number of setups and the cutting times shown.

46


1997

the aspects of RFPE that are performed manually are the ones that are felt to be the easiest to automate.

Finally, an as-yet-untapped advantage of encapsulation techniques in general is the high damping properties of materials like Rigidax. * This quality was taken advantage of only peripherally to machine component 8 in Figure 16. An important avenue of future research is to model the behavior of the encapsulation material. As explained earlier, finite element models are currently under development to predict drift and the static and dynamic behavior of an encapsulated blank.

Conclusions This paper presented the initial technological

aspects of a new universal workholding technique, Reference Free Part Encapsulation, to machine components on a milling machine. Specifically, the advantages of this technology are:

A wide variety of shapes can be machined with this technique.

Delicate overhangs and slender sections like flexures can be machined integrally with the component.

The need to square the workpiece separately is eliminated. In fact, the technique takes care of all indexing needs during any change of setup in machining.

Process planning requirements are greatly simplified.

RFPE can be automated to the extent that the machining process can be made completely "hands-free."

Tolerances achieved thus far are in the range of 1 O0 microns. Future work, for example, involving new filler materials, is needed to improve tolerances.

* An initial result of the DOE's TEAM component may be seen on the World Wide Web at http://kingkong.me.berkeley.edu

Acknowledgments The authors would like to thank Rekha

Ranganathan and Jamie Stori for gathering some of the experimental data described in this paper. Many other colleagues contributed to this effort by sharing their experiences in using RFPE. The authors thank Mark MacKenzie, Suschiel Gandhi, Jamie Stori, Steven Schofield, Jane MacFarlane, Rekha Ranganathan, Chuck Smith, and Matt Giere. This project was partially supported by the National Science Foundation and the Lawrence Berkeley Laboratory.

References 1. M.R. Cutkosky, E. Kurokawa, and P.K. Wright, "Programmable

Conformable Clamps," Proceedings of IJCAI-7 (1982), pp766-772. 2. WE. Boyes, ed., Low Cost Jigs, Fixtures & Gages for Limited

Production (Dearborn, MI: Society of Manufacturing Engineers, 1985). 3. J.E. Stangrom, "Electrorheological Fluids," Physical Technology

(v14, 1983), pp290-296. 4. M.V. Gandhi, B.S. Thompson, and D.J. Mass, "Adaptable Fixture

Design: An Analytical and Experimental Study of Fluidized Bed Fixturing," ASME Journal of Mechanisms, Transmissions and Automation in Design (vl08, 1986), pp155-121. 5. L. Coes, "Workholder for Irregular Shaped Workpieces," U.S. Patent

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Adaptive Fixtures Operated by Robot Manipulators," Proceedings of the ASME Japan-U.S.A. Symposium on Flexible Automation (1988), pp467-474. 7. K.E. Hazen and P.K. Wright, "Workholding Automation: Innovations

in Analysis, Design and Planning," Manufacturing Review (v3, n4, 1990), pp224--237. 8. J 3?. Black, The Design of the Facto~ with a Future (New York:

McGraw-Hill, 1991). 9. E. Sachs, M. Cima, P. Williams, D. Brancazio, and J. Comic, "Three

Dimensional Printing: Rapid Tooling and Prototypes Directly from a CAD Model;' Journal of Engineering for Industry (v 114, n4, 1992), pp481-488. 10. I. Greenfeld, EB. Hansen, and P.K. Wright, "Self Sustaining Open- System Machine Tools," Proceedings of the North American Manufacturing Research Conference (vlT, 1989), pp304--310. l l . S.E. Sarma and P.K. Wright, "Making a Design Domain Deterministic: The Case of DFM," submitted to Research in Engineering Design (1996). 12. Mark MacKenzie, personal communication, 1994. 13. S.E. Sarma, "A Methodology for Integrating CAD and CAM in Milling," PhD dissertation (Berkeley, CA: University of California- Berkeley, 1995).

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reference free part encapsulation

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