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Conventional Methods for Rapid Prototyping

Richard A. Wysk

Dopaco Distinguished Professor

Industrial and Systems Engineering

North Carolina State University

Agenda

• Why is Rapid Manufacturing (RM) important in medicne?

• What is RM/RP?• Limitations of RP• Economics of RM/RP• New directions in RM• Sustainable RM• Observations and conclusions

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Broad focus -- Engineering integration

ENGINEERING -- the planning, designing, construction (manufacture), or management of machinery, roads, bridges, etc..

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Traditional Engineering

A Vision of Integrated Engineering Systems (cont.)

INTEGRATE– 1. to make or become whole or complete.– 2. to bring parts together as a whole. – 3. to remove barriers imposing segregation.

A Vision of Integrated Engineering Systems (cont.)

INTEGRATED ENGINEERING – planning, designing, construction and

management of a product.

Engineering Integration

Product Engineering

Process Engineering

Production Engineering

A Vision of Integrated Engineering Systems (cont.)

INTEGRATION ENGINEERING – tools and techniques that

can be used to assist in combining planning, design, construction and management of a product.

A Vision of Integrated Engineering Systems (cont.)

• INTEGRATED ENGINEERING – planning, designing, construction and management of a

product.

Engineering Integration

• Performing all business activities in unison• Analyzing all engineering functions

concurrently• Making wise real-time economic decisions• Team concepts

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• Prototyping is critically important during product/process design– Reduce time to market– Early detection of errors– Assist concurrent manufacturing engineering

• Prototypes are used to convey a products’:– Form– Fit– Function

• Prototype building can be a time-consuming process requiring a highly skilled craftsperson– Time spent testing prototypes is valuable– Time spent constructing them is not…

• “Rapid Prototyping” (RP) methods have emerged – (Solid Freeform Fabrication, Additive Manufacturing, Layered Manufacturing)

Need for model accuracy increases

What does this have to do with Rapid Prototyping (RP)

Rapid Manufacturing definitions• Rapid Manufacturing (RM)

– Direct from CAD model without tooling• No process engineering

– Short lead time– Increased product fidelity– Ready to use end products

• Additive processes– Traditional Rapid prototyping (RP) process

• 3D printer, SLA, FDM, SLS…– No geometry limitation– Push button manner operation– Restricted in material, accuracy, and surface finish15

Image from http://www.tctmagazine.com/x/guideArchiveArticle.html?id=10839

Stereolithography (SLA)

Stereolithography is a common rapid manufacturing and rapid prototyping technology for producing parts with high accuracy and good surface finish. A device that performs stereolithography is called an SLA or Stereolithography Apparatus.

Stereolithography is an additive fabrication process utilizing a vat of liquid UV-curable photopolymer "resin" and a UV laser to build parts a layer at a time. On each layer, the laser beam traces a part cross-section pattern on the surface of the liquid resin.

Selective Laser Sintering (SLS)

SLS can produce parts from a relatively wide range of commercially available powder materials, including polymers (nylon, also glass-filled or with other fillers, and polystyrene), metals (steel, titanium, alloy mixtures, and composites) and green sand. The physical process can be full melting, partial melting, or liquid-phase sintering. And, depending on the material, up to 100% density can be achieved with material properties comparable to those from conventional manufacturing methods. In many cases large numbers of parts can be packed within the powder bed, allowing very high productivity.

Fused Deposition Modeling (FDM)

• Fused deposition modeling, which is often referred to by its initials FDM, is a type of rapid prototyping or rapid manufacturing (RP) technology commonly used within engineering design. The FDM technology is marketed commercially by Stratasys Inc.

• Like most other RP processes (such as 3D Printing and stereolithography) FDM works on an "additive" principle by laying down material in layers. A plastic filament or metal wire is unwound from a coil and supplies material to an extrusion nozzle which can turn on and off the flow. The nozzle is heated to melt the material and can be moved in both horizontal and vertical directions by a numerically controlled mechanism, directly controlled by a Computer Aided Design software package. In a similar manner to stereolithography, the model is built up from layers as the material hardens immediately after extrusion from the nozzle.

• Several materials are available with different trade-offs between strength and temperature. As well as Acrylonitrile butadiene styrene (ABS) polymer, the FDM technology can also be used with polycarbonates, polycaprolactone, and waxes. A "water-soluble" material can be used for making temporary supports while manufacturing is in progress. Marketed under the name WaterWorks by Stratasys this soluble support material is actually dissolved in a heated sodium hydroxide solution with the assistance of ultrasonic agitation.

Laminated Object Manufacturing (LOM)

Laminated Object Manufacturing (LOM) is a rapid prototyping system developed by Helisys Inc. (Cubic Technologies is now the successor organization of Helisys) In it, layers of adhesive-coated paper, plastic, or metal laminates are successively glued together and cut to shape with a knife or laser cutter.

Electron Beam Melting (EBM)• Electron Beam Melting (EBM) is a type of rapid

prototyping for metal parts. The technology manufactures parts by melting metal powder layer per layer with an electron beam in a high vacuum. Unlike some metal sintering techniques, the parts are fully solid, void-free, and extremely strong. Electron Beam Melting is also referred to as Electron Beam Machining.

• High speed electrons .5-.8 times the speed of light are bombarded on the surface of the work material generating enough heat to melt the surface of the part and cause the material to locally vaporize. EBM does require a vacuum, meaning that the workpiece is limited in size to the vacuum used. The surface finish on the part is much better than that of other manufacturing processes. EBM can be used on metals, non-metals, ceramics, and composites.

Types of RP Systems

Prototyping Technologies Base Materials

Selective laser sintering (SLS) Thermoplastics, metals powders

Fused Deposition Modeling (FDM) Thermoplastics, Eutectic metals.

Stereolithography (SLA) photopolymer

Laminated Object Manufacturing (LOM) Paper

Electron Beam Melting (EBM) Titanium alloys

3D Printing (3DP) Various materials

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So what’s the problem?

• Rapid Prototyping?– Technology for producing accurate parts directly from CAD

models in a few hours with little need for human intervention.– Pham, et al, 1997

• Prototype?– A first full-scale and usually functional form of a new type or

design of a construction (as an airplane)– Webster’s, 1998

• Model?– A representation in relief or 3 dimensions in plaster, papier-mache,

wood, plastic, or other material of a surface or solid– Webster’s, 1986

physical models

How can we automatically create toolpath and fixture plans for CNC?

Problems with Additive RP

• Long processing times• Energy use• Expensive product ?!?

– Not intended as a production technique

• Functional product ?!?– May not quite meet material or dimensional

requirements

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CNC MachiningComputer numerical control, and refers

specifically to a computer "controller" that reads G-code instructions and drives a machine tool, a powered mechanical device typically used to fabricate components by the selective removal of material. CNC does numerically directed interpolation of a cutting tool in the work envelope of a machine. The operating parameters of the CNC can be altered via a software load program.

Economics

Product cost = engineering cost + materials cost + manufacturing cost

Product cost /part = engineering cost / total # of parts + materials cost / part +

manufacturing cost / part

This is the cost for all parts that will be made and sold.

This is the cost for each part that will be made and sold.

Engineering cost

• Product design (Ced)– Cost of engineering design

• Process design (Cpc)– Cost of process planning

• How is the part to be made

– Cost of fixtures and tooling

• Production design (Cpd)– Cost of setting up production

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Material cost

• In most cases this is somewhat dependent of the number of parts– Economies of scale– Efficiencies of scale

• Additive or subtractive– Fractional or bulk materials

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Engineering cost

CE = Ced / nt + Cpc / nt + Cpd / nb

total parts total parts parts in a batch

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Manufacturing cost

• One time costs– Process planning and design– Fixture engineering and fabrication

• Set up cost (Cset)– Cost to set up a process

• Processing cost (Cpsc)– Cost of processing a part

• Production cost (Cpdc)– Cost of tooling and perishables

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Manufacturing cost

CM = Cone / nt + Cset / nb + Cpsc + Cpdc / ntool

Total parts parts in a batch each part tool cost by parts/tool

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Economic Analysis• A general cost model

• Tooling, Planning, Design31

Eng

CostsMaterial

CostsManuf.

Costs Total Cost

Planning

Design

Tooling

Process

SubtractiveProcess

Additive Process

So how can engineering costs be reduced for CNC machining?

Machine cost Fixture cost Process planning cost

• CNC-RP Method: A part is machined on a 3-Axis mill with a rotary indexer and tailstock using layer-based toolpaths from numerous orientations about an axis of rotation.

TableOpposing

3-jaw chucks

Rotary indexer

Round stock

End mill

Axis of rotation

TableOpposing

3-jaw chucks

Rotary indexer

Round stock

End mill

Axis of rotation

STEPS TO CREATE A PART ( MT. Bike Suspension Component)

1. First orientation of part section is machined

(Side View)

1. First orientation of part section is machined

(Side View) (Side View)

3. Third orientation is machined3. Third orientation is machined

4. Fourth orientation is machined4. Fourth orientation is machined4. Fourth orientation is machined

Rotate Stock

2. Second orientation is machined

Rotate StockRotate Stock

2. Second orientation is machined2. Second orientation is machined

1. First orientation of part section is machined

(Side View)

1. First orientation of part section is machined

(Side View) (Side View)

3. Third orientation is machined3. Third orientation is machined

4. Fourth orientation is machined4. Fourth orientation is machined4. Fourth orientation is machined

Rotate Stock

2. Second orientation is machined

Rotate StockRotate Stock

2. Second orientation is machined2. Second orientation is machined

STEPS TO CREATE A PART ( MT. Bike Suspension Component)

5. Left support section is machined5. Left support section is machined

6. Right support section is machined6. Right support section is machined

7. Temporary supports are removed7. Temporary supports are removed

8. Part is severed from stock at supports8. Part is severed from stock at supports

Part fixtured with final 2 sacrificial supportsPart fixtured with final 2 sacrificial supports

Finished Steel Part

4”

Finished Steel Part

4”4”4”

Part fixtured with final 2 sacrificial supportsPart fixtured with final 2 sacrificial supports

Finished Steel Part

4”

Finished Steel Part

4”4”4”

Material: Steel

Layer depth: 0.001” (0.025mm)

Process/fixture planning time: Minutes

Processing time ~20 hours

Setups/Orientation Planning

VISIBILITY

50o

155o

228o 335o

50o

155o

228o 335o

MACHINABILITY (for a given tool geometry)

Non-machinable Region

Non-machinable Region

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Methodology• Overview:

– Visible surfaces of the part are machined from each orientation about an axis of rotation

– Long, small diameter flat end tool with equal flute and shank diameter used.– Sacrificial supports (temporary features) added to the solid model and created in-

process– Begin with round stock material, clamped between two opposing chucks

• Example:

x

y

z

x

y

z

Toolpath layers at 180º orientation

y

z

Toolpath layers at 0º orientation

y

z

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Research Problems

• Setup/Orientation– How many rotations (setup orientations) about the axis of rotation are required?– Where are they?

• Toolpath planning– For each orientation, how can we automatically generate toolpaths?– What diameter and length tools should be used?– In what order should the toolpaths be executed?

• Fixture planning– How can we automatically generate sacrificial supports?– What diameter and length should they be?

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Rapid Prototyping• Basics:

– Solid model (CAD) is converted to STL format• Facetted representation where surface is approximated by triangles• Intersect the STL model with parallel planes to create cross sections

– Create each cross section, adding on top of preceding one

x

y

z

CAD (ProE) STL “slicing” operation

2-D cross section

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Rapid Prototyping• Fixtures are created in-process (Sacrificial Supports)

– Secure model to the build platform– Support overhanging features

• Remove fixture materials in post-process step

Model material

Support material

Build Platform

FDM Model with/without supports

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• A problem of tool accessibility

• Approximated as a problem of visibility (line of sight)

• A Visibility map is generated via a layer-based approach

• Tool access is restricted to directions in the slice plane (2D problem)

• Goal is to generate the data necessary to determine a minimum set of rotations required to machine the entire surface

Set of segments on a slice visible from one tool access direction

Determining the number of rotations

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Approaches to 2D visibility mapping

• Shortest Euclidean paths - Lee and Preparata, 1984

• Convex ropes - Peshkin and Sanderson, 1986

• 2D visibility cones - Stewart, 1999

Issues:

• Computing S.E.P.s/VCs for polygons with holes

• Granularity of STL files, may need to add collinear points to polygon segments

• Would need to retriangulate

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(a) Visibility for the segment= [Θa,Θb,]

(b) Visibility for the segment= [Θa,Θb,], [Θc,Θd,]

Θa Θb

Θc

Θd Θa

Θb

• Visibility for each polygonal chain is determined by calculating the polar angle range that each segment of the chain can be seen.

• Since there can be multiple chains on each slice, we must consider the visibility blocked by all other chains.

Solution approach

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Pi+1

P: , S:

Pi+1Pi-1

LCHP RCHP

LCHP RCHP

Pi

Pi

not visible

• We have a polygon P and its convex hull S

• For any point Pi not on S, the visible range can be found by investigating points from the

adjacent CCW convex hull point to the adjacent CW convex hull point

• These points will be denoted the “left” and “right” convex hull points of Pi, LCHP(Pi) and

RCHP(Pi), respectively.

• It is only necessary to calculate the polar angles from Pi to the points in the set [LCHP,

RCHP], excluding Pi.

• The set is divided into, S1 and S2 where: ],[:2

],[:1

1

1

RCHPPS

PLCHPS

i

i

Step one: Visibility with respect to own chain

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• The visible range for a point is bounded by the minimum polar angle from Pi to points in S1 and the maximum polar angle from Pi to points in S2.

• This is the visible range for the point Pi with respect to the boundary of its

own chain, and is denoted V(Pi).

Where:](),([)(

12YPMinXPMaxPiV i

SYi

SX

V(Pi)

Pi

V(Pi): [43.82 ,121.31]

S1

S2

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• Consider the segment defined by points in P, u and v, where:

u: Pi and v: Pi+1

• The intersection of visibility ranges for the points u and v and the 180º range above the segment define a feasible range of polar angles in which the segment could be reached.

],[],[],[)( uvvvuuvu LVRVLVRVLVRVVV

LVuLVv RVu

RVv

vuv+1

u-1vu uv

• The sets S1 and S2 are redefined:

• The ends of the visibility range are:

)](),1[(:2

)]1(),([:1

vRCHPvS

uuLCHPS

)]([)(2vxMaxuvRV

Sx

)]([)(

1uyMinuvLV

Sy

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Problem Surfaces

(a) RV is outside of the 180º range, (b) Both RV and LV are out of the 180º range, (c) No visibility due to overlapping, (d) Visibility to the entire segment is not possible since RV > LV.

(a) (b)

(c) (d)

u v

I1

I2

u v

I1 I2

u v

I1

I2 u vI1

I2

LVRV

RV

LVRV

LV

RV LV

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Step two: Visibility blocked by all other chains on the slice

• V( )j* is the visibility with respect to the chain j on which resides,

denoted j*.

• For all obstacle chains , the polar range blocked by the chain is denoted VB( )j.

• The set of visible ranges for the segment is defined:

• Visibility blocked to the segment is the union of the visibility blocked by chain j to point u and the visibility blocked by chain j to point v, intersected with the 180º range above segment

• The set of angles blocked to the segment where:

• The set of angles blocked to points u and v where:

uv uv

*\ jJj

uv

jj uvVBuvVuvVIS )()()( *

uv

uv

]},[]])([])({[[)( vuuvvVBuVBuvVB jjj

],[)( uuj LBRBuVB ],[)( vvj LBRBvVB

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RBu

LBu RBvLBv

u v

• Considering the condition that blocked visibility is only for blockage in the 180º range above the segment, it can easily be seen that the set:

],[],[],[)( vuvvuuvu LBRBLBRBLBRBVBVB

• RBu is simply the minimum polar

angle from u to all points on the blocker chain

• LBv is the maximum polar angle from

v to all points on Pj, where Pj is the

set of points for the blocker chain.

)]([ uxMinRBjPx

u

)]([ vyMaxLBjPy

v

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jj uvVBuvVuvVIS )()()( *Recall:

• For each segment the collection of visible ranges given in polar angle about the axis of rotation:

rbababatjkVIS ],,,...[],,[,],,[: 21 where: rMAX = n

• From the data in [VIS] we can formulate a set corresponding to the segments visible from a given angle.

}],[ range, somefor )(){( tjkrbabsatjks VISSEG

.

.

.

VIS1,1,1 VIS2,1,1

VIStjk

VISqnp

.

.

.

(Θa,Θb)1, (Θa,Θb)2, …(Θa,Θb)n

.

.

.

.

.

.

(Θa,Θb)1, (Θa,Θb)2, …(Θa,Θb)n

(Θa,Θb)1, (Θa,Θb)2, …(Θa,Θb)n

(Θa,Θb)1, (Θa,Θb)2, …(Θa,Θb)n

.

.

.

Θ1 Θ2

Θs

Θ359

.

.

.

SEG1,1,1, SEG2,1,1, SEG1,5,3…

.

.

.

.

.

.

SEG13,1,2, SEG14,1,2, …

SEGtjk. . . .

SEGtjk. . . .

The Minimum Set Cover problem:

Given: A collection of subsets Θs of a finite set SEG (the set of all segments)

Solution: A set cover for SEG, i.e., a subset S’ S such that every element in SEG belongs to at least one member of Θs for .

'Ss

Visibility – Machinability Analysis

50o 155o

228o 335o Axis of Rotation

Fig. 15 Machining result of a “Jack” model

(a) “Jack” model (b) Machined “Jack”

Non-machined regions

Non-machined regions Inch

Predicted vs. Measured Machinability on sample part

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CNC RP Methodology• Creation of complex parts using a series of thin layers (slices) of 3-axis

toolpaths generated at numerous orientations rotated about an axis of the part

• Toolpath planning based on “layering” methods used by other RP systems

• “Slice” represents visible cross-sectional area to be machined about (subtractive) rather than actual cross section to be deposited (additive)

• Slice thickness is the depth of cut for the 2½-D toolpaths

• Tool used is a flat end mill cutter with equal flute and shank diameter (or shank diameter < flute diameter)

• Stock material will be cylindrical, therefore toolpath z-zero location will be same for all orientations

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Flat end mill cutter

Methodology (cont.)

“Staircase” effect

Region not visible from current orientation

Set of visible slices from current orientation

Toolpath planning using this approach is done with ease in current CAM software (MasterCAM rough surface pocketing)

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Methodology (cont.)

• Fixturing accomplished through temporary feature(s) (cylinders) appended to the solid model prior to toolpath planning

• Cylinders attached to solid model along the axis of rotation

• Incrementally created during machining operation as the model is rotated

• Model remains secured to stock material then removed (similar to support structures in current RP methods)

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Fixture Planning• Approach uses “sacrificial supports” to retain the prototype within the stock material• Round stock clamped between opposing chucks• As prototype is rotated b/w toolpaths sacrificial supports are incrementally created• Supports cut away to remove finished part• Current approach assumes model surfaces exist along axis of rotation

– Only one fixture support cylinder used on each end– No change to visibility calculations

Problems:

Where do cylinders begin/end?

What diameter?

• Medical RP, one of the major territories for RP application– Manufacturing of dimensionally accurate physical models of

the human anatomy derived from medical image data using a variety of rapid prototyping (RP) technologies

– CNC-RP? • Typical bio/medical Material

– Titanium– Stainless steel– Cobalt alloy

• Advantage of Wire Electric Discharge Machining(WEDM)– Cut any electrical conductive material regardless hardness– Ignorable cutting force– Capable to produce complex part

Satisfy material requirement

Wire EDM Rapid Prototyping

Motivation(Con’d)

• WEDM is different from traditional machining process

Point contact

•Wire EDM

•Laser•Waterjet

Linear Surface

Motivation(Con’d)

• Visibility problems are different–“Can we see it” vs. “Can we access it using a

straight line”

Can we see it?

Tool orientation

Can we access it?

wire orientation

Can we make it?

How to make it?

(setup)

How to make it?

(Toolpath, NC code)

Wire EDM RP

• Investigate the manufacturability –Part Geometry–6-axis Wire EDM–Rigid machining part–No internal through features

• Find the B-axis orientation–Try to minimize number of B-axis orientation

Can we make it?

Wire EDM RP

How to make it?

(Toolpath, NC code)

• Toolpath generation–Discrete Toolpath for B-axis and other 5-axis–STEP-NC

• Fixture Design–Ignorable cutting force : Clamp part

How to make it?

(Toolpath, NC code)

Wire EDM RP

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RP versus CNC-RP and WireEDM-RP• RP processes are very flexible and very capable• However:

– RP processes rely on specialized materials– Limited accuracy in some cases

• CNC Machining is:– Subtractive process – Accurate– Capable of using many common manufacturing materials

• CNC Machining is NOT:– Automated– Easily usable except by highly skilled technicians

• CNC machining cannot create all parts• No hollow parts• No severely undercut features

• The time consuming tasks of process and fixture planning are major factors which prohibit CNC machining from being used as a Rapid Prototyping Process

– Wang et al, 1999

Functional prototypes?

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Conclusions

• For prototyping, the process is dominated by engineering cost– Product engineering, Process engineering, production engineering

• RP has come a long way– Usable products– Process and production engineering coasts are minimal

• Conventional methods are on their way back– CNC RP– Wire EDM RP

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Example- The “Jack”

1”

Material: 6061 AluminumTool: 1/8” Flat end millMachine: Haas VF-O, 3-Axis millLayer thickness: 0.005”Speed: 7500rpm, Feed: 350 ipm Machining time: 3 hours

Prototype after 2 of 4 rotations

Toolpath and Fixture planning time: < 15 minutes!

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Questions?!?

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