Conventional Methods for Rapid
Prototyping
Richard A. Wysk
Dopaco Distinguished Professor
Industrial and Systems Engineering
North Carolina State University
1
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
2
Broad focus -- Engineering
integration
ENGINEERING -- the planning, designing,
construction (manufacture), or management
of machinery, roads, bridges, etc..
3
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
What does this have to do with
Rapid Prototyping (RP)
•
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
Need for model
– Function
accuracy increases
•
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)
14
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
15
– Restricted in material, accuracy, and surface finish
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 adhesivecoated 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
So what’s the problem?
• Rapid Prototyping?
physical models
– 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
How can we automatically create toolpath and fixture plans
for CNC?
22
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
23
CNC Machining
Computer 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
This is the cost for all parts that will be made and sold.
Product cost /part = engineering cost / total #
of parts + materials cost / part +
manufacturing cost / part
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
26
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
27
Engineering cost
CE = Ced / nt + Cpc / nt + Cpd / nb
total parts
total parts parts in a batch
28
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
29
Manufacturing cost
CM = Cone / nt + Cset / nb + Cpsc +
Total parts
parts in a batch
each part
Cpdc / ntool
tool cost by parts/tool
30
Economic Analysis
• A general cost model
Eng
Costs
Materi
al
Costs
Manuf.
Costs
Planning
Tooling
Design
Process
Total
Cost
Subtractive
Process
Additive
Process
31
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.
End mill
Round stock
Axis of rotation
Table
Rotary indexer
Opposing
3-jaw chucks
STEPS TO CREATE A PART
( MT. Bike Suspension Component)
2. Second orientation is machined
(Side View)
1. First orientation of part section is machined 3. Third orientation is machined
Rotate Stock
2. Second orientation is machined
4. Fourth orientation is machined
STEPS TO CREATE A PART
( MT. Bike Suspension Component)
5. Left support section is machined
7. Temporary supports are removed
6. Right support section is machined
8. Part is severed from stock at supports
Process/fixture planning time: Minutes
Processing time ~20 hours
Part fixtured with final 2 sacrificial supports
Part fixtured with final 2 sacrificial supports
4”
Material: Steel
4”
Layer depth: 0.001” (0.025mm)
Finished Steel Part
Finished Steel Part
Setups/Orientation Planning
50o
155o
228o
50
o
155o
335o
228o
VISIBILITY
335o
MACHINABILITY
(for a given tool geometry)
Non-machinable
Non-machinable
Region
Region
•
•
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 inprocess
– Begin with round stock material, clamped between two opposing chucks
Example:
z
z
y
x
y
Toolpath layers at 0º orientation
z
y
Toolpath layers at 180º orientation
z
y
x
38
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?
39
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
z
y
x
CAD (ProE)
STL
“slicing”
operation
2-D cross
section
40
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
41
Determining the number of rotations
•
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
42
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
43
Solution approach
• 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.
Θb
Θc
Θb
Θa
Θa
(a) Visibility for the segment=
[Θa,Θb,]
Θd
(b) Visibility for the segment=
[Θa,Θb,], [Θc,Θd,]
44
Step one: Visibility with respect to own chain
•
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.
S1 : [ LCHP, Pi 1 ]
The set is divided into, S1 and S2 where:
S 2 : [ Pi 1 , RCHP ]
•
LCHP
RCHP
Pi+1
Pi
P:
S:
,
not
visible
RCHP
LCHP
Pi-1
Pi+1
Pi
45
•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:
V ( Pi)  [ Max( Pi X ), Min ( Pi Y ]
X S 2
Y S 1
V(Pi): [43.82 ,121.31]
V(Pi)
S1
Pi
S2
46
• 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.
(Vu  Vv )  [ RV u , LVu ]  [ RV v , LVv ]  [ RV v , LVu ]
RVv
LVv LVu
RVu
u-1
vu
v+1
u
uv
v
•The sets S1 and S2 are redefined:
S1 : [ LCHP (u ), (u  1)]
S 2 : [(v  1), RCHP (v)]
•The ends of the visibility range are: RV (uv)  [ Max(vx)]
xS 2
LV (uv)  [Min (uy)]
yS1
47
Problem Surfaces
LV
I1
u v I2
u v I2
RV
(a)
RV
I1
LV
(b)
RV
LV
RV
LV
I2
I1
u
(c)
I2
I1
v
u
v
(d)
(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.
48
Step two: Visibility blocked by all other chains on the slice
•
V( uv)j* is the visibility with respect to the chain j on which
denoted j*.
•
For all obstacle chains
denoted VB( uv )j.
•
The set of visible ranges for the segment is defined:
j  J \  j *
uv
resides,
, the polar range blocked by the chain is
VIS (uv)  V (uv) j*  VB(uv) j
•
Visibility blocked to the segment uv 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 uv
•
The set of angles blocked to the segment where:
VB(uv) j  {[[VB(u ) j ]  [VB(v) j ]]  [uv, vu]}
•
The set of angles blocked to points u and v where:
VB(u ) j  [ RB u , LBu ]
VB(v) j  [ RB v , LBv ]
49
• 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:
(VBu  VBv )  [ RB u , LBu ]  [ RB v , LBv ]  [ RB u , LBv ]
LBu
RBv
LBv
RBu
• 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.
RBu  [ Min (ux)] LBv  [ Max(vy)]
xP
j
uv
yPj
50
Recall: VIS (uv)  V (uv) j* 
VB(uv)
j
•For each segment the collection of visible ranges given in polar angle about the
axis of rotation:
VIS tjk : [ a ,  b , ]1 , [ a ,  b , ] 2 ,...[ a ,  b , ]r
where: rMAX = n
•From the data in [VIS] we can formulate a set corresponding to the segments visible
from a given angle.
 s  {( SEGtjk ) ( a   s   b ) for some range, [ a ,  b ] r  VIS tjk }
VIS1,1,1 (Θa,Θb)1, (Θa,Θb)2, …(Θa,Θb)n
VIS2,1,1 (Θa,Θb)1, (Θa,Θb)2, …(Θa,Θb)n
Θ1
Θ2
SEG1,1,1, SEG2,1,1, SEG1,5,3…
(Θa,Θb)1, (Θa,Θb)2, …(Θa,Θb)n
Θs
SEG
tjk. . . .
.
.
.
.
VIStjk
.
.
.
.
.
.
.
.
.
VISqnp (Θa,Θb)1, (Θa,Θb)2, …(Θa,Θb)n
.
.
.
.
.
.
Θ359
SEG13,1,2, SEG14,1,2, …
.
.
.
.
.
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 s  S ' .
51
l
Visibility – Machinability Analysis
Non-machined regions
Inch
Non-machined regions
50o
Predicted vs. Measured Machinability on sample
part
335o
(b) Machined “Jack”
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
53
Methodology (cont.)
Flat end mill cutter
“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)
54
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)
55
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?
56
Wire EDM Rapid Prototyping
• 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
• WEDM is different from traditional
machining process
Point contact
Linear
Surface
• Wire EDM
• Laser
• Waterjet
• Visibility problems are different
Can we see it?
Can we access it?
Tool orientation
wire orientation
– “Can we see it” vs. “Can we access it using a
straight line”
Wire EDM RP
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
Can we make it?
How to make it?
(Toolpath, NC code)
• Find the B-axis orientation
– Try to minimize number of B-axis orientation
Wire EDM RP
• Toolpath generation
How to make it?
(Toolpath, NC code)
– Discrete Toolpath for B-axis and other 5-axis
– STEP-NC
• Fixture Design
– Ignorable cutting force : Clamp part
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
Functional prototypes?
•
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
•
63
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
64
Example- The “Jack”
Material: 6061 Aluminum
Tool: 1/8” Flat end mill
Machine: Haas VF-O, 3-Axis mill
Layer thickness: 0.005”
Speed: 7500rpm, Feed: 350 ipm
Machining time: 3 hours
Prototype after
2 of 4 rotations
1”
Toolpath and Fixture planning time: < 15 minutes!
65
66
Questions?!?
68
69
70