Transcript of CNC RP Presentation
Numerical Controlled Machining
– Reduce time to market
– Early detection of errors
– Assist concurrent manufacturing engineering
– 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
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 A pparatus.
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
technology was developed
by S. Scott Crump in the late 1980s and was commercialized in
1990. 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.
(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 .
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Electron Beam Melting (EBM) • Electron Beam Melting (EBM) is a type
of rapid
prototyping for metal parts. It is often classified as a
rapid manufacturing method. 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.
Stereolithography (SLA) photopolymer
Laminated Object Manufacturing
3D Printing (3DP) Various materials
number of parts
fixture, retrieve tooling , etc.)
for feature operation j (chuck, fixture, etc..)
the machining/processing time for feature j
tool change time/part
L/UL
setup + t
j + t
j + t
Production time per piece
as:
Production cost per piece, C p
Ct is the perishable tooling cost
np/t is the number of pieces that can be produced per tool
Csetup is the setup resource cost for the part
(fixture, jig, steady-rest, etc)
• 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
for CNC?
CE = Ced / nt + C pc / nt + C pd / n b
total parts total parts parts in a batch
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– Fixture engineering and fabrication
• Set up cost (Cset) – Cost to set up a process
• Processing cost (C psc) – Cost of processing a
part
• Production cost (C pdc) – Cost of tooling and
perishables
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Manufacturing cost
CM = Cone / nt + Cset / n b + C psc + C pdc //
ntool
Total parts parts in a batch each part tool cost by
parts/tool
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reduced for CNC machining?
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• 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.
Table Opposing
3-jaw chucks
Rotary indexer
Round stock
End mill
1. First orientation of part section is machined
(Side View)
Rotate StockRotate Stock
7. Temporary supports are removed
7. Temporary supports are removed
from each of a set of orientations using
layer-based toolpaths
The number of rotations
required to machine a
geometric complexity
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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)
current orientation
Toolpath planning using this approach is done with ease in current
CAM
software (MasterCAM rough surface pocketing )
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)
– 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
– Support overhanging features
Model material
Support material
Build Platform
• However:
– Limited accuracy in some cases
• CNC Machining is:
• CNC Machining is NOT:
• 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
– Machined layers using robotic arm/machine tool
– Layers laminated in a stack
• Merz, et al, 1994
– Rapid tooling
– Process planning is simplified by layer-based
approach
– Fixtures are created in process
• The approach to CNC-RP will have to relax many of the traditional
constraints
– Efficient machining is not a major driver
(Traditional feeds/speeds not used)
– Not feature-based (Not necessary to machine
entire feature in one setup orientation)
– Surface finish not as critical (Allow staircase
effect)
• Goal of this research is to develop a method for CNC rapid
prototyping such that:
– Toolpath planning, sequencing, tool sizing is
automated
– Fixture design is created in-process, flexible, and
allows access to almost all
surfaces
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
y
z
y
z
• Setup/Orientation
– How many rotations (setup orientations) about the
axis of rotation are required?
– Where are they?
– What diameter and length tools should be used?
– In what order should the toolpaths be executed?
• Fixture planning
• 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
from one tool access direction
Determining the number of rotations
• Shortest Euclidean paths - Lee and Preparata, 1984
• Convex ropes - Peshkin and Sanderson, 1986
• 2D visibility cones - Stewart, 1999
Issues:
need to add collinear points to
polygon segments
[Θa,Θb,]
[Θa,Θb,], [Θc,Θd,]
• 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
• We have a polygon P and its convex hull S
• For any point P i 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 P i, LCHP ( P i) and
RCHP ( P i), respectively.
• It is only necessary to calculate the polar angles
from P i to the points in the set
[ LCHP ,
RCHP ], excluding P i.
• The set is divided into, S1 and S2 where: ],[:2
],[:1
1
1
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•The visible range for a point is bounded by the minimum polar
angle from P i to points in S1 and the maximum polar
angle from P i to points in S2.
•This is the visible range for the point P i with respect
to the boundary of its
own chain, and is denoted V(P i ).
Where:
](),([)( 12
Y P Min X P Max PiV
i S Y
i S X
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• Consider the segment defined by points in P, u and v,
where:
u: P i and v: P i+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.
RV v
)](),1[(:2
)]([)( 1
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 .
I 2
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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
in the 180º range above the segment,
it can easily be seen that the set:
],[],[],[)( vuvvuuvu
LB RB LB RB LB RBVBVB
• 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 P j , where P j is the
set of points for the blocker chain.
)]([ ux Min RB j P x
u )]([ vy Max LB j P y
v
j j uvVBuvV uvVIS )()()( * Recall:
•For each segment the collection of visible ranges given in polar
angle about the
axis of rotation:
r bababatjk VIS ],,,...[],,[,],,[: 21 where:
r MAX = n
•From the data in [VIS ] we can formulate a set corresponding
to the segments visible
from a given angle.
.
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The Minimum Set Cover problem:
Given: A collection of subsets Θ s 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 Θ s for .'S s
320º
49º140º
228º
C.H.
A.C.
Facets
S lice ( in ) #sgmts
time( s ) #sgmts time( s )
#sgmts time( s ) #sgmts
time( s ) #sgmts time( s )
0.0025 19,566 22.750 27,285
25.812 36,199 29.390 49,975
36.623 69,212 47.122
0.0050 9,772 11.230 13,553 12.875
18,178 14.671 25,044 18.640
34,458 23.389
0.0100 4,850 5.687 6,781 6.515
9,054 7.405 12,476 9.297
17,306 11.843
0.0200 2,375 2.875 3,409 3.312
4,597 3.907 6,269 4.859
8,683 6.281
0.0400 1,182 1.453 1,655
1.718 2,159 2.032 2,974 2.453
4,123 3.141
1990 3686
STL Resolution
• Set cover problem solved as integer linear program using
LINDO:
The “Jack”…
– Machine visible surfaces from approach
direction
– 2½-D pocketing, easily generated using current CAM
software (MasterCAM, rough surface pocketing )
– A gouge-free approach, given flute and shank diameter
are same (or shank < flute)
– Investigated as a rough machining approach -
Balasubramanium, 1999
• Can approach finish machining using very small depths of
cut
• We assume that tool length, not diameter will be active
constraint
– To avoid collision, tool length > maximum swept
diameter of part (Same as stock diameter)
Toolpath Planning
• Stock diameter/Tool length can be found from slice data used in
VISI algorithm
– For each slice, find diameter of the set of
points
– Set stock diameter to MAX
– Ds = MAXDIAM(CHP(slice points)) for all slices
k
– Set tool length to diameter of the stock Lt =
Ds
• Toolpath sequencing is a significant problem
– Need to avoid “thin web” conditions
– Can occur during one toolpath or from successive
toolpaths
Depth of cut(max) = -Ds
Where Ds= Stock Diameter
• For each successive toolpath
planned in sequence, undesirable
orientations to be avoided:
• Preparatory toolpath sequence to avoid thin material
conditions
• Removes bulk of stock material prior to processing remainder of
toolpaths
• Choose from orientations in the solution set, or add new
Model
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:
• Start/end of cylinder
– Need to have room for tool diameter to pass b/w
end of part and stock
– Cylinder end protruding into the part must be fully
“embedded”
• Use slice geometry to calculate depth of penetration where
cylinder is fully attached
Part length
Pd ? Lf
Fixture Planning • Determine first slice where fixture cylinder
diameter is contained within the boundary
chain of the part ( Circle with center at axis of rotation )
Slice k=1 (0.005”) Slice k=1 (0.010”) Slice k=1 (0.015”)
Part slice boundary
Fixture cylinder diameter
– Cylinders must limit deflection (torsion) caused by
machining forces
• Approach
– Negligible bending
– Model as a statically indeterminate torsional
shaft
Ft
Thrust force
– Ensures collision avoidance Dh
c = L p + 2a + 2b + 2Lf
Layer thickness: 0.005”
Machining time: 3 hours
• 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?
– Cut any electrical conductive material regardless
hardness
– Ignorable cutting force
machining process
Point contact
• Wire EDM
• Visibility problems are different
– “Can we see it” vs. “Can we access it using a
straight line”
– STEP-NC
– Layer thickness 0.005”
– Process time ~3hours
320º
49º140º
228º
cost
– Usable products
– CNC RP
Conclusions -- continued
• The methods developed (CNC-RP and Wire EDM – RP)
represent a deliberate approach at making CNC machining usable by
engineers and designers, not just machinists
• Capable of producing fully functional prototypes in the
appropriate material
• Wide spread availability of CNC machines provides fast, low-cost
integration to current product design processes
• Quick changeover from RP to Production setup will enable higher
utilization of machines
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References:
• Wang, F.C., L. Marchetti, P.K. Wright, “Rapid Prototyping Using
Machining”, SME Technical
Paper, PE99-118, 1999
• Chen, Y.H., Song, Y., “The development of a layer based machining
system”, Computer Aided
Design, Vol. 33, pp. 331-342, 2001
• Merz, R., Prinz, F.B., Ramaswami, K., Terk, M., Weiss, L.E.,
“Shape Deposition Manufacturing”,
Proceedings of the Solid Freeform Fabrication Symposium, University
of Texas at Austin, pp. 1-8,
1994
• Walczyk, D.F., Hardt, D.E., “Rapid tooling for sheet metal
forming using profiled edge laminations-
design principles and demonstration”, Journal of Manufacturing
Science and Engineering,
Transactions of the ASME, Vol. 120, No. 2, pp. 746-754, November
1998
• Vouzelaud, F.A., Bagchi, A. & Sferro, P.F., (1992), Adaptive
Laminated Machining for Prototyping
of Dies and Molds, Proceedings of the 3rd Solid Freeform
Fabrication Symposium, pp. 291-300,
August 1992
• Lennings, L., “Selecting Either Layered manufacturing or CNC
machining to build your prototype”,
SME Technical Paper, Rapid Prototyping Association, PE00-171,
2000
• Peshkin, M.A., Sanderson, A.C., “Reachable Grasps on a Polygon:
The Convex Rope Algorithm”,
IEEE Journal of Robotics and Automation, Vol. RA-2, No. 1, March
1986
• Lee, D. T., Preparata, F. P., "Euclidean Shortest Paths in the
Presence of rectilinear Barriers",
Networks, Vol. 14, No. 3, pp. 393-410, 1984.
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Computers and Graphics, Vol. 23, No. 5,
pp. 693-702, 1999
• Balasubramaniam, M., “Tool Selection and Path Planning for 3-Axis
Rough Cutting”, Thesis,
Department of Mechanical Engineering, The Massachusetts Institute
of Technology, June 1999
• Tang, K., Woo, T.C., Gan, J., “Maximum Intersection of Spherical
Polygons and Workpiece
Orientation for 4- and 5-Axis Machining”, Journal of Mechanical
Design, Vol. 114, pp. 477-485,
September 1992