Segmentation of Dimensionally-Large Rapid
Prototyping Objects
Y. Tang, H. -T. Loh, J. -Y. -H. Fuh, Y. -S. Wong, S. -H. Lee
AbstractAn algorithm was developed to enable efficient
segmentation of dimensionally-large objects into smaller
components that can be fabricated within the given Rapid
Prototyping (RP) machine workspace. The algorithm uses
vertical and horizontal flat plane cuts, as well as feature -based
volume decomposition. Due considerations were given to the
optimisation of the surface accuracy, the build time, the strength
and the number of segments generated by the segmentation
process. A computer-aided design (CAD) application programme
that interfaces with Unigraphics (UG) was also developed to
allow import of objects in Standard Triangulated Language
(STL) files into UG without loss of accuracy. In addition, the
application software provides the functions that facilitate the
implementation of the segmentation algorithm in UG. Two case
studies were carried out using the algorithm in a Selective Laser
Sintering (SLS) RP system. The resulting objects had properties
that matched the research objectives with which the proposed
algorithm was validated.
Index termsComputer-aided Design (CAD), Rapid
Prototyping (RP), Selective Laser Sintering (SLS), Segmentation
I. INSTRODUCTION
C
ONVENTIONAL Rapid Prototyping (RP) processes, e.g.
Stereolithography (SLA), Selective Laser Sintering
(SLS), 3-Dimensional Printing (3DP), and Fused Deposition
Modelling (FDM), etc., enable the fabrication of an object
through a fully additive process of building thin layers of the
object. When the object is too large to fit within the RP
machine workspace, the conventional way is to fabricate the
object in a bigger RP machine with a larger workspace.
Manuscript received October 30, 2002. This work was jointly supported
by Singapore-MIT Alliance and Department of Mechanical Engineering,
National University of Singapore.
Y. Tang is with Singapore-MIT Alliance, National University of
Singapore, E4-04-10, 4 Engineering Drive 3, Singapore 117576 (phone: 6568744857; fax: 65-67795922; e-mail: smatyx@nus.edu.sg).
H. -T. Loh is with the Mechanical Engineering Department, National
University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 (e-mail:
mpelht@nus.edu.sg).
J. -Y. –H. Fuh is with the Mechanical Engineering Department, National
University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 (e-mail:
mpefyh@nus.edu.sg).
Y. -S. Wong is with the Mechanical Engineering Department, National
University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 (e-mail:
mpewys@nus.edu.sg).
S. -H. Lee was a final year student of 2002 with the Mechanical
Engineering Department, National University of Singapore, 10 Kent Ridge
Crescent, Singapore 119260.
But having a larger workspace is not always feasible, because
machine cost escalates exponentially with the increasing
workspace dimension, and the workspace can only be
increased up to a certain size due to technical limitations.
Alternatively, the object can be segmented into smaller
components that are then fabricated in the existing RP
machine, following which they are reassembled back into a
whole object. Personnel experienced in RP area currently
performs this type of object segmentation manually and on an
ad hoc basis. But such operations are not always reliable and
can be very complicated and time -consuming when the object
has complex features. Hence it is necessary to develop a more
rigorous and algorithmic framework so that the process can be
easily carried out and not be so dependent on the skill and
knowledge of the experienced operator.
Segmentation algorithms for large objects have been
developed in certain newer RP systems by Horváth et al [1]
and Beascoechea-Gangoiti et al [2]. But such algorithms are
not directly applicable to conventional RP systems due to
fundamental differences between the conventional and the
newer RP systems. For conventional RP systems, Ilinkin et al
[3] developed a segmentation algorithm with an optimal
decomposition of both convex and non-convex objects using a
single horizontal flat cutting plane, with considerations for
minimisation of support volume for the SLA process and the
ability to control the number of decomposed parts. However,
the algorithm does not discuss about larger objects that require
more than one cutting plane, or cutting planes in other
directions for the purpose of achieving the desired segment
size. Qian et al [4,5] developed a feature-based segmentation
method such that each feature can be fabricated in its optimal
orientation. This allows fabrication of parts with a higher
precision, higher quality and in less time. This work also
introduces the interaction-free volume decomposition
algorithm, which enables elimination of staircase interaction
effects between adjacent feature components. Similar work
was developed by Wang et al [6], in which a big or complex
CAD model is decomposed into several simpler or smaller
components according to their geometrical features. The
components are then optimised to their individual orientations,
and packed optimally within the RP workspace. Although
these feature-based decompositions are applicable to
decomposing a large object and thus enabling its fabrication, it
is observed that pure decomposition by features may generate
components with thin walls that are fragile, and corners with
very acute angles that are difficult to build. Also, there is no
guarantee that the object segment will fit within the RP
workspace. Another research work developed by Chang and
Prinz et al [7] uses the layer decomposition principles for a
new additive/subtractive RP technology called Shape
Decomposition Machining (SDM), in which an object is
fabricated through the additive deposition of materials for
free-form layers of the object with undercuts, and the
machining of the freeform layers without undercuts. The
object segmentation in this case can generate different freeform layers consisting of entirely undercut surfaces or nonundercut surfaces. Surface geometry, particularly the undercut
and non-undercut surfaces, are used to determine the parting
surfaces for each layer, hence allowing for free-form parting
surfaces instead of only flat planar parting surfaces. Though
this freeform object decomposition methodology is specific
for SDM only, some relevant insights about object
segmentation can still be obtained. Another relevant aspect
about object segmentation is the optimisation of object
orientation for fabrication in RP systems.
This paper develops a segmentation algorithm that
combines the advantages of both feature volume
decomposition method and flat planar parting surfaces
method, while reducing or eliminating their disadvantages.
The segmentation will generate a minimum number of object
segments, produce an object with the optimal surface and
dimensional accuracy, and require minimum build time. The
segmentation algorithm should be robust enough for
application to different conventional RP systems, with a
minimal amount of customization needed. The segmentation
should not generate new thin segments or walls that are
structurally weak, and should allow for easy location of
segments during assembly of segments into the object. The
final assembled object should also have good overall strength
and dimensional accuracy, especially at the parting surfaces of
the segments. The implementation of the algorithm will be
carried out using a customized CAD application program,
which will allow import, manipulation and export of Standard
Triangulated Language (STL) object file. The CAD
application should also provide functions to facilitate
interactive segmentation of the large object with the proposed
algorithm. Two case studies will then be performed to
demonstrate and study the feasibility of the proposed
algorithm.
II. SEGMENTATION ALGORITHM
To enable detailed formulation of the algorithm, this
novel algorithm was developed based on a particular
conventional RP process, namely the SLS process. The
algorithm consists of the following eight steps: Feature,
Union, Isolation, Orientation, Feature Interaction Volume
(FIV), X, Y, Z-cuts, Recombination, and Packing.
A. Feature
Under a CAD environment, decompose the object model
into separate and distinct feature components, using the
existing boundary lines between features as the parting loops
of the parting surface as much as possible. After feature
decomposition, identify critical surfaces that have maximum
possible surface accuracy. Examples of critical surfaces are
cylindrical and conic surfaces. Other examples of critical
surfaces are curved surfaces that require maximal surface
accuracy, as part of the surfaces’ functional requirements.
Following this, identify the feature components with critical
surfaces as critical surface features (Fc), and identify the
remaining feature components, which have non-critical
surfaces, as non-critical surface features (Fnc). Performance of
this feature step will result in a feature decomposition of the
object, and the identification of features with maximal surface
accuracy requirement. Corresponding parting surfaces will be
generated too.
B. Union
For each parting surface on the critical surface of an Fc,
check whether the corresponding parting surface belongs to the
Fnc. If it happens, union the Fc and Fnc with this parting
surface. On the other hand, if the corresponding parting
surface belongs to another Fc, do not union these two Fc but
take a note of this parting surface. The rest of the Fnc that are
not connected to any Fc will remain as separate feature
components.
C. Isolation
For each Fc that has an adjacent Fnc united to it, isolate
and separate this Fc from the adjacent Fnc again using flat
cutting planes on the body of each Fnc, ensuring that each Fc
shall still remain whole and uncut, as well as the cuts are only
performed on the body of each attached Fnc. These cuts will
produce modified Fc with portions of the previously attached
Fnc, as well as modified Fnc with a new shape. The selection
of flat cutting planes shall follow the rules that cut planes are
normal to the z, y or x axes and with a minimum portion of
Fnc left to attach Fc. Then, unite the adjacent Fnc together at
the corresponding parting surface of each pair of adjacent Fnc.
All the Fnc that can be united together will form a new and
larger Fnc component. All Fnc that cannot be united with
other Fnc will remain as a separate Fnc component. All
original and modified Fc will remain unchanged during the
union of the Fnc components.
D. Orientation
For each original or modified Fc component, orient it to
the maximum build accuracy orientation. For each of the
remaining Fnc components, orient it to the best build time or
build accuracy orientation. Finally individually rotate each Fc
and Fnc components about the z-axis such that the longest
horizontal dimension of each component is aligned along the
workspace x-axis.
E. Feature Interaction Volume (FIV)
For each Fc or Fnc, if the surface normal at any point on
the parting surface is neither perpendicular nor parallel to the z
build direction, staircase interaction will be resulted because of
the layered manufacturing mechanism of RP. Hence, a Feature
Interaction Volume (FIV) is needed for the corresponding two
components. The FIV is a solid connection between features
generated from the parting surface. All the planar, uniformly
curved or freeform parting surfaces on all the components
should be checked for necessity of FIV formation. The
method of generating a FIV is projecting the original paring
surface to the bottom surface of each feature, then cut the
object to the smallest volume. After all required FIV has been
formed, orient each FIV to the orientation with the normal of
parting surface perpendicular or parallel to the z build
direction.
F. X, Y, Z-cuts
As a result of above feature-based segmentation, a set of
components of Fc, Fnc or FIV will be produced. But these
components may be larger than the RP working space. Hence
all components should be further segmented by X, Y, Z cuts
to ensure that all sub-components after the segmentation are
fit within the size limitation of the RP machine in X, Y, Z
direction.
G. Recombination
For each pair of corresponding parting surfaces to all
sub-components, if the surface normal of all points on both
parting surfaces are all perpendicular or parallel to the z build
direction, label the pair of parting surfaces as a Possible
Recombination Surface (PRS) pair. Then selectively
recombine sub-components of the PRS pair such that the
resulting object segments can still fit within the RP
workspace, and the overall number of object segment is
minimised. FIV can also be recombined if above conditions
are met.
H. Packing
Optimally pack all sub components into one RP
workspace boundary to prepare for fabrication.
and Fnc identification), union, orientation, isolation, XYZcutting, recombination, and packing are performed for this
object segmentation. FIV is not necessary here because there is
no staircase interaction between any two segments.
Optimisations of the object requirements are also considered
based on the surface accuracy, build time, part strength and the
number of segments throughout the segmentation process. The
segmentation process of case study 2 is shown in Fig. 2
(a)∼(f). Unlike case study 1, case study 2 has no Fnc at all.
Hence there is no union step for this case. But it is necessary to
generate FIV here because for some parting surfaces, the
staircases are unavoidable.
After segmentation, both the case study 1 and 2 were
implemented in a SLS system (shown in Fig. 3), i.e., all
segmented and packed components were sintered by the
selective laser sintering machine using a plastic material. Then
the built components were assembled to the original large
objects. It is observed that the final assembled objects have a
good overall accuracy, a short build time and a high strength.
V. CONCLUTION
A novel algorithm of large-object segmentation for the SLS
RP system has been successfully developed. The algorithm
combines two segmentation methodologies: flat plane
segmentation and feature-based segmentation. The algorithm
has successfully utilised the advantages of these two
methodologies while minimising their inherent disadvantages.
With the self-developed CAD application RP-T, the algorithm
has been successfully implemented in UG CAD system based
on the two case study objects. The two objects were then
successfully fabricated using an in-house SLS system. The
properties of these two objects were acceptable and met the
user’s requirements and expectations, and hence showing that
the segmentation algorithm is practically feasible.
III. CAD APPLICATION
A CAD system, called Rapid Prototyping Toolkit (RPT), has been developed to facilitate object segmentation using
the proposed algorithm. RP-T was developed on a HP
Visualise 2000 graphical workstation with HP-UX operating
system, and interfaces with Unigraphics (UG) version 15.0.3.
RP-T consists of the following two modules: STL import
module and Interactive Object Segmentation Assistance
Module (IOSAM). The STL import module imports a STL
file into UG using boundary representation (B-Rep), without
loss of accuracy during the import. This allows for
manipulation of the STL object in UG environment. The
IOSAM is to provide functions to perform the object
segmentation process using the algorithm described as above.
IV. CASE STUDY
Two case studies were conducted to testify the
segmentation algorithm. The segmentation process of case
study 1 is shown in Fig. 1 (a)∼(k), where feature analysis (Fc
REFERENCES
[1] Horváth, I., Broek, J. J., Rusák, Z., Kuczogi, G., Vergeest, J. S. M.,
“Morphological Segmentation of Objects for Thick-layered
Manufacturing”, Proceedings of 1999 ASME Design for Manufacturing
Conference, Las Vegas, CA, September 1999, pp. 18-24
[2] Beascoechea-Gangoiti, A., 3D packing for solid freeform fabrication: A
heuristic approach, Ph.D. thesis, Rensselaer Polytechnic Institute, Troy,
NY, 1996
[3] Ilinkin, I., Janardan, R., Majhi, J., Schwerdt, J., Smid, M., Sriram, R., “A
Decomposition-Based Approach to Layered Manufacturing”, Proceedings
of the Seventh International Workshop on Algorithms and Data
Structures, Providence, RI, 2001, pp. 211-216
[4] Qian, X., Dutta, D., “Feature based fabrication in layered manufacturing”,
Transactions of the ASME, Journal of Mechanical Design, Vol.123, No.
3, pp. 337-345, 2001
[5] Qian, X., Dutta, D., “Features in the Manufacturing of Heterogeneous
objects,” Proceedings of the Solid Freeform Fabrication Symposium,
University of Texas at Austin, Austin, Texas, 1999, pp. 689-696
[6] Wang, W. L., Fuh, J. Y. H., Wong, Y. S., Miyazawa, T., “Make-up
Fabrication of Big or Complex Parts using the SLA Process”, Proceedings
of the Solid Freeform Fabrication Symposium, University of Texas at
Austin, Austin, Texas, 1996, pp. 299-306
[7] Chang, Y. C., Pinilla, J. M., Kao, J. H., Dong, J., Ramaswami, K., Prinz,
F. B., “Automated Layer Decomposition for Additive/Subtractive Solid
Freeform Fabrication”, Proceedings of the Solid Freeform Fabrication
Symposium, University of Texas at Austin,
Austin, Texas, 1999, pp 111-120
United Fc and Fnc
FIGURES
United Fc and Fnc
(c)
Isolated modified Fc
Horizontal
planar cut
(a)
Vertical
planar cut
(d)
Critical
surface
Fc
Critical
Fnc surface
Fnc
(b)
United adjacent Fnc
(e)
Oriented Fnc
Oriented Fc
Build
direction
X-cut
(i)
(f)
Z-cut
Z-cut
Recombined
components
Z-cut
(j)
(g)
No y-cut needed
RP machine
workspace boundary
Build
direction
Z
(k)
(h)
Fig. 1 Segmentation process of case study 1: (a) Original object, (b) Two Fc
identified. Rest of features is Fnc, (c) Fc united with neighboring Fnc, (d)
Isolate all Fc from Fnc with planar cuts, (e) Union all Fnc, (f) Orient Fc to
required optimal orientation. Orient Fnc for optimal build time/accuracy, (g)
Do z-cuts for each component. Three z-cuts are made, (h) Do y-cuts on each
sub-component. No y-cut is needed, (i) Do x-cuts on each sub-component.
One x-cut is needed, (j) Perform re-combination of sub components where
possible, (k) All sub-components reoriented as necessary and packed into RP
machine workspace
Fc
FIV
Fc
FIV
Fc
Fc
FIV
Fc
FIV
(d)
(a)
All Fc
Fc and
adjacent
Fc do not
unite
Z
Z-cut
Parting
surface
joined to
Fc on
two
sides
Build
direction
(e)
All Fc
(b)
Z
FIV
Build
direction
(f)
FIV
(c)
Fig. 2 Segmentation process for case study 2 (a) Original object, (b) Feature
identification. All features are Fc. Skip Fc union with Fnc, since no Fnc exist.
Skip isolation of Fc, since all Fc are already isolated, (c) Design FIV. Four
pairs of corresponding parting surfaces joins to Fc on both sides; four FIV are
required, (d) Orient all Fc and FIV to required build orientation. Skip
orientation of Fnc as there is no Fnc, (e) Perform z-cuts. Y and x-cuts are
skipped as they are not needed, (f) Recombination not needed. Finally pack
and orient sub-components
(a)
(d)
Fig. 3 Implementation of Case Study 1 and 2: (a) Segmented and SLS Built
Components of Case Study 1, (b) Segmented and SLS Built Components of
Case Study 2, (c) Final Assembled Object of Case Study 1, (d) Final
Assembled Object of Case Study 2
(b)
(c)