More than ever, it is critical that products are designed and manufactured right the first time. Design for Manufacturing (DFM) methodology has been recognized as one of the most effective ways to short product lifecycle time and reduce manufacturing cost. The main function of DFM in the detailed design stage is analyzing the manufacturability of the part. Various existing manufacturability evaluation methods have their limitations. In this paper, a new approach to DFM for the complicated parts is proposed. Instead of checking the manufacturability following the design, the in-process model resulting from the manufacturing process simulation is used to generate process dependent geometry surfaces at the design stage. The definition of the manufacturing process dependent geometry is given, and the methodology for creation of in-process model is presented in details.

1. Manufacturing Process Simulation Based Geometrical Design for
Complicated Parts
C.F. Zhu, P. Liu, W.F. Lu, X. Ding, Y. Lu
Singapore Institute of Manufacturing Technology, Singapore
1. INTRODUCTION
To satisfy ever increasing requirements of shorter
product lifecycle time, higher product quality and
lower product costs, it is critical that products are
designed and manufactured right at the first time.
Design for Manufacturing (DFM) methodology has
been recognized as one of the most effective ways to
improve product development process. Various
approaches to DFM have been adopted in industry.
Team approach is the most common practice in
companies with design and manufacturing
departments. It brings manufacturing engineers
working together with designers to assess
manufacturability. DFM manuals are also widely
used. The generic and company specific guidelines
are provided for designers to consider
manufacturability during the design process.
However, manufacturability is only considered
superficially in many cases, due to the high working
pressure, the limited availability of experienced
employees, and mainly the poor access to required
knowledge (van Vliet et al 1999).
With the emergence of CAD/CAM technology,
computer based DFM approaches have been paid
attention by both academia and industry. It is
expected to provide designers with manufacturability
analysis software tools that contain manufacturing
knowledge/expertise working in tandem with CAD
systems to give feedback on manufacturability as the
design evolves. In the last decade, numerous
researches have been carried out in searching for the
methods of analyzing the manufacturability of
product design, Zhao and Shah classified these
methods into three categories, i.e. heuristic,
analytical and plan-based (Zhao and Shah 2005).
In heuristic methods, rules are used to identify
infeasible design attributes that impose
manufacturability problems. Rules may include
general guidelines that are domain independent as
well as process specific. An example is the rule-
based design adviser for sheet metal fabrication (Yeh
et al 1996).
Analytical methods are based on the physics,
kinematics, or mechanics of the process. Moldflow
is a commercial design validation tool that predicts
whether an injection molded plastic parts will fill
based on geometry, injection location, type of
material and processing conditions
(http://www.moldflow.com/stp/).
In plan-based methods, process plans are
automatically generated for manufacturability
analysis. A great deal of work has been done in
automatic process planning using feature technology
and geometry reasoning (Shah et al 1990, Dissinger
ABSTRACT: A new approach to Design for Manufacturing (DFM) for the complicated parts is proposed in
this paper. Instead of checking the manufacturability following the design, the in-process model resulting
from the manufacturing process simulation is used to generate process dependent geometry surfaces at the
design stage. The definition of the manufacturing process dependent geometry is given. The extended Z map
representation of the in-process model is described and its application to manufacturing process simulation
based geometrical design for the complicated forging parts is presented.
Keywords: Design for Manufacturing; Process dependent geometry; Manufacturing process simulation

2. and Magrab 1995, Wang and Bourne, 1997,
Ramama and Rao 2002, Chen et al 2003).
Although many efforts have been made on the
manufacturability evaluation, these methods
inevitably have their limitations. The
manufacturability of a part cannot be guaranteed
even it if satisfies all the rules, let alone the rules are
very difficult to collect and generalize. Process
planning is still facing challenges in automation, as
it is highly domain specific and experience oriented.
Current analytical methods are not sufficient to the
complicated parts with process dependent geometry.
In this paper, a new approach to DFM for the
complicated parts – manufacturing process
simulation based geometrical design is proposed. In
Section 2, the definition of manufacturing process
dependent geometry is given, followed by the
discussion of the manufacturing process simulation
approach. Section 4 and 5 describe the extended Z
map representation of the in-process model and its
application to manufacturing process simulation
based geometrical design for the complicated
forging part. Finally conclusions are made and future
perspectives are given.
2. MANUFACTURING PROCESS DEPENDENT
GEOMETRY
From the pure design perspective, designers concern
only with the functional geometry (the faces where
parts meet and through which force and motion are
transmitted) and the connective geometry (the
surfaces that connect the functional geometry to
make complete solids) in the design process
(Stahovic 1995). However, they have to concern
with some kind of geometry from the manufacturing
perspective as well. The draft angle is an example to
plastics parts. Without the draft angle, the plastic
parts cannot be pull out of the injection mold cavity
during the fabrication process. In general, such kind
of geometry is for the purpose of manufacture and
usually constrained by the manufacturing processes.
For instance, sheet metal housings are quite different
from injection molded housings. This kind of
geometry is defined as manufacturing process
dependent geometry in this study. The complicated
parts discussed in this paper refer to the mechanical
parts with the manufacturing process dependent
geometry surfaces.
3. MANUFACTURING PROCESS
SIMULATION APPROACH
Parts are usually designed with DFM considerations.
Two-step design process is a common practice. The
first step, namely process independent design, is to
create functional and connective geometry. The
second step, process dependent design, is to create
manufacturing process dependent geometry. Process
independent design is common across many
different process domains. In this paper, forging is
taken as an example of manufacturing process.
Draft angle and machining allowance are two
types of manufacturing process dependent geometry
for forging parts. It is not an easy work to add draft
angle and machining allowance in 3D CAD models,
as offset feature only works for simple faces.
Conventionally, upon completing part design, a
prototype of tooling is designed and produced. The
try part is fabricated using the tooling (Figure 1a).
The try and modification iterations lead to long cycle
time and high manufacturing costs.
With the support of DFM tools, the
manufacturing analysis is conducted during the
design process. The process dependent geometry can
be modified before the prototype of tooling is
produced (Figure 1b). This approach can decrease
but not eliminate the try and modification iterations.
In the new approach, instead of checking
manufacturability following the geometrical design,
in-process model is used in this study to generate the
process dependent geometry (Figure 1c). The in-
process model represents the intermediate state of
the part in the machining process. It provides users
with a 3D geometry that reflects the results of
machining operations at each step, which allows the
user to visually verify that the machining operations
have been defined accurately and correctly.
In the research, the in-process model is created
through the manufacturing process simulation,
which virtualizes the trial production. By associating
machining information early with the part geometry,
this approach facilitates concurrent integration of
design and manufacturing with minimal risk.
Therefore the manufacturability of the parts can be
improved, and the development cycle time and
manufacturing cost can be reduced.

3. Figure 1 DFM in Design Process
4. EXTENDED Z MAP REPRESENTATION OF
IN-PROCESS MODEL
A variety of modeling methods is suggested by
researchers for the geometric simulation of milling
process (Lee and Ko 2002). Each solid modeling
method including CSG, B-rep and decomposition
has its disadvantages relative to the others in terms
of accuracy, robustness data structure and
computation time. Because of its simplicity in the
data structure and fast computation time, Z map
model is used by most commercial CAM software.
Z-maps are conventionally employed as a format
for representing objects. Data from the z-maps are
readily extracted for use as coordinates in numerical
control (NC) machines. A z-map uses a standard grid
format with a plurality of grid points equally spaced
apart to form a matrix. General discussion of
conventional z-maps can be found in (Choi 1991).
When using the z-maps, an increase in grid
resolution for defining finer details translates into an
increase in the quantity of grid points required.
Therefore, a presence of intricate features on a
portion of a surface of the object, which requires a
high grid resolution, results in a substantial increase
in the number of grid points required by the z-map.
Furthermore, the standard grid format of
conventional z-maps requires the surface of the
object to be segregated into at least two portions, for
example a top face and a bottom face, and
represented separately by multiple z-maps resulting
in high object management complexity.
A computer model generated from the
conventional z-maps further requires a high level of
processing power for rendering the computer model
when the computer model is displaced virtually in a
computing environment. The high level of
processing power results in a long processing time
when a user changes the orientation of the computer
model for viewing by the user via a graphical user
interface.
Hence, this clearly affirms a need for an object
representation model for addressing the foregoing
disadvantages of conventional z-maps used for
object representation.
In this study, at least one grid on a z-map is
segregated into sub-cells. Only grids corresponding
to intricate features on the surface of an object are
assigned sub-cells to improve representation of
object features. Figure 2a shows the front sectional
view, while Figure 2b illustrates the plan view of the
z-map grid with sub-cells 52.
The size of the grid can be reduced by sub-cells
to achieve greater precision of XY dimension, but it
is still limited by the size of sub-cells. For a sub-cell
of 0.1mm, the best precision is 0.1mm in XY plane.
Ok
Process independent design
Process dependent design
Produce a
prototype of the
tooling
Try
production
Modify
process
dependent
geometry
Ok
yes
no
Process independent design
Process dependent design
Produce a
prototype of the
tooling
Try
production
Modify
process
dependent
geometry
Ok
yes
no
yes
Checking manufacturability
no
Process independent design
Process dependent design
Produce a
prototype of the
tooling
Try
production
Modify
process
dependent
geometry
OK
yes
no
yes
Checking manufacturability
no
In-process model
(a) (b) (c
Ok
Manufacturing process simulation

4. The real world objects are not always uniform in XY
dimension and can be any shape. Nodes are used to
enhance sub-cell’s precisions in representation of
object face. For example, one edge of the sub-cell
may have two overlapping nodes to represent a
vertical face. The nodes of a sub-cell may not be
uniformly distributed over XY plane. Figure 2c
shows an exploded plan view of a portion of the z-
map grid with nodes 54.
Figure 2 Extended Z Map
In addition, a colour index is assigned to each grid
point on the z-map grid and stored in a reference list
containing cells corresponding to each grid point on
the z-map. A computer model of the object is pre-
rendered using the reference list onto a plurality of
display lists corresponding to different portions of
the computer model of the object. By recalling the
required display list for display when the computer
model is virtually displaced, the time lag for
displaying the computer model to a user is
substantially reduced.
The extended Z map representation method
comprising the steps of providing geometric data of
an object having a surface, the object constituting
one of a physical object and a virtual object, and the
geometric data being indicative of the surface of the
object; generating a reference plane having a z-axis
being generally perpendicular to the reference plane;
constructing a z-map grid, the z-map grid being
planar and coincident with the reference plane;
constructing a first z-map of a first portion of the
surface of the object, the first z-map being generated
with reference to the z-map grid; and constructing a
second z-map of a second portion of the surface of
the object, the second z-map being generated with
reference to the z-map grid.
33
The detailed description of the extended Z map
method can be found in two patents (Liu et al 2002).
5. IMPLEMENTATION
Traditionally the copy mill is commonly used in
forging die machining (Figure 3). To machine die
cavity, the physical model of a concept part is
constructed manually first and then used to control
the movement of the cutting tool by moving the
probe over its surface. The ball nose cutter with draft
angle is used for milling so that the draft angle is
created by cutter slant and the machining allowance
is created by cutter corner radius.
Figure 3 Copy mill machining die cavity
Concept
part
probe
Die cavity
42a/42b
50
34
22
36
26
38/40
3252
(a) 44
42a/42b
44
52 52 52
44
48
(b)
42a/42b44 52 52
54
54
54
(c)

5. Simulating the forging die machining process, the
in-process model of the part can be obtained by
sweeping a cutter along the face of the concept part
and getting the envelope of the cutter. The
simulation based geometrical design is performed in
the following procedures:
Build 3D concept part model
Create in-process model
Convert in-process model into part model
5.1 Concept part modeling
3D CAD modeling is a mature technology and a
number of commercial software is available such as
ACIS, SolidWorks, Pro-Engineer wherein B-rep
model is widely employed. In this research, Inventor
from AutoDesk is used for building the concept part
model. Figure 4 depicts the concept part model.
Figure 4 Concept Part Model
5.2. Creation of in-process model
The in-process model is created from the simulation
of the forging die machining process. An efficient,
stable and effective algorithm of swept volume
generation for machining simulation is developed
with the extended Z map method. Figure 5 illustrates
the in-process model.
The benefit of this method is simplicity. It
simplifies the world into dots and a few thousands of
lines of C++ code. Other model needs complex
topological structure and several million lines of
code. The simplicity also means stability and never-
fail Boolean operation.
The extended Z map representation is in fact a
multiple resolution display model. Using sub-cells
not only reduces computing memory, but also
reduces display time. The sub-cells comprise only
one tenth of the total grids, so the display speed
could be ten times faster. In addition, the color index
can be used to enhance the rendering performance.
Figure 5 In-process Model
5.3 Model conversion and reconstruction
Volume model based CAD is still in infancy. It is
required to convert the space discretion in-process
model into conventional B-rep model, which is the
cornel of commercial CAD systems. The extended Z
map model using the sub-cell simplifies this
conversion.
Figure 6 Finished Part Design
The Z map can be seen as a well-organized
polygon mesh, since the topological relationships are
clearly defined in the XY grid. It is very easy to
convert Z map model to a traditional B rep model.
But the model will be huge due to the double cost of

6. B-rep wing structure that needs a group of pointers
to link to the right-left faces of an edge. The
extended Z map model using sub-cells successfully
reduced the number of Z map data, so the resulted B-
rep data could ten times smaller than the
conventional way. To further convert swept envelop
faces into part features, techniques of solid model
reconstruction are required. There are many
researches in this topic. Roy and Xu’s (1999) work
on the model conversion and reconstruction
algorithms between polyhedral boundary models and
feature models can be used in our research. Many
commercial CAD systems already build this function,
such as FeatureWorks. The finished part design is
displayed in Figure 6.
6. CONCLUSIONS
In this paper, a new approach to DFM for the
complicated parts – manufacturing process
simulation based geometrical design is presented. It
has been shown that the approach is very effective
for the complicated parts with the process dependent
geometry such as forging and plastic parts.
The approach can also be applied to the complex
parts with sophisticated surfaces and Micro
Electronic Mechanical System (MEMS) parts.
Compared with traditional mechanical parts, MEMS
parts are much more process–dependent. It is not
possible to alter micro machining process to match
design geometry. The manufacturing process
simulation based geometric design approach can
play more critical role in future MEMS design.
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