SESAME is a software package that
combines the functions of computer-aided design (CAD), process planning
and manufacture for the production of engineering components. It handles
all stages from conceptual design through to the production of numerical
code for cutting machines. Advantages include a feature-based design
system which is more intuitive than traditional CAD programs, and
rapid feedback from process planning to design ('simultaneous engineering'),
to ensure that the final design can be manufactured without unnecessary
difficulty. Software based on SESAME modules is already being marketed
by the lead partner.
The manufacturing industry is moving away
from mass production towards customised manufacture of relatively
small batches of specialised goods. For companies to compete in
this fast-changing market, they will have to drastically reduce
their investment of time and money in product design and development.
The ideal solution would be a single computer system to handle all
processes from conceptual design through to the manufacture of the
In this project, a team of academic and industrial partners has
come close to realising this ideal. SESAME - Simultaneous Engineering
System for Applications in Mechanical Engineering - is a software
package that combines the functions of computer-aided design (CAD),
process planning (CAPP) and manufacture (CAM).
Traditionally, the design of a component is finished before it is
passed to the process planning department. If the part poses difficulties
in manufacture, it is sent back to be redesigned. In SESAME, the
manufacturing processes are planned as the design proceeds (hence
'simultaneous engineering'), so that problems can be solved as they
arise. As a result, the designer can be confident that the component
can be manufactured without problems.
SESAME consists of two main parts - a feature-based design system
and a CAPP-CAM system.
An engineering component can be thought of as built up from a number
of design features, each of which has a function. A feature may
be a slot, a drilled and threaded hole, a bearing seat, a flange,
and so on. Research by the SESAME team has focussed on features
which are so-called 'two-and-a-half-dimensional', that is, prismatic
or rotational in form and produced on milling or drilling machines
or lathes. In SESAME, the component is assumed to start out as a
block, or a casting, in which features are formed by cutting away
material. Each feature then corresponds to a volume to be removed
by one or more actions of the appropriate cutting machine.
Features are defined not only in terms of their size and shape,
but also their tolerances, surface finish and interdependency with
other features. As each feature is treated as a conceptual whole,
rather than a collection of lines and surfaces, it is relatively
simple to alter a design. Features can be freely moved, resized
or exchanged, and the program will modify its internal model to
accommodate the changes.
In contrast, conventional CAD programs represent components as assemblies
of simple geometrical shapes such as cubes, cylinders and cones.
It can be very difficult to alter the design, because these shapes
are purely descriptive and have no functional significance.
The designer builds a complete design for the component by selecting
and specifying features on the screen. SESAME contains a pre-defined
library of primitive features (such as slots and holes) and standard
engineering features (corresponding to ISO specifications, such
as threaded holes or circlip grooves). User-defined features which
are particular to the industry or the company (sheet metal features,
for example, are quite different to those in solid components),
may be easily created and added to the pre-defined library. The
designer can modify the design at will and experiment with different
solutions, viewing the result on the screen each time.
When the design is ready, the specification is passed to the CAPP-CAM
system. Here it may be processed by up to three different modules.
The most innovative is the automatic process planner. It first checks
if the design is realisable, by looking for geometric inconsistencies
such as intersecting features, obstructions, inaccessible voids,
disconnected parts, and so on. It also ensures that there is sufficient
room for the machine tools to move in and out. Next, each design
feature is assigned to one or more manufacturing features, building
up a network of all possible manufacturing processes. Each feature
has a corresponding microcycle, which is the sequence of machine
operations required to execute the feature. The microcycle chosen
depends on the specified dimensions, tolerance and surface finish,
as well as implicit considerations, such as the proximity of features
to each other. Any problems arising at this stage are automatically
referred back to the designer.
Once all possible processing paths have been mapped out, the best
solution is chosen by a genetic algorithm. In a process analogous
to biological evolution, a number of possible solutions are bred
together to produce offspring which are in turn used to produce
the next generation. At each stage the best solutions, defined by
lowest manufacturing cost, are selected for breeding. The result
is the most economical process for manufacturing the component.
If the user prefers, or if the automatic process planner is unable
to produce a complete plan, the interactive feature planner can
be brought into play. Here, the user constructs the manufacturing
plan by guiding SESAME in selecting manufacturing options where
more than one process is available.
If need be, a third module, the manual feature planner, can be used
for fine-tuning the manufacturing plan. It is essentially a traditional
CAM system, in which the process plan can be altered by the user.
Operations can be added, deleted or modified as required.
Finally, whichever module it comes from, the process plan is automatically
translated into numerical control (NC) code used to give explicit
instructions to the machine tools to make the component.
Ideal for small companies
SESAME is capable of supplying cost-effective solutions for one-off
designs or small numbers of products. It is ideal for small companies
who cannot afford a specialised CAPP department, and lack the resources
to capture manufacturing know-how with conventional software tools.
It should be particularly useful in the automotive and aircraft
industries and for the manufacture of machine tools.
The Italian machine-tool manufacturers Spring were the end-users
for SESAME, supplying specifications for features and testing the
complete system. The University of Edinburgh designed the automatic
process planner, while the University of Hannover developed the
interactive feature planner. Project manager Strässle produced
the feature-based design system and the NC system.
Although SESAME has been a success in its own right, the development
of a marketable system has been hampered by the loss of a crucial
software component which has been withdrawn by its US suppliers.
Strässle has, however, used the feature-based design system
of SESAME in their Feature M package (part of its Konsys 2000 suite),
with pre-defined libraries of primitive, functional and industry
standard features. In addition, the system enables the user to easily
define new features to supplement the pre-defined library. A sheet
metal working module is also available.