ComputerAided_Design_Engineering_amp_Manufactur.pdf
ComputerAided_Design_Engineering_amp_Manufactur.pdf
ComputerAided_Design_Engineering_amp_Manufactur.pdf
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Product Model Representation for Computer-Aided Measurement<br />
The product design representation for a part must carry enough information and be structured such<br />
that it is amenable for computer interpretation for automated inspection planning. Three levels of<br />
information are required:<br />
• Geometrical information pertaining to shape, volume, and surfaces. Such information is required<br />
for visualization of the part model, extraction of surface data for measurement, accessibility<br />
analysis of contact probes, and path planning for inspection on the CMM.<br />
• Measurement evaluation as defined by the GD&T specifications.<br />
• High-level engineering information pertaining to shape or region on the part. Usually such regions<br />
are linked to specific manufacturing or measurement processes.<br />
It is quite established at this stage that a solid modeling scheme is capable of providing complete and<br />
unambiguous representation for the above purpose. There are many solid representation schemes available<br />
such as constructive solid geometry (CSG) and boundary representation (BREP). In CSG, solids are<br />
represented by a collection of geometric primitives, such as box, cylinder, wedges, etc., associated together<br />
through a binary tree. The nodes of the tree represent Boolean operations such as union, intersection,<br />
difference, and rigid transformation, while the leaves of the tree represent the primitives. CSG models<br />
represent the volume of the objects very well but do not explicitly store the edge information that most<br />
CAD applications require. Representation and evaluation of complex surfaces are the problem areas for<br />
the CSG model. BREP models store information about the surfaces, curves, and points that form the<br />
boundary of the object. Since surface information is well known, direct manipulation of the surfaces<br />
is possible for applications in path planning and digitizing surface points in CMM planning. BREP<br />
models tend to be very difficult to create compared to CSG and their data files tend to be very big.<br />
Hence, it is common to find that many commercial packages have CSG as a means to create the model,<br />
but the actual model is evaluated in terms of BREP.<br />
The second level of information required for CAIPS-CMM is the GD&T specifications. The specifications<br />
will help to determine the CMM evaluation functions to be applied to geometric elements<br />
that are measured. For ex<strong>amp</strong>le, a circularity tolerance specification for a cylinder element will only<br />
require a circle evaluation function, whereas a cylindricity tolerance specification will require a cylinder<br />
evaluation function. Most CAD/CAM systems do not have a consistent approach for linking the GD&T<br />
information to the product model. Some systems allow for the GD&T specifications to be added as<br />
descriptive notes associated with certain geometry, while others allow for the GD&T specifications to<br />
be integral to the CAD model.<br />
A product model that is created based purely on geometrical information has little engineering<br />
significance in terms of engineering analysis and planning for manufacturing and inspection. A<br />
feature-based modeling technique has been developed to provide a more natural interface between<br />
the designer and other engineering applications such as process planning. A feature can essentially<br />
be defined as a frame of information organized such that tasks like planning for manufacture and<br />
inspection can be carried out more easily. From this definition, it can be seen that many different<br />
features can be defined based on the different applications as different shape and engineering<br />
knowledge are required. For ex<strong>amp</strong>le, a machinable feature is mappable to a generic shape that has<br />
engineering significance in terms of machining, while a technological feature contains non-geometric<br />
attributes related to function and performance of an entity in the feature-based model. Features are<br />
likened to the basic building blocks for a product model and it is quite apparent that more than one type<br />
of feature is required. A more extensive review on features for design and manufacture can be found in<br />
Shah. 53 It should be emphasized that feature-based modeling alone is not sufficient for engineering<br />
applications as geometric reasoning and processing still have to be carried out using solid model<br />
information. Hence it is quite important to have a hybrid system that has both solid (usually BREP)<br />
and feature information.<br />
© 2001 by CRC Press LLC