ComputerAided_Design_Engineering_amp_Manufactur.pdf
ComputerAided_Design_Engineering_amp_Manufactur.pdf
ComputerAided_Design_Engineering_amp_Manufactur.pdf
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Several techniques of defining a feature were presented earlier. Of these, the first definition—‘‘feature is<br />
a part surface generated after machining operation(s)”—is adopted in the present work, because (a)<br />
it is simple to define surface features; (b) it is necessary to provide a natural way of working with<br />
features; and (c) the definition of rotational features in terms of removal volumes is difficult.<br />
Part Representation-Data Structures<br />
The design of data structures for representing the part information (global, geometrical, and technological<br />
details) depends on several factors:<br />
• The type and number of form features considered<br />
• The relationship of the form features with the primitive features<br />
• The general arrangement of the features while constructing a part<br />
• The nature and number of technological details considered<br />
• The modeling environment (CSG, B-rep, wire frame etc.)<br />
• The application data.<br />
Global data can be thought of as common data that can be accessed by every part feature. Global data<br />
(e.g., component name, material name, material code, drawing code, revision number, process planner’s<br />
name, department name, date, etc.) need to be represented for the completeness of the model. Some of<br />
these data are required to make planning decisions, while other data are used in generating companyspecific<br />
process plans.<br />
This type of data is purely non-geometrical. This is also non-repetitive in nature as only one set of<br />
data exists for a given part. Hence, it is stored as a separate entity in a predetermined order. This can be<br />
extended to contain any other relevant data, if the need arises.<br />
With geometrical data,<br />
a large number of rotational parts from several industries are analyzed and<br />
the constituent geometric features identified. These features can be grouped into two categories based<br />
on the geometry. These external features (turn, groove, taper, etc.) and internal features (bore, internal<br />
groove, etc.) are shown in Figures 5.14 and 5.15. Since the scope of the present work is limited to rotational<br />
parts, 2-D wire frame representation is adopted as it is adequate for visualizing the rotational parts<br />
without any ambiguity. In this representation, the point, line, and arc constitute the primitive features<br />
upon which the form features are built. Based on these considerations, the parameters required for<br />
representing the external and internal rotational features evolve. These are shown in Tables 5.1 and 5.2,<br />
respectively. The structures shown in the tables are basically the explicit representation of feature coordinates<br />
with a code attached to them.<br />
Technological data—the<br />
integral representation of the technological details (tolerances and surface<br />
finish) with the geometry—assume significance in the wake of feature-based models. In feature-based<br />
systems, a part can be thought of as an ordered compilation of features. In this context, it can be observed<br />
that each feature has tolerances associated with its size parameters such as length and diameter. Also,<br />
there are form tolerances on features, such as cylindricity, circularity, etc. Apart from these, there will be<br />
relational tolerances which involve more than one feature.<br />
In the present work, these technological details are categorized into two groups, (a) intrafeature<br />
tolerances and (b) interfeature tolerances, to ensure a simple way of attaching tolerances to the features.<br />
This division is based on the number of features involved in defining a particular piece of technological<br />
data. The classification of the technological data adopted in the present study is shown in Figure 5.16.<br />
Intrafeature tolerances. It can be observed that many of the tolerances specified are directly related to<br />
only one feature at a time. For ex<strong>amp</strong>le, all form tolerances fall into this category. These are feature-specific<br />
and are frequently applied to single features or portions of a feature. These tolerances are specified without<br />
a common datum reference because the features are not controled in relation to another feature. Intrafeature<br />
tolerances can be attached to the feature, along with the geometry, as additional attributes.