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# Parametric Dimensioning

From the user's point of view parametrics means the control (manipulation) of geometry by manipulation of suitable parameters, i.e. values, which influence the geometry in any way. This may be, for example, the value of dimensions or co-ordinates.
SISCAD-P is conceived as a relational system. Thus, the possibility to parametrically modify the geometry is an integral part of the core system and not an additional module. Due to the relational data structure all necessary information is saved within the internal CAD model.
The parametric concept bases on 3 columns:

- constructional relations
- constraints
- variables, functions, formulae

Constructional relations are relations between geometrical objects that describe the way how one or more objects have been created and which other objects or parameters have been involved in this construction.
These relations between the objects automatically occur during the construction process and may be saved by the system. Any later manipulation will be considerably influenced due to these relations.
Constraints determine certain positional and dimensional constraints of an object. Possible constraints can be fundamental geometrical properties, as for example, parallelism of two lines or more complex properties, as for example the determination of the position of a point on the center of a line.
In general, the objects concerned are by far not as restricted when using constraints for any later manipulation as by saving constructional relations. The constraint parallelism of two lines , for example only determines that these lines are parallel and remain so even after a manipulation. The position of the lines to each other or the length of them, however, is not determined; thus it is free.
Variables, functions and formulae form the 3rd column of the parametrics concept of SISCAD-P.
Variables are wildcards for any parameter and can be called with their variable name. When using a variable for the construction of an object the current content of the variable is inserted.
To determine parameters you may not only use simple variables, but also functions and complex arithmetic expressions (formulae) can be entered.
Each of these 3 columns is suited for a particular range of applications. You may decide for each case, which method is the most sensible one for your individual application.
The following sections describe these methods. The meaning of parametric dimensions is particularly emphasized.

Next: Constructional Relations Up: SISCAD-P Parametrics Previous: Contents