Conventional types of dimensioning (e.g. reference dimensioning in SISCAD-P)
are associative, i.e. after the geometry has been modified the dimension
values automatically adapt.
Parametric dimensioning delivers the associativity in the opposite
direction.
Definition:
A parametric measure is a measure which can only be changed by modification of its dimension value. If this is the case, the dimensioned objects automatically adapt. Other manipulations, as for example moving or rotating do not effect the dimension value as well as the geometry.
This type of dimensioning is the prerequisite for a parametric system.
Working with Parametric Measures
To be able to create parametric measures, 2 requirements have to be met:
MODIFY PART PARM OP=<id>;PRC=1to explicitly activate the parametrics.
Default is that a reference dimensioning is created, i.e. parametrics is
inactive and the measure is green.
The syntactic rules for the creation of parametric measures resp. reference
measures are the same, but not all available types of dimensioning are
possible for parametric dimensions.
For easier differentiation parametric measures are displayed in yellow and
reference measures in green. They are also assigned to different layers.
To parametrically dimension geometrical objects, they must not be exactly
determined, i.e. the should have some degrees of freedom left. If they are
exactly determined, SISCAD-P does not create a parametric measure, because it
would not be possible anymore to react to a modification of the nominal size.
Example (parametrically dimensioned lines):
2 finite lines (g1 and g2) with a common starting point are
constructed right-angledly. Parametric constraints or
constructional relations are not saved.
Now you can create a parametric angular measure between these two lines.
The corresponding command reads
CREATE ANGULARDIM BTWL REFL1=<id>;REFL2=<id>;PDL=<pos>!
Figure 6: Right-angled lines with parametric angular measure
The color of the measure is yellow to see that this is a parametric
measure.
Because of the modification of the nominal size to 45 (for this purpose you
have to enter the command: MODIFY NOMINALSIZE OP=<id>;VALUE=45! )
the lines adapt so that the desired value is reached.
Figure 7: The lines after the modification of the nominal size
But:
If, before the creation of the angular measure the constraint
PERPENDICULAR has been saved for the two lines (REMEMBER
PERPENDICULAR), the attempt to create a parametric angular
measure causes the following prompt (or a similar one) to appear:
"The geometry would be in an inconsistent or overdetermined condition after this action. The geometry or dimensioning that is the reason for this will be displayed."
The command is not executed. Actually there would not be an inconsistent
condition after having entered the angular measure, but the system recognizes
that it cannot react to a modification of the nominal size.