Computer Aided Design XE Week 2 - Computer-aided Design

Parametric Design/Modelling

What is Parametric Modelling?

Parametric modelling is the relation of many geometries to one dimension and the ability to modify those geometries by altering the dimension.

Before the advent of parametrics, editing the shape was not an easy task for designers. For example, to modify a 3D solid, the designer had to change the length, the breadth and the height. However, with parametric modelling, the designer need only alter one parameter; the other two parameters get adjusted automatically. So, parametric models focus on the steps in creating a shape and parameterize them.” – www.designtechsys.com

Solidworks and Fusion 360 are examples of Parametric modelling – they are driven by specific dimensions and use constraints and calculations to produce very precise 2D sketches and 3D models.

 

Just quickly, ‘cos it’s interesting.

My favourite and one of the best and earliest example of Parametric Modelling was by Antoni Gaudí when he was developing his design for the Sagrada Família. The design of the Sagrada Família (and Gothic architecture generally) uses catenary arches which are arches that are derived from the inverse of a catenary curve – “the curve a hanging chain or cable assumes under its own weight when supported only at its ends.”

Gaudí used this principle in a remarkable way to develop the support structure and geometry of his design. He secured and hung strings and hung strings from those strings. Then he added lead shot to weight them down at structural intersections to create catenary arches – all suspended from the ceiling. All these string and weights eventually looked like a chandelier until viewed in a mirror where it revealed a geometrically accurate model of the design.

Gaudi Parametric design

 

Gaudi Parametric design

 

By using this technique Gaudí was able to simulate how an adjustment to a specific dimension, i.e. the length of string or the position of the anchor points or weights, would effect the overall geometry – parametric modelling.

More info about Gaudí  and how he used parametric modelling to design the Sagrada Família can be found at the world’s best design podcast –  99% Invisible

 

The opposite to Parametric modelling is Direct modelling

Here there are no specific dimensions required, you start with a 3D model that can be manipulated into a desired form. Autodesk Maya is an example of Direct modelling CAD software, mostly – pretty much all modern CAD software use features of both modelling methods.

The way I like to consider the difference is imagine you’re wanting to make a model of a house. You could draw measured sketches and carefully cut and stick cardboard pieces together to create the 3D model. Or you grab some modelling clay and start squashing and forming. Either way you’ll end up with a model of a house, but the possible uses of that model may vary.

THIS article explains the difference in much greater detail and expands on what the possible implications of choosing one method over another can have with regards to a CAD model’s history and edit-ability.

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