Page 34 - MetalForming August 2014

Trying to get the “Best of Both”
Growing number of attempts to merge these very different methods
Feature & Parameter Manipulation
Parametric History-Based Approach
Direct Geometry Manipulation
Geometry Editing
Recording
The ordeal, structured model
Not Recording
Just geometry
Parametric Control
Direct History-Free Approach
3D Technologies for
Stamping- Die Design
Understand the differences between parametric and direct modeling to find the right method,
or combination of methods, for the die-design job at hand.
Metal stampers have a variety of software tools available to them: design tools (CAD), sim- ulation and analysis tools (CAE), and manufacturing tools (CAM). On the design side, the most often used design tool is referred to as parametric or his- tory-based modeling. Another tech- nology quickly gaining acceptance: direct modeling.
A Brief History Lesson
In the early days of mechanical CAD, software developers selected one of two techniques for developing geome- try at the accuracy levels required to support mechanical design: construct solid geometry (CSG), and boundary representation (b-rep). The CSG tech- nique to create and modify geometry uses primitives such as cylinders, blocks and cones with Boolean opera- tions such as union, intersection and combines. The primitives and Booleans were organized in a structured tree. To
Paul Hamilton is director, CAD seg- ment go-to-market, North America, PTC Inc., Needham, MA: 781/370-5000; www.ptc.com
BY PAUL HAMILTON
modify the model, an existing primitive or Boolean, or both, could be edited or replaced. The designer could replay or regenerate the tree structure, result- ing in a different 3D object.
The b-rep technique of representing 3D shapes describes the connectivity (topology) of vertices, edges and sur- face boundaries to form faces, which connect to form solid bodies. It pro- vided flexibility in the types of geome- try described, but proved somewhat rigid when it came to editing the geom-
etry. On the other hand, CSG tradi- tionally has proved somewhat limited in the types of geometry that could be represented, while providing editing flexibility.
In the early to mid-1980s, work by Sam Geisberg (a Russian immigrant and mathematics genius) and others sought to combine the CSG tree structure with b-rep primitives, so that the b-rep primitives could be created with sketches. Any primitive shape could be defined and used in
32 MetalForming/August 2014
www.metalformingmagazine.com
Fig. 1