Sunday, 22 February 2009

Structural Modelling In Rhino3D

Here's some thoughts and opinions that I've formed in the 3 years I've been using Rhino to model structure and construction that might hopefully inform and assist any newer Rhino users reading this.  This is intended as a brief introduction to some of these topics, many of which I expect will form the basis of future posts.

Many users like to operate Rhino primarily using the toolbars via their mouse, but of course commands can also be run from the menus, or by typing commands (or shortcut aliases) in the command window.  Primarily this is the way I work, and the blog posts will generally refer to commands by their command name (hopefully this allows readers to find references in help files easier), with some menu references.  If you're a convert from CAD programs such as AutoCAD or Microstation, you'll probably be reluctant to learn a whole new suite of commands (however similar they may be) for Rhino.  The good news is, you won't have to.  I would highly reccommend using an rhino alias to allow you to use command names/shortcuts that you are already familiar with, and most importantly allows you to use multiple cad environments simultaneously without frustration.  To do this, use the menu Tools-Options-RhinoOptions-Aliases.  A simple example is assigning an alias id to '_evaluatept 

Rhino commands and tools work to a tolerance that is specified by the user within each Rhino Document, and it's quite important that designers are aware of this and have given thought to the tolerance before they start modelling.  Problems due to a "loose" tolerance can emerge at later stage of project design, particularly when the model is used as an input to other uses such as rapid prototyping, or Finite Element Analysis.  I would highly recommend setting your tolerance as tight as you dare, and relaxing it when necessary if commands such as the boolean operations, intersection, splitting etc are failing.

The default tolerance is specified in your Rhino Template files.  Because Rhino is used in so many industries and purposes, the installed defaults should be edited for your purpose.  You can save your own defualts by opening a new document in your required units (ie metre, or milimetre etc), go to 

Here's the Rhino wiki entry on tolerance.


It's common practise in CAD to model construction projects in worldwide position (ie OS coordinate system).  In Rhino, I'd strongly recommend keeping your model in space close to the origin by using a local project axes system.  You will likely observe display problems if you use a coordinate system with coordinates of a large value such as those typical in OS.  This is because Rhino uses numbers of "single" precision for display mesh positions.   But you can set up named construction planes (similar to User Coordinate Systems in  AutoCAD) that will enable you to interogate or specify coordinates in your model in large coordinate systems.

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