Thursday, 31 March 2016

Exporting for 3D print

The subject of this blog is one of those topics that rarely gets covered in depth. In practice I am frequently telling customers to move their data to the origin before exporting for 3D print. But why is this? 

Many BIM, CAD and 3D modelling programs have a very large drawing space. An example of why this would be useful is when designing a very large structure like a road or railway. Similarly a large coordinate space allows buildings to be designed at their correct location in relation to a city grid.

When exporting 3D models for applications such as 3D printing a problem can occur causing the exported data to become deformed as shown in the image below. 

Bad STL export of sphere from Rhino when
deliberately modelling far from origin

The problem seems to be that most 3D modelling programs are based on geometric modelling Kernels such as ACIS or Parasolid. These work fine when close to the origin but lose accuracy outside of the kernel's modelling space. 

Confusingly many of the applications functionality is unaffected by modelling outside the kernels modelling space but certain functions either fail completely or result in degraded data.

In MicroStation for example the coordinate system (Working Area) will go well beyond a million km from the origin but the Solids Area is only a 4.2km cube. The 4.2km limit being set by the Parasolid modelling kernel used in MicroStation. When you draw more than 2.1km from the origin the lower resolution may not be immediately apparent but may manifest downstream, such as when you export to STL etc.

This is not a problem restricted to MicroStation. It is a problem with most 3D modelling packages. As a consequence it is always best practice to model near the origin whenever possible. 

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