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Where can I find how to draw circular flexible ductwork in BricsCAD 2D or 3D
Charles Alan Butler (CAB) has created a Lisp program for this: Flex Duct Creator. It is available at theswamp.org (you have to log in to be able to download the zip-file).
An alternative is to use a special linetype as demonstrated here.
Two options for creating a simplified 3D duct.
1. For a simple run use LOFT with the setting ‘normal to both ends’ (set LOFTNORMALS = 4). You need a CIRCLE at each end to create the duct, no need to create a guiding line.
2. For more involved runs you can use DMTHICKEN in conjunction with a SPLINE, making sure that the spline tangency at each end is parallel to your connection. Using a SPLINE allows you to go up and down in the Z axis.
I also found previously found DMTHICKEN was useful for modelling metal spouting.
Here is the ZIP file.