Model symmetry methodology

Experienced user of surface modelling in parametrics, reasonably new/returning to Rhino
Observations and suggestions on the following gratefully recieved;

Because of the complexity of my usual models I prefer to build symmetrical to a mid plane and then mirror when finished.

To ensure my curves are normal to my symmetry plane I’m using grid snap or ortho to ensure the end control vector is perpendicular at the connection point to the sym plane.

I build my curves on the symmetry plane and extrude them to the mirror side to give me a reference to match my surface edges to, ensuring my surface boundary conditions are normal at the sym plane.

Setting curves or boundaries ‘normal’ doesn’t seem to be in the Rhino toolbox? so I presume the philosophy is to have a reference - curve or surface - for G1/2?

Cheers, Sean

Hi Sean, welcome back to Rhino! Do you mean rows of points across your mirror plane staying tangent with one another? You could use Symmetry with History enabled if the srf edge is untrimmed if so. Your method of creating an extrusion to match to is also useful in some cases. Most of the time, I use grid snaps or hold down Shift (Ortho) to keep the next point drawn in line with the previous one and then stay away from editing ctrl pts next to the mirror plane. A last thought that may help is to use Mirror and then MatchSrf with ‘average’ on to create the continuity between the two srfs. Let me know if I can help with anything else.

Hi Sean, Brian- also, V5 has a new tool : SetSurfaceTangent - you can set untrimmed surface edge to have a specific tangent direction- it can be macro-ed to set vertical up vertical down, +/-X +/- Y etc. Quick n easy and no Mirroring or MatchSrf/average needed.


Nice!.. I missed that one.

I am so glad that finally there is the ‘SetSurfTangent direction’ tool available.
Preiously I couldnt biuld my surfaces as I needed them to be… had to always do it manually if not too much control points on the way (so I mostly use ‘Surf from 2, 3, 4 edges’ or ‘Sweep2Rails’ commands to build somehow complex surfaces).

Also make sure that you always set your edge points coplanar (select them precisely) and set them with either the free “PetersTools” plugin or within Rhino ‘SetX,Y,ZCoordinates’ command to be coplanar to a plane or construction plane, becuase there could be very subtle displacements of the boundary edge points when tweaking a surface inner CVs. I always have wondered why my model had those naked edges when surfaces had been offset but I do always check the positions and tangent (with the SetSurfTangent tool). Especially for dome shaped surfaces that have symmetry along one or two axis!