Definitions

Surface – a non solid, 3D construction feature

Surface Patch – an individual area of a surface boundered  by a single chain of edges

Surface Quilt – a number of patches joined together

Curvature – with respect to a spline curve or surface with a constantly changing radius: at any point on the curve/surface its curvature is 1 divided by the radius at that point. Therefore, a nearly flat area of a surface/curve (very large radius) has a very small curvature – see splines

 

Primitives as surfaces

Standard geometry creation features such as extrude, revolve, sweep and blend can be switched to open surface geometry rather than solids.  If a driving sketch is an open rather than a closed loop then it will have to be a surface or thin feature.

 

Surface Display

The one-sided outer edges of a surface feature are displayed in orange.

The two-sided inner edges are displayed in magenta (purple).

Therefore a quilt will be displayed as a number of magenta lines inside an orange boundary.

 

Surface Continuities

Make sure you know what a spline curve is [HERE] and what curvature is [HERE]

Continuity between surface patches is important for both aesthetic and functional reasons. Poor continuity can show creases and show each individual patch’s boundaries. Continuity between curves and surfaces can be expressed as geometric (G0, G1, G2) continuity.

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G0 Continuity: Positional continuity. Two curves that share an endpoint, two surfaces that share a boundary are G0 continuous.

G1 Continuity: Tangential continuity. Two curves that share an endpoint, two surfaces that share a boundary are G1 continuous when the normals at the join/boundary are exactly aligned in direction – at that point they are travelling in the same direction.

G2 Continuity: Curvature continuity. Two curves that share an endpoint, two surfaces that share a boundary are G2 continuous when they have the same curvature values where they meet.

 

 

Beyond parametric modellers you will also see G3 continuity – acceleration (rate of change of curvature)

 

Midplane/Symmetry Continuity

If your model is symmetrical then it will generally be quicker and more robust to model half of it and then mirror the whole model – at least to the point where it becomes asymmetrical.

To achieve continuity across the midplane:

All curves and resultant surfaces must be normal to the midplane.

 

Surface Classification

Surfaces within a model are often classified according to their aesthetic importance in the final product.

The fundamental outer surfaces which are most prominent in a product are often classed as the A surfaces – those which need most aesthetic consideration, ie. the top surfaces of the mouse in your hand.

The surfaces which are generally hidden but may still be seen by the user, ie. the bottom of the mouse, are classed as the B surfaces.

The C surfaces are then the internal, always hidden surfaces which need no aesthetic consideration.