Curvature continuous blends are the new age replacement for lead-in + fillet construction of the traditional world. While lot of CAD packages have G2/G3 continuous fillet capabilities there are some critical points to be addressed before successfully using such blends.

Refer the following pictures.

The first figure shows a standard fillet of 10mm in Alias. As expected only tangency is achieved.

The second figure shows a lead fillet. The fillet will be created using a degree 3 ( in V direction) surface. Although theoretically the fillet is tangent continuous, the amount of curvature discontinuity is very minimal. And the curvature plot show a very nice acceleration.

The third figure shows a curvature continuous blend. Will need a minimum degree 5 surface to construct them.

The fourth figure shows a g3 continuous blend, where the minimum radius value is 10mm. Note that the tangent length required to attain a radius value is 20 mm which is twice that of the fillet value in lead, g2 and g3 blends.

For a similar radius of curvature, those fillet needs twice the tangent length or in other words distance between the transition edges. Which also implies that the minimum radius of curvature of the base surface should be more than the tangent value to build the fillets successfully. Inversely, the actual radius of curvature value of a g2/g3 continuous fillet will be about half that of tangent offset value. Therefore it makes no sense in specifying a g2/g3 fillet of 3mm or lesser tangent offset value, the actual value will be very close to a sharp edge. Moreover, in the final product, most of the small tangent continuous geometry will automatically become smooth or continuous because of various manufacturing processes.

The other potential problem is oscillations in G3 blends. G3 blends are constructed by seven degree surfaces. Higher order blends are prone to oscillation, unless the base surfaces are built as tight as possible.

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