Autodesk Inventor surfacing. An introduction to G3 Surface continuity

Thanks very much to David Harrington, John Evans and the team at AUGIworld for allowing me to re-publish this article. To read it in full, check out the May 2014 Issue of AUGIworld.

Hard Surface modelling is possibly the most challenging discipline within 3D CAD  – and the most rewarding, once you’ve got the way of it…

With a bit of surfacing theory you too can relish meeting the challenge with Autodesk Inventor!

The Pink Panther LimoImage from

It was a choice between the Pink Panther Mobile and Lady Penelope’s Roller. I love Lady P, but the Panther’s Limo doesn’t get as much press ;)

Previously in this series, we learned about G2 surface continuity.

G3 Continuity + Tangency + Curvature + Acceleration

Chauffeurs  don’t blast around like an F1 driver. Every move must be considered and graceful to prevent the precious cargo being rattled about (let’s pretend that the precious cargo is you and me!).

The limo driver must accelerate incredibly gently, take corners without sliding us passengers across the seats and anticipate breaking early enough to be able to come to a complete stop without so much as a judder.

G3 curvature continuity includes the concept of ‘Acceleration’ of curvature. That is – the amount of change in the curvature, over the length of the curve.


Now – this one is a bit more tricky. Inventor doesn’t have a G3 continuity sketch constraint. There is no G3 option in the fillet tool (or any other surfacing tool in Inventor). However, you can create G3 relationships – you just need to know how to recognise one when you see one.

Let’s take another look at our 2D curvature combs. To achieve G3 continuity, the curvature combs should be heading in the same direction (Tangency), be the same length (Curvature) and, crucially, should also be tangent (Acceleration – or rate of change).


With a little careful tweaking, we can coax a G3 relationship out of our input geometry. The curvature analysis of a G3 junction should remind you of a ride in our Limo – super smoooth acceleration all the way through.

Finally, read surface continuity a conclusion

or read this article in full in the May 2014 Issue of AUGIworld.

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