Y-Fillets in Class A Surfaces

Y-Fillets in Class A Surfaces

This tutorial shows an effective method to create fillet blending and wash-out areas in digital surfacing in order to achieve a high quality blended region (Class A surfaces). The author is Dharamjeet Singh, Styling Manager at Maruti Suzuki India.

The following tutorial is courtesy of Dharamjeet Singh, Manager-Styling at Maruti Suzuki India Ltd. He explains how to face the creation of Y-fillets in the 3D modeling process when creating Class A surfaces – a common situation in the automotive styling or product design process.

The tutorial refers to the Alias Design software, but the same concepts can be applied to other 3D NURBS-based digital modellers.

Y Fillet TutorialThese types of regions are faced by all the digital modelers while creating Class A surface models.

The general practice to solve this region is involves the creation of a parent surface and the trimming of this the parent surface and then creation of a secondary surface over it.

The point marked by the arrow represents the location where the curvature from the upper fillet surface transforms into the lower fillet surface.

At that location the curvature inflects which creates a number of problems for making tertiary surfaces.

How to solve this region for the best quality, curvature and highlights? The method which I prefer and which works all the time is explained in the following steps.

Y Fillet Tutorial Y Fillet Tutorial
Step 1 Step 2

Step 1 – Create a surface simply building a bridge curve and using the Birail command. Make sure that at A and B locations the surface is matched collinear with parent surfaces.

Match the new surface with the parent at the location C. You might need to add a couple of spans to meet the requirements. That is acceptable.

Step 2 – Create the bridge curve from A to B. The location of B and the shape of bridge curve can be choosen as per individual’s taste. This curve will be used to trim the newly created surface.

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Step 3 Step 4

Step 3 – This is how the trimmed surface will look like. I am sure most of the digital sculptors are very much aware about this method. Now I will explain how to proceed from here and solve the area, especially in the A region.

Step 4 – Use the Birail command to create a secondary surface. Use A and B side edges as generators, C and D as rail edges. The Birail command is used because it creates a uniform CV structure from A to B.

While creating the surface with the Birail command try to keep G2 continuity at edge C with the minimum number of CVs. Let the A and B have position continuity (G0).

Y Fillet Tutorial 4b Y Fillet Tutorial
Step 5 Step 6

Step 5 – The following is important. Once G2 is achieved at edge C, pick G2 condition for edge D in the Birail command. You might receive errors like shown in the picture. That is Ok.

You will not be able to achieve G2 condition at edge D. It is because of the inflection point on the intersection of edge A and D, but this is not a problem.

Try increasing the span if you need to and keep checking for tangency condition at edge D. The aim is to have tangency condition in the region indicated by blue arrowed line.

Step 6 – Split the secondary surface into 2 surfaces in the location where G1 (tangency) condition starts failing. The new surfaces are Surface 1 and Surface 2. Use the Match command to match Surface 1 to Edge C, B and D using the partial option. Achieve G2 continuity (curvature continuity) at all the 3 edges.

Match Surface 2 to Edges A and E using the Match command. Use collinear, explicit off and partial off while matching. Do not worry about discontinuities at edge D. Match surface 2 to Edge C using partial on, explicit on.

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Step 7 Step 8

Step 7 – Create a duplicate curve on edge 1 and extend it to obtain "Curve 1". Trim the curve as shown in picture. For the Curve 2, create a line, increase the CVs to 4 and match it to edge 2. Use these curves to trim surface 2 and parent surface.

Step 8 – Use the Birail command to create the surface shown in picture. Use match command to achieve G2 continuities at edge 2, 3 and 4. Use match command to achieve G1 continuity at edge 1

Note: The G1 continuity at edge 1 will fulfill the required level of quality and highlights.

The same happens with consecutive surfaces in a running fillet where – even if G2 is not reached among the fillet surfaces – the highlights are still good.

The reason is that the U or V curvature of the fillet surfaces change in the same way as the fillet runs along the edges.

About the Author

Dharamjeet Singh works with Maruti Suzuki India Ltd as Manager-Styling.


(Image and Text Courtesy: Dharamjeet Singh)

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  1. Anddreson Tianth

    this is the best tutorial about, it can be applied to Rhino3d. that’s interesting.


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