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# The Magic of Spline on Surface

Thursday June 19, 2014 at 3:15pm
Want to create a free form cut out in a 3D surface? The little known 'Spline on Surface' is a great tool for this. Here is how to create an egg with the top chopped off - ready for the soldiers!

A question was emailed in yesterday from a customer who wanted to know if we could cut some free form holes in a spherical part. There are several ways that we could try this with cut extrudes and, for a cylindrical part, the Wrap feature would work, but there is another neat way that is seldom used - probably as it is not taught on many of the basic training courses.

To illustrate, let's model an egg - with the top chopped off. This is hardly heavy engineering but we have a lot of customers who design crockery and point of sales furniture so modelling an egg might not be as wacky as it seems.

We start with a half ellipse and revolve a solid around the centre line - simple. Then comes the new sketch tool. This is only available in a 3D sketch - but we don't even have to create the sketch! Just pick Tools > Sketch Entities and 'Spline on Surface'....

It automatically puts us into a 3D sketch and enables the spline entity sketch tool. The special characteristic is that it is now expecting to add 'On Plane' relations to all spline points. We simply click on faces of an existing surface or solid body and the spline stays on the surface as shown below ...

We then exit the sketch and use a 'Split Line' to add a split on the surface of the solid.

The next step is to add a 'Delete Face' feature to remove the top half of the egg. This, of course, turns the solid into a surface. To turn it back into a solid we use a 'Thicken' feature with the thickness of the egg white. We then add a dull appearance to the outside face to look like an egg shell and a white appearance to the rest of the body...

The penultimate step is to model the egg yolk. We can do this with an 'Offset Surface' set to zero thickness on the inside of the egg white and a 'Fill Surface' on the top. We can also add a couple of control curves to get the shape of the egg yolk just right. Knitting the 'Offset Surface' and the 'Fill Surface' allows us to turn it into a solid.

Finally a simple 'Revolve' creates the cup and a render gives us the finished product...

I am not an expert at surfacing but this is so easy to do that even I managed it without any fuss.

By Andy Fulcher

Technical Manager

Solid Solutions Management Ltd

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