Hiding Edges During Intersection in SketchUp

Bonnie Roskes
Independent Writer and Consultant

Say you have two objects and you want to trim each object using the other object. In other words, you want to use Object A to trim Object B, and then use Object B to trim Object A. This is not so straightforward, because once Object B is trimmed, SketchUp no longer considers it to intersect Object A.

The example used to illustrate this trick is a skylight. Start with a box and insert a Sphere component (found in the Shapes folder of the Component Browser). Move and scale the sphere so that the top sticks out of the box.

Two intersection cuts are needed. First, the top of the box needs to be cut to insert the skylight. Second, the sphere needs to be trimmed to remove the section inside the box.

For the first cut, right-click on the top face of the box and select Intersect with Model.

This creates intersection edges where the face meets the sphere.

Hide the sphere (do not delete it) so that you can see the intersection edges.

Make the skylight hole by removing the circular face within the top face.

The next step is to cut the sphere. Unhide it and open it for editing (right-click the sphere and select Edit Component, or just double-click the sphere).

With the sphere open for editing, right-click on it and run Intersect with Model again. No edges are created.

Why does this happen? Because the top face of the box has already been cut, and no longer passes through the sphere. So nothing is being intersected.

Here's the solution: First end the component editing. Then select and hide each of the intersection edges previously created on the top face. There should be several small edges (segments of the intersection circle), and you can select groups of edges at a time, using left-to-right selection windows.

Now open the sphere for editing again, and run Intersect with Model. This time, the intersection edges are created on the sphere. Because the first set of intersection edges was hidden, SketchUp was tricked into thinking that the top face was still whole.

Now you can delete the lower part of the sphere.

If you assign a transparent material to the partial sphere, you can see that both the sphere and top face of the box are now cut.

Note that if you had exploded the sphere in the beginning, only one set of intersection edges would be needed. These edges could be used to trim both the face and the sphere. But imagine a case in which you're creating dozens of skylights—it's much easier to keep the component intact so that when you edit one, you edit them all.

About the Author

Bonnie Roskes is the author of a number of books on SketchUp, including the recent "Mastering Piranesi: version 4" and "The SketchUp Book: version 5." She is the founder of F1, a technical documentation and training company, which does project work for several CAD/CAM companies such as updating manuals, online help, and creating tutorials. In addition to SketchUp, she has authored books on OneCNC, and Piranesi (soon-to-be released). Bonnie has a BS and MS in structural engineering, and worked for several years in bridge design and analysis before moving to tech writing. She can be reached at: bonnie@f1help.biz.

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