Understanding and Using Open and Closed Sketches

As the common saying goes: work smarter, not harder. It’s easier to do if you keep your 3D CAD skills current. Dave Martin, Creo, Windchill, and Mathcad instructor and consultant, presented several Creo Parametric modeling best practices and useful techniques that can help you quickly “level-up” your Creo design skills in a recent webcast.

One of Martin’s top tips was to understand the differences between open sketches and closed sketches. He explained that by understanding how open and closed sketches behave differently, you can use them strategically based on your needs.

Before we take a closer look at the differences between closed sketches and open sketches, it’s worth noting that Creo 9 includes enhanced inspection tools for those working with Sketcher.

The inspection tools now make it easier to identify and highlight potentially problematic sketch geometry. Options to Highlight Intersections and Highlight Junctions are available. And the existing Highlight Overlaps tool has been enhanced so that it remains active, identifying the overlapping geometry while you sketch.

But, back to the differences between closed and open sketches.

When the Shade Closed Loops option is enabled, a closed sketch displays as shaded loop, which indicates the sketch forms a closed loop. Start points and endpoints are the same. Here’s how it looks:

An open sketch has red colored endpoints (as shown below), indicating they are not the same and a closed loop is not formed. 

The example below contains two closed sketches and two open sketches. You’ll notice that when the closed sketches are extruded, they maintain their cross-section shape, while simply gaining depth. After the open sketches are converted to solid geometry, when they are extruded, they follow the geometry of the part to which they intersect (the other extruded part in the example). Their cross-section shape also changes as they gain depth.

In this example, you can see that gaps may form as closed sketches are extruded. Whereas, when an open sketch is extruded, no gaps form as the geometry of an open sketch stays attached to the feature to which it intersects.

In general, closed sketches tend to be more robust from a regeneration stability and design change perspective. However, open sketches can provide benefits. For example, to get to a certain desired geometric result more easily, like what is illustrated in the example above.

This is just one of many tips Martin demonstrated in his presentation, Modeling Best Practices. To see them all, check out the replay.