You must carefully consider how to define a sketch since several other features use sketches in their creation.
• Considerations include:
– Sketch/Reference plane selection
– Reference selection
– Dimension scheme
– Sketched geometry type
• Open sketch results may differ from closed sketch results
Design intent is captured in Sketcher by selecting references and by sketching, constraining, and dimensioning entities. It is important to capture design intent in sketches since several other features originate from sketches.
Consequently, you must carefully consider how to define a sketch and then capture it. You can always modify the sketch’s design intent, but it is easiest to do when you have planned for future changes.
Considerations When Capturing Design Intent in Sketches
When you create a new sketch in Creo Parametric, consider the options available for capturing design intent in each of the following areas, and in some of the examples listed. Your decisions in these areas at the time of sketch creation impact the overall model downstream when you wish to modify it.
• Sketch/Reference Plane — Should these selected references be default datum planes or a construction plane created with an adjustable offset or angle? Perhaps the sketch plane should be on a surface created from another feature?
• References — Remember that when you select additional sketching references or dimension to existing geometry, you are selecting the
parents for your sketch. If the references you select update, so does the sketch. Consider whether you want your sketcher references to be default datums or another feature. In Figure 1, the sketch feature references the angled surface of existing geometry, so if that angled surface updates, the sketch also updates. Additionally, the sketch references the existing hole. Therefore, if the hole location updates, the sketch’s location updates as well.
• Dimensioning scheme — When dimensioning circles and arcs, should the dimension be a radius or diameter? Should the sketch be dimensioned with an X-Y scheme or a radius-angle scheme? Deciding whether the sketch must pivot can help you determine which scheme to use. Consider which dimensions you might want to modify at a later time if the design changes.
• Constraints — You must decide which constraints to use, and to which reference you wish to constrain because you are again creating parents when selecting constraint references. How should the sketch entities react to each other? Should they be parallel, perpendicular, or tangent?
Should the sketch be symmetrical? If so, you need a centerline. Do you want arc and circle centers to remain aligned? In Figure 1, the sketch’s
construction line between each arc center is constrained to be parallel to the angled surface. Therefore, if the angle of the existing surface changes, so too must the angle of the sketch. Similarly, if the existing hole diameter is modified, this sketch’s upper arc diameter also changes because it is constrained to be of equal radius.
• Sketched geometry type — When sketching arcs, for example, you should use the arc type which helps you obtain your desired design intent.
Remember to use construction geometry or sketched datum points to your advantage.
Open Sketches Versus Closed Sketches
There are two different techniques for creating sketch features:
• Closed-section sketch — The sketched geometry forms a closed loop.
• Open-section sketch — The sketch geometry does not form a closed loop.
Closed-section sketches are the more robust of the two options and should therefore be used whenever possible. However, your desired design intent should ultimately dictate which type of sketch section is created. The yellow extrude features shown in Figure 2 are created from closed-section sketches, whereas the yellow extrude features created in Figure 3 are created from open-section sketches. The geometry created using an open-section sketch causes the resulting geometry to follow the 3-D contour of a surface. The endpoints of the open-section sketch must be constrained to the surface edge. The geometry created using the closed-section sketches ignores the 3-D contour of the surface and simply extends the geometry upward.
There are two specific rules regarding open-section versus closed-section sketches in regards to feature requirements:
• Rib features require an open-section sketch.
• You must create the first extrude or revolve feature using a closed-section sketch.