The ability to easily define precise 2D sketch geometry is at the core of any parametric modelling system. The activities that take place during this stage determine how intelligent, flexible, and robust the design can become. The grouping of features, the placement of both geometric and dimension constraints, and all external references play key roles in this. The methods used to dimension and constrain sketch geometry will determine the design intent of the feature and ultimately how easily the part can be modified. It is better to have less complicated sketch geometry and more features than complicated sketch geometry and fewer features. This is because it is easier to modify, suppress, reorder, or delete sketch features if they are contained in a separate sketch. To this end, sketch geometry should be separated into logical groups so that it can be controlled more effectively.
Sketch relations such as horizontal, vertical, perpendicular and tangency can be captured automatically while creating sketch geometry. Additional geometric constraints can then be added if required. The two-dimensional sketch can then be fully constrained or defined by adding parametric dimensions that control the size of the sketch geometry prior to being used to create a 3D feature. Sketches are made up of three parts: sketch entities, geometric relationships, and sketch dimensions. These components are combined to define a sketch and the key is to put them together in sequence so they define the design's intent. Using the following sequence makes the sketching process much easier.
- Firstly, create the sketch geometry. This should be the same shape as the final geometry, but can be slightly larger or smaller than the final form. The sketch should be close to the final size, so that when the dimensions are added, it does not distort excessively.
- Secondly, add any required additional geometric relationships that were not automatically added.
- Thirdly, locate the sketch to existing or reference geometry (such as planes, axes, the origin, and the like).
- Fourthly, drag the sketch geometry to see if the origin and geometric relationships are adequately captured.
- Fifthly, add the dimensions to size the sketch entities to fully define the sketch. Remember a sketch object can only be dimensioned once.
- Build design intent into sketch geometry while sketching by using the appropriate tools such as tangent arcs, axes of symmetry and coincidence to automatically capture relationships.
- Do not put too much detail into one sketch. It is better to have simple sketches and more features.
- While under-defined sketches can be useful early in the design process, it is good practice to fully constrain and place sketch geometry. This ensures that the sketch will modify as intended, and the dimensions can be reused during the creation of drawings.
- Use centre lines and symmetric relations for sketching symmetric parts.
- Sketch cylindrical and conical parts and features so that the axis coincides with the model origin.
- Copy from existing sketches where it can save time.
- Sketch in 2D and 3D. Sketching usually takes place in 2D, but by slightly rotating the model to give a 3D view after starting, it is often easier to see where one is sketching and to select the correct model references.
- When dimensioning sketch geometry, it is usually better to start with the smallest dimension and work your way to the largest.
- Dimension feature sketches to default datum planes, construction datum's or datum axes rather than part edges whenever possible.
- Use open and closed sections appropriately.