Why is it so important to fully define a sketch?
Simply put, designs with fully-defined sketches are much easier to change. When dimensions exist in your sketch, you can edit them with a double-click on the feature (or even easier with Instant3D!). When relations and dimensions exist in your sketch, the sketch updates much more predictably when something changes. Many people believe that under-defined sketches slow down performance, although I don’t have any data to support that. But one thing’s for sure, fully-defining your sketch helps you capture design intent, and capturing design intent is one of the greatest advantages of a parametric model.
SolidWorks helps you simplify your sketch process by offering to define a sketch for you!
Tools > Dimensions > Fully Define Sketch… allows you to add dimensions and relations to a sketch without so many clicks!
Fully Define Sketch allows you to select which entities you want it to “define” for you; so you can be as manual or as automated as you wish.
Relations and Dimensions are calculated separately… so you can chose to add dimensions with no relations, or vice versa. (in which case, your sketch will not be fully defined… but it will save you many clicks.)
When using Fully Define to add your dimensions, you have three Dimensions Schemes from which to choose: Chain, Baseline, and Ordinate.
Bonus: You can select different Dimension Schemes for Vertical and Horizontal dimensions!
The datum you select for each Dimension Scheme can be a model edge, a point, (such as the sketch Origin), or a sketched construction line. Remember that if you use a sketched reference line (centerline), you must manually create a relation or dimension from the centerline to the Origin.
Good to know: if you have two equal sized circles, Fully Define Sketch will call add the diameter’s callout, and add an equal relation between the two circles.