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When using Onshape or any other CAD 3D modelling software is a good practice to fully define your sketch before transforming it into a solid part. But why, is it really that important?

Ok, picture the following scenario. You are putting a bookcase together. You managed to screw the main frame structure so it can stand on its own and finally fit the shelves on without screwing them as the manual specifies. They perfectly fit wedged between the two vertically standing side panels of your bookcase and you feel like they can stay in place. But do they? Does the current condition satisfy the result? What happens when you decide to use the bottom drawer on that bookcase, will it affect the rest of the structure? What if you try to relocate the whole bookcase to another place, will the shelves stay together?

If you can spot the problem in the above scenario then you will understand why it is important to have all your geometric conditions fully constrained. It is a method to avoid uncontrollable errors like accidentally dragging a line and making it longer or shorter than it should be or changing the radius on a sketch fillet and eventually and unwillingly create an undesirable sketch profile and hence solid part further down the road. You may say: “ Yes but I am under control of my sketch entities as I can spot the changes quickly and move to correct any undesirable conditions”.

In a small design with few sketch entities that may be, but when you have a sketch profile that involves a large combination of arcs, lines, circles, angles etc. then things become complicated and spotting the errors may be a Herculean task.

In Onshape an under defined sketch profile is characterised by the blue entities visible. They can be a number of sketch entities or even end points present. A fully defined sketch is characterised by having all the entities of your profile sketch in black colour as seen in the featured photo at the top. In Onshape you can fully define sketch entities in three ways:

  1. By dimensioning them
  2. Set constraint relationships
  3. Combination of both.

To find out if a sketch is under defined look for any sketch entity that is blue. To answer the questions as to why it is under-defined grab the sketch entity in question and move it around to determine which of its degrees of freedom are not constrained.

As in most traditional CAD packages, in Onshape as well you have a sketch profile that is over defined where some of the geometrical constraints, dimensional constraints or both are conflicting. Onshape highlights that condition by colouring your entire sketch in red. It also assists you finding the conflicting constraints by highlighting them in red as well.

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