To recover a sketch that changes in an unexpected way:

Use Edit menu > Undo, or CTRL + Z on your keyboard.

Sometimes the Sketch collapses to a small dot.

Use Edit menu > Undo, or CTRL + Z on your keyboard.

Sometimes the Sketch expands to the size of a planet.

Use Edit menu > Undo, or CTRL + Z on your keyboard.

Sometimes, a sketch-element is BLACK but you know it should be BLUE. You should be able to add a dimension or constraint to it, but you cannot.

Or, a sketch-element is BLUE when it should be BLACK.

The best solution is to do one of these actions:

•Delete the sketch-element and add it again.

•Shift-Click the sketch-element so it and the constraints that you have added to it show in the Selection-Window

•Right-click a constraint, or the sketch-element, and select Delete Element in the shortcut menu.

OR

•Use Edit menu > Undo or CTRL + Z

Then add a different constraint.

You can add the same constraint again, or use a different constraint, to see if the sketch-element turns BLACK.

In our defense, the Part-Editor is continuously improved.

Sometimes, a sketch-element is BLUE but you cannot add more constraints.

You might be able to drag a sketch-element that is blue, but when you come to add a new constraint, MechDesigner does not accept it.

In this case, a new constraint may remove two degrees-of-freedom, but the sketch has only one remaining degree-of-freedom.

Example:

Add Coincident Constraint between the center-Point of a Circle and a Point at the end of a Line.

If: the center-Point of the Circle has a Coincident Constraint with the Line

Then: it is not possible to also add a Coincident-Constraint with the start-Point or end-Point of the Line

1.SHIFT + Click the Line or the center-Point of the Circle so the Coincident-Constraint shows in the Selection-Window

2.Delete the Coincident Constraint using the Selection-Window