4.4.4      Sketcher Constraints

Objects do not need to be drawn exactly as intending to, because they will be defined later on by constraints. Objects can be drawn loosely, and as long as they are unconstrained, can be modified. They are in effect "floating" and can be moved, stretched, rotated, scaled, and so on. This gives great flexibility in the design process.

Constraints are used to limit the degrees of freedom of an object. For example, a line without constraints has 4 Degrees Of Freedom (abbreviated as "DOF"): it can be moved horizontally or vertically, can be stretched, and can be rotated.

Applying a horizontal or vertical constraint, or an angle constraint (relative to another line or to one of the axes), will limit its capacity to rotate, thus leaving it with 3 degrees of freedom. Locking one of its points in relation to the origin will remove another 2 degrees of freedom. And applying a dimension constraint will remove the last degree of freedom. The line is then considered fully-constrained.

Multiple objects can be constrained between one another. Two lines can be joined through one of their points with the coincident point constraint. An angle can be set between them, or they can be set perpendicular. A line can be tangent to an arc or a circle, and so on. A complex Sketch with multiple objects will have a number of different solutions, and making it fully-constrained means that just one of these possible solutions has been reached based on the applied constraints.

Constraints are used to define lengths, set rules between sketch elements, and to lock the sketch along the vertical and horizontal axes.

Constraints not associated with numerical data:

 - Coincident: affixes a point onto (coincident with) one or more other points.

 - Point On Object: affixes a point onto another object such as a line, arc, or axis.

 - Vertical: constrains the selected lines or polyline elements to a true vertical orientation. More than one object can be selected before applying this constraint.

 - Horizontal: constrains the selected lines or polyline elements to a true horizontal orientation. More than one object can be selected before applying this constraint.

 - Parallel: constrains two or more lines parallel to one another.

 - Perpendicular: constrains two lines perpendicular to one another, or constrains a line perpendicular to an arc endpoint.

 - Tangent: creates a tangent constraint between two selected entities, or a co-linear constraint between two line segments. A line segment does not have to lie directly on an arc or circle to be constrained tangent to that arc or circle.

 - Equal Length: constrains two selected entities equal to one another. If used on circles or arcs their radii will be set equal.

 - Symmetric: constrains two points symmetrically about a line, or constrains the first two selected points symmetrically about a third selected point.

Constraints associated with numerical data:

 - Lock: constrains the selected item by setting vertical and horizontal distances relative to the origin, thereby locking the location of that item. (These constraint distances can be edited later.)

 - Horizontal Distance: fixes the horizontal distance between two points or line endpoints. If only one item is selected, the distance is set to the origin.

 - Vertical Distance: fixes the vertical distance between 2 points or line endpoints. If only one item is selected, the distance is set to the origin.

 - Length: defines the distance of a selected line by constraining its length, or defines the distance between two points by constraining the distance between them.

 - Radius: defines the radius of a selected arc or circle by constraining the radius.

 - Internal Angle: defines the internal angle between two selected lines.

See also General Overview, Sketcher Operations, Sketcher Geometries, Sketch Tools and Good Practises chapters for more information.