Sketcher ConstrainTangent

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Sketcher ConstrainTangent

Menu location
Sketch → Sketcher constraints → Constrain tangent
Workbenches
Sketcher
Default shortcut
T
Introduced in version
-
See also
Sketcher Constraint point on object

Description

The Sketcher ConstrainTangent tool constrains two edges, or an edge and an axis, to be tangent. Lines are treated as infinite, and arcs are treated as full circles/ellipses. The constraint can also connect two edges, forcing them to be tangent at the joint, making the joint smooth. If two lines are selected, or a line and the endpoint of another line, they are made collinear.

Usage

  1. Do one of the following (optional if the tool will run in continue mode):
    • Select two edges. The edges can be any edge a B-spline.
    • Select two endpoints belonging to different edges.
    • Select an edge and the endpoint of another edge (in any order).
    • Select a point and two edges (idem).
  2. There are several ways to invoke the tool:
    • Press the Constrain tangent button.
    • Select the Sketch → Sketcher constraints → Constrain tangent option from the menu.
    • introduced in version 0.22: Right-click in the 3D view and select the Constrain → Constrain tangent option from the context menu.
    • Use the keyboard shortcut: T.
  3. If the tool runs in continue mode:
    1. The cursor changes to a white cross with the tool icon.
    2. Optionally keep selecting elements:
      • Select two edges (see above).
      • Select a point and two edges (in that order).
      • Select an edge, a point and another edge (idem).
    3. To finish, right-click or press Esc, or start a another constraints or geometries tool.

Between two curves (direct tangency)

Two curves will be made tangent, and the point of tangency will be implicit. This mode is applied if two curves were selected.

Accepted selection:

  • line + line, circle, arc, ellipse, arc-of-ellipse
  • circle, arc + circle, arc

If direct tangency between selected curves is not supported (e.g. between a circle and an ellipse), a helper point will be added to sketch automatically, and tangency-via-point will be applied.

It is not recommended to reconstruct the point of tangency by creating a point and constraining it to lie on both curves. It will work, but the convergence will be seriously slower, jumpier, and will require about twice as many iterations to converge than normal. Use other modes of this constraint if the point of tangency is needed.

Between two endpoints (point-to-point tangency)

In this mode, the endpoints are made coincident, and the joint is made tangent (C1-smooth, or "sharp", depending on the placement of curves before the constraint is applied). This mode is applied when two endpoints of two curves were selected. If you want this kind of tangency you must not use concidence plus the tangency between the curves/lines. The solver cannot create stable solutions for this combination and replaces the constraints appropriately.

Accepted selection:

  • endpoint of line/arc/arc-of-ellipse + endpoint of line/arc/arc-of-ellipse (i.e., two endpoints of any two curves)

Between curve and endpoint (point-to-curve tangency)

In this mode, an endpoint of one curve is constrained to lie on the other curve, and the curves are forced tangent at the point. This mode is applied when a curve and an endpoint of another curve were selected.

Accepted selection:

  • line, circle, arc, ellipse, arc-of-ellipse + endpoint of line/arc/arc-of-ellipse (i.e., any curve + endpoint of any curve)


Between two curves at point (tangent-via-point) (v0.15)

In this mode, two curves are made tangent, and the point of tangency is tracked. This mode is applied when two curves and a point were selected.

Accepted selection:

  • any line/curve + any line/curve + any point

"Any point" can be a lone point, or a point of something, e.g. a center of a circle, an endpoint of an arc, or the origin.

For the constraint to work correctly, the point must be on both curves. So, as the constraint is invoked, the point will be automatically constrained onto both curves (helper constraints will be added, if necessary), and the curves will be forced tangent at the point. These helper constraints are plain regular constraints. They can be added manually, or deleted.

Compared to direct tangency, this constraint is slower, because there are more degrees of freedom involved, but if the point of tangency is needed, it is the recommended mode because it offers better convergence compared to direct tangency + point on two curves.

The placement of the point before the constraint is applied is a hint for the solver for where the tangency should be. With this constraint, one can constrain two ellipses to touch each other in two places.

Between two lines (collinear)

Accepted selection:

  • any line/vertex + any line/vertex

Scripting

Tangent Constraint can be created from macros and from the Python console by using the following:

# direct tangency
Sketch.addConstraint(Sketcher.Constraint('Tangent',icurve1,icurve2))

# point-to-point tangency
Sketch.addConstraint(Sketcher.Constraint('Tangent',icurve1,pointpos1,icurve2,pointpos2))

# point-to-curve tangency
Sketch.addConstraint(Sketcher.Constraint('Tangent',icurve1,pointpos1,icurve2))

# tangent-via-point (plain constraint, helpers are not added automatically)
Sketch.addConstraint(Sketcher.Constraint('TangentViaPoint',icurve1,icurve2,geoidpoint,pointpos))

where:

  • Sketch is a sketch object
  • icurve1, icurve2 are two integers identifying the curves to be made tangent. The integers are indexes in the sketch (the value, returned by Sketch.addGeometry).
  • pointpos1, pointpos2 should be 1 for start point and 2 for end point.
  • geoidpoint and pointpos in TangentViaPoint are the indexes specifying the point of tangency.

The Sketcher scripting page explains the values which can be used for incurve1, incurve2, pointpos1, pointpos2, geoidpoint and pointpos and contains further examples on how to create constraints from Python scripts.