Sketcher Micro Tutorial - Constraint Practices
Tutorial |
Topic |
---|
Sketcher |
Level |
Beginner |
Time to complete |
30 minutes |
Authors |
Mark Stephen (Quick61) and vocx |
FreeCAD version |
0.19 |
Example files |
Sketcher Constraints practices |
See also |
None |
Introduction
This tutorial was originally written by Quick61, and it was rewritten and reillustrated by vocx.
This tutorial is designed to help the new user become familiar with the best practices of constraining a Sketch in the workflow of the Sketcher Workbench.
There is a general rule with constraints: the fewer amount of datum constraints (dimensions), the better.
It is preferable to use a geometric constraint in place of a dimensional one if possible. This has to do with the internal workings of the Sketcher's constraint solver.
Setup
1. Open FreeCAD, create a new empty document with File → New.
- 1.1. Switch to the Sketcher Workbench from the workbench selector, or the menu View → Workbench → Sketcher.
Some actions to remember:
- Press the right mouse button, or press Esc in the keyboard once, to deselect the active tool in edit mode.
- To exit the sketch edit mode, press the Close button in the task panel, or press Esc twice in the keyboard.
- To enter again edit mode, double click on the sketch in the tree view, or select it, and then click on Edit sketch.
Create a sketch
2. Click on New sketch.
- 2.1. Choose the sketch orientation, that is, one of the base XY, XZ, or YZ planes. We will use the default plane and options.
- 2.2. Click OK to start constructing the sketch.
Note: in the task panel expand the Edit controls section, and make sure the Auto constraints option is disabled. Also turn off the grid snap, and hide the grid.
First approach: datum constraints
3. We will draw a fully constrained square, centered at the origin.
- 3.1. Click on Create polyline, then trace four lines in the general shape of a rectangle around the origin.
Unconstrained rectangular sketch.
- 3.2. Select one horizontal line, and press Horizontal distance, then enter
20 mm
. - 3.3. Select the other horizontal line, and repeat the constraint with the same distance.
- 3.4. Select one vertical line, and press Vertical distance, then enter
20 mm
. - 3.5. Select the other vertical line, and repeat the constraint with the same distance.
- 3.6. Select one bottom corner point (a), and the origin of the sketch, and press Horizontal distance, then enter
10 mm
. - 3.7. Select the top corner point (b) above the previous corner point (a), and the origin of the sketch, and repeat the horizontal constraint with the same distance.
- 3.8. Select the other bottom corner point (c), and the origin of the sketch, and press Vertical distance, then enter
10 mm
. - 3.9. Select the top corner point (b) again, and the origin of the sketch, and repeat the vertical constraint with the same distance.
Left: datum constraints for the sides. Right: additional datum constraints for the interior distances.
Looking at the Constraints section in the task panel, we see that the constraints are too many; they also clutter the view of the sketch. These constraints are also computationally intensive for the solver; while this is not an issue with a simple shape, it can become one with more complex shapes.
A better way: datum and geometric constraints
4. We will draw the same square fully constrained, and centered at the origin. When you create the new sketch, make sure the Auto constraints option is disabled.
- 4.1. Click on Create polyline, then trace four lines in the general shape of a rectangle around the origin.
- 4.2. Select one horizontal line, and press Horizontal.
- 4.3. Select the other horizontal line, and repeat the constraint.
- 4.4. Select one vertical line, and press Vertical.
- 4.5. Select the other vertical line, and repeat the constraint.
Geometrical horizontal and vertical constraints.
- 4.6. Select one horizontal line, and press Horizontal distance, then enter
20 mm
. We see that the other horizontal line changes size at the same time. - 4.7. Select one vertical line, and press Vertical distance, then enter
20 mm
. We see that the other vertical line changes size at the same time. - 4.8. Select one bottom corner point (a), and the origin of the sketch, and press Horizontal distance, then enter
10 mm
. - 4.9. Select the top corner point (b) above the previous corner point (a), and the origin of the sketch, and press Vertical distance, then enter
10 mm
.
Left: datum constraints for only two sides. Right: additional datum constraints for only two interior distances.
This is a better constrained sketch than the first one. The horizontal and vertical geometric constraints allow us to use fewer datum constraints, so our sketch looks less cluttered.
Optimum scheme: mostly geometric constraints
5. We will draw the same square fully constrained, and centered at the origin. When you create the new sketch, make sure the Auto constraints option is disabled.
- 5.1. Click on Create polyline, then trace four lines in the general shape of a rectangle around the origin.
- 5.2. Select one horizontal line, and press Horizontal.
- 5.3. Select the other horizontal line, and repeat the constraint.
- 5.4. Select one vertical line, and press Vertical.
- 5.5. Select the other vertical line, and repeat the constraint.
Geometrical horizontal and vertical constraints.
- 5.6. Select one bottom corner point (a), then the top corner point that is diagonally opposite, and then the origin of the sketch; then press Symmetric. The two selected points will be equidistant from the origin.
- 5.7. Select two adjacent sides of the rectangle (connected at one corner), and press Equal length. Notice that due to the symmetry of the corner points, all sides are now of the same size.
Left: symmetric constraint for only two corner points. Right: additional equal length distances for only two adjacent sides.
- 5.8. Select one horizontal line, and press Horizontal distance, then enter
20 mm
. Due to the symmetric and length equality constraints previously added, we see that all sides become equal at the same time.
All geometric constraints applied, and a single datum constraint for a side.
This is the best way to constrain this sketch, as we only used one datum (dimensional) constraint.
Additional resources
- General: Create sketch, Edit sketch, Attach sketch, Reorient sketch, Validate sketch, Merge sketches, Mirror sketch, Leave sketch, View sketch, View section, Toggle grid, Toggle snap, Configure rendering order, Stop operation
- Sketcher geometries: Point, Polyline, Line, Arc, Arc by 3 points, Arc of ellipse, Arc of hyperbola, Arc of parabola, Circle, Circle by 3 points, Ellipse, Ellipse by 3 points, Rectangle, Centered rectangle, Rounded rectangle, Triangle, Square, Pentagon, Hexagon, Heptagon, Octagon, Regular polygon, Slot, Arc slot, B-spline by control points, Periodic B-spline by control points, B-spline by knots, Periodic B-spline by knots, Toggle construction geometry
- Sketcher constraints:
- Dimensional constraints: Dimension, Horizontal distance, Vertical distance, Distance, Auto radius/diameter, Radius, Diameter, Angle, Lock
- Geometric constraints: Coincident (unified), Coincident, Point on object, Horizontal/vertical, Horizontal, Vertical, Parallel, Perpendicular, Tangent or collinear, Equal, Symmetric, Block
- Other constraints: Refraction (Snell's law)
- Constraint tools: Toggle driving/reference constraint, Activate/deactivate constraint
- Sketcher tools: Fillet, Chamfer, Trim, Split, Extend, External geometry, Carbon copy, Select origin, Select horizontal axis, Select vertical axis, Array transform, Polar transform, Scale transform, Offset geometry, Symmetry, Remove axes alignment, Delete all geometry, Delete all constraints
- Sketcher B-spline tools: Convert geometry to B-spline, Increase B-spline degree, Decrease B-spline degree, Increase knot multiplicity, Decrease knot multiplicity, Insert knot, Join curves
- Sketcher visual: Select unconstrained DoF, Select associated constraints, Select associated geometry, Select redundant constraints, Select conflicting constraints, Show/hide circular helper for arcs, Show/hide B-spline degree, Show/hide B-spline control polygon, Show/hide B-spline curvature comb, Show/hide B-spline knot multiplicity, Show/hide B-spline control point weight, Show/hide internal geometry, Switch virtual space
- Additional: Sketcher Dialog, Preferences, Sketcher scripting
- Getting started
- Installation: Download, Windows, Linux, Mac, Additional components, Docker, AppImage, Ubuntu Snap
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- Help: Tutorials, Video tutorials
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