Difference between revisions of "Sketcher Workbench/sv"

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''A basic, fully-constrained sketch‎.''
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=== Basics of constraint sketching ===
 
=== Basics of constraint sketching ===

Revision as of 22:41, 3 August 2018

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The Sketcher Workbench is used to create 2D geometries intended for use in the Part Design Workbench and other workbenches. Generally a 2D geometry is considered the starting-point for most CAD models - a simple 2D sketch can be "extruded" into a 3D shape, further 2D sketches can be used to create pockets in the surface of this shape and sketches can be used to define 'pads' (extrusions) on the surface of 3D objects. Along with Boolean operations, the Sketcher forms the core of generative solid shape design.

The Sketcher workbench itself features constraints - allowing 2D shapes to be constrained to precise geometrical definitions. And a constraint solver which calculates the constrained-extent of 2D geometry and allows interactive exploration of sketch degrees-of-freedom.

FC ConstrainedSketch.png

A basic, fully-constrained sketch‎.

Basics of constraint sketching

To explain how the Sketcher works, it may be useful to compare it to the "traditional" way of drafting.

Traditional Drafting

The traditional way of CAD drafting inherits from the old drawing board. Orthogonal (2D) views are drawn manually and intended for producing technical drawings (also known as blueprints). Objects are drawn precisely to the intended size or dimension. If you want to draw an horizontal line 100mm in length starting at (0,0), you activate the line tool, either click on the screen or input the (0,0) coordinates for the first point, then make a second click or input the second point coordinates at (100,0). Or you will draw your line without regard to its position, and move it afterwards. When you've finished drawing your geometries, you add dimensions to them.

Constraint Sketching

The Sketcher moves away from this logic. Objects do not need to be drawn exactly as you intend 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.

What are constraints?

Instead of dimensions, 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, it can be stretched, and it 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.

There are two kinds of constraints: geometric and dimensional. They are detailed in the 'The tools' section below.

What the Sketcher is not good for

The Sketcher is not intended for producing 2D blueprints. Once sketches are used to generate a solid feature, they are automatically hidden. Constraints are only visible in Sketch edit mode.

If you only need to produce 2D views for print, and don't want to create 3D models, check out the Draft workbench (keep in mind though that the Draft workbench can also be useful to create 2D geometry not available in the Sketcher at this time, like B-Splines.)

Sketching Workflow

A Sketch is always 2-dimensional (2D). To create a solid, a 2D Sketch of a single enclosed area is created and then either Padded or Revolved to add the 3rd dimension, creating a 3D solid from the 2D Sketch.

If the Sketch has segments that cross one another, places where a Point is not directly on a segment, or places where there are gaps between endpoints of adjacent segments, Pad or Revolve won't create a solid. The exception to this rule is that it doesn't apply to Construction (blue) Geometry.

Inside the enclosed area we can have smaller non-overlapping areas. These will become voids when the 3D solid is created.

The tools

The Sketcher Workbench tools are all located in the Sketcher menu that appears when you load the Sketcher Workbench.

Sketcher Geometries

These are tools for creating objects.

  • Sketcher CreatePoint.png Point: Draws a point
  • Sketcher Arc.png Arc: Draws an arc segment from center, radius, start angle and end angle
  • Sketcher Circle.png Circle: Draws a circle from center and radius
  • Sketcher Line.png 2-point Line: Draws a line segment from 2 points
  • Sketcher CreatePolyline.png Polyline (multiple-point line): Draws a line made of multiple line segments
  • Sketcher CreateRectangle.png Rectangle: Draws a rectangle from 2 opposite points
  • Sketcher CreateFillet.png Fillet: Makes a fillet between two lines joined at one point. Select both lines or click on the corner point, then activate the tool.
  • Sketcher Trimming.png Trimming: Trims a line, circle or arc with respect to the clicked point.
  • Sketcher External.png External Geometry: Creates an edge linked to external geometry.
  • Sketcher AlterConstruction.png Construction Mode: Toggles an element to/from construction mode. A construction object will not be used in a 3D geometry operation.

Sketcher Constraints

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

  • Sketcher ConstrainLock.png Lock: Creates a lock constraint on the selected item by setting vertical and horizontal dimensions relative to the origin (dimensions can be edited afterwards).
  • Sketcher ConstrainCoincident.png Coincident: Creates a coincident (point-on-point) constraint between two selected points.
  • Sketcher ConstrainPointOnObject.png Point On Object: Creates a point-on-object constraint on selected items.
  • Sketcher ConstrainDistanceX.png Horizontal Distance: Fixes the horizontal distance between 2 points or line ends. If only one item is selected, the distance is set to the origin.
  • Sketcher ConstrainDistanceY.png Vertical Distance: Fixes the vertical distance between 2 points or line ends. If only one item is selected, the distance is set to the origin.
  • Constraint Vertical.png Vertical: Creates a vertical constraint to the selected lines or polylines elements. More than one object can be selected.
  • Constraint Horizontal.png Horizontal: Creates a horizontal constraint to the selected lines or polylines elements. More than one object can be selected.
  • Sketcher ConstrainDistance.png Length: Creates a length constraint on a selected line.
  • Sketcher ConstrainRadius.png Radius: Creates a radius constraint on a selected arc or circle.
  • Sketcher ConstrainParallel.png Parallel: Creates a parallel constraint between two selected lines.
  • Sketcher ConstrainPerpendicular.png Perpendicular: Creates a perpendicular constraint between two selected lines.
  • Constraint InternalAngle.png InternalAngle: Creates an internal angle constraint between two selected lines.
  • Constraint Tangent.png Tangent: Creates a tangent constraint between two selected entities, or a colinear constraint between two line segments.
  • Sketcher ConstrainEqual.png Equal Length: Creates an equality constraint between two selected entities. If used on circle or arcs, the radius will be set equal.
  • Sketcher ConstrainSymmetric.png Symmetric: Creates‎ a symmetric constraint between 2 points with respect to a line.

Other

  • Sketcher NewSketch.png New Sketch: Creates‎ a new sketch on a selected face or plane. If none were selected, the default work plane XY will be used.
  • Sketcher ViewSketch.png View sketch: Sets the model view perpendicular to the sketch plane.
  • Sketcher MapSketch.png Map sketch: maps a sketch to the previously selected face of a solid.
  • Sketcher LeaveSketch.png Leave Sketch: Leave the Sketch editing mode.
  • Sketcher Reorient: Allows you to change the position of a sketch
  • Valida sketch: It allows you to check if there are in the tolerance of different points and to match them.


Best Practices

Every CAD user develops his own way of working over time, but there are some useful general principles to follow.

  • A series of simple sketches is easier to manage than a single complex one. For example, a first sketch can be created for the base 3D feature (either a pad or a revolve), while a second one can contain holes or cutouts (pockets). Some details can be left out, to be realized later on as 3D features. You can choose to avoid fillets in your sketch if there are too many, and add them as a 3D feature.
  • Always create a closed profile, or your sketch won't produce a solid, but rather a set of open faces. If you don't want some of the objects to be included in the solid creation, turn them to construction elements with the Construction Mode tool.
  • Use the auto constraints feature to limit the number of constraints you'll have to add manually.
  • As a general rule, apply geometric constraints first, then dimensional constraints, and lock your sketch last. But remember: rules are made to be broken. If you're having trouble manipulating your sketch, it may be useful to constrain a few objects first before completing your profile.
  • If possible, center your sketch to the origin (0,0) with the lock constraint. If your sketch is not symmetric, locate one of its points to the origin, or choose nice round numbers for the lock distances. In v0.12, external constraints (constraining the sketch to existing 3D geometry like edges or to other sketches) are not implemented. This means that to locate following sketches geometry to your first sketch, you'll need to set distances relative to your first sketch manually. A lock constraint of (25,75) from the origin is more easily remembered than (23.47,73.02).
  • If you have the possibility to choose between the Length constraint and the Horizontal or Vertical Distance constraints, prefer the latter. Horizontal and Vertical Distance constraints are computationally cheaper.
  • In general, the best constraints to use are: Horizontal and Vertical Constraints; Horizontal and Vertical Length Constraints; Point-to-Point Tangency. If possible, limit the use of these: the general Length Constraint; Edge-to-Edge Tangency; Fix Point Onto a Line Constraint; Symmetry Constraint.


Tutorials