草图工作台

From FreeCAD Documentation
Revision as of 12:46, 10 October 2017 by Starofrainnight (talk | contribs)
一个基本的,全约束的素描

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.

素描工作台本身具有约束条件 - 允许将2D形状约束到精确的几何定义。以及一个约束求解器,它计算二维几何约束范围,并允许对素描自由度的交互式探索。

约束素描的基础

为了解释素描器的工作原理,将其与“传统的”起草方式进行比较可能是有用的。

传统起草

CAD绘图的传统方式继承于旧的[1][2] 手动绘制,用于生产技术图纸(也称为蓝图)。对象被精确地绘制到预期的尺寸或尺寸上。如果要从(0,0)开始绘制长度为100mm的水平线,您可以激活线条工具,点击屏幕或输入第一个点的(0,0)坐标,然后再次点击或在(100,0)处输入第二点坐标。或者你会画你的线,而不考虑它的位置,然后移动它。绘制几何图形后,可以向其添加尺寸。

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.

Template:Sketcher Tools

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