手册:产品设计建模

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This page is a translated version of the page Manual:Modeling for product design and the translation is 23% complete.
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产品设计最初是商业术语,但在 3D 领域,通常意味着建模的时候,心里想着将来 3D 打印,或更一般地说,由机器(例如 3D 打印机或 CNC 机床)制造出来。

在 3D 世界打印物体时,最重要的是保证你的对象是 实体 的,因为它们将成为真实的,坚固的物体,这是显而易见的。当然,没有什么阻止它们在里面有空洞。但是总要明确哪个点在材料的里面,哪个点在外面。因为 3D 打印机或 CNC 机床需要确切地知道什么地方是填充/有材料,什么地方不填充/没有材料。因此,在 FreeCAD 中,PartDesign Workbench 是构建此类作品的完美工具,因为它会始终保证对象的实体性和可构建。

为了说明 PartDesign 的工作方式,我们练习建模著名的 Lego 玩具的这款模型。

Exercise lego 01.jpg

乐高积木有个很酷的特色——容易在互联网上获得零件的尺寸,至少对于标准件而言是这样。建模之后,在 3D 打印机上打印,都很容易。带着点耐心(3D 打印通常需要大量的调整和微调),可以制作出完全兼容的部件,完美地匹配原始乐高积木。下面的示例将制作比原始尺寸大 1.5 倍的零件。

这里专门使用 SketcherPartDesign 工具。由于Sketcher 工作台中的所有工具都包含在 PartDesign 工作台里了,我们可以就在 PartDesign 中工作,不需要在两者之间来回切换。

PartDesign 对象完全基于 Sketches。Sketch 是一个 2D 对象,由线段(线,圆弧或椭圆弧)和约束组成。这些约束可以应用于线段,或其端点和中心点,并强制几何元素满足某些规则。例如,可以在线段上施加垂直约束以强制它保持垂直,或者在端点上施加位置(锁定)约束以禁止它移动。当草图的约束数量刚好时,草图的任何点都无法再移动了,称为完全约束的草图。当存在冗余约束时,移除某些约束也不会令几何体允许移动,称为过度约束。这应该避免,如果出现这种情况,FreeCAD 会通知你。

Sketches have an edit mode, where their geometry and constraints can be changed. When you are done with editing, and leave edit mode, sketches behave like any other FreeCAD object, and can be used as building blocks for all the Part Design tools, but also in other workbenches, such as Part or Arch. The Draft workbench also has a tool that converts Draft objects to Sketches, and vice-versa.

  • Let's start by modeling a cubic shape that will be the base of our Lego brick. Later on we will carve the insides, and add the 8 dots on top of it. So let's start this by making a rectangular sketch that we will then extrude:
  • Switch to the PartDesign Workbench
  • Click on the Sketcher NewSketch.svg New Sketch button. A dialog will appear asking where you want to lie the sketch, choose the XY plane, which is the "ground" plane. The sketch will be created and will immediately be switched to edit mode, and the view will be rotated to look at your sketch orthogonally.
  • Now we can draw a rectangle, by selecting the Sketcher CreateRectangle.svg Rectangle tool and clicking 2 corner points. You can place the two points anywhere, since their correct location will be set in the next step.
  • You will notice that a couple of constraints have automatically been added to our rectangle: the vertical segments have received a vertical constraint, the horizontal ones a horizontal constraint, and each corner a point-on-point constraint that glues the segments together. You can experiment moving the rectangle around by dragging its lines with the mouse, all the geometry will keep obeying the constraints.

Exercise lego 02.jpg

  • Now, let's add three more constraints:
    • Select one of the vertical segments and add a Constraint VerticalDistance.svg Vertical Distance Constraint. Give it a size of 23.7mm.
    • Select one of the horizontal segments and add a Constraint HorizontalDistance.svg Horizontal Distance Constraint. Make it 47.7mm.
    • Finally, select one of the corner points, then the origin point (which is the dot at the crossing of the red and green axes), then add a Constraint PointOnPoint.svg Coincident Constraint. The rectangle will then jump to the origin point, and your sketch will turn green, meaning it is now fully constrained. You can try moving its lines or points, nothing will move anymore.

Exercise lego 03.jpg

Note that the last point-on-point constraint was not absolutely necessary. You are never forced to work with fully constrained sketches. However, if we are going to print this block in 3D, it will be necessary to maintain our piece close to the origin point (which will be the center of the space where the printer head can move). By adding that constraint we are making sure that our piece will always stay "anchored" to that origin point.

  • Our base sketch is now ready, we can leave edit mode by pressing the Close button on top of its task panel, or simply by pressing the Escape key. If needed later on, we can reenter edit mode anytime by double-clicking the sketch in the tree view.
  • Let's extrude it by using the PartDesign Pad.svg Pad tool, and giving it a distance of 14.4mm. The other options can be left at their default values:

Exercise lego 04.jpg

The Pad behaves very much like the Extrude tool that we used in the previous chapter. There are a couple of differences, though, the main one being that a pad cannot be moved. It is attached forever to its sketch. If you want to change the position of the pad, you must move the base sketch. In the current context, where we want to be sure nothing will move out of position, this is an additional security.

  • We will now carve the inside of the block, using the PartDesign Pocket.svg Pocket tool, which is the PartDesign version of Part Cut. To make a pocket, we will create a sketch on the bottom face of our block, which will be used to remove a part of the block.
  • With the bottom face selected, press the Sketcher NewSketch.svg New sketch button.
  • Draw a rectangle on the face.

Exercise lego 05.jpg

  • We will now constrain the rectangle in relation to the bottom face. To do this, we need to "import" some edges of the face with the Sketcher External.svg External geometry tool. Use this tool on the two vertical lines of the bottom face:

Exercise lego 06.jpg

You will notice that only edges from the base face can be added by this tool. When you create a sketch with a face selected, a relation is created between that face and the sketch, which is important for further operations. You can always remap a sketch to another face later with the Sketcher MapSketch.svg Map sketch tool.

  • The external geometry is not "real", it will be hidden when we leave edit mode. But we can use it to place constraints. Place the 4 following constraints:

Exercise lego 07.jpg

  • Leave edit mode and we can now perform the pocket operation: With the sketch selected, press the PartDesign Pocket.svg Pocket button. Give it a length of 12.6mm, which will leave the upper face of our pad with a thickness of 1.8mm (remember, the total height of our pad was 14.4mm).

Exercise lego 08.jpg

  • We will now attack the 8 dots on the top face. To do this, since they are a repetition of a same feature, we will use the handy PartDesign LinearPattern.svg Linear pattern tool of the Part Design Workbench, which allows to model once and repeat the shape.
  • Start by selecting the top face of our block
  • Create a Sketcher NewSketch.svg New sketch.
  • Create two Sketcher Circle.svg circles.
  • For each circle, select it and add a Sketcher ConstrainRadius.png Radius Constraint of 3.6mm to each of them
  • Import the left edge of the base face with the Sketcher External.svg External geometry tool.
  • Place two vertical constraints and two horizontal constraints of 6mm between the center point of each circle and the corner points of the imported edge, so each circle has its center at 6mm from the border of the face:

Exercise lego 09.jpg

  • Notice how, once again, when you lock the position and dimension of everything in your sketch, it becomes fully constrained. This always keeps you on the safe side. You could change the first sketch now, everything we did afterwards would keep tight.
  • Leave edit mode, select this new sketch, and create a PartDesign Pad.svg Pad of 2.7mm:

Exercise lego 10.jpg

  • Notice that, as earlier with the pocket, since we used the top face of our base block as a base for this latest sketch, any PartDesign operation we do with this sketch will correctly be built on top of the base shape: The two dots are not independent objects, they have been extruded directly from our brick. This is the great advantage of working with the Part Design Workbench, as long as you take care of always building one step on top of the previous one, you are actually building one final solid object.
  • We can now duplicate our two dots four times, so we get eight. Select the latest Pad we just created.
  • Press the PartDesign LinearPattern.svg Linear pattern button.
  • Give it a length of 36mm (which is the total "span" we want our copies to fit in), in the "horizontal sketch axis" direction, and make it 4 occurrences:

Exercise lego 11.jpg

  • Once again, see that this is not just a duplication of an object, it is a *feature* of our shape that has been duplicated, the final object is still only one solid object.
  • Now let's work on the three "tubes" that fill the void we created on the bottom face. We have several possibilities: create a sketch with three circles, pad it then pocket it three times, or create a base sketch with one circle inside the other and pad it to form the complete tube already, or even other combinations. Like always in FreeCAD, there are many ways to achieve the same result. Sometimes one way will not work the way we want, and we must try other ways. Here, we will take the safest approach, and do things one step at a time.
  • Select the face that is at the bottom of the hollow space we carved earlier inside the block.
  • Create a new sketch, add a circle with a radius of 4.8825mm, import the left border of the face, and constrain it vertically and horizontally at 10.2mm from the upper corner of the face:

Exercise lego 12.jpg

If you have trouble to select features hiding part of the model can help. To hide a feature select it from tree view and press Space-key to toggle visibility.

  • Leave edit mode, and pad this sketch with a distance of 12.6mm
  • Create a linear pattern from this last pad, give it a length of 24mm and 3 occurrences. We now have three filled tubes filling the hollow space:

Exercise lego 13.jpg

  • Now let's make the final holes. Select the circular face of the first of our three "pins"
  • Create a new sketch, import the circular border of our face, create a circle with a radius constraint of 3.6mm, and add a Constraint PointOnPoint.svg Point on Point Constraint between the center of the imported circle and our new circle. We now have a perfectly centered circle,and once again fully constrained:

Exercise lego 14.jpg

  • Leave edit mode, and create a pocket from this sketch, with a length of 12.6mm
  • Create a linear pattern from this pocket, with a length of 24mm and 3 occurrences. That's the last step, our piece of lego is now complete, so we can give it a nice color to mark our victory!

Exercise lego 15.jpg

You will notice that our modeling history (what appears in the tree view) has become quite long. This is precious because every single step of what we did can be changed later on. Adapting this model for another kind of brick, for example one with 2x2 dots, instead of 2x4, would be a piece of cake, we would just need to change a couple of dimensions and the number of occurrences in linear patterns. We could as easily create bigger pieces that don't exist in the original Lego game.

But we could also want to get rid of the history, for example if we are going to model a castle with this brick, and we don't want to have this whole history repeated 500 times in our file.

There are two simple ways to get rid of the history, one is using the Create simple copy tool from the Part Workbench, which will create a copy of our piece that doesn't depend anymore on the history (you can delete the whole history afterwards), the other way is exporting the piece as a STEP file and reimporting it.

Assembling

But the best of both worlds also exists, which is the Assembly2 Workbench, an addon that can be installed from the FreeCAD-addons repository. This Workbench is named "2" because there is also an official built-in Assembly Workbench in development, which is not ready yet. The Assembly2 Workbench, however, already works very well to construct assemblies, and also features a couple of object-to-object constraints which you can use to constrain the position of one object in relation to another. In the example below, however, it will be quicker and easier to position the pieces using Draft Move.svg Draft Move and Draft Rotate.svg Draft Rotate than using the Assembly2 constraints.

  • Save the file as it is now
  • Install the Assembly2 Workbench and restart FreeCAD
  • Create a new empty document
  • Switch to the Assembly2 workbench
  • Press the Import a part from another FreeCAD document button
  • Select the file we saved above
  • The final piece will be imported in the current document. The Assembly2 workbench will determine automatically what is the final piece in our file that needs to be used, and the new object stays linked to the file. If we go back and modify the contents of the first file, we can press the Update parts imported into the assembly button to update the pieces here.
  • By using the Import a part from another FreeCAD document button several times, and moving and rotating the pieces (with the Draft tools or by manipulating their Placement property), we can quickly create a small assembly:

Exercise lego 16.jpg

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