Tutorial di Sketcher

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Introduzione

Questo tutorial ha lo scopo di introdurre il lettore al flusso di lavoro di base dell'ambiente Schizzo. Creeremo un disegno simile a quello mostrato qui sotto.

This tutorial is meant to introduce the reader to the basic workflow of the Workbench Sketcher.svg Sketcher Workbench.

L'ambiente Sketcher esiste come modulo autonomo, ma è accessibile anche dall'ambiente PartDesign quando è necessario per creare un profilo.

The reader will practice:

  • Creating construction geometry
  • Creating real geometry
  • Applying geometric constraints
  • Applying datum constraints
  • Obtaining a closed profile

For a more in depth description of the sketcher, read the Sketcher reference.

00 Sk01 Sketcher fully constrained final.png

Sketcher tutorial result.png

Requisiti

  • FreeCAD versione 0.16 o superiore

1. Open FreeCAD, create a new empty document with File → Std New.svg 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 Sketcher EditSketch.svg Edit sketch.

Procedura

Creare uno Schizzo

  1. Creare un nuovo documento
  2. Passare nell'ambiente Sketcher utilizzando il menu a discesa o cliccando su Visualizza> Workbench> Sketcher.
  3. Selezionare Sketcher NewSketch.png Nuovo schizzo

2. Click on Sketcher NewSketch.svg New sketch.

2.1. Choose the sketch orientation, that is, one of the base XY, XZ, or YZ planes. Also choose if you want an inverted orientation, and an offset from the base plane.
2.2. We will use the default plane and options.
2.3. Click OK to start constructing the sketch.

All'interno di Sketcher è possibile utilizzare la maggior parte degli strumenti che questo ambiente mette a disposizione.

Nella Vista combinata, espandere le opzioni Modifica controlli e abilitare gli Autovincoli.

01 Sk01 Sketcher Task panel.png

Upper part of the task panel of the sketcher.


Creare la geometria

La geometria di costruzione dello schizzo

La geometria di costruzione viene utilizzata come guida per creare dei profili complessi. Per accedervi, è necessario attivare la modalità Costruzione.

  1. Selezionare Sketcher ToggleConstruction.png Costruzione
  2. Selezionare Sketcher Line.png Linea da 2 punti
  3. Nello schizzo avvicinare il cursore al punto di origine, il punto viene evidenziato e vicino al cursore appare questa iconaSketcher ConstrainCoincident.png.
  4. Selezionare il punto e tracciare una linea in diagonale con una lunghezza arbitraria.
  5. Ripetere questa procedura fino a creare cinque linee di costruzione. Accertarsi che siano disposte a raggera, come nella figura iniziale.
  6. Per uscire dalla modalità di costruzione, è sufficiente cliccare nuovamente su Sketcher ToggleConstruction.png Costruzione

Note: up to this point the line tool is still active. This means we can keep clicking on the 3D view to draw as many lines as we want. If we wish to exit this tool, we can press the right mouse button, or press Esc in the keyboard once. By doing this the pointer won't create lines any more, it will just be a pointer allowing us to select the objects we just created. In this pointer mode we can pick and drag the endpoints of each line to adjust its placement.

Note 2: do not press Esc a second time as this will exit the sketch edit mode. If you do this, re-enter the edit mode by double clicking on the sketch in the tree view.

Take a look at the task panel again. The Solver messages section already indicates that the sketch is under-constrained, and it mentions the number of degrees of freedom.

Look at the Constraints and Elements sections to see the new listed constraints and lines. Once your sketches have many elements, it may be difficult to select them in the 3D view, so you can use these lists to select the object that you wish exactly.

02 Sk01 Sketcher construction.png

Construction lines forming a star shape with its center in the origin.


La geometria dello schizzo

La geometria dello schizzo viene utilizzata per creare i profili chiusi necessari per eseguire le operazioni 3D nell'ambiente PartDesign.

  1. Selezionare Sketcher Circle.png Cerchio
  2. Posizionare il suo centro nell'origine dello schizzo.
  3. Estendere la circonferenza per una lunghezza arbitraria.
  1. Selezionare Sketcher Arc.png Arco
  2. Avvicinarsi al punto finale di una linea di costruzione.
  3. Impostare il centro dell'arco coincidente con il punto finale.
  4. Selezionare una posizione arbitraria del cursore da impostare come inizio dell'arco, cliccando una volta.
  5. Estendere arbitrariamente l'arco, assicurandosi che la circonferenza si apra verso l'esterno (lo spazio vuoto sia rivolto verso il cerchio creato in precedenza).
  6. Ripetere questi passaggi per ogni linea di costruzione.

Outer arcs

4. Create a circle.

4.1. Click on Sketcher Circle.svg Create circle.
4.2. Click on the origin of the sketch to position its center point.
4.3. Click anywhere in the 3D view to set the circumference radius as a distance from the origin. Make it approximately 8 mm. Again the dimension will be fixed later.

5. Create a series of arcs.

5.1. Click on Sketcher Arc.svg Create arc.
5.2. Approach the endpoint of one of the construction lines, and click on it. This will set the center point of the circular arc to be Constraint PointOnPoint.svg coincident with this line's endpoint.
5.3. Click once in the 3D view at an arbitrary location to set simultaneously the radius of the arc, and the first endpoint of it. Define an approximate radius of 8 mm.
5.4. Move the pointer in an anti-clockwise direction to trace an arc that has its concavity pointing towards the origin of the sketch. Click to set the final endpoint of the arc, defining a circular arc that approximately sweeps 180° or half a circle.
5.5. Repeat these steps with each construction line, so that each of them has a circular arc at its tip. We will call these O-arcs for outwards-arcs.

03 Sk01 Sketcher outer arcs.png

Circular arcs added at the endpoints of the construction lines. Also a central circle.


Inner arcs

  1. Creare un arco tra ogni coppia dei precedenti archi, con la loro circonferenza rivolta verso il cerchio.

To summarize, the O-arcs should have their curvature pointing outwards, and their concavity pointing towards the origin of the sketch; the I-arcs should have their curvature pointing inwards, and their concavity pointing away from the same origin.

04 Sk01 Sketcher inner arcs.png

Circular arcs added between the first set of arcs placed.


Constraints

Take a look at the task panel again. Due to the new geometrical elements that we have drawn, the Solver messages section indicates even more degrees of freedom. A degree of freedom (DOF) indicates a possible movement of one element. For example, a point can be moved both in horizontal and vertical directions, so it has two degrees of freedom. A line is defined by two points, therefore in total it has four degrees of freedom. If we fix one of those points, then the entire system has only two degrees of freedom available; if we additionally fix the horizontal movement of the remaining point, we only have one degree of freedom left; and if we also fix the vertical movement of this point, then the last degree of freedom disappears, and the line cannot move from its position any more.

I vincoli

I vincoli sono usati per determinare i gradi di libertà dei punti e delle curve all'interno di Sketcher.

There are two principal types of constraints:

  • Geometric constraints define characteristics of the shapes without specifying exact dimensions, for example, horizontality, verticality, parallelism, perpendicularity, and tangency.
  • Datum constraints define characteristics of the shapes by specifying dimensions, for example, a numeric length or an angle.
I vincoli geometrici

Essi sono utilizzati per stabilire le relazioni tra i punti e le curve senza usare le dimensioni.

Equal length and radius

  1. Selezionare tutte e cinque le linee di costruzione.
  2. Selezionare Sketcher ConstrainEqual.png Uguale lunghezza
7.3. Select all five O-arcs, those centered on an endpoint of a construction line.
7.4. Press Constraint EqualLength.svg Equal length.
7.5. Repeat with all I-arcs, those between the O-arcs.
Note: again the constraints are chained. Therefore all O-arcs will have the same radius, and all I-arcs will have the same radius. At this moment, the specific value of these lengths is not fixed. You may use the pointer to drag a point and see how the sketch is updated while respecting the constraints in place.
  1. Selezionare la linea di costruzione che è più vicina all'asse verticale.
  2. Selezionare Constraint Vertical.png Verticale

Note: as you add constraints, overlay symbols indicating the type of constraint appear over the geometry in the 3D view. If these symbols obfuscate your view, you can hide them by unchecking the constraint in the task panel. Also note that the number of degrees of freedom decreases after adding each constraint.

Note 2: if you wish to temporarily disable the constraint, you may select it and press Sketcher ToggleActiveConstraint.svg Toggle active constraint. When you want to apply it again, press again the same button.

05a Sk01 Sketcher equality constraints lines.png 05b Sk01 Sketcher equality constraints O-arcs.png

05c Sk01 Sketcher equality constraints I-arcs.png

Sketch with equality constraints applied to the construction lines, and to the two sets of arcs.


Tangency

  1. Selezionare il punto finale in un arco e il più vicino punto finale di un altro arco.
  2. Selezionare Constraint Tangent.png Tangente
  3. Ripetere l'operazione per ogni punto finale, fino a quando il profilo viene chiuso.

Note: applying the tangential constraint very often will move the geometry around in order to produce a smooth connection. You may have to use the pointer to reposition the points a bit before applying the next tangential constraint. Try placing the endpoints in such a way that two arcs aren't too far apart, so they can be connected with a short line rather than a long line.

A questo punto, il profilo è chiuso e può essere regolato con le dimensioni desiderate.

06 Sk01 Sketcher tangency constraints.png

Sketch with tangential constraints applied to the arcs, which closes the shape.


I vincoli di dati

Questi vincoli sono utilizzati per specificare la distanza tra i punti in una determinata direzione e le dimensioni delle curve.

These constraints specify the numerical distances between two points, and angles between two lines.

Distances and angles

  1. Selezionare la linea di costruzione vincolata verticalmente.
  2. Selezionare Sketcher ConstrainDistanceY.png Lunghezza verticale
  3. Impostare la lunghezza a 30 mm.
  1. Selezionare la linea di costruzione verticale e la linea più vicina ad essa
  2. Selezionare Constraint InternalAngle.png Angolo interno
  3. Impostare l'angolo a 72°
  4. Ripetere la stessa procedura per ciascuna coppia di linee adiacenti.

07a Sk01 Sketcher length constraint.png 07b Sk01 Sketcher angle constraint.png

Sketch with length constraint applied to one vertical construction line (left), and angle constraints to three pairs of construction lines (right).


Radius

  1. Selezionare uno degli archi centrati sul punto finale di una linea.
  2. Selezionare Sketcher ConstrainRadius.png Raggio
  3. Impostare il raggio a 8 mm.
  4. Fare la stessa cosa per un arco non centrato su un punto finale. Impostare la distanza a 11 mm.
  5. Impostare il raggio del cerchio centrale a 10 mm.

08a Sk01 Sketcher radius 1 constraint.png 08b Sk01 Sketcher radius 2 constraint.png

Sketch with radius constraints applied to the outwards arcs (left), and inwards arcs (right).


11.7. Finally, select the circle in the center of the sketch, press Constraint Radius.svg Radius, and set the value to 8 mm.

Alla fine si dovrebbe ottenere uno schizzo completamente vincolato. Questo è confermato dal cambiamento di colore di tutte le curve.

09 Sk01 Sketcher fully constrained.png

Sketch with all geometrical and datum constraints applied.


Extrusion

12. Now that we have a fully constrained sketch, it can be used to create a solid body.

12.1. Exit the sketch edit mode by pressing the Close button, or pressing Esc twice. The sketch should appear in the tree view and the 3D view.
12.2. Switch to the PartDesign Workbench.
12.3. With the sketch selected in the tree view, press PartDesign Body.svg PartDesign Body, choose the default XY-plane, and press OK. The sketch should appear now inside the Body.
12.4. Select the sketch, and then press PartDesign Pad.svg PartDesign Pad, choose the default options, and press OK to create a solid extrusion.

09b Sk01 Sketcher fully constrained clean.png 10 Sk01 Sketcher solid extrusion.png

Left: fully constrained sketch with only the most important constraints showing. Right: solid extrusion produced with PartDesign Pad.


Additional information

For a more in depth description of the sketcher, visit the Sketcher Workbench documentation and also read the Sketcher reference.

Constraining a sketch can be done in many different ways. In general, it is recommended to use geometrical constraints first, and minimize the number of datum constraints, as this simplifies the task of the internal constraint solver. To investigate this, repeat this example, now adding the constraints in different order.

  • First constrain the construction lines before drawing the arcs.
  • Or constrain the size of the arcs before making them tangent.
  • Or set the angle of the construction lines before adding more elements.
  • Try using other construction geometry.