Manuale:Usare i fogli di calcolo

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FreeCAD dispone di un altro ambiente di lavoro interessante da esplorare: l'ambiente Spreadsheet. Questo ambiente permette di creare direttamente in FreeCAD un foglio di calcolo come quelli fatti con Excel o con LibreOffice. Questi fogli di calcolo possono essere popolati con dei dati estratti dal modello, e possono anche eseguire una serie di calcoli tra i valori. I fogli di calcolo possono essere esportati come file CSV, che possono essere importati in qualsiasi altra applicazione che gestisca i foglio di calcolo.

In FreeCAD, però, fogli di calcolo hanno un'utilità aggiuntiva: le loro celle possono ricevere un nome, e possono quindi essere referenziate da qualsiasi campo supportato dal motore delle espressioni. Questo trasforma fogli di calcolo in potenti strutture di controllo, in cui i valori inseriti nelle specifiche celle possono guidare le dimensioni del modello. C'è solo una cosa da tenere a mente, dato che FreeCAD vieta le dipendenze circolari tra oggetti, uno stesso foglio non può essere utilizzato per impostare una proprietà di un oggetto e allo stesso tempo recuperare il valore della proprietà dallo stesso oggetto. Ciò renderebbe il foglio di calcolo e l'oggetto dipendenti l'uno dall'altro.

Nel seguente esempio, creeremo un paio di oggetti, recupereremo alcune delle loro proprietà in un foglio di calcolo, e quindi utilizzeremo il foglio di calcolo per guidare direttamente le proprietà di altri oggetti.

Leggere le proprietà

  • Start by switching to the Part Workbench, and create a couple of objects: a Part Box.png box, a Part Cylinder.png cylinder and a Part Sphere.png sphere.
  • Edit their Placement property (or use the Draft Move.png Draft Move tool) to place them a little apart, so we can watch better the effects of what we'll do:

Exercise spreadsheet 01.jpg

  • Now, lt's extract some information about these objects. Switch to the Spreadsheet Workbench
  • Press the Spreadsheet Create.png New Spreadsheet button
  • Double-click the new Spreadsheet object in the tree view. The spreadsheet editor opens:

Exercise spreadsheet 02.jpg

The spreadsheet editor of FreeCAD, although it is not as complete and powerful as the more complete spreadsheet applications we listed above, has nevertheless most of the basic tools and functions that are commonly used, such as the possibility to change the aspect of the cells (size, color, alignment), join and split cells, use formulas such as =2+2, or reference other cells with =B1.

In FreeCAD, to these common behaviours, has been added one very interesting: The possibility to reference not only other cells, but other objects from the document, and retrieve values from their properties. For example, let's retrieve a couple of properties from the 3 objects we created above. Properties are what we can see in the properties editor window, under the Data tab, when an object is selected.

  • Let's start by entering a couple of texts in the cells A1, A2 amd A3, so we remember what is what later on, for example Cube Length, Cylinder Radius and Sphere Radius. To enter text, just write in the "Contents" filed above the spreadsheet, or double-click a cell.
  • Now let's retrieve the actual length of our cube. In cell B1, type =Cube.Length. You will notice that the spreadhseet has an autocompletion mechanism, which is actually the same as the expression editor we used in the previous chapter.
  • Do the same for cell B2 (=Cylinder.Radius) and B3 (=Sphere.Radius).

Exercise spreadsheet 03.jpg

  • Although these results are expressed with their units, the values can be manipulated as any number, try for example entering in cell C1: =B1*2.
  • We can now change one of these values in the propertties editor, and the change will be immediately reflected in the spreadsheet. For example, let's change the length of our cube to 20mm:

Exercise spreadsheet 04.jpg

The Spreadsheet Workbench page will describe more in detail all the possible operations and functions that you can use in spreadsheets.

Writing properties

Another very interesting use of the Spreadsheet Workbench in FreeCAD is to do the contrary of what we have been doing until now: Instead of reading the values of properties of 3D objects, we can also assign values to these objects. Remember, however, one of the fundamental rules of FreeCAD: Circular dependencies are forbidden. We can therefore not use the same spreadsheet to read and write values to a 3D object. That would make the object depend on the spreadsheet, which would also depend on the object. Instead, we will create another spreadsheet.

  • We can now close the spreadsheet tab (under the 3D view). This is not mandatory, there is no problem in keeping several spreadsheet windows open.
  • Press the Spreadsheet Create.png New Spreadsheet button again
  • Change the name of the new spreadsheet to something more meaningful, suchas Input (do this by right-clicking the new spreadsheet object, and choosing Rename).
  • Double-click the Input spreadsheet to open the spreadsheet editor.
  • In cell A1, let's put a descriptive text, for example: "Cube dimensions"
  • In cell B1, write =5mm (using the = sign makes sure the value is interpreted as a unit value, not a text).
  • Now to be able to use this value outside the spreadsheet, we need to give a name, or alias, to the B1 cell. Right-click the cells, click Properties and select the Alias tab. Give it a name, such as cubedims:

Exercise spreadsheet 05.jpg

  • Press OK, then close the spreadsheet tab
  • Select the cube object
  • In the properties editor, click the little Bound-expression-unset.png expression icon at the right side of the Length field. This will open the expressions editor, where you can write Spreadsheet001.cubedims. Repeat this for Height and Width:

Exercise spreadsheet 06.jpg

You might wonder why we had to use "Spreadsheet001" instead of "Input" in the expression above. This is beacause each object, in a FreeCAD document, has an internal name, which is unique in the document, and a label, which is what appears in the tree view. If you uncheck the appropriate option in the preferences settings, FreeCAD will allow you to give the same label to more than one object. This is why all operations that must identify an object with absolutely no doubt, will use the internal name instead of the label, which could designate more than one object. The easiest way to know the internal name of an object is by keeping the selection panel (menu Edit->Panels) open, it will always indicate the internal name of a selected object:

Exercise spreadsheet 07.jpg

By using cell aliases in spreadsheets, we are able to use a spreadsheet to store "master values" in a FreeCAD document. This can be used, for example, to have a model of a piece of certain dimensions, and to store these dimensions in a spreadsheet. It becomes then very easy to produce another model with different dimensions, it is just a matter of opening the file and changing a couple of dimensions in the spreadsheet.

Finally, note that the constraints inside a sketch can also receive the value of a spreadsheet cell:

Exercise spreadsheet 08.jpg

You can also give aliases to dimensional constraints (horizontal, vertical or distance) in a sketch (you can then use that value from outside the sketch as well):

Exercise spreadsheet 09.jpg


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