Assembly3 Workbench/it

Introduzione
Assembly3 è un banco di lavoro esterno che viene utilizzato per eseguire l'assemblaggio di diversi corpi contenuti in un unico file o in più documenti. Il banco da lavoro si basa su diverse modifiche delle funzioni principali effettuate per la versione di FreeCAD 0.19 (ad es. App Link), quindi il banco da lavoro Assembly3 non può essere utilizzato con le versioni precedenti.

Le caratteristiche principali del banco da lavoro Assembly3 sono
 * dynamic/interactive solver. Ciò significa che è possibile spostare le parti con il mouse mentre il risolutore limita il movimento. Ciò consente, ad esempio, di collegare una ruota ad un asse e di ruotare la ruota in modo interattivo con il mouse.
 * links. Ciò significa che è possibile utilizzare un solo pezzo, ad es. una vite più volte in un assemblaggio (in luoghi diversi) senza duplicare la geometria.
 * external links. E' possibile avere un documento freecad che contiene solo un assemblaggio e non parti. Tutte le parti potrebbero essere in file singoli. I file potrebbero anche essere in una libreria o in qualsiasi altra parte del file system. L'unico requisito è che il file deve essere caricato al momento del collegamento. Dopo che il collegamento è stato fatto, il file deve essere aperto per fare gli aggiornamenti ai collegamenti che coinvolgono il file. Assembly3 risolve questo problema aprendo i file in background.
 * hierarchical assemblies. Come nella vita reale, un assemblaggio meccanico può essere costituito da sottoinsiemi. Questi potrebbero consistere di nuovo in sottoinsiemi e così via.
 * assembly freeze. Poiché la CPU può gestire solo un numero limitato di vincoli simultanei in tempo reale, il congelamento di un assieme permette di utilizzare i vincoli anche per assiemi di grandi dimensioni. Congelando gli assiemi finiti o i vincoli che non devono rimanere dinamici (ad es. parti saldate, bullonate o incollate), questi vengono esclusi dai calcoli di aggiornamento e considerati geometria fissa dal solutore Assembly3.
 * Si noti che altri approcci offrono soluzioni diverse a questo problema, per esempio Assembly4_workbench_icon.svg Assembly4 Workbench.

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Barre degli strumenti
A partire dal 2020 il banco da lavoro Assembly3 è dotato delle seguenti barre degli strumenti.


 * Assembly3_ToolbarMain.png
 * Barra degli strumenti principale


 * La barra degli strumenti principale contiene strumenti per
 * Aggiungere una cartella assembly
 * Importare file STEP
 * Strumenti per spostare le parti in 3D
 * Controlli per l'aspetto e il solutore.
 * Strumenti per ordinare gli elementi dell'albero


 * Assembly3_ToolbarConstraints_1.jpg
 * Barra dei vincoli principali. Alcuni strumenti sono di fatto un menu per altri strumenti.


 * Assembly3_ToolbarConstraints_2.jpg
 * Barra dei vincoli supplementari. È possibile abilitare questa funzione selezionando l'icona "..." nella barra dei vincoli principale.


 * Le Barre dei vincoli saranno l'interfaccia principale utilizzata per l'assemblaggio delle parti. Sono grigie di default, ma vengono attivate una volta selezionata almeno una faccia, una linea o un punto di una parte. Generalmente si selezionano gli elementi che devono essere uniti e poi si seleziona il tipo di vincolo. Le diverse cornici colorate contrassegnano le diverse caratteristiche dei vincoli: se è possibile aggiungere più di 2 elementi in 2D/3D. Una descrizione dettagliata si trova nel wiki di Gibhub.


 * "Bloccare" il primissimo vincolo. In ogni assemblaggio una parte deve essere bloccata per agire come parte base. Bloccare non significa altro che vincolare questa parte in 6DOF alla sua posizione e orientamento attuali.


 * Assembly3_ToolbarNavigation.jpg
 * Barra degli strumenti di navigazione
 * Queste funzioni sono utili quando si lavora con un assemblaggio con una gerarchia di file esterni collegati
 * Selezionare l'oggetto pezzo corrispondente nel gruppo di relazioni
 * Selezionare l'oggetto collegato
 * Selezionare l'oggetto link più profondo


 * Assembly3_ToolbarMeasurement.jpg
 * Barra degli strumenti di misura
 * La Barra degli strumenti di misura aggiunge funzioni per misurare le distanze tra due punti, un punto e una retta o un punto e una faccia. Lo strumento Misura angolo traccia l'angolo tra due facce o rette. Non esiste una funzione per misurare un raggio o un diametro.
 * Gli strumenti di misura sopravvivono ai cambiamenti del pezzo, ad esempio la distanza tra i bordi di un cubo quando il cubo viene ridimensionato. Come i vincoli, i calcoli sono fatti in tempo reale e aggiornati ad ogni cambiamento. Dietro le quinte, la funzione è molto simile alla funzione vincoli. La distanza o l'angolo viene calcolata tra elementi come per ivincoli. Il visualizzatore nell'albero funziona allo stesso modo.

Come al solito è possibile modificare le barre degli strumenti e aggiungere o rimuovere singoli strumenti. Accertarsi di controllare nel menu Assembly3 le funzioni che potrebbero non essere presenti nelle barre degli strumenti.

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Vincoli
Il progettista utilizza i vincoli per ottenere il risultato desiderato nella relazione tra due parti. L'arte è la selezione dei giusti vincoli più adatti ad affrontare ogni problema. Ogni DOF eliminato dovrebbe in teoria essere eliminato solo una volta tra due oggetti, ma in pratica con molti strumenti CAD i vincoli selezionati causano combinazioni di vincoli eccessivi, spesso compensati da algoritmi complessi, a volte no. Assembly3 utilizza algoritmi per rilevare e compensare gli eccessi di vincoli, ma chiaramente non sono ancora molto maturi. Quindi in pratica per i vincoli di Assembly3 si evitano problemi essendo consapevoli di quanti gradi di libertà (DOF) sono stati utilizzati e quali devono ancora essere bloccati dai vincoli. Nessuna parte dovrebbe avere una connessione di vincoli utilizzati maggiore di 6DOF.


 * Nota: Se il risolutore incontra una combinazione che non può essere risolta, darà un errore. È molto difficile per il risolutore scoprire cosa ha causato il problema, quindi tipicamente da questo errore dato non sarà chiaro dove è il problema. In assemblaggi più grandi questo può portare a ricerche complesse del problema. Purtroppo non c'è un modo semplice per evitarlo. Tuttavia, aiuta essere pienamente consapevoli di come funziona il sistema (.e.g. vedi Elementi qui sotto), utilizzare nomi chiari per tutti i componenti coinvolti e aggiungere vincoli aggiuntivi solo quando il risolutore risolve l'assemblaggio corrente. Molto utile per rintracciare un problema è la funzione "ContexMenu/Deactivate" di ogni Vincolo.

Assembly3 I vincoli definiscono le restrizioni nella posizione o nell'orientamento tra due Elementi. Alcuni vincoli funzionano anche con più di due Elementi. Un Elemento può essere una faccia, una linea o un bordo o un punto di una parte. Generalmente i vincoli sono definiti selezionando gli Elementi e poi selezionare il vincolo dai Vincoli toolbar.

Other
 * Fixes 6 DOF, leaves 0 DOF:
 * Lock: The lock constraint fixes all DOFs for a face. It should be used for one base part in each assembly. You may also want to enable the "MoveLock" function (in the tool bar) so that the part can not be moved accidentially. Normally it does not matter which face/line/point you use to fix a part. Also note that the lock is only valid for the direct assembly, i.e. in case of a sub-assembly the parent assembly would still require a locked part on its own.
 * Attachment: Makes both elements coordinate systems equal for all axes. This is computation wise the most inexpensive function and should be used where ever possible. Note that you could use the element properties to compensate for offsets and angles if the two elements are not perfectly aligned.
 * Fixes 5 DOF, leaves 1 DOF:
 * Plane Coincident: fixes Tx,Ty,Tz, Rx,Ry. Only Rz is free. There remains the rotation around the normal passing through the ‘‘center of the plane’’.
 * Fixes 4 DOF, leaves 2 DOF:
 * Axial Alignment: fixes Tx,Ty, Rx,Ry. Only Tz, Rz are free. There remains the rotation around the axis of the shape and the translation along this same axis. Two PointOnLine constraints (if the two points are different) give the same result. The '‘Colinear'’ constraint too.
 * PointOnLine: This eliminates the translation and rotation along the normals to the reference line. Only the translation and rotation along the line axis is allowed.
 * Fixes 3 DOF, leaves 3 DOF:
 * Same Orientation: fixes Rx,Rz,Rz. All T's remain free.
 * Points Coincident: fixes Tx,Ty,Tz. All R's remain free.
 * PointOnPoint constraint eliminates the 3 translations.
 * Plane Alignment: fixes Tz, Rx,Ry. In plane motion and Rz. This eliminates the translation along the normal to the reference plane and the two rotations around the axes of this plane.
 * Fixes 2 DOF, leaves 4 DOF:
 * Multi Parallel: fixes Rx,Ry. all T's and Rz remain. This eliminates the two rotations around the axes of the reference plane.
 * Fixes 1 DOF, leaves 5 DOF:
 * Points in Plane: Fixes Tz. This eliminates the translation along the normal to the reference plane.
 * Points Distance: fixes the distance between the Element origins.
 * This gives you more freedom than Points in Plane
 * Points on Circle: fixes Tz and partially Tx,Ty. Freezes the point translation (or several points) on a circle or disk area. You must pick the circle second. This leaves all rotations free and gives limited translation in the circle reference plane.

'': Note: In the following list Tx,Ty,Tz and Rx,Ry,Rz are used to describe translations and rotations about the reference coordinate systems of the involved Element's. This is not always exact or fully defined, e.g. when a line is involved it is not defined if it runs in X, Y or any angle in betweeen. The system is used for bevity and easy comparison in favour of a correct but more complex definition. So Z is generally the normal direction of any faces involved. Please feel free to modify this with a better approach with improved readability.''

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Elements
Elements is a specific term used in the Assembly3 workbench and it is important to understand Elements for understanding how Assembly3 should be used.

It is helpul to think of an Element as a general word for a 'selectable item' of a part, i.e. a face, an edge, a circle or a corner or other point. The items you select to constrain them, are those Elements. In the tree an Assembly folder has three sub-folders. Beside 'Parts' and 'Constraints' there is a folder named 'Elements', which is emtpy as long as there are no constraints added. When adding a constraint the constraint itself gets two (or more) leafs, these are the selected 'Elements'. Also these get added in the 'Elements' folder which is just a list of all Elements used in the assembly. Its a good idea to change their names (with F2 key), especially in bigger assemblies.

Lets look at an example
 * Create a new file and add from the Part workbench a cube and a cylinder. We will stack the cylinder onto the tube. First we fix the base part, in out case the cube. Select the bottom face of the cube and select the "Locked" constraints (first icon in the Constraints toolbar). Select the top face of the cylinder and the top face of the cube. Then select the "Plane Coincident" constraint. Now the cylinder is moved into the cube and in the tree a new leaf with two childe nodes was added under 'Constraints'. Additionally the same two child nodes were added under 'Elements'. If you cylinder is inside the cube instread of on top of the cube, lets correct that first: select the child node under 'Constraints' that shows the cylinder face and with a right click in the context menu select 'Flip Part'. Now the cylinder is stacked onto the box.

The key think to understand is that the constraint operates on links to Elements in the list in the 'Elements' tree folder. This allows to keep the constraint structure intact while changing the parts. This is very difficult to see without an example.

Lets get back to the example above
 * Note: make sure you added the "Lock" constraint to the cube or this will look confusing
 * In the CAD window select another face of the cube. Now we only work in the tree view. Go with your mouse in the tree where the cube should be selected. Drag&Drop the cube to the 'Elements' folder. Drop it on the 'Elements' name, not anywhere else in the folder - why we see later. You should see that another Element is added to the 'ELements' list. Now select in the 'Constraints' folder the child node of the cube face in out "Plane Coincident" constraint and delete it. The Constraint will show an exclamation mark since its missing one Element. Note that by deleting the Element in the Constraint we did not delete it in the list. That is because in the constraint was only a link to the Element in the list. Now take the newly added Element in the 'Elements' list and drag&drop it onto the "Plane Coincident" constraint. Now the cylider moves to the other face we selected. We might need to select 'context menu/flip part' again if the cylinder is again inside the cube.

The example showed that without removing the constraint we can change the Elements that are used for the constraint. The same way we can move the cylinder to a totally different part. After playing around with this example a bit more, you will note some additional things as
 * If you rename an Element in the list, the name will be changed in all Constraints.
 * you can use one Element in the list in several constraints.
 * You can use the Property Window of an Element to add Offsets. In the example this could move the cylinder around on the cube face.
 * you can use the "Show Element Coordinate System" button in the main toolbar to see what 'ContextMenu/Flip Part' and 'ContextMenu/Flip Element' are doing. Be sure to look what happens in the Property Window.
 * you can add a constraint in a totally different order: First add some Elements to the 'Elements List' (naming is useful, e.g. "Cube Top Face" or "Cube Front Face"), then add a constraint without selecting anything - it will be an empty constraint. Then drag Elements from the 'Elements' list. The result is the same than what we did in the first example. After doing that exercise the nature of how constraints work with Elements should be clear.
 * you can change an existing constraint between existing elements by just select a different item in the PropertyWindow/ConstraintType property.

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Compatibility
Assembly3 was inspired by Assembly2, but it is not compatible with it. If you have older models made in Assembly2, you should stay with FreeCAD 0.16 and use Assembly2 there.

New models developed with Assembly3 should only be opened and edited with this workbench.

Although they may have similar tools, Assembly3 is not compatible with A2plus nor Assembly4. Models created with these workbenches should be opened only with their respective workbench.

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Testing
The Assembly3 Workbench is under development and is not yet available (April 2020) through the Addon Manager, but it is expected that this will happen at some point.

You can test it in two ways:
 * A special fork of FreeCAD made by realthunder; see FreeCAD_assembly3 releases. This fork is based on a particular commit of the master branch of FreeCAD, but it also has additional features currently not present in the master branch. Due to this fork being based on a particular development snapshot, it does not have the latest features merged daily to the master branch.
 * The development AppImage; this is based on the current master branch, and includes the dependencies needed for working with Assembly3 such as the SolveSpace solver.

Since the AppImage only works for Linux, for Windows users at the moment the only option to test Assembly3 is the first option (realthunder's fork).

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Get Started
There are many ways to create an assembly with Assembly3. Here is the most simple one you can do.


 * Assembly3_Example-GettingStarted.jpg
 * Final Result of the Getting Started Example. In the image the Assembly3 Worksbench is selected, so its multiple toolbars are visible. Note that the vertical "TabBar" left of the tree view is an AddOn Workbench that is not contained in standard FreeCAD (but can be installed with the Addon-Manager).


 * Create a new FreeCAD file
 * Select assembly Workbench. Select CreateAssembly (first icon)
 * Select Part Workbench and add a cylinder and a cube
 * Save the file with any filename you like. Close and open and the file.
 * The tree view should look like this


 * Now Draw&Drop with the mouse both Cylinder and Cube onto the Parts folder. They are moved into that folder.
 * That is the quickest way. Please note that a better way is to open the Context menu on both and select ContetxMenu/LinkActions/MakeLink. This adds two link files. Then Drag/Drop the link files to the Parts folder. For simple cases like this it does not really matter.
 * Click both top surfaces of Cylinder and Cube (keep Ctrl pressed)
 * Select assembly Workbench. Select "PlaceCoincident" from the Constraint Toolbar.
 * Now the parts should be joined into each other and your tree should look like this

We omitted one important step that should be done in larger assemblies: locking a base part. That means define one part that should not be moved by constraints. In your case we use the cube for that: Done. If you like you can move the "Locked" constraint upwards in the tree. Use the "MoveItemUp" button on the Main Toolbar for that.
 * Right click "_Element" (any of the two) and select "Flip Part".
 * Now the Cylinder should be on top of the box. If the whole thing is upside down, go back and select "Flip Part" on the other Element.
 * Select the lower face of the cube. Only the lower face, not the whole cube.
 * select the "Lock" constraint from the constraint tool bar
 * The finished assembly tree should look like in the image above

Note: all new external files must be saved, closed and re-opend at least once, so that Assembly3 can find it. Without doing that FreeCAD can not give a file handle to the Assembly3 Workbench and it can not find the new part. When all parts are in the same file, you should save and re-open the file.

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Add an Offset
Assembly3 does not offer Offset with the constaints in the way the A2plus Workbench or other CAD tools do. Instead it offers a more general and flexible system to add offsets translations but also angles.


 * Add the offset in the properties of one Elements of a Constraint.
 * you can choose which one of the two you want to use.

Example:
 * Add 2 cubes to an assembly and select their side faces.
 * select "PlaneCoincident". The cubes will be attached inside each other.
 * select one Element and ContextMenu/Flip Part. The cubes will be attached side-by-side.
 * select one Element property Offset/Position/Zz and set to 5mm. The cubes will be 5mm apart.
 * Test with other axes or the angle/axis fields. Also verify that you get the same result when using the other Element.

This is the same approach for all other constraints.

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Solve a Solver Failure
This often happens when parts are over-constraints, i.e. more than 6 DOF are locked.

The easiest way to find the problem is to click relevant constraints in the tree and select ContextMenu/Disable and re-calculate. It is helpful to know the last added constraints before the solver failed and just undo them.

Note: as Assembly3 tries to compensate for over-constraint parts behind the scenes, sometimes the problem is just triggered by a new constraint but the root cause is somewhere different. Before deleting all and starting again, remember that you can re-use Elements. If you named them you can identify the required elements and re-build the constraints without using the 3D view at all. See Elements seciton above.

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Tips & Tricks

 * Using hierarchical assemblies helps in avoiding solver issues and keeping you model fluid. You can freeze a subassembly with one click and save CPU resources easily. Also note that Assembly3 does not need to keep external files for frozes subassemblies open. For large assemblies with large MB files this can be a severe advantage.
 * Using external files makes it easier to re-use a parts or do part versioning with systems like git or subversion. The workflow in FreeCAD with Assembly feels quite the same as with files that have all parts in the same file. For exchanging files often with other parties, single files might be more convenient.
 * Multiply linked parts. If you added a link into the assembly, it will have a property value named "Element Count", default 0. If you set this to 3 you get 3 instances of that part. They will be added into a subfolder and can be used like fully separte parts. Use this feature to keep the data footprint of your file low, because the part is saved only once. Each instance only contain the differences.
 * Insert multiple Screws with one click. Check out the Assembly3 Wiki on the Github site. This is not only a stunning function (even a bit magic), but really really useful.


 * Using the TabBar Workbench speeds up working with assembly. This adds a Toolbar with one button for each workbench. You can sort the toolbar and can put it where every you want it. Many people put it vertically on the left side just beside the tree view. Of you have Assembly3, Part, PartDesign and other often used workbenches close to the top switching between them becomes extremely easy.

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Links

 * App Link object that makes Assembly3 work.
 * FreeCAD_assembly3 repository and documentation.
 * Assembly3 preview, big discussion thread.
 * Test tutorial for Assembly 3 WB by jpg87.
 * Current Assembly Status
 * External workbenches
 * External workbench list
 * Old Assembly project development plan, to get acquainted with the history of the issue.