Autodesk Mechanical Desktop Piston Tutorial

7-1

E X E R C I S E 7

Clean CAD Export From Mechanical Desktop

This demonstration shows how easy it can be: You define and control your geometry inside Mechanical Desktop and export to Working Model to add the physical components which drive the motion. This demonstration is made easy because the piston was assembled in Mechanical Desktop with the idea in mind that it would move inside of Working Model. This concept of Assembling With Motion In Mind is an important part of this training course and is discussed in detail in one of the following demonstrations.

New Concepts Introduced

CAD Integration

7-2 Exercise 7 Clean CAD Export From Mechanical Desktop

CAD Associativity

7.1 Export Piston Model from Mechanical DesktopThe Working Model CAD integration program exports all information from Mechanical Desktop that is pertinent to a Working Model simulation. This includes a mapping of both parts and assembly constraints. All parts defined in Mechanical Desktop come over as Working Model bodies. The export for each part is very thorough in that it includes the geometry, location, color, mass properties, and name. The CAD assembly constraints map over as equivalent physical constraints. The importance here is that the location and orientation of the constraint be correct. The integration never presumes the connection is driven; for example, it would not presume a CAD constraint to be a motor, it would instead map the constraint over as a non-driven revolute joint. If it is to be a motor, it can be modified quite simply inside Working Model as will be shown in this demonstration.

Open Piston Model in Mechanical Desktop:1. Launch Mechanical Desktop.

2. Select Open... from the File menu, and move to the directory Corporate Demo (Piston).

This directory is part of the Working Model 3D installation. Its path is shown in Figure 7-1.

Figure 7-1Path to Piston Model.

3. Open the file Pistasm.dwg.

7.1 Export Piston Model from Mechanical Desktop 7-3

Export to Working Model 1. Select Simulate Motion from the Motion menu.

The installation of Working Model 3D modifies the Mechanical Desktop work space such that both a Motion menu and a Simulate Motion Toolbar appear. Selection of the menu drops down the list shown in Figure 7-2. You should select Simulate Motion from this list. This initiates the CAD export. All parts and constraints in the Mechanical Desktop document are mapped over to a Working Model file; Working Model is then launched automatically with this file open, as is shown in Figure 7-3. This same export can be initiated also by choosing the simulate motion button from the toolbar which is shown in the left margin.

Figure 7-2Selection of Simulate Motion begins export to working Model

Figure 7-3Selection of Simulate Motion begins export to working Model


7.2 Add Physics in Working ModelOnce inside Working Model, we adjust the orientation of the World Coordinate System such that gravity acts along the piston head motion axis. We then complete the construction of the simulation model by adding two physical components. The first is a motor to drive the rotation of the crankshaft relative to an anchored body. The second component is a rigid slot joint to constrain the piston to a vertical motion only. This constraint accounts for the motion restriction imposed on the piston by the inside wall of the piston cylinder.

Reorient Global Coordinate SystemDrawings are often made with little regard to orientation because in a CAD program there is no physical significance to any of the three drawing axes (X,Y or Z). In a physics simulation like Working Model, where gravity must exist, a choice is made that gravitational acceleration is directed along the negative Z-axis. The problem occurs, therefore, that assemblies come into Working Model with an incorrect orientation relative to gravity. In this demonstration we correct the orientation of the piston such that the motion of the piston head occurs along the gravity axis.

Attach a Coord to the Body Anchor_1

1. From the View menu, choose Look At -> Front View.

A view like that shown in Figure 7-4 appears. Notice from this view that the z-axis points up; this is the axis along which gravity acts. Our objective with this maneuver is to align the piston with the Z-axis.

7.2 Add Physics in Working Model 7-5

Figure 7-4Initial Front View of Exported Piston

2. Place a coord arbitrarily onto the sketch grid.

We will attach this coord to the body called Anchor_1 and position it through the properties window.

3. Through Properties window Appearance tab, name the new coord The New Global Coordinate System.

4. Add to this selection the body Anchor_1 by holding the control key down and by clicking the left mouse button on this object in the object list.

5. Attach the selected coord to the selected body.

Locate mouse over the highlighted word Anchor_1 in the object list and click once on the right mouse button. A list like that shown in Figure 7-5 appears. Choose Attach Coord to Body.

6. Through the Properties window Position tab assign the coord The New Global Coordinate System the position shown below.

X = 0 Rx = -90

Y = 0 Ry = 0

Z = 0 Rz = 0


Figure 7-5Attach Coord to Body is accessible through right mouse click

Attach World Coordinate System to New Coord

1. Select the coord The New Global Coordinate System.

2. Select Attach Coordinate System... from the Grid menu.

A dialog like that shown in Figure 7-6 appears.

Figure 7-6Attach Coordinate System Dialog

3. Select the option, Attach to: The New Global Coordinate System.


4. Select the button marked Make the attachment permanent.

A dialog stating that you cannot reverse this appears. Select OK.

5. From the View menu, choose Look At -> Front View.

The view shows the piston aligned vertically as is shown in Figure 7-7.

Figure 7-7Front View Shows Piston Aligned with the Gravity Axis

Driving the Motion with a MotorIn this section we convert into a motor the revolute constraint connecting the bodies Anchor_1 and Crank_1.

1. Through the Object List select the body Anchor_1.

2. Select the Revolute constraint connected to Anchor_1.

With Anchor_1 as the selected object, all objects connected to it appear in the connections manager. As is shown in the figure to the left, there should be only one constraint on Anchor_1 and it should be called Constraint[59].

3. Open the properties window for this constraint and choose the Constraint tab.


4. Select the constraint type to be a Revolute Motor from the constraint list to the left in this tab, as is shown in Figure 7-8.

Figure 7-8Constraint Tab of properties window

Run SimulationRun simulation to make sure that constraints function as intended.

1. Select run button.

The motor drives the rotation of the crank shaft. The motion shows that the connections are correctly defined between the connecting rod and the crankshaft and between the connecting rod and the piston head. The piston head tumbles, however, because we have not yet enforced the constraint that the combustion cylinder imposes on a real piston; that is that the piston head is confined to a non-rotating, vertical motion only.

2. Let simulation run 50 frames before selecting stop button.

Apply a Rigid Slot Joint to Top Surface of Piston1. Use Box Zoom and Rotate tools to attain a view of top surface of piston

like that shown in Figure 7-9.

2. Use coord tool to place a coord on the top surface of the piston, as is shown in Figure 7-9.


Figure 7-9Location of Slot Joint Coord on Top Surface of Piston Head

3. Select newly created coord and open the Create Constraint dialog.

Create Constraint dialog appears as is shown in Figure 7-10.

Figure 7-10Create Constraint dialog


4. From the constraint list to the left side of the Create Constraint dialog, choose rigid joint on a slot.

The complete piston appears with a rigid slot joint attaching the top surface of the Piston Head to the background as is shown in Figure 7-11.

Figure 7-11Rigid Slot Joint attaches Piston Head to the background

5. Select Go Home from View menu.

A view with the original magnification and orientation appears.

Measure Constraint Load on Slot JointHere, we make a measurement on the force required of the slot joint to impose the motion constraint on the piston head. This sort of measurement might be made to determine the type of wear and tear to expect for the piston head and for the inside wall of the combustion cylinder.

1. Select the rigid slot joint.


It should appear in the Object List as Constraint[76].

2. Select Force on Piston Head-1 expressed in Coord[75] from the Measure menu.

This creates a meter measuring the force required by the constraint. The force component references of Fx, Fy, and Fz refer to the coordinate axes defined for Coord[75].

Run SimulationConfirm that the model functions as intended by running the simulation for 200 frames. Observe the variation of the constraint force with the motion of the piston.


Piston should move as expected. Constraint force measurement should appear as is shown in Figure 7-12.


Figure 7-12Measurement of force carried by slot joint


7.3 Modify Geometry Using CAD Associativity Up to this point we have defined geometry in Mechanical Desktop, exported it to Working Model, and have added physical components to drive the motion of this simulation. Now, we show how the programs continue to be integrated with associativity. Through associativity, changes in the geometry of a simulation can be made in Mechanical Desktop without altering the physical components previously defined in Working Model. In this final part of the demonstration, we investigate the effect of lengthening the connecting rod.

Modify Geometry in Mechanical DesktopWith Working Model still open, we reenter Mechanical Desktop to lengthen the connecting rod.

1. Launch Mechanical Desktop and open file Pistasm.dwg if it is not currently launched with this file open.

2. Inside Mechanical Desktop, expand list of components for Pistasm assembly so that it appears as is shown in Figure 7-13.

Figure 7-13Expanded Display for Piston Assembly in Mechanical Desktop

7.3 Modify Geometry Using CAD Associativity 7-13

3. Select CON_ROD_1 from this list.

4. Select the Part Modeling Button in Mechanical Desktop.

5. Select the Edit Feature Button.

6. Mechanical Desktop asks you to select a feature for editing, Locate mouse over connecting rod in workspace and click left mouse button.

Mechanical Desktop displays dimensions for the base extrusion of the connecting rod. The vertical dimension appearing to the left of the connecting rod indicates the length of the connecting rod.

Figure 7-14Right Mouse click on Base Extrude

7. Select vertical dimension indicating length of the connecting rod.

Mechanical Desktop requests: Enter new value for dimension .

8. Enter a value of 250.

9. Update the part drawing by selecting the Update Part button.

10. Update the assembly drawing by selecting the Update Assembly button.


The drawing appears with the lengthened connecting rod integrated into the assembly as is shown in Figure 7-15.

Figure 7-15Updated Assembly in Mechanical Desktop

Re-export to Working Model 3D1. Select Simulate Motion from the Motion menu of Mechanical

Desktop.

The Working Model simulation is automatically updated to include the new geometry definition as is shown in Figure 7-16. Through Associativity, the file is updated and does not require redefinition of the motor and rigid slot joint, even though they were originally created in Working Model.

7.3 Modify Geometry Using CAD Associativity 7-15

Figure 7-16Piston Geometry Modified for Working Model through Associativity

Run SimulationRun the simulation with newly defined geometry.


The motor drives the motion as expected.



Exercise 7: Clean CAD Export from Mechanical Desktop7.1 Export Piston Model from Mechanical Desktop7.2 Add Physics in Working Model7.3 Modify Geometry Using CAD Associativity

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Autodesk Mechanical Desktop Piston Tutorial