By Stephen M. Samuel PE
Synchronous modeling is an amazing technology. It’s like a great big “Get out of jail” card for CAD modeling. As amazing as parametric modeling is and as powerful and stable as the NX CAD system is, there are times when parametric models with a multitude of features begin to do things that the original model creator just failed to anticipate. Also, when models are being edited by someone other than the original creator they may not understand the way a model has been created. This can cause the editor to make a parametric change to the model that effects something other than was intended. Like anything else that has a lot of moving parts the model can get “jammed” up. When a model becomes all jammed up and a designer would normally spend a huge amount of time deciphering a list of features that are refusing to regenerate properly, they can switch to a synchronous modeling technique and in a few seconds get a model to do just what they want.
Synchronous modeling is also great because a majority of the commands are fully parametric. When you use the “move face” command or the “reuse” command, the parametric expressions that you enter are stored in the expressions list just like any other parametric command. You can comment them, edit them and re-name the variables just like any other command.
There are those who criticize synchronous modeling because they think it may lead to sloppy modeling technique. The common wisdom is that if you use synchronous modeling it will lead to a confusing model because faces of solids can end up in different places then where the sketches would normally place them. The example that is used is that of a simple extruded rectangle that forms a rectangular box. Then a subsequent move face is imposed on the top of the box. The finished product then has a top face that is no longer controlled by the sketch and extrude alone. Although there are CAD users that would be confused by this, anyone who is familiar with synchronous modeling would be able to click through the various features and figure this out in a heartbeat. In fact, the parametric synchronous modeling commands are very similar to many of the commands that have been accepted for years. For example, the “offset face” command, the “draft” command, the “trim” command. All these commands are capable of having a similar effect on the above mentioned block, as the synchronous modeling “move face” command. In the end analysis, any command that is extremely powerful also has the power to confuse and confound the untrained user. Innovators are usually in a race against time. In this continual race we must be given the most powerful tools and we must gain the wisdom to use them well. We can only do this by great training, experience and making a mistake every now and then.
New power has been added to the “Edit cross section” synchronous modeling command in NX8. It can now be used in the “History ” mode. The command allows a user to create a datum plane that intersects a solid, then where the plane intersects with the surfaces of the solid, sketch curves are created. Once the sketch curves are created, the solid surfaces can be manipulated by placing dimensions and constraints on the sketch curves. This like many of the other synchronous modeling commands is very powerful. It can allow a user to redefine the design intent of a model. This is a blessing to those who are aggressively trying to create transformational designs that are innovative and get to the market first. It is a technique that enables those who bravely transcend the status quo.
For example, a vessel is created in a simple way. A block is created whose walls are drafted, then the shape is blended, then shelled. A designer later finds out that the wall on the right has to be moved out and tilted to meet up with another portion of the assembly.
Figure 1. Ordinary creation of a vessel
The Synchronous modeling “Edit cross section” method is chosen for its ability to edit shapes in an extremely flexible and powerful sketch based way. A center datum plane is created and the “Edit Cross Section” command is invoked.
Figure 2. Center datum plane is created and the Edit Corss Section command is invoked
The two surfaces of the wall on the right are selected, then the datum plane. The sketch button is selected and the intersection curves of the walls and datum plane appear. The user can then use a variety of sketch commands to create a new construction method for the vessel. In the case bellow, a gage point that measures one inch below the top surface is created. The gage point is also moved such that it is 9 inches away from the left edge and the two intersection lines are angled 30 degrees. The two surfaces of the solid wall change position in order to comply with the sketch.
Figure 3. The shape of the vessel is edited using the Edit Cross Section command
Figure 4. The finished product
Congratulations to the NX8 software development team at Siemens. Synchronous modeling is a truly amazing invention. It saves an incredible amount of time in the design process and allows designers to focus on their design more than the foibles of CAD commands.