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Archive for November, 2009

This blog post was inspired by a question I received via email. The question was about how to create a section view in an assembly.  There was a picture attached to the email that showed an assembly  that seemed to have been cut along two planes, but only partially, and  it appeared that one chunk  of the assembly had been removed to reveal the inside, while the rest of the assembly remained visible, similar to cutting a slice of a pie.  I was in a bit of a hurry, so I tried to explain myself really quickly, but then I thought that it was actually a very good question, and it may help a few other readers out there if I answer it here, as well.

For illustration purposes, I’m going to use this old model of a drill jig. Suppose I want to create a section view of this assembly by making a partial cut along the Front and Right planes and removing a section of the drill jig’s handle and  block, thus revealing the screw inside.  If I tried to use Section View  (from the Heads Up Toolbar in the Graphics Area or  from View, Display, Section View), it would slice all components in the assembly all along  a couple of planes parallel to the Front and Right planes, leaving just a piece of it, as you can see in the following image. That’s not really what the section view of the assembly in that picture was supposed to look like.

So, let’s try a different approach using Assembly Features.  First of all, I created a new configuration and called it Section View.  Now, on the assembly’s Top plane, I sketched a couple of lines right where I needed the cut for the section, as you can see in this image. Notice that this sketch in on the assembly’s Top plane, not at the part level.

Once my sketch was ready, I clicked on Assembly Features from the Assembly toolbar and selected Extruded Cut from the list to cut the components in the assembly using my sketch (you can also find this command from Insert, Assembly Feature, Cut, Extruded Cut).

Notice that this extrude cut command works pretty much in the same way it does for parts, but rest assure that the components themselves won’t be affected by this feature outside of the assembly. As a start condition, I decided to use offset, so the cut would begin a short distance from underneath the sketch plane. Under Direction 1, I reversed the direction to extrude trough all in the upward direction.  This way it won’t cut all through the bottom of the block, but only through the top. I used Feature Scope at the bottom of the Property Manager to select that only the handle and the block, but not the screw, would be affected by the cut. You can see all my selections in the following image.

Here in this other image you can see the section view that was created this way. In the Feature Manager, a new cut extrude feature has been added at the assembly level. This feature only exists inside the assembly; if you open your parts in a separate window, they’ll remain unchanged.

It seems to me like I’ve been out of touch with everything lately. I have certainly neglected this blog and haven’t even kept up with the entire buzz about SolidWorks related news, forums and blogs.  I apologize to those that have sent questions through email asking for help, and never received an answer. I wasn’t ignoring you, I was simply too busy working on a non-modeling, but SolidWorks related little job that happened to fall into my hands, and that needed immediate and undivided attention.  I’m sorry I can’t tell you details about it. What I can say is that it’s was fun and challenging.  At times, it was a bit stressful, but I did learn a lot from it, though, and I believe this blog will eventually benefit from that experience.

Now I’m trying to catch up with the rest of the world, although I know that’s just impossible. I did notice, however, that this year DS SolidWorks came up with something new: contests! And the prize is free admission to SolidWorks World 2010! That’s so cool!

I think their second contest is currently going on and it’s open to those that have never been to SolidWorks World.  From what I understand, all you have to do is leave a comment on their blog describing a first SolidWorks related experience of your own. It can be your first user group meeting, your first experience using SolidWorks, your first finished model. Hmmm, I wonder if writing about the first time you overdefined a model, or your first crash would also be acceptable…  Never mind that! If you have never been to SolidWorks World and you want to get in for free, courtesy of DS SolidWorks, then head over to that blog and write your heart away.

A few days ago, my friend Chris Thompson, founder and owner of Appian Way Technologies, took a look at my model of the safety latch and suggested the following changes in order to improve the simulation study.

First of all, he added small fillets to the areas of the latch where stress concentrations are expected, at the “root” of the latch. He also “cut” the model in half, in order to take advantage of its symmetry through the use of symmetry constraints, which can be found among the Advanced Fixtures available in SolidWorks Simulation.

The symmetry fixtures will simulate the half of the latch that was cut from the model. Having this fixture in place will prevent any displacements across the plane of symmetry, but allow displacements on the plane of symmetry. The idea behind this is to reduce the number of equations necessary, as well as the solving time. In order to use this constraint, right click on Fixtures, and select Advanced Fixtures, Symmetry. He selected the left planar face of the latch to define the plane of symmetry, as you can see in the following image.

Chris also talked to me about the possibility of improving results by any of two options: manually refining the mesh and using mesh controls, or making use of the h-adaptive solution method, which is available only for static analysis and solid elements. Why is this going to improve results? Well, simply because any solution obtained through FEA will depend on our choices for discretization (a.k.a. meshing). Different choices for meshes will also cause different discretization errors, and we can estimate these errors by making systematic (planned and gradual) changes to the mesh and analyzing the impact of such changes in the results of our study. This is often called a convergence process. The way we can do this is by simply starting with a study that uses an average element size mesh, and then, in subsequent studies, gradually refine the global mesh (reduce the size of the elements), while keeping an eye on any changes in stress and strain in the whole model or in areas of interest (in this case the fillets). We’ll know the process is converging when any further refinement of the mesh produces insignificant changes in the magnitude of the results. This can be a long and tedious process.

Further manual refinement consists of applying mesh controls to the areas of interest in the model. Basically, mesh controls allow us to refine the mesh locally, only in those areas of interest where we expect high concentration of stress, while the rest of the model is meshed using a much larger element size, thus reducing the number of equations and time needed to solve the study, at least when compared to global mesh refining. Mesh controls can be applied to edges, vertices, faces or entire components of assemblies, and they need to be applied before meshing the entire model.  The way to apply mesh controls is by right clicking on the mesh icon in the Simulation Study tree and select Apply Mesh Control.

Here in this image you can appreciate the way Chris applied a mesh control to that couple of fillets. He selected the two faces and used an element size of 0.029 in and a Ratio of 1.5.  This Ratio parameter simply specifies the ratio between element sizes in consecutive transitional layers when going from the global mesh element size to the local mesh element size. A Ratio of 1.5 is usually default.

Chris also applied mesh controls to the curved face of the cutout you see on the bottom of the latch, where stresses also concentrate, and to that edge on the tip of the latch, that he created by means of a split line, and used to define the Use Reference Geometry Advanced Fixture that I applied in the original study to make sure the latch had that 5 mm displacement, remember?

He then meshed the rest of the model using the default mesh element size. Notice in this image the transition between mesh element sizes in different areas of the model.

So that’s the manual way to do it, but this refinement process can also be automated, by using the h-adaptive Solution Method. By the way, the “h” refers to the size of the element, so the convergence process through mesh refinement is actually called “h convergence process”, since the size of the elements is gradually reduced.

To make use of the h-adaptive solution method right click on the name of the study in the Simulation Study tree and select Properties, then select the Adaptive tab, and under Adaptive method option select h-adaptive.  You have a few options to choose from here.  From the help document, “Target Accuracy sets the accuracy level for the strain energy norm in the model, which is not the same as stress accuracy level.” A default value of 98% means that the convergence process will stop if the difference in the strain energy norm between two loops drops below 2%. Accuracy Bias instructs the solver how to concentrate on getting stress results: Local (all the way to the left) will cause the solver to concentrate on getting accurate peak stress results for those very localized areas with high strain energy errors (the fillets) by highly refining the mesh in those areas, while Global (all the way to the right) will cause the solver to ignore high, localized strain energy errors and concentrate on getting accurate overall stress results for the whole model.  The maximum number of loops will tell the solver how many times to repeat the process of mesh refinement. Looping will end when Target Accuracy is achieved or when the maximum number of loops is reached. If Mesh Coarsening is selected, it simply means that during the mesh refining process our original mesh can actually be made coarser in some areas of the model, as the solver sees fit. This way the mesh will be refined only where needed.

This is the mesh that my friend Chris achieved for the latch by using the h-adaptive solution method with default values and a maximum number of loops of 3.

As my friend pointed out to me, the h-adaptive method is useful not only to save us from the tedious process of manual mesh refinement, but also for those times when we’re not exactly sure where the areas of high concentration of stresses will be.

Thanks, Chris!