Week 3 - 2D meshing for sheet metal

NOTE:- model ilink is attach in doc file, refer document for screenshots & images.

Week 3 Challenge

Link for the model:- week3challenge

Aim:-In This week 3 Challenge Hood model has to be mesh with good mesh flow, prior to that geometrical error has to be removed from the model, proper mid-surface has to be extracted & mesh those mid surface with given quality parameter and eliminate all the off elements. Assigning PID to the shell elements.

Procedure:-

• Importing the given hood model which is in Step file (.stp) into the Ansa. Importing can be done by clicking on file-> open -> select hood model-> click open as shown in figure below.

• As shown in the figure model is imported into the Ansa, it is having the sub assembly namely Hinge, Inner, Outer &Latch sub assembly.

• After the model is imported in the Ansa first and step we need to do is Cad topology check up. Checking whether all the surfaces are stich properly, an enclosed volume is form or not.
• Ansa provide checks manager with the predefined templates in it, one of the template is check geometry as shown if fig below
• Once click on the geometry another window pop up on the screen as shown below, here various option are available for checking any discrepancy in the geometry.

• Click on the box as shown in the figure to find out most common geometry errors during importing the cad file into ansa. Before execute make sure that the visible option is on. That will check error for the component which are visible on the screen.
• On execute error will be appear on the pop up box specifying the type of error, it’s id number and description of the error.
• As shown in above image red color highlighted are the errors, total 51 error has appeared on the component, most common error encountered in any cad geometry are Needle faces, unchecked faces. Which can be auto-fixed as shown.
• Needle faces are those small and sharp faces which are present in the geometry that can be eliminated by deleting those faces.
• In some case manual fix has to be done where auto fixing is not applicable. Errors can be highlighted by clicking the arrow mark at top right corner.
• Next is to identify the cons that the right cons are placed at right place for example red cons means there is no sharing surface, yellow cons means there is adjacent sharing surface, cyan cons means there is three surface sharing the edges.
• Whole assembly cant be done at time & it wont be convenient so we should always go sub assembly wise.

• As shown above latch component geometry check was done for it and no error was found since its simple geometry. now after geometry check up is done we go for toggling unnecessary cons present in the geometry as its disturb the mesh flow and also create the quality issues.
• As we toggle any con it turns into orange color in topo module that means it wont affect the mesh flow and it will be neglected in mesh module.
• Always try to avoid toggling the important cons. For example in fillet there should be two capturing the feature areas. Similarly if there is any feature then cons highlighting those the region should not be toggle unless its impossible to mesh in those areas.
• We need to check the thickness of the component at every critical location to check whether the component is having uniform thickness or not. Here the component is having uniform thickness all over the places of 1.2mm.
• After completing the toggle of cons we go for mid surface extraction of the component. There are various command available to extract the mid surface. Most promising is offset command.
• As we can see in above image that the component is fully grey in color and other side is yellow in color. There should be only one color on one side that is recommended. This can be done by orient command in faces options.

• Once the orientation and clean up is done we can move towards extracting the mid surface of latch component. Generally there are many method to do but we’ll follow offset method by picking the surface manually.

• Now image as we can see as the orientation is in grey color arrow mark so in upward if it was in yellow color then it will in inward direction. Now to offset the faces we need to know the thickness of the component that can be measure from utilities bar present top of centre.
• So while offsetting the component direction we need to be kept in mind here -1.2/2 mm distance is apply to offset the faces that will be in the centre of the component.
• As shown below the mid surface of latch component is extracted

• Once the component is extracted, we again check for the geometrical error if any distorted faces present then we should remove it from there.
• Next step is to assign the property to the mid surface. Here for Nastran deck PID card will be PSHELL. Properties for the mid surface can be done by using SET PID Command from the faces option.
• Similarly we need to assign the PID for every mid surface having different thickness. As shown on below image the pid is assign to each and every different thk mid surface. In some cases is more convenient to segregate mid surface in different pid in order to get proper separation.

• In automatic offset geometry must be fully enclosed to get mid surfaces like in skin command.
• Next step is meshing of the component so for that we need to go in mesh module. So before meshing the geometry we need to set meshing parameters & quality parameter given to us.

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• Also check the element percentage box in the PRESENTATION PARAMETER, keep detail in demand at all in detail.
• Before starting the mesh, perimeter length has to be set same as target length this command is found in perimeter / length. By doing this all the perimeter grids will be set at 5 mm distance with each other.
• Now we have to go area by area and need to check whether the perimeter length is matching the target length. If it’s not matching then we have to modify the region
• Modifying can be done by deleting unwanted hot points present in the component and adjusting the grid points. If still its notable to do then defeaturing comes into the picture.
• Defeaturing is done only when the perimeter length is falling beyond the minimum element quality criteria. While defeaturing the we should not change the feature significance.
• In latch component no defeaturing was needed as its falls under the given limits.
• Next steps is to check the edges of the hole it should be even, holes which are near to the edges they should be rearrange such that the edge of the holes becomes parallel to each other. This operation can be perform by the help of slide operation present in the perimeter command section.
• After sliding the holes to the parallel to the edges we need to mark that nodes so it does not come back again to it’s original position by hitting the mark/unmark command.
• For creating the mesh there are various algorithm available to us like best, batch, spot mesh. I have mesh the component with best mesh.

• After performing the best mesh some quality issues will come up that can be removed by performing spilt, swap, paste operation and by reconstructing and smooth the mesh. Further more if you want to get good mesh flow then we should divided the faces into the small section to get good mesh flow.

Similar procedure was followed for next sub assemblies. Some screenshot are given below

• As shown in fig b the cons over the plane surface is been suppressed in order to get good mesh flow.

• The outer panel was mesh for one half and second half was copied using transform command -> copy->Default symmetry, then after element where stich with help of paste command.

• Some of the critical region where the where defeaturing was done are shown below
• Applied thickness to the shell from quality criteria => Presentation parameter=> Draw shell as solid.

LEARNINGS FROM THIS CHALLENGE

• Understand ANSA GUI with proper handling and switching to one module to another module.
• Assigning PID &MID to mid surface.
• Learned lots of command which are frequently used for geometry clean up as well as for meshing the component, creating the shortcut for the same command used frequently
• Learned abut the Various Drawing styles, understood the function of ent mode, pid mode, mid mode.
• Learned about the various faces draw option how switch from one user mode to other, hiding the unwanted part using hybrid i-not command
• Learned about quality criteria for 2d elements like warpage, aspect ratio, jacobian.
• Learned about function of various mid surfacing tool such as offset, skin.
• Learned about function Geometry check up tool.
• Learned about the procedure need to be follow before starting the mesh, various defeaturing techniques to avoid the quality issues.
• Learned how to deal with tria in the mesh flow by using the commands like swap, split, paste.
• Mirror the elements by doing one side and copying it other side, also learned about the mesh associative with mid surface.

Conclusion:-

In this week 3 challenges I have completed hood model meshing with given quality criteria, with this challenge I would like to conclude that in any pre processing tool we have to follow the predetermine steps in order to get good quality of mesh on the component that also saves users time and also it’s best practise to follow in order to get better efficiency then others. Feature area has to take extra care as there will be multiple cons over those region and also results of component mostly depends over those region. With proper understanding of the commands one can save lot of minor time which at last results in to huge time saving.