Week 3 - 2D meshing for sheet metal

Meshing of Car Hood using ANSA

Aim: - For the given Hood model, we have to clear the geometrical errors that are present in the model, we have to extract the Mid-surface of all the components and have to mesh the mid surface by fulfilling the quality criteria given in the problem statement with the help of ANSA.

Procedure: -

• Basic checks of geometry
• Geometry cleanup
• Mid surface Extraction
• Meshing the Mid surface

Step 1: -

Basic checks of Geometry: -

• First, we will import the CAD Geometry to ANSA and will check whether the CAD model format is in supportable format or not. Supportable formats are

STEP (*. STEP/ *.step/*.stp/*.STP)

IGES (*.IGES/*.iges/*.IGS/*.igs)

STL (*.stl)

• Second, we have to ensure that the dimensions of all parts in the model, importing in ANSA must be same, and it is recommended to be in mm.
• Most important thing is graphics check. Here we have to ensure that the Hot points and CONS are clearly visible and no problem should be there in visibility. If problem is there, we can put this command “-performance_mode 3”, in the popup box while opening ANSA to get good Graphics.

TIP: - If we need to get Good Resolution of the hot points, do the following

Click F11 >> Go to Presentation parameters >> scroll the bar of “all details” to left >> click on apply

Step 2: -

Geometry cleanup: -

• First, we have to assign the material to the given model without letting any problem to arise further. Here, assigning means nothing but just ticking the box of the created material in MID tab.
• Now, we have to perform checks on the geometry using “checks” Geometry checks are, checking for Single cons, triple cons, cracks, needle faces, collapsed CONS, unchecked faces.

Go to checks >> Geometry >> select checks to be performed >> Execute

Before Geometry cleanup

• The above errors can be cleared in two ways. Either by going to error by error and dealing with it, or by using the inbuilt ANSA “fix” option for removing all errors at once. I used the “fix” option, as it is easy to use when there are lot of errors and doing manually consumes so much time.

Select all the errors using “CTRL+A” >> right mouse click >> click on FIX

• Sometimes the ANSA might show some errors after some time even after using FIX option, then we have to clear the errors manually. This is the drawback of FIX
• Manually, most of errors can be cleared using basic release CONS & Hot points, TOPO, paste options etc.
• Now, we have to do visual check, which means the outer surface or positive surface should be “Grey” and the inner surface or negative surface should be “yellow”.

Using Orient >> visible, we can orient the surfaces.

• we also have to check the entire model, if there are any thickness changes. So, here we will go to feature by feature, surface by surface and randomly measure the thickness.
• Finally, we will check whether the features are correctly placed or not. How we will check?

Go to Auxiliaries >> Fine >> click on the features

Here we will know the true shape and placement of the feature

After Geometry Cleanup

Step 3: -

Mid surface Generation:

• Before mid-surface Generation, we have to ensure that we have created different locks for components based on their PID’s. It will be easy for us further, while doing mid surface generation and meshing, component by component.
• We can generate the mid surface in 2 types, either by using “Faces >> offset” or “mid surface >> skin”
• I used “mid surface >> skin” option, for generating the mid surface for all the components, as we have so many features and surfaces.
• Mid surface Generation

Go to Faces >> Mid surface >> skin >> Select entire component >> middle click >> give offset value >> middle click.

• While, generating mid surface using Skin option, we will have 3 colours, red, blue, and Green. Red indicates the bottom surface of the component; green indicates the top surface and Blue indicates the thickness regions of the component.
• If we generate mid surface using this option, the PSHELL card is created by itself, and the Thickness will be automatically assigned to the mid surface.

Mid surfaces of all components: -

Inner extract Mid surface

outer extract Mid surface

Latch support Mid surface

Hinge support Mid surface

Step 4: -

Meshing: -

• After generating the mid surface and before meshing also, we have to check for the geometrical errors and has to do basic things such as, performing Geometry check, surface orientation. Only after having zero errors, we have to proceed for meshing.
• We will perform meshing on the extracted mid surface components one by one, which already stored in locks.
• Now, before meshing, we have to perform some operations such as creating and removing construction lines on the component, defeaturing of some features, to create a good mesh flow and to avoid the Tria’s in the component.
• We have to follow the Quality criteria given in the question,

Quality Criteria	value
Target / Average Length	5
Minimum length	2
Maximum length	7
Aspect ratio	3
Warpage	15
Skewness	45
Jacobian	0.7
Min Quad Angle	45
Max Quad Angle	135
Min Tria Angle	30
Max Tria Angle	120
Tria percentage	15

• And after set Quality parameters such as, average length, min length, max length, first order, mixed type, etc.,
• Don’t forget to activate the “element %” in quality criteria >> Performance parameters.
• Now, we can start doing meshing. For meshing,

Go to perimeter >> Length >> select entire component >> set element length to 5

• Now, Go to Meshing >> Mesh Generation >> Best mesh >> select entire component >> middle click.
• To achieve the good element quality, we can use options such as Reconstruction, split, paste, swap, join, cut, edge2perimeter, Num +/-, Mv surface etc.,

Meshed Components: -

Inner extract

Outer extract

Latch support

Hinge support

Entire Meshed Hood model

• Now, we have to apply the " Draw shell as solid " command to the meshed hood component.

Click F11 >> Go to Presentation parameters >> Tick the " Draw shell as solid " command box.

Learning Outcomes: -

• Knowing the ANSA GUI and its tips for better visulisation.
• To get to practice all the commands, in TOPO and Mesh Module.
• to know the cleaning of geometry, both manually and automatically.
• Mid surface extraction for sheet metal components using both offset and skin commands.
• How to get a good mesh flow by adjusting the construction lines and some defeaturing with low triangle percentage.
• Managing the element quality using different commands in mesh module.
• Assigning the thickness to the components

Conclusion: -

• Properly extracting the Mid surface of the components.
• Generating good mesh flow.
• Achieving zero OFF elements.
• Tria's near the holes and Free edges are eliminated.
• Activated the "Draw shell as solid" command