Week 4

Aim- To pre-process (mesh) a given bottle cap model with given quality criteria and to provide thickness afterwards.

Procedure-

Step 1-

First of all the model was downloaded and opened in ANSA’19 and we performed a geometry check to look for TOPO errors in the CAD model provided.

And as expected it had some minor TOPO errors like unchecked faces ,needle faces etc which was automatically fixed by using the fix option of ANSA and afterwards for quality purpose we also applied TOPO to let ANSA automatically rectify the topo errors.

FIG 1 GEO CHECK FOR THE MODEL (errors)

FIG 2 TOPO ERRORS FIXED

Step 2-

After that a close inspection was made to understand the cad for better capturing of mid surface (manually).

Step 3-

After that we started capturing the mid surface manually using the offset operator. While using the offset operator one thing we had to make sure was of the side where the mid surface was going to be appear and if required we also had to use the – sign to reverse the direction of the mid surface.

FIG 3 USING THE OFFSET OPERATOR

Step 4 –

The procedure was applied for most of the geometry available but as the model had some complex geometry in places like thread and ribs, it was difficult for us to normally use the offset option over such places. So to capture those features we used different procedure.

They are described below-

• There was a tapered section at the top of the cap.

FIG 4.1 TAPERED SECTION (before)

Tapered section before

To capture this feature we first had to project the marked double cons on the inner side of the tapered section and after that we used the middle option under the curves module to generate a curve at the mid of those to cons and we created mid surface over that region by creating a new surface between two middle curves.

FIG 4.2 TAPERED SECTION (after)

• After that we went for the ribs of mid section.

FIG 5.1 RIBS (before)

As you can see in the picture, this feature can’t be extracted normally using the offset tool.

To extract the mid surface of this rib we first extracted the mid surface of base region (marked by blue color) using the offset option and then projected the double con(marked by red color) on the extracted mid surface .after that we created a middle curve between the 2 cons (marked by black color) and after that we created a new surface between the generated curves. And thus we were able to capture mid surface over these region.

FIG 5.2 RIBS (after)

• Third complex part was to capture thread which was present on the inner surface of the 2 cylindrical surfaces in the interior of the CAD model provided.

FIG 6.1 THREAD (before)

Now to capture these types of surfaces we create a middle curve from the top most two curves ( marked with black color) present on the top of the thread. Then we project that middle curve to surface where thread is placed and then we create a new surface using the middle curve generated and the curve projected.

FIG 6.2 MIDDLE CURVE GEN AND PROJECTED

FIG 6.3 THREAD (After in mid surface)

Now there were some features which we could easily copy rather than wasting time to extract mid srfc for them beacause of there symetry. They were ribs of base and top .

FIG 7 FEATURES WHICH CAN BE COPIED

The copy operator was used for this purpose and two COG were created first using the two cicrcular regions in the base and then in the copy module the rotation operator was selceted followed by selecting the entity and providing the number of steps we had to copy and the corresponding angle between the two simialr consecutive ribs.

FIG 8 USING COPY OPERATOR

The final model will look like this.

FIG 9 MID SURFACE EXTRACTION COMPLETED

Complete mid srfc

After capturing the threads we were done with the mid surface extraction and we were good to go for mesh and hence we switched to mesh module for the sam

Mesh module

Step 5-

We switched to mesh module from the topo module and under mesh generating segment there were variety of mesh algorithms like FREE mesh ,spot mesh ,best mesh, batch mesh etc.

Now as per the geometric location, use, results we had to choose among these algorithms and find out the best mesh which also fulfils our quality criterion requirements.

Quality criteria were as following-

Minimum Length	0.5
Maximum Length	3
Aspect	3
Warpage	15
Skewness	45
Jacobian	0.7
Minimum Quad Angle	45
Maximum Quad Angle	135
Minimum Tria Angle	30
Maximum Tria Angle	120
Tria Percentage	15

These values are in mm and the average meshing length is kept to be 1mm.

These criteria’s were to be fed in the quality criterion module of ansa.

Step 6-

After that we had now to rearrange the construction lines so that the mesh should fit better and should look uniform. For that we had to toggle unnecessary construction lines and at some places where required create lines using the cut operator.

Basically the concept was to mesh in a manner where complex areas to be given priority in meshing sequence.

Step 7 –

Once we were done meshing the model, we then switched to hidden mode from the right bottom part which enables us to visualise the elements which are failing the quality criteria by colouring them with respective colours.

Step 8 –

Now after switching to hidden we could easily see the elements which were failing the quality criteria and now we had to fix them.

For fixing them we used a couple of operators like-

• Split- to spilt element into 2 using nodes or edge
• Swap- swap to consecutive elements with each other
• Join- join to consecutive elements
• Paste – to paste 2 nodes
• Move grid – to move element edge using node
• Edge2perimeter – to change the edge of elements into a cone
• Smooth – to smoothen the elements
• Reconstruct – to reconstruct the mesh in a particular area

So these were the operators we used frequently with one after another as per requirement and were able to get rid of elements which were failing quality criteria i.e. off elements.

There was no sequence of using them; we just had to use them one after another after looking for off elements we were dealing with.

Step 9 –

We tried to get rid of the off elements as much as we could but there were some complex areas where there were triple cons intersecting which cannot be toggled also, so as a result we still had some off elements in these regions only because there was no space to rectify them and we had to leave them as it is.

FIG 10 FINAl MODEL (with some off elements)

Step 10 –

After working on the model we now assigned respective PIDs for the different parts of the model and we also gave respective thickness to them .

FIG 11 PID ASSIGNED MODEL

Thus we were able to pre-process this model.

Outcomes-

• We got to learn about how to fix TOPO errors.
• We got to know how to manually extract mid surface.
• We got to learn about how an ideal mesh should like

1. There shouldn’t be enough tri elements(although the quality was allowed to 15% but in industry usually we want it to be less than 5%)
2. There shouldn’t be tri elements scattered everywhere, they should be placed in a uniform manner.
3. Two tri elements shouldn’t share same edge or node.

• We got to know how to capture ribs and threads
• We got to know how to capture tapered sections.

google drive link for the model - https://drive.google.com/file/d/1oStdh-9_Fwbs6tdCK387i_sa5FbWCPKt/view?usp=sharing