Creo Community Challenge October 2023: Isogrid on a Curved Surface

The October 2023 Creo Community Challenge was to make an isogrid—a repeating rib structure used in aerospace that adds strength with low additional mass. Although it may seem simple, it can be difficult to manufacture and model in CAD, especially placed on curved surfaces typical of flight structures and OMLs (Outer Mold Lines).

This month, we had seven people make around 15 submissions (some submissions contained more than one model). Let’s dive into it!

Common challenge: Large part management

One of the biggest challenges in modeling this geometry is that you’re probably going to use multiple patterns, using the General option. This results in large file sizes and long regeneration times. Some zipped files exceeded the PTC Community site’s 47-megabyte file size limit. Some people submitted files with suppressed features or everything in Insert Mode. One submission from Thomas Braxton made all the features Read Only to avoid regeneration. Normally when I evaluate models, I use Edit Definition a lot. This time, the Model Tree and Feature Information command were my friends.

The submissions

Thomas Braxton was the first person to submit a model, and he ended up submitting three. The first used a core modeling approach of projecting Trajectory Ribs down into the dome. There was a pattern of On-Point Holes and a Reference Pattern of Rounds. Finally, a Solidify feature was used to scoop out the excess geometry so that the isogrids had a constant height. 

Thomas’ second submission was the isogrid on a cylindrical surface. This was achieved by modeling “in the flat.” Their process went as follows: Create an Extrude for the base. Create a triangular Extrude that removes material, then make an Axis Pattern for the hexagonal cell. Use a Fill Pattern to place the isogrid across the base. Then something I had not considered: an AutoRound feature on all concave edges. Dimension Pattern the center post holes, then make it cylindrical with a Toroidal Bend.

Thomas’ third submission was an isogrid on a conical surface with a completely different approach. For this one, I recommend you walk through the Model Tree yourself. This was quite complex. It involved surface modeling, multiibody modeling, Sweeps, Offset with Replace, Mirror, Copy and Paste, AutoRound, Copy Geometry within the same model, and, of course, Patterns. This part could be a tutorial in an Advanced Part Modeling class.

On the aesthetic side, I liked Thomas’ use of Annotation Features and Local Groups to organize the Model Tree. Always make your models user friendly. You never know who will have to use them later or how long it will be until you touch them

Fiddix submitted two models. The first created an isogrid on a curved surface, starting by modeling in the flat like others. A triangle with rounded corners created a cut. They rounded the Intent Edges. They created an internal Copy Geometry of the cut which was Axis Patterned to form the hexagonal cell. A Fill Pattern formed the isogrid across the entire base. The flat was made curved with a Toroidal Bend, and finally a Revolved Cut made it look like the nose cone on an aerospace vehicle.

Fiddix’s second model was an isogrid on a conical surface with a new approach. The base cone was created with a Revolve. Then the crisscross of the isogrids was made with patterns of right-handed and left-handed Helical Sweeps! The model was reduced to a sliver with cuts created via Solidify with Datum Planes (a favorite technique of mine) to facilitate an AutoRound. An internal Copy Geometry feature was patterned to restore the model to 360 degrees. My mind is blown by the creativity and strategy in our user community.

The Model Tree for Jignesh Vadalia’s submission at first appears incredibly short with only 8 features—until you realize that one feature is a pattern of a Local Group with 96 instances. His technique was to model everything in the flat and then use a Flatten-Quilt Deformation feature to wrap the geometry around a surface, which itself was bent in three directions with a Warp feature and then flattened and thickened to form the base feature. The isogrids were created with an Extrude for the center post and a Trajectory Rib with good use of Sketch References to facilitate patterning—a very novel approach. I thought someone might use a Warp feature, but not in this way. I did not expect to see Flatten Quilt and Flatten-Quilt Deformation features.

Olivier submitted two models, one with the isogrid on the interior, the other with the isogrid on the exterior of a seemingly revolved ellipse. But the egg shape was not created with a Revolve feature. He created a hexagonal “cell” of the isogrid, duplicated it with a Fill Pattern, then curved the geometry with a Toroidal Bend. Then a few Mirror features completed the egg.

Here’s the ingenious part: it looks like changing the sketch plane of one Extrude feature was all that was needed to flip the isogrid from being internal in one part to external in the other.

Kevin Dirth submitted two models. The first placed the isogrids on a revolved surface defined by a spline curve. His approach defined points, curves, Boundary Blends, Extends, Thickens, and patterns of internal Copy Geometry features to construct the webs. He used patterns of Sweeps to place the center posts and holes. A smart approach to get that geometry on a curved surface. Kevin’s model exemplifies how isogrids are deceptive because they are much harder to model than they look. On the PTC Community post, Kevin said that he created Mapkeys to automate the repeated operations in his “brute force” approach. Then Kevin went so far as to 3D print his model! Wow! 

Kevin’s second model placed the isogrid on the radome model that I supplied in the initial challenge post. He took a similar construction approach with points, axes, curves, Boundary Blends, Extends, Thickens, and patterns, topped off by a Solidify feature for a constant height. I am impressed that his isogrids extend all the way to the nose of the radome. I honestly didn’t know if that would be possible.

EF 10127429 opted for the first challenge: create an isogrid on a planar surface. (We want to people to submit entries, so we endeavor to have a level of participation that they can hopefully complete in a couple of hours or less.) They created an extrude with four of the six sides of a hexagonal isogrid cell, then patterned that with the Direction option. There are lots of isogrids on flat panels in aerospace, so this technique can be useful to the community. 

Pettersson submitted five models! The first placed the isogrid on the radome by modeling in the flat and then using Spinal Bends to wrap it onto the curved surface. They could have stopped there, but continued with a combination of Extrudes, Copy and Paste, Extends, Solidifies, and multibody modeling Merges to get the strengthening structure all the way to the nose.

Pettersson’s second model placed the isogrid on a curved surface defined by a Sweep with a curved trajectory and curved section. They modeled in the flat like others and then placed the isogrid on the curved surface with a Flatten-Quilt Deformation. I’m impressed by the economy of the model. The geometry was created with only 42 total features, and there was only one pattern with two instances. They mention having to manipulate the model accuracy, which people should keep in the back of their minds if they’re ever having trouble getting features to regenerate.

The third and fourth models were additional takes on the radome. The third used patterns of two Sweeps to define a slice and another Sweep to define the zigzag of the “ribs.” There were a bunch of editing features to transition the geometry from the main body to the nose, then several patterns addressed the center posts and holes. The slice is then mirrored and patterned to fill the interior with the isogrids.

The fourth model has a nice uniformity I find pleasing. Whereas the third model approached the isogrids by developing a slice along the length and patterning around the axis, this one generates an isogrid ring around the circumference and patterns it along the length. It’s interesting to flip back and forth between the three radome models to compare the isogrids: three approaches by the same person to the same problem.

The final model made my jaw drop. The isogrids were placed on a cylindrical surface with a single pattern of Extrudes. With the pattern collapsed, it looks like just two features in the Model Tree. The Extrude does have some embedded datums, which is a nice way of keeping the Model Tree uncluttered. In the comments, Petterson explains the trick was in using some Relations in the sketch to move the triangles up and down. The Relations use reference sketch dimensions and the modulus function. That’s really taking advantage of all the tools available in Creo. This is another model that could be used as a teaching tool in advanced part modeling.

What can we learn?

This is the longest blog post I’ve ever written for PTC, but these people spent a lot of time and brain power on the challenge, so they deserve the recognition and more. Like with the Mathcad challenges, I’m impressed by the variety of approaches and tools that people take to the same problem. This was not an easy problem to tackle. People used surfacing and multibody. There was a wide variety of commands, including Trajectory Ribs, Variable Section Sweeps, Helical Sweeps, Boundary Blends, Autorounds, Toroidal Bends, Spinal Bends, Flatten Quilt, Flatten-Quilt Deformation, and Warp. Just about every kind of Pattern was used, including Dimension, Direction, Axis, Fill, Point, and Reference. 

My personal takeaways are:

• Designs that look simple at first glance can sometimes be complex to achieve.
• You want to consider multiple different approaches to a problem, and you should include tools that you might not select with your first choice.
• You should solicit input from multiple people, as their knowledge, skills, and experience can take you in a variety of directions.

Take a look at these models. I guarantee you will learn from these people. Check out the next challenge focused on simulation!