Dear Reader,

In accordance with my timeline, I have explored the capabilities of FreeCAD’s FEM Module along with other auxiliary modules in order to find a reliable software program that enables easy application and analysis of stress on a 3D object.

I explored the basic functionality on a trivial yet essential structure as shown in the following image:

Here were the displacement and stress results on individual nodes of the mesh:

In this test, I used auto generated nodes for the mesh. FreeCAD also appears to support more customized methods for defining structural nodes:

However, I’m still in the process of figuring this feature out. Also, I’m working on designing a specific 3D object that would be simple to stress test given a specific set of available equipment.

Stay tuned for updates.

Thanks for reading,

-Xiang

Dear Reader,

The sphere packing software currently generates 3D internal structural meshes for hollow 3D objects. However, our current usage of sphere packing does not provide a major improvement over simple manual generation of uniform truss structures. The true advantage to using sphere packing is actually the ability to selectively manipulate local regions of the internal structure. Of course, a human engineer could manually design a complex structure with desirable properties in specific parts of a 3D object, but this is time consuming. My current goal is to create software that can automate or aid a human in this process.

In order to take advantage of sphere packing’s capability, I am moving the project to the next phase. I’ve been doing research on stress in mechanical engineering to gain a better understanding of how structures are designed to counter or use stress. Unfortunately as far as my novice eyes can see, it seems that the structures generally do not involve complex 3D graphs. Also there’s a large amount of information on how to calculate stresses on objects, but there is much less information on how to augment structures to counter stress besides using varying materials.

Regardless, I am going to perform an experiment to determine if I can structurally augment 3D meshes to resist stress forces, specifically compression, tension, and shear.

In the first phase, the inputs are the STL file of a 3D object and the sphere packing parameters. The output was a 3D graph that served as the internal mesh of the object. In the second phase, the input will be the 3D graph and data describing forces applied to each vertex of the graph. The output will be an altered graph that should resist the forces better.

To obtain the forces applied to each vertex of the graph, I will convert the graph to solid form as a STL file and use a finite element analysis tool that will enable a user to selectively apply forces to the entire object. I am thinking of using an FEM module for FreeCAD; although I have yet to explore the capabilities of the module.

Next, I will add functionality to the sphere packing software where the vertex forces data can be used to refine the graph. The specific refinement scheme still needs to be resolved, but I do have an idea that I am going to implement. I will explain in greater details once the implementation is complete, but the general idea is based off of cell growth in biology where in my case the cells are spheres. This is based on the assumption that a dense graph can withstand more stress than a sparse graph. Think of osteoporosis for example:

Once the refined graph for the original object has been generated, I will conduct physical tests to determine its structural limits. I will conduct the same tests on the unrefined graph, the full solid object, and a hollow shell of the object as controls.

To keep myself on track I present my Timeline:

March 27th, 2017 – Find and utilize suitable software for simulation of forces on 3D objects to calculate forces on specific nodes of a graph.

April 7th, 2017 – Finish implementation of refinement functionality in sphere packing.

April 14th, 2017 – Finish designing experimental structures and print as objects for testing.

April 21st, 2017 – Complete physical stress tests on objects.

April 24th, 2017 – Write up report on experiment.

I shall keep you posted on progress.

Thanks for reading,

-Xiang

Dear Reader,

This week we’re able to see some physical prints of the stl infills generated through the process described my last week’s post. The prints and pictures are all courtesy of Professor John Bowers.

Here are photos of a print using the shell included:

Thanks for tuning in this week.

-Xiang

Dear Reader,

This past week I have read over the official OpenSCAD documentation and achieved a limited but sufficient understanding of the OpenSCAD scripting language.

To the Sphere Packing software, I have added a new file output for the graph edges as well as a file output for the mesh triangles.

Unfortunately, since it doesn’t seem like OpenSCAD has a direct way of pulling input from general files aside from standard 3D object files such as STL, OFF, and AMF, I have to manually copy and paste the file outputs from Sphere Packing into an OpenSCAD file. On the bright side, I have made the file outputs to be vectors of points that are already in scad format so copying and pasting is all that really needs to be done. I have written a simple scad script file that takes the graph data and mesh data and generates corresponding solids.

Here is a picture of an example solid for a graph from Sphere Packing:

The solid above took several minutes to render since the level of detail is relatively high. Next, I constructed a hollow shell from the triangles of the original mesh to fuse with the internal graph structure as a cover. The results are demonstrated below:

These solids are all union-ed together to create one solid which is then exported as an STL file. The object can be sliced with any 3D slicer program and then printed using existing 3D printers.

Hopefully, I’ll get to print a physical model this week.

Thanks for reading,

Xiang

Dear Reader,

A major goal of my Sphere Packing research is to actualize the mathematics and computer models into novel techniques for the mesh generation portion of the 3D printing process. To this end, we need a way to complete the full process starting from an STL model of a 3D object and ending with a 3D printed object. With the current software, we have already established a way to generate the meshes; however, we do not yet have a way to print the meshes. As partially mentioned in the previous post, we came up with two possible approaches for printing the meshes. The first method that we will try involves the conversion of a mesh into a solid using OpenSCAD.

A simple illustration of this idea is shown below:

Having the solid, we could then use traditional slicer software to generate the G-code for printing the object.

Also, we can use OpenSCAD’s shape union and intersection functionalities to combine the shell of the original object with the internal infill mesh to create a print object that would have the shape of the original object but with a semi-hollow internal support structure consisting of the mesh from sphere packing.

The general idea is produce a lightweight solid with a strong internal structure that can withstand great stress. An analogy to a natural structure with this quality would be bird bones which are necessarily lightweight for flight yet they can withstand the constant pressures from flight motions.

The specific implementation of this procedure is in progress, so stay tuned. ~

Thanks for reading!

-Xiang

Dear Reader,

Since the last time I posted, I have added the ability to generate a different lattice structure to the Sphere Packing software. This new lattice structure is called the hexagonal close packed (hcp) lattice. It is one of the tightest ways to pack equally sized spheres, with the other way being the face-centered cubic (fcc) lattice which only differs in the way that consecutive layers are aligned.

In the above image, a bird’s-eye view of hcp packing (left) and fcc packing (right) is shown. The underlying hexagonal arrangement of white spheres representing layer 1 resembles the tightest way of packing equally sized circles in a plane also known as a penny-packing. Every layer of the hcp and fcc lattices has this identical hexagonal arrangement. The black spheres representing layer 2 are placed over the gaps created between three underlying spheres. Note that layer 2 and layer 3 are not fully drawn out so that the lower layers are not obscured completely. Also note that layer 2 is identical for both hcp and fcc lattices. The difference is from layer 3 which lines up with layer 1 in the hcp lattice, but is shifted in the fcc lattice.

Anyways, back to the main point. Hexagonal close-packed lattices are now available to use in the sphere packing software. Earlier there was only body centered cubic lattices:

Now here are the hcp lattices:

With this new type of lattice we hope to gain insights into how a different set of combinatorics affects sphere packing.

Moving forward, I would like to 3D print the meshes generated by the sphere packing. The difficulty lies in the fact that the meshes are not solids, so traditional 3D slicer programs can not work with the mesh output directly. Thus, one approach would be to convert the mesh into a solid. As it has been suggested to me, this could possibly be done using OpenSCAD. Another approach would be to skip the slicer step completely and directly generate gcode from the mesh using a toolpath planning algorithm. I will first try the method of converting the mesh into a solid since it seems easier than developing a new algorithm for toolpath planning in 3D space. I will keep you posted on the results.

Thanks for reading!

-Xiang

Dear reader,

I wish to propose a new method of generating new infill meshes for 3D printing objects. However, let me first give a brief background. Over the summer I worked as a software developer at Oak Ridge to create a suite of tools that apply the mathematical concepts of Circle Packing to generate graphs (Computer Science graphs) that were later used as the meshes for infills. The overall goal was to create meshes that used less printing material but had sufficient physical integrity to still perform its role as a support structure. The end result would ideally be a strong mesh that was still light-weight. An image that illustrates this is shown below:

I will also add here that we wanted a way to “densify” the mesh in custom regions in order to selectively increase the physical strength in specific areas of the print. In the image above for one layer of a 3D airplane wing, based on the assumption that the edges would experience more stress, we selectively refined the mesh in 2 layers around the edge using a technique developed by Ken Stephenson, a math professor at the University of Tennessee.

Later on, the project leader for this area of research at the Lab, Greg Dreifus, suggested that we could maybe extend the concepts to 3D. Specifically, he wondered if we could build 3D meshes instead of 2D meshes by using Sphere Packing instead of Circle Packing. In order to demonstrate the idea, I worked on a new software that generates a Sphere Packing given a 3D object encoded in an STL file. This new software is based largely on a Circle Packing implementation by John Bowers, a computer science professor at JMU. An example of the software’s capability is shown below:

We hope to further develop the software into a tool that can produce meshes of varying density in custom regions of the 3D object.

Thanks for reading,

Xiang