Octet Truss Cube: Printability Update

This little octet truss example cube moved from "First to Try" to "Successfully Printed" on my Shapeways store. They estimate a greater than 80% success rate on the print (thanks to whoever ordered the part!).

I knew it could be printed since I had previously printed versions of this design, but Shapeways updated their print-ability guidelines since then. Nice to see it still works!

Fun with Machines that Bend

I really like his 3-D printed titanium part at about the 8 minute mark, and the chainsaw clutch at minute 10 is pretty neat too.


The eight "P's" of compliant mechanisms:

1. Part count: reduced parts count with bending parts instead of hinges and springs
2. Production processes: lower price through processes like injection molding
3. Price: lower because of reduced parts count and affordable processes with reduced assembly
4. Precise motion: no backlash (yea!),
5. Performance: no need for lubricants, reduced wear
6. Proportions: can be made at small scale with photolithography
7. Portable: lightweight
8. Predictable: the operation of the mechanism can be well-known and reliable

OpenLSTO plus InverseCSG

I was recently excited to learn about the OpenLSTO and InverseCSG projects, and that got me thinking: can we automate topology optimization interpretation for a 3D part with open source tools?

Topology optimization results are usually a discrete set of density voxels (as from ToPy) or a triangulated mesh (as from OpenLSTO). There is an interpretation step often required to take this result and turn it into something that you can fabricate or incorporate into further design activities. In the case of OpenLSTO you are getting what your manufacturing chain needs (an stl file) if you are 3D printing.

Interpreting the results of a topology optimization can be a time consuming manual process for a designer. While the steps to interpret a 2D topology optimization result can already be automated with a complete open source tool-chain, 3D is harder. I demonstrated in this post how the 2D bitmap output of ToPy can be traced to generate dxf files that you can import and manipulate in a CAD program. On the other hand, here’s an example I did that demonstrates the more manual process for a 3D part.

OpenLSTO: New Open Source Topology Optimization Code

Optimized 3D Cantilever from OpenLSTO Tutorial

I was excited to see this short mention of a new open source topology optimization code in the Aerospace America Year in Review.

In July, University of California, San Diego published open-source level set topology optimization software. This new software routinely runs 10 million element models by adapting and tailoring the level set method, making design for additive manufacturing immediately accessible.
New computing tools, international collaboration spell design progress

The software site for UC San Diego's Multiscale, Multiphysics optimization lab has the basic license information, and links to documentation and downloads. The source code is up on github as well.

Topology Optimization with ToPy: Pure Bending

From The Design of Michell Optimal Structures

Here is an interesting paper from 1962 on the design of optimal structures: The Design of Michell Optimal Structures. One of the examples is for pure bending as shown in the figure above. I thought this would be a neat load-case to try in ToPy.

Topology Optimization for Coupled Thermo-Fluidic Problems

Interesting video of a talk by Ole Sigmund on optimizing topology for fluid mixing or heat transfer.

Installing ToPy in Fedora 26

This post summarizes the steps to install ToPy in Fedora 26.

Deep Learning to Accelerate Topology Optimization

Topology Optimization Data Set for CNN Training

Neural networks for topology optimization is an interesting paper I read on arXiv that illustrates how to speed up the topology optimization calculations by using a deep learning convolution neural network. The data sets for training the network are generate in ToPy, which is an Open Source topology optimization tool.

Fully Scripted Open Source Topology Optimization

Helical Extruder Gear for Printrbot with Optimized Topology

I've used a couple different methods for stringing together open source tools to do topology optimization, but they have all required some interactive user input. Here are some previous posts demonstrating those manual methods:

• Open Source Topology Optimization for 3D Printing
• Octet Truss for Topology Optimization
• Prusa Mendel RepRap Frame Vertex Topology Optimization Using ToPy
• 3-in Tack Strip Bracket TopOpt

Those approaches are fine if you've got time to fiddle with interactive software, but I wanted to do some parametric studies, so I need an automated approach that would be scalable to lots and lots of optimizations.

FreeFem++ Topology Optimization Scripts

There are lots of open source topology optimization options out there (e.g. 99 line code, ToPy) that I've written about before. One that I haven't posted about yet is a collection of FreeFem++ scripts by Allaire, et al. that illustrate a variety of topology optimization approaches and problems. FreeFem++ is a partial differential equation solver based on the finite element method. FreeFem++problems are defined in scripts that use a high level language. FreeFem++ itself is written in C++.

3-in Tack Strip Bracket TopOpt

I found a useful bracket on thingiverse for mounting things on a 3-in tack strip. Of course I thought this was a perfect opportunity for a bit of topology optimization. All of the design files and the stl (rendered above) are available on GitHub. The part is also on thingiverse.

Here's a video showing the progress of the optimization:


Rendered with a wave texture in Cycles to give the layered look it would have from an FDM machine, not quite right, but pretty close:

GE Jet Engine Bracket Challenge Winners

Topologically Optimized Bracket by KMWE + TU Delft

The winners for the GrabCAD GE Bracket Design challenge have been manufactured and tested. See all the pictures here. The team I mentioned previously that was using topology optimization went ahead and manufactured their bracket themselves. What a great demonstration of metal additive manufacturing (3d printing) and topology optimization. I hope the GrabCAD folks get the promised detailed feedback for all the entries up soon.

Free (not Libre) TopOpt App

This little app from the TopOpt research group at the Technical University of Denmark is lots of fun to play with. Versions available:

• iOS AppStore
  
• Android Google Play
  
• On the web
  
• pre-compiled binaries for Linux
  

You can move around boundary conditions and forces to see what 2-D arrangement of material is the stiffest. There is also an option to export the geometry to an stl file for 3-D printing.

I couldn't find links to any source for this implementation, but it is based on the methods in the 99- and 88-line codes I've written about previously.

GE Jet Engine Bracket Challenge: Phase I Winners

I wrote previously about some neat entries in the GE Engine Bracket Challenge on GrabCAD that used topology optimization. As reported by GE, they have picked their winners from Phase I. Phase I consisted of simulating the submitted parts in several different load cases and ranking them by how much weight the designer was able to shave off.

Located around the world, finalists include:

• Ármin Fendrik, based in Hungary, is a third-year university student and this entry is among his first 3D printing designs.
• Thomas Johansson, Ph.D, based in Sweden, is a consultant for a Swedish hyper-car manufacturer and is a champion snowmobile drag racer.
• Nic Adams, based in Australia, supported the installation of a pathology lab automation system in a Sydney hospital, which includes a robotic handling system that helps analyze hundreds of test tubes each day.
• M Arie Kurniawan, based in Indonesia, is co-founder of an engineering firm that provides high quality mechanical engineering, design optimization and product design services.
• Sebastien Vavassori, based in the United Kingdom, is a stress engineer for a leading European space manufacturer and service provider.
• Piotr Mikulski, based in Poland, works as a rapid prototyping systems specialist for a Polish-Swiss joint-venture that provides industrial and machining services.
• Andreas Anedda, based in Italy, is a postgraduate university student and holds three patents.
• Alexis Costa is based in France.
• Mandli Peter is based in Hungary.
• Fidel Chirtes is based in Romania.

These winner from Phase I will have their designs "printed" in Titanium and then tested by GE to determine the winners for Phase II.

3-D Printing in DoD: Who's Dragging Their Feet?

I found this article, Why is the Pentagon Dragging Its Feet on 3D Printing, by way of Small Wars Journal. It has some interesting information. The Army is deploying mobile Fab Labs, which seems like a mini MIT FabLab in a shipping container. I think this is a really neat idea. How this can be characterized as feet dragging, I'm not sure. The feet dragging accusation is based on some hand-waving from an article on Disruptive Thinkers, and another article that seems to be worried that there is no Pentagon overlord in charge of an additive manufacturing strategy:

With possible dwindling budgets on the horizon, a clear strategy and cohesive approach is essential to create efficiencies in the area of research and development as well as eliminating duplicative efforts. In order for DoD to take advantage of what is anticipated to be an explosion in the commercial sector within the next ten years, the Department must take an active approach, partnering with the private sector to keep up with this relatively nascent technology and shaping/guiding it towards the desired end state the department has in mind.

One step towards a clear strategy and cohesive approach is for DoD to designate an AM Czar within the Department. They could serve as a single point for all things AM and not the myriad of technical advisory boards that currently exist. This office could then work with policy makers to execute and monitor a strategy which will allow DoD to take full advantage of this technology. Logically, this office would interface directly with the National Additive Manufacturing and Innovation Institute (NAMII) as DoD's representative
3-D Printing Revolution in Military Logistics

I think an "additive manufacturing Czar" sounds like a terrible idea (so I'm sure it will secure funding for some beltway bandits to do a study). I know my recent success with qualifying a particular additive manufacturing process and supplier for use in 3D printing wind tunnel models did not need a Pentagon king-pin to tell me about DoD's strategy for additive manufacturing. Using this technology just made sense as a way to solve my problem: get a complex wind-tunnel model rapidly, and at an affordable cost. I did not receive top-down direction or guidance to use AM, I simply took the initiative to solve my problem. After reading that article I'm left wondering, just how exactly is waiting on direction from the very heights of the bureaucracy supposed to lead to innovation?

Topology Optimization in GE Jet Engine Bracket Challenge Entry

There is an interesting contest on GrabCAD for designing a lighter weight engine bracket for a GE engine the winner of which GE will produce and test using an additive manufacturing method (maybe DMLS). One of the contestants used PareTO software (methods based on these matlab scripts I linked previously) to design a pretty nice looking bracket.

Update: There are more entries that are using topology optimization.

• KMWE and TU Delft Team Entry. This comment the team makes is interesting: "Since the optimised topology models are in stl we have started to first create a volume model with stp extension so we meet the competition recuirements. This takes a lot of time!" This bottle neck in the work-flow is similar to the problem CFD analysts have with structured grid generation. While many (most) 3D printers will take an stl format file (which is just a triangulate surface), you still really want the normal CAD formats (parametric) for a couple reasons. Usually for the metal printing processes you have to add support material. This is done more easily / accurately with something other than an stl. Also you want to be able to use the part in larger assemblies, and this is likely to go better using a native CAD format.
• GE Jet Engine Bracket v1.5, Topology Optimized Bracket - V4, by Igor Lins e Silva
• GE-jet engine bracket-opti-design-phase 1, by Cheng.Li
• Engine Bracket V2.1, by Igor Lins e Silva
• GE Challenge, by Charlie Pyott. I like this one because he uses a lattice, which reminds me of the octet truss things I was working on previously. Charlie also has a website with other interesting designs.

Octet Truss Improved Time Cost

Previously, I did a quick and dirty scaling study on how long it takes to make a 3D cube of octet truss unit cells in BRLCAD. I posted some questions about the results to the list and got some good recommendations on speeding things up. The approach I used previously was just unioning a big, flat list of primitives. The main recomendation to get speed-up was to introduce a bit of spatial organization in the way the primitives are grouped. I did this by making each unit cell a region, and then making an assembly combination of the unit cells. This is only two levels of hierarchy, so ultimately the scaling is still quadratic. The speed-up is pretty dramatic though. To get the \( N log(N) \) scaling mentioned on the email list would require an octree structure with an adaptive number of subdivision levels. Here's a plot showing the improved time scaling:

Octet Truss Memory, Time and Dollar Costs

Cost for 3D Octet Truss Arrays

	i	j	k	time (s)	stl (bytes)	WSF ($)
1	1	1	1	1.47	73384
2	2	2	2	28.74	435384	2.61
3	3	3	3	173.32	1342184	4.99
4	4	4	4	873.20	3052984	9.46
5	5	5	5	2952.67	5826984	16.69
6	6	6	6	6694.16	9923484	27.32

This is a follow-up to the previous post on using the octet truss for topology optimization. The memory cost of performing the union of all the truss members in BRLCAD to generate stl files for printing was too large for the ToPy dogleg example so I generated a set of stl files for a range of arrays of octet truss unit cells. Then I uploaded them to shapeways to see how much they would cost to print in the white-strong-flexible nylon material. The table shows how long the python script took to execute, the size of the resulting stl file, and the cost to print the part on Shapeways in the white-strong-flexible material.

Octet Truss for Topology Optimization

Random Octet Truss Array (on shapeways)

This post demonstrate a work-flow for topology optimization using open source tools (with mixed success). The approach uses an unpenalized method (see wiki) that maps the material density output from the optimizer to unit-cells based on the octet truss (inspired by this white paper, see pp10). As some further motivation for this approach, the students working on the record-setting human powered helicopter demonstrated that multi-scale trusses (trusses with elements made of smaller trusses) were a very efficient structural concept (see the comments for further references on multi-scale structures). One of the benefits of not penalizing (using variable density solutions rather than trying to achieve predominantly solid-void solutions) is that we don't need to spend time doing parameter continuation on the penalization exponent.

Open Source Topology Optimization for 3D Printing

Rough Hex Output & Smooth Surface Reconstruction for Dogleg

This post describes a set of open source tools for simple topology optimization for parts destined for 3D printing. The two Matlab codes (both are plain vanilla Matlab, so they work successfully in Octave too) are good introductions to topology optimization. The papers that go along with the codes provide great documentation and examples of tweaking and changing the scripts to treat various problems. The python implementation is more capable (and a few more lines of code).