To perform TopOp requires four known constraints; A boundary in which the optimisation of the model will take place, a place at which the model will be supported, a hot steaming load case acting upon the model, and modelled elements upon which these forces will act.
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| Here’s one I made earlier. |
The boundary is the black box. The computer can place any ‘material’ it likes within this space.
The red crosses indicate the area at which the part is supported.
The blue lines indicate the load, in this case pointing straight down with a force of x Nm.
Elements of the design that the forces are acting upon. In this case it is the circles filled with blue lines
The rest of the decisions to make are arbitrary, but for your design, perhaps necessary. For instance, areas in which no material can be added, or from which no material can be taken away. Once these inputs are plugged in and the program is run, it will produce an optimised shape from which a design can be produced (Fig 5.) Using 2D optimisation as in this case, will produce an outline from which a 3D form can be derived.
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the optimisation process
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| Fig 6. |
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| Fig 5 |
It is a shame that the 3D optimization provides such bad meshes, as the shapes it produces are wild. I would like to reiterate just how indecipherable this program can be. If you’re struggling, get in touch and we can try and figure it out.
The distinctly organic form that these models have gives an insight into how the program works. In nature, organic structures are grown by the animal or plant. The energy required to form life must also sustain it, and so conservation of energy is basically the most important requirement for a successful organism. As an organism evolves over successive generations it optimises itself in response to biomechanical stresses, and gravity, by using the least amount of energy to grow the structure that will resist those forces. TopOp doesn’t grow parts from nothing as it requires a pre-defined model, but the optimised shapes it produces have what we perceive to be organic forms because organic forms themselves are optimised.TopOp essentially works in reverse by taking a defined model and simulating stresses applied to it, and then removes material which doesn’t affect the performance of the part within tolerable levels***.
With the CAD sorted it was time to get physical, and the only man I know who can make my fantasies a reality is Odin. The all-father now walks the earth selling pencil sharpeners, but despite that he is my friend, and a born craftsman. I’d link you to his blog, but weirdly he doesn’t have one. CNC milling is a topic I've spoken about before on this blog, but it’s worth pointing out that you can’t just push a button and make a part. You actually have to push loads of buttons. Odin is an incredible button pusher and lets me watch while he one inch punches his milling machine in all the right places until finally it decides to do all the work and spits out the Part. This Part is only the interface between an enve seat clamp, and a post, which is a tube, that goes in another, slightly larger tube called a seat tube, duh.
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| How does he do it? |
I had originally intended the interface between the Part and the tube (which goes in the seat tube) to be made with a thermal shrink fit. The Part would have been put in the fridge and left to thermally contract whilst the tube would be put in the oven and thermally expanded. The expansion and contraction are very small but they allow parts with extremely tight tolerances just enough room to be put together. When the temperatures of both parts subsequently equalised they would, in this case, be permanently bonded as there would be no way to heat them separately, creating a very strong (and interesting) joint. This didn’t happen. The orientation of the Part in relation to the bit (known as the barbershop pole in the industry) meant that only half of the Part could be milled before it was manually flipped and the other side was completed. It is very difficult to manually flip the part and put it in the exact same place as it was before which introduces inaccuracies that ruin any possibility of producing the tolerances required for a thermal shrink fit (Fig 7.) We could have introduced a third orientation and placed the future nubbin vertically to the barbershop pole which would have produced an accurate cylinder in one operation as opposed to two.****
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| The second half being milled. |
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The Part encased in Carbonite
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In reality the tube I'd bought was way bigger than the nubbin I'd modelled anyway so we turned a shim and shoved the Part into the tube, bonding it with Loctite. Metal glue doesn’t even sound like a real thing so I wasn’t expecting this to hold but this wasn’t where the seatpost failed. Turns out the diameter of the tube was wayyyyyy thinner than on a regular seatpost. This was not helped by the massive layback built into the seatpost which further leverages the tube until it folds in half at about the same point on your two week bike trip! |
| What happens when you forget to water your seatpost |
*I would usually blame myself, but I think in this instance I think I'm being generous.
**Although the terms generative design and topology optimization are often used interchangeably, they are different. Generative design can be thought of as a package of optimisation strategies that accounts for a variety of factors when generating designs, one of which is topology optimisation. It’s one of those, not all rectangles are squares, but all squares are rectangles.
***What the software actually does is break the boundary area into a certain number of cubes or squares (depending on whether it’s a 3D or 2D optimization respectively) determined by the resolution of the optimization (picture of resolution). More blocks/ squares equates to a more detailed model. The software then simulates an applied load to each block/square and determines how removing this block affects the rest of the model's ability to resist that load.Over a number of iterations the simulation gradually establishes which parts of the boundary area are most necessary in sustaining the load applied. This breakdown of a model into a certain number of individual elements upon which analysis is performed is called Finite Element Analysis or FEM.
**** I’m aware that for most readers this level of detail isn’t particularly interesting, but although i'm doing this for fun i'd like to be paid to do it. These bits are so that the people who know, know that i know they know i know. you know?
