Reverse Engineering a Motorcycle Foot Peg – Part 4

Last Updated on by DesignPoint Team

Part 4: Preparing for 3D Printing

Welcome back to our reverse engineering series. So far, we’ve covered:

Today, we cover my favorite part…preparing for 3D Printing!

Before we jump in, I want you to think back to your first encounter with 3D printing. Maybe it was during college and you were one of a select few that had exposure to a mysteriously cool device. Maybe it was some trade show where you got to see it live. Maybe all you’ve ever seen are YouTube videos. Whenever and however it was, if you’re anything like me,  and you can recollect that instance…you’ll agree that it was a magical moment. It was almost like Star Trek was finally coming to life and we could create whatever we wanted.Well, hold off Captain Kirk, no one’s beaming up just yet. But this is one giant leap for engineering kind, especially with how quickly the technology has evolved over the past couple of years.

As many of you know, the project we’re working through in this series is a broken foot peg of a BMW motorcycle. This foot peg is made by a foreign manufacturer so finding a cheap off-the-shelf part was not an option. A couple questions come to mind:

  • How are we going to recreate this legacy component?
  • What kind of cost would it take to make a mold and have this cast?
  • What about just taking a big hunk of metal and machining away the parts that we don’t want?

If only there was a solution to manufacture at a low volume, intricate geometry, with structural integrity and aesthetic requirements…. Oh, wait there is! 3D printing with Markforged is light years beyond the early days of 3D printing. It is no longer like playing with sandcastles that could break at any moment. These structures are ready for all kinds of tough environments. Let’s discover the possibilities together!

Completed Model of Foot Peg and Footrest

Evaluating Design

When 3D printing something, it is important to understand how the 3D printer works. Here are the steps in a nutshell. There’s going to be a specific program setup for your printer that takes your universal CAD file (like a STL file) and evaluates the geometry. You can scale the part if it’s not the correct size. Once it is scaled properly, then you just need to orient the part. Which direction is the “right” direction you might ask? Well that’s the direction the item is “built” by the 3D printer. The 3D printer is equipped with a nozzle where material is melted and then added to the shape in layers. It’s almost like decorating Christmas cookies with frosting, except there’s very little cookie by the time you’re done and all you have is a frosting sculpture! This process of building the part from the bottom up is a major factor in determining the “correct” orientation, but not the only one. Considering hole tolerances and surface finishes is also important when choosing a print orientation. The best build direction takes this into account. Consider the following two orientation examples, which one would you choose?

Foot Pad Printing Orientations for the Markforged Mark Two

Either one works! This part is simple enough and there aren’t any glaring overhangs of material that would be helped by having a different layout. But the differences between the two builds are the surface finishes and the accuracy of the holes. Usually, it’s a trade-off between the two options. The orientation where the footrest is lying flat (left) would produce better holes since they are parallel with the build direction, but the top surface finish would be marred by the support material. The upright orientation (right) would need very little support material and produce the cleanest finish, but the holes would require support material. Which one is the right one? Depends on the piece and the end goal. Either one works for our Markforged Mark Two printer.

Adjusting Design

Metal Foot Peg Shown from Bottom View

Choosing the orientation for the foot peg is a little bit trickier. The shape has some irregularity to it and doesn’t lay flat in any orientation. This was an issue we ran into at the beginning of this project. I decided to orient the foot peg with the rider in mind. But now that we’re printing this model, this issue is at the forefront again. There are multiple holes in the part and the aesthetic value of the project weighs on which surface we can allow to have support material and which one we want to be smooth.

Orientations for Printing the Metal Foot Peg with the Markforged Metal X

Out of the 3, my favorite to work with would be where the model’s back bracket is lying flat on the build plate and the rest of the peg is almost resting on its side (top-right). This one looks like it will require the least amount of support material but only if we can smooth out the bottom and remove any areas that are not flat. This leads to altering the design at the finished product stage. This is where things get tough and you must either add material or remove material to the actual design. DO NOT try to go back and adjust your original sketches and geometry. Not unless you modeled the part with perfect design intent, which for an open-ended project like this is highly unlikely.

The actual orientation chosen for the build was the top left picture. This is because the accuracy of the bolt holes is more important than the surface finish. Having the bolt holes parallel with the build direction produces the most accurate holes.

Now that we’ve taken the time to look at the design and account for the 3D printer, it’s time to build this and see how it turned out. This will be the next and final chapter of our series. Make sure to subscribe to our mailing list so you don’t miss the finale!

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