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MAKE: Intern's Corner
Every other week, MAKE’s awesome interns tell about the projects they’re building in the Make: Labs, the trouble they’ve gotten into, and what they’ll make next.

By Kris Magri, engineering intern

How I designed Makey, Part II: Creating the “stretchy” robot body in Inventor

When designing Makey the Robot for MAKE, Volume 19, I ran into a problem that plagues all kinds of designers — how to continually redesign a body to accommodate changes in whatever’s crammed inside it?


Once I’d sketched out Makey’s configuration and modeled the major parts in Autodesk Inventor 3D modeling software, I really got into some of Inventor’s awesome features. Inventor has three basic design types you work with: sketches, parts, and assemblies. Up to this point I had designed each individual component, including Makey’s robot body, as a part, as shown in Figure A.

Fig. A: Makey’s sheet metal body, near-final version, shown as a single part in Autodesk Inventor. Because I designed it as a component of an assembly, all the mounting holes and dropouts are perfectly aligned to internal robot components; if I move the components, Inventor automatically moves the holes.

Once I had these parts modeled, I placed them together into an assembly, as in Figure B. Then, I attempted to stretch the robot body as needed by making that part “Adaptive” inside the assembly. (That’s what Inventor calls “stretchy” parts, and it’s a powerful feature.)

robot innards 0.JPG
Fig. B: Makey’s body shown as part of an assembly in Inventor, constrained to the edges of the motors (at bottom, in blue). If I move the motors, the body automatically stretches to accommodate the new motor positions. Similarly, I constrained the battery boxes (at top, in tan) to the body, so wherever the body stretches, the battery boxes follow automatically. Nice!

Also, I cut holes into the body where I needed them for mounting the motors. This was the wrong approach! It seemed to work, but when I looked at the robot body as a part, outside of the assembly, the holes I had made weren’t shown. They had simply vanished.

The reason for this is that Inventor can’t know ahead of time how you’re going to use a part. You could design one part that could be used in multiple assemblies, so if you alter the base part in any way inside one particular assembly, the alteration exists only in the assembly, but the base part is unchanged. Thus, my changes didn’t “take hold.”

The key was to create the robot body from inside the assembly. You can actually be inside an assembly and make a brand-new part. To do this, in the Assembly Panel area, instead of selecting Place Component, choose Create Component.

I ended up first creating what I called a “base plate,” which existed solely to help me anchor all the parts, including the robot body. It would not be a part I would actually fabricate. I then placed the base plate, the motors, the Arduino, and the batteries into an assembly, using Place Component, and assembled it all by anchoring everything to the base plate (using constraints). This was pretty much what I had been doing before.

Now, still inside the assembly, I created a new part, via Create Component, which would become the robot body. I selected the material type Sheet Metal.ipt, since it’s a sheet metal part, and created each bend and flange step by step, inside the assembly. This robot body now “belonged” to the assembly, and was adaptive inside the assembly. Any editing of it, from that point on, was always initiated from within the assembly.

Instead of making the body a specific width, I just made everything extra large with no dimensions. Once the body was formed, I finished editing, and now I was back inside the assembly with my new robot body. I then constrained the side of the body to an existing “edge” from another part, for instance, the sides of the motors (Figure B). When the constraint went into effect, the sides of the body “snapped” into place next to the motors. To make holes, I projected the motor mount holes onto the robot body, again edited the robot body part (from within the assembly), cut holes there, and then the holes “stayed put,” so to speak.

Success at last — I had modeled a fully adaptive robot body that I could easily modify to accommodate all the robot components I would be cramming inside it.

Next up: The battle to fit the brains inside.

More: How I designed Makey the robot, Part I: The first design

Keith Hammond

Keith Hammond

I’m projects editor of MAKE magazine.

  • unclemymy

    Kris, I’m a mechanical engineer and have had to teach myself Inventor. Truly helpful instructions are hard to come by, because they’re usually written by the folks that wrote the software, and I often can’t tell what the hell they’re talking about. You’re real-life explanations of the tools and actions are so much easier to understand. You should consider writing an Inventor instruction book – using your technique of explaining it to the novice or non-CAD person. It would help a large number of people.

    • Kris Magri

      Wow, thanks, here I thought I was the only one scratching my head in puzzlement…

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