Monday, February 25, 2013

Open Manufacturing: A Home for the Beagle Bone

People often take for granted the amount of engineering that goes into simple everyday products and items. Very few people know or care why that door handle has a gusset or the reasons washers even exist. Even the most mundane of things have considerable engineering thought go into them. It was decided that I Heart Engineering needed a custom case for mounting a Beagle Bone to the TurtleBot. This is an attempt to walk readers through some of the thoughts and considerations that go into building something so small and seemingly trivial. After all, a case is just another box right?

The first thing is to frame what you want to build by constructing a list of constraints and intentions for guidance. In our case, we decided it needed to consider following:

  1. Access to all of the ports with special consideration given to SD Card removal.
  2. Maintaining minimal dimensions 
  3. Able to be mounted to the TurtleBot using a universal set of mounting holes.
  4. 3D Printable Design 
Doing this creates the "world" that your design lives in and as such the tools you may consider using and the initial ideas for construction.

The next step is to obtain or create a 3D Cad replica of the Beagle Bone board. If one has to create their own model for testing, it's best to note that only the major dimensions of the board and components necessary to the case are needed, but they must be accurate.  These include the board itself, it's ports, mounting holes, and anything that one may feel needs to be built around to complete the case. 

Now that we have a physical and 3D model of the Beagle Bone, we can start to draw out the general outline and order of feature construction. I personally rough out a general model idea on paper and then begin creating in a CAD program of choice with the frame. Testing and experimentation are important parts of the design process. I constructed a rough case frame that wraps around the Beagle Bone and printed the piece to test the tolerances and fit. Doing this is beneficial to designers as it saves time over the long term versus running into an unaccounted error later in the process. The idea uses empirical analysis to verify quantitative and qualitative data.
Early test frame for tolerance checking.
After verifying the dimensions, I can now construct the bottom frame of the case. Here is where I asked: well just how high should the case be? To solve this problem, one has to consider the space between the bottom of the board and any components on its underside. In this case, it's ~6mm.  So, one can aim for the bottom half of the case to be 8-10mm or slightly more, but certainly not less. 

So you've built the bottom half of the case frame, now what? Well, you have to get the Beagle Bone mounted inside the frame somehow. Let's consider the 4 mounting holes on the board. I decided to construct bosses extruded from the bottom of the case coming up to the underside of the board. However, one cannot simply create a boss to your desire; this board has components on both sides that limit the amount of usable space. The design also has to be 3D printable which dictates a minimum thickness AND you have to consider the size of potential screw heads. This left only 2 mounting holes with sufficient space on either side for the design in mind.

Test Boss used to verify clearances
The same process is repeated as before, we create a test boss to test the tolerances and verify dimensions, but this time another problem arises. The bosses aren't very rigid and are susceptible to damage. How can one make them more stable and rigid? Well, it turns out the mounting holes are conveniently located close the inside frame of the wall. Therefore, we can build the bosses out from the inside wall, integrating them into the
case frame itself and solving the problem.

After another round of test and fit, we see that the Beagle Bone Board may still teeter and totter on the inside due to use of only 2 mounting holes. Depending on whether you view the amount of deflection as tolerable or not, you can opt to build two bosses located at the unused holes to provide more support. Remember, one has to take into account the actual diameter of usable space for each hole when constructing the bosses.

Internals of bottom half across revisions [left to right.] 

Now we can begin to cut out the access points for the ports on the Beagle Bone board that affect the bottom half of the case. The key thing here is to once again verify that your board dimensions are correct or the test prints will be useless.

Shot of  front side, bottom half port access points across revisions  [left to right.] 
Next, I constructed the mounting points and bottom support layout for the bottom of the case. Thankfully, a much simpler process.  However, we want to save on material to reduce cost and print time. So the bottom of the case does not have to be completely solid. Now we are creating a horizontal support structure that acts as the bottom of the case. One could either go with a simple geometrically rigid structure or use some physics to calculate some assumed forces and optimize accordingly.

Bottom of Case changes across revisions [left to right]

Building the top half of the case is a much simpler endeavor as it's a mirror of the bottom frame. For it, one only needs to consider the access points for ports and maximum vertical height. The access points are already defined and it was decided that the case should extend no more than 10mm from the highest board component.

What we have yet to discuss is exactly how this case fits together.  There are various joinery methods available to close the case. It was personal preference that motivate the decision to use a press fit method to restrict all lateral movement and some vertical movement. The ultimate endeavor is to implement a locking mechanism designed into the case itself that is 3D printable. However, a satisfactory resolution is not ready yet.  Therefore, two ports were added on the left and right sides of the case for use with a small velcro strap to prevent all vertical lift. At the cost of an integrated solution, the case can now be opened and closed with ease.

Close up of  Assembly Secured to TurtleBot 2
The final touches and adjustments necessary for a salable product remain. 3D Printing settings must be created to ensure consistent quality prints. The case top will be taller for accommodating a second potential board stack.  Dimensions may be altered to be better fit the installation of threaded inserts in certain locations. And closing methods may be altered for a better fit.

NOTE: .stl files for printing your own case should be up on Thingiverse on Tuesday afternoon EST.

Update 2/26/2013: Files uploaded to Thingiverse


Anonymous said...

Interesting read. Could you give a time estimate how long such a project takes? How many revisions towards a usable - salable product and how long does a redesign and reprint take?

Ziggfield said...

Absolutely, those details are coming in the second part of this blog post where I discuss just those things.