Saturday, March 30, 2013

CADing: Preparing for Water Jet

Since I didn't really elaborate on the CADing aspect of the laser cut acrylic orbit wheel prototype in the previous post, here's the story behind it:

I've been CADing with Autodesk Inventor since 2008 when we built little puzzle cubes in my Introduction to Engineering and Robotics at Lathrop High School. Many people at MIT use SolidWorks, but I can't give up that nice little cube in the top right corner of Autodesk. Eager to start my new project, I decided to go with Autodesk. Maybe I will do my next project in SolidWorks.

I'm spending a lot of time refining my CAD files before printing them on the water jet. I can't use the free scrap stock in MITERS' pile of metal, because there aren't any water jetable 1/4 thick 8 inch squares. By "water jetable" I mean that there must be enough metal on the edges of the part in order to clamp it down to the machine. Therefore, I want to make the Autodesk files as detailed as possible to avoid wasting the nice 8" wide aluminum stock I got from McMaster.

As of now, the orbit wheel consists of 9 major aluminum (to be) water jetted parts.

The front and back plates took the most planning, because I had to plan out how my croc would fit inside the wheel and bearings. After putting together the t-nutted laser cut orbit wheel, I decided to make my "final" prototype without t-nuts. This change led to getting rid of the tongue and grooves on the front and back plates.

I had to estimate the motor screw holes for the front plate, because the motor is still on its way from HobbyKing's International Warehouse. I extended the holes in order to provide some way to adjust how the gear teeth mesh. Other than that, the front and back plates are nearly identical.

The inside parts were relatively easy (rectangles):

The plate on the left is the inside side plate (there will be two of these). And the one on the right is the inside top plate.

Here's the bottom plate:

I made the bottom plate and support plates able to interlock, because I didn't want to put four screws through the support plates. (They also interlock with the back and front plates). 

I decided to make the support plates, because the acrylic orbit wheel's bottom plate broke during my travels to and from Alaska for Winter Break. Even though aluminum is much sturdier than acrylic, I wasn't sure how the aluminum would hold up if it ever scraped on the ground.

Besides the fact that the bottom plate is broken, the laser cut prototype still helps with visualizing where I can fit the batteries.

I need to fit at least 5 A123 cells in series to get approximately 16.5V for the motors I ordered. I really want to be able to fit ten cells and make two 5S packs to hook up in parallel to get more distance out of this electric "vehicle". This would give me about 4600 mAh. With this arrangement, I can fit 10 cells. Due to the motor on one side, I might need to put 6 cells on the opposite side and 4 cells on the side with the motor. However, this will make the wiring really ugly.

Here's a picture of one of my first CADed battery holders:

I decided to put 5 cells on each side to solve the wiring problem. This also opens up some more room in case the motor controller doesn't fit inside underneath the ring gear.

With this battery layout in mind, I CADed some A123 cell clips and since I didn't have to wait on a water jet I printed a few out on the laser cutter.

I made one side flat so I could slip them over the batteries. I had to play around with the spacing, because I also plan to put heat shrink around the batteries to protect them in a pack. Notice, one side is slightly shorter than the other to allow the clip to grip on the batteries better.

I asked around for a safe material that I could laser cut, but that would also be strong enough to hold batteries. At first I thought I might be able to use PVC sheets, but that is especially bad to cut with a laser. Ed Moriarty (runs the Saturday Thing at MIT's Edgerton Center) said that laser cutters could cut ABS, but it makes the lense very unhappy. I could use the band saw to cut them out, but that takes more time and it's not as pretty. MITERS had some acrylic scraps which seemed to be a good starting point.

The "0.25" black acrylic (which is really 6 mm) was too small and too weak to drill and tap for holding batteries. So, I found some 12 mm clear acryclic which worked a bit better. 

I can't do much now until I get some parts water jetted. Until then, here are a few pretty pictures of the complete assembly in Autodesk:

This is an orthographic view of the complete assembly. The black cylinders are A123 cells, and the oval with four circles in the front is where the motor goes. My ankle will be on the otherside. I usually build in this view and with this level of detail. However, there are a bunch of visual aspects to play around with. 

I experimented with ray tracing, perspective, and shadows below. 

Here's a nice shiny ray traced image with perspective. My computer made loud fan sounds, but they didn't last too long. This view shows the orbit wheel without batteries and shows the interlocking supports for the bottom plate.

I like this one the best:

Ready to water jet! Now I just need to find someone to get me into either CSAIL's (Computer Science and Artificial Intelligence Laboratory) or Media Lab's water jet.

Know anyone?

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