The Car

I have the mk.2  car back!  It went together very well, though it was perhaps too tight in some places – the short loops and forks that hold the drive wheels to the gears needed to be shaved a little.  I added 0.1mm to the perimeter of all the shapes, and on thinking about it, it seems likely that it should have been more like 0.08 mm or so.  Needing to make adjustments that tiny is a strange business indeed!

Anyway, here is the finished product in all its lasered glory:

This time there were only two things amiss:  First, I didn’t account for the width of the AA batteries in the bottom compartment in addition to the holder for the power transfer gear, so it can only hold 1x AA in the bottom bay and 2x AA in the top – although you could probably pack a bunch of those squishy Lithium ion batteries in with no problem.  The other problem lies in trying to make the design too cramped – on each side, one wheel is attached directly to the motor, and one wheel is attached via a wooden axle, to a point behind the motor that powers the opposite side.  The current design is so compact, though, that there isn’t really enough room behind the motor for a solid fixture, and it has a tendency to fall off.

Widening the design by 3mm would solve both of those problems and the whole plan would stay well within the 181 x 181 mm limits set by Ponoko though, so I’m pretty pleased with the design.  Ideally I’d like to be able to keep the base design small enough that it can form the basis of a miniature robotics platform, and be able to have room on a P1 board to put holders for sensors or a small robotic arm. Stay tuned!

Arduino Enclosure

I also needed to make an enclosure for the Arduino Duemilanove as part of another project I’ve been tinkering with. After looking online I found surprisingly few examples anywhere – in particular a total absence of  compact, fully-enclosing cases, so I designed one and got it cut.  At first I was going to get it made in 3mm MDF as well but Heather suggested that an acrylic would be more pleasing and I agree.  It looks like this:

There are pass-through holes for the three holes in the Duemilanove board,  so it could be bolted to another object.  I offset the pats for this design by 0.1mm as well, and unfortunately, acrylic is a lot more brittle than MDF, which has meant a couple of small parts snapping off when I tried to force them.  Other than that, though, the design is perfect and the tightness has meant it keeps together really well!

The other good thing about sticking the Arduino in an enclosure is that you can stick other things to it – in this case I wanted to guarantee the orientation and position of a compass module on top of the unit, which is encased in the small box on top.

Once I made the enclosure design (and was able to verify that it actually works), I decided that it would be worth looking into making the interlocking parts more automatically.  I was inspired by Jared Tarbell’s Cubes to start thinking about how to generate the map automatically.  It’s still very much a sketch, but the principle is straightforward enough to automate most – if not all- of the process.

Lay out the lines of a shape by clicking, adjust the tooth width with the slider and hit ‘clear’ to clear the shape.  Enjoy!

I’m getting a better feel for how pieces should fit together, and how much or how little of a plane is necessary to keep in a design.  This is the initial exploded view of the car.  It uses two motors, one driving the left side and the other driving the right.  There’s a simple 1:1 gearing mechanism to get both wheels on each side to travel in the same direction.  It has kind of a payload on top that can hold 4x AA batteries, so each motor can be powered by two of them.

There’s no power switch or onboard computer yet.  It’s too small to comfortably fit the Arduino board on to, so I think I’ll put a miniaturized controller on – there are StickDuinos, Boarduinos and a number of other extremely miniaturized variants that would do nicely.

I haven’t actually had it cut and assembled yet, but I was so pleased with the previz system I put together for it that I had to show it off.  Behold, the realtime animated assembly in Unity3d!

Move the mouse around to orbit the assembly – hit pause / play to toggle playback, and use ‘slower’ and ‘faster’ to change the playback speed.

Please view the full post to see the Unity content.

With the gold-ish material on it, it looks surprisingly steampunk.  I kind of like it, although the real thing will be made of MDF.  I look forward to seeing it!

I wasn’t sure what to start with though – something too easy and you won’t learn anything about the nature of the medium  - cutting width, material tolerances, that sort of thing.  On the other hand, though, if you reach too far  you won’t end up learning anything either – if you try to design an ornate interlocking castle without some appreciation of how things actually work you’ll end up with an elaborate mess.  So I tried to strike a balance somewhere in the middle.  I tried to design a remote control car with rack-and-pinion steering (though I didn’t know what that was until I looked it up either)

The first thing to consider with laser cutting things is that you only get flat sheets and holes carved into a single dimension – so when you have a mechanism where two parts have to be hinged at right angles, you’ll need to make that hinge  happen at right angles to both of those surfaces.  I’m not an engineer so it’s a novel experience trying to figure these things out. Once I had an overall idea of how the car was going to fit toegether, I could start doing more precise drawing.

Vector graphics applications are a wonderful thing.  Using Illustrator, I was able to draw the shapes I wanted, measure them up against one another and fit them on to the specified sizes.  I was hoping to keep the thing as small as possible, so it’s on an 18x18cm board with a lot of room left over!  I used some of that room to try and figure out how a bearing assembly might work made out of cut board.

Next I had to wait. Ponoko say they’ll clear their queue of designs inside 28 days, and mine was done within 11 so I was pretty happy with that.

Then, it arrived! Actually I picked it up but it was fantastic to see.  Unfortunately at the time of first unwrapping the sheet with all 114(!) parts laid out together I didn’t have a camera with me, so that moment is lost to history.  Once I did manage to get everything assembled, though, it looked like this:

Unfortunately once it was assembled, it turned out that the motors I had planned to use were too big, and the drive axle for the rear wheels was too short.  The bearings worked out better than expected, though, and at about a third of a millimetre, the beam width will be easy to account for in future designs.  I’m looking forward to prototype #2! Between this and the Arduino micro-controller, I think it’ll be possible to do some pretty fun stuff.