Prepare to make a whole bunch of sawdust.
Now that we have the three sections of the robot cut to length, the next step will be using a router to create the cavities inside the main chassis piece (the longest one) that will hold the motors and electronics.
Measure and Mark the Design
The first step here will again be measuring the cuts. The measurements are a little bit more complex than before but I again created a diagram to show the dimensions:
So, you will basically need to measure out the different sections from the cutting pattern and copy them onto the board. I like to use a marker to color in the sections that will be removed. When you start cutting with the router, visibility will be quite limited and sawdust will be flying all over the place so it is best to make the cutting pattern as obvious as possible.
Cut the Cavity to 20mm Depth
There are three main cavities we will be cutting out with a router: one on each side for the motors and a central one for the electronics. Because the motors are a fair bit thicker than the electronics PCBs, their cavities will be deeper and the one for the electronics. So, the strategy we will use will be to first cut out the entire area of the three cavities to a depth of 20mm. This is the finished depth for the electronics section. Then, we will cut the motor cavities deeper.
So, start by inserting a straight bit into your router and setting the depth to 20mm.
When it comes to cutting out the design, I find it easiest to first cut the outlines of the shapes and then go back and hollow out the cavities.
The router is a great tool in general, but one of the things it excels at doing is generating sawdust. The sawdust will tend to collect in the cavities you are cutting and it will eventually start to interfere with the router. So, while you are cutting out the cavities, make sure to stop and clear out the sawdust whenever it gets too thick. You can turn the wood over and bang it into the table or something, but by the end, the wood will get very thin and fragile. So, I took an old paint brush and used it to sweep out the sawdust.
Once you get all the outlines cut, the rest is a matter of going back and forth with the router to clear out the interior of the cavities.
One thing to watch out for as you get close to removing all the material on the inside of the cavities is that you may get a little chunk of wood that breaks off at the end. It will then have a tendency to get kicked around under the router. If this happens, just switch the router off and remove the little piece before you continue cutting.
Cut the 35mm Depth Motor Cavities
Now that we have the entire cavity cut to a depth of 20mm, we will go back and deepen the motor cavities on the sides of the board to 35mm. So, to start, set the depth of the bit on the router to 35mm.
Next, we will need to re-draw the lines that divide the motor cavities from the electronics cavity since they got cut off by the router on the first pass.
Then, use the same technique as before to cut the motor cavities on the ends of the board down to a depth of 35mm. Now, this time, when removing the material from the inside of the cavity after going around the outside edge, be extremely careful and go slow. We are cutting almost the hole thickness of the board so the thin spot that remains is very fragile. If the router grabs the wood too hard, it can easy break through the bottom of the board (that will actually be the top of the robot). When you are done, you will end up with two levels of cavities inside the board.