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Building Bootstrap without a 3D printer
If you don’t have access to a 3D printer you can still build Bootstrap. This variant will be illustrated with 6mm (1/4″) plywood for the chassis, but any stiff material of the same thickness that can be easily cut and drilled will work. Be careful with expanded PVC sheet (aka Sintra, PVC foam board). It’s great stuff but it gives off toxic fumes when it reaches its very low melting point. Work it with slow speed tools in a well ventilated area and wear a proper respirator if you are drilling or cutting at high speeds (and especially if you are using a Dremel type tool–which is not recommended because it will melt rather than cut the plastic).
In addition to a 150mm (6″) square blank for the chassis, you will need will need to fabricate a bumper and purchase the extended gear motor brackets and 1/2″ castor ball & mount from Pololu, and a HC-SR04 ultrasonic sensor mount as specified at the bottom of the materials page.
Once you have the materials, download the following .zip file:
The archive contains two files. The first is a .pdf template which can be printed for hand cutting and drilling. The second is a .svg vector file which you can use if you are cutting your chassis with a CNC machine or laser cutter. For clarity the .svg file does not contain the counter sinks (pocket operations) required for some screws. You can add these in the software for your machine by editing the .svg file before generating your g-code, using the information in the .pdf file as a guide.
If you are cutting the chassis by hand, print the .pdf template file, trim it to fit your chassis blank after making sure your printer produced the outline in the correct dimensions as specified on the template (use “print actual size” not “print to fit page”), and then temporarily attach it to the bottom of your chassis with spray adhesive or a glue stick–anything removable.
It’s your choice as to which side of your chassis blank you use as the bottom of the robot, depending on if you want the switch to be on the left or right once it is flipped over. A right side switch is shown in the main instructions and a left side switch (which will require longer runs of wire) is shown here. If you choose to put the power switch on the left side assemble the underside components with the wired side of the battery pack and diode on the left (switch) side of the chassis.
To process the chassis blank it’s best to start by drilling the holes with a 2mm (5/64″) drill bit and then drilling the required counter sinks as specified in the .pdf file. It is important to drill these holes very accurately–go slow and use a drill press if you have access to one. Be especially careful with the counter sinks, it is easy to drill all the way through the material, at which point you will have to start over with a new blank. Once you have the holes and counter sinks done, drill a hole for the switch. A 6mm (1/4″) bit will do this. Next saw out the slot around the switch hole and the outline of the robot. A scroll saw with a narrow blade will be fastest, but it can be done by hand with a coping saw. Accuracy on the sawing is not as important as with the drilling. Finally, saw out the rectangular opening in the middle. The best way to do this is to drill the corners first with a bit wide enough to accommodate your saw blade and then insert the blade in one corner go around the rectangle to finish the job. You don’t have to be super accurate but don’t make it overly long if you are planning to add a servo to that slot.
A prepared chassis is illustrated in Figure 7.1. It was cut on a CNC machine from 6mm plywood. The counter sink holes were manually added with a drill.
Drilling holes for the ultrasonic sensor mount is next. These mounts vary somewhat among manufacturers so holes are not included in the template. It’s probably easiest to drill holes in your chassis to match those in the mount, although it can be done the other way around. The mount needs to be centered at the front of the rectangular hole in the chassis. There is a tight clearance between the pins for the jumper wires on the back of the sensor and the edge of the prototyping board. Thus it is a good idea to “dry fit” the prototyping board, the mounted ultrasonic sensor and the switches by placing them over their proper mounting holes before drilling. Figure 7.2 shows these components temporarily in place on top of the chassis.
The Pololu castor wheel with its mounting hardware fits into the narrowly spaced holes at the back of the chassis (where the 3D printed castor mount fits into the widely spaced holes). In addition to the material supplied in the kit from Pololu, you will need two nylon standoffs, two 12mm screws, and 2 nuts for the screws. Figure 7.3 shows the Pololu castor installed in a partially completed Bootstrap. If you find that this arrangement does not allow your Bootstrap to sit level after the wheels are installed, you can try different arrangements of the included spacers and nuts until you are satisfied.
The Pololu gear motor brackets are essentially identical to the 3D printed units and mount the same way as described in the primary assembly instructions.
The final component is the front bumper. As discussed at the bottom of the materials page, aluminum flashing works best for this, although other materials can be used. Strong double sided tape is a good method for mounting the bumper to the level switches. Be sure to mount the top edge of the bumper flush with the top edge of the switch levers and no higher or it will interfere with the ultrasonic sensor. If you do find that the sensor is detecting the top of your bumper when finished you can raise the sensor by placing nuts under its mounts, tilting it back a bit in its mount, or possibly by lowering the bumper. A Boostrap mock up with a flashing bumper is pictured in Figure 7.4
Upgrades to the basic Bootstrap