4. Wooden Rods Make Bumpers; Rubber Grommets Protect Wires

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As you’ve just seen, there are switches attached to both ends of the robot candy-container body. The switches allow the robot to detect obstacles and change direction.

If the switches were left alone, they could still physically detect walls and large obstacles. Additional switches could be added to detect smaller objects or oddly-shaped objects. A better solution is to attach a stiff material across a pair of switches to provide wider coverage in front and behind the robot.

For the Flip-Flop Robot’s bumpers, I selected a 3/16-inch diameter wooden rod that I picked up at a local hardware store. I brought a switch with me into the store in order to determine the diameter of rod that best fit into the curve of the switch’s lever arm.

An aluminum rod or brass tube would have made excellent choices, but I thought wood fit better with the back-to-basics nature of this article. For that same reason, I opted not to drill holes in the lever arms, but instead to glue the wooden rod onto the switches.

Excess epoxy securely holds wooden-rod bumper.

Excess epoxy securely holds a wooden-rod bumper.

Carpenters wood glue or household white glue is probably too weak to hold up to repeated collisions. In addition, those types of glue usually require a porous material in order to adhere. The metal lever arms are not porous.

Instead, use a two-part epoxy that lists both wood and metal on the label. I bought Devcon High-Strength All-Purpose 5-Minute Clear Epoxy (S-208-20845). Note: Be sure to wear rubber gloves when working around uncured epoxy.

To ensure good adhesion, clean the metal arms with isopropyl alcohol and let them dry before applying the epoxy. This should remove any oils on the surface of the metal.

Helping hands and quarters apply pressure while epoxy cures on the wooden bumper.

Helping hands and quarters apply pressure while epoxy cures on the wooden bumpers.

It is important to give the epoxy adequate time to cure before putting the bumper to use. Epoxy passes through a sticky jam-like state before hardening. If you play with the bumpers prematurely, or they move or slide around accidentally, you'll end up with a gloppy, unattractive, uneven mess.

I have a “helping hands” holding device that sits in a mini-vise on my desk for soldering. Attaching the robot body in the alligator clips allows the bottom bumper to rest against the switch levers while curing. Gravity holds the rod in place. Meanwhile, the top rod has three quarters shoved between it and the top of the robot’s body to force the rod against the switches. This setup allows both bumpers to cure at the same time.

The epoxy instructions said that it would reach full strength in 1 hour. I swear that I didn’t touch the bumpers until that time had passed, because I’ve had so many prior problems with fouling uncured parts due to impatience.

Rubber Grommets

Years ago, I was playing cards with my friend Pete Weigand in a newly rented apartment. The cable installer had done a horrible job routing the cable along a baseboard for what seemed like the entire length of the downstairs. Pete commented that the installer could have saved a lot of work by simply drilling a hole in the kitchen wall and installing a “rubber grommet.”

I had never heard that term before and didn’t know what it was. It sounded hilarious! I remarked that it would be the perfect name for a rock band. “Ladies and gentlemen, please welcome Tony and the Rubber Grommets.”

Grommets are like eyelets on shoes. They reinforce and protect the hole. Additionally, they often provide a smooth surface to protect whatever is passing through the hole.

Hole in tin for rubber grommet to protect switch wires.

A hole drilled in the side of a candy tin and a rubber grommet to protect the switch wires that will pass through.

In order to place the candy-car lid back on top of the robot, the wires from the switches are going to need to pass through the metal sides. Sheet metal leaves an unsightly, jagged hole when drilled through.

A rubber grommet is pressed in the hole to make the hole more aesthetically pleasing and to prevent the insulation from being accidentally stripped from the wires. Note the groove in the center of the outer perimeter of the grommet. The groove holds the grommet in place while the ends of the grommet wrap around each side of the hole in the wall.

Rubber grommet installed.

Rubber grommet installed.

Above you can see the grommet in place with the wires passing through it. It is certainly more attractive and professional looking than the bare hole.

Notice in the overhead shot (right side of the above photo) that the grommet extends on both sides of the sheet metal of the candy container.

Okay. That’s it for the mechanics and machining of the robot shell. The next page describes the circuitry that reads the switches, remembers the last directional state, and controls the motor.