I’ve built quite a few robots over the years. Some of the robots have broken and a few were never quite tweaked to full operational capability. I decided to spend a weekend inspecting my robots; making any minor repairs or improvements as necessary.
Like all robot builders, when I look at a robot that I made several years ago (or sometimes several hours ago), I see a lot of mistakes and room for improvement. I could spend months rebuilding and retrofitting many of my earlier designs. It took every ounce of self-control to avoid the perfection trap. I vowed not to replace whole boards or redesign whole body parts. Instead, I focused on repairs (glue, solder, paint, filing, drilling) and part replacements (screws, capacitors, diodes, motors) only to the extent necessary to get the robot in good working condition again.Sweet
Sweet is a line-following robot featured at Robot Room.
At the rear of the robot, a pushbutton switch sticks out as the robot’s tailpipe. Pushing the tailpipe switch causes the robot to begin following the line course.
At the time Sweet was built, I used hot-melt glue to secure the switch in place. I’ve tried using hot-melt glue on several occasions and every time I’ve been disappointed with the results. Hot-melt glue is messy to apply. It also has a tendency to peel off of non-porous surfaces, such as plastics and metals — which is exactly what happened in this case.
It’s unfair to blame hot-melt glue in this particular case since Sweet is made from a candy container with a slick inside. I mean, the very purpose of the inside of this container is to prevent food from sticking to it. Nearly any type of adhesive, from tape to epoxy, will fail to adhere.
A mechanical fastener, such as a bolt, is a better choice to hold the switch in place.
I had to make a decision as to whether to remove the hot-melt glue from the switch and to glue the switch to a piece of machined plastic or metal that could be more easily attached with screws. (A good example of this technique appears on Figure 16-17 of Intermediate Robot Building.)
Well, I took the lazy route and decided to simply drill two holes through the metal container and the hot-melt glue. That was a mistake! If you’ve every drilled through plastic, you’ve probably accidentally melted plastic chips (debris) into the flutes (channels) of the drill by drilling too fast. (Drilling plastic at too fast of an RPM results in significant amounts of friction-induced heat, causing the plastic to melt rather than cut.)
Hot-melt glue is actually just thermoplastic engineered to melt at a lower temperature. So guess what happens when you try to drill through hot-melt glue? It melts.
I quickly gummed up one of my nicer drill bits — even at what I thought was a reasonably slow speed. After picking out as much of the glue as possible from drill flutes, I was able to get the rest out by drilling through a block of scrap wood. Even so, I am suspicious that the drill retains some hot-melt glue at a microscopic level or in some unseen crevice.
Believe it or not, I chose a #43 drill for #4-40 screws. That means I originally intended to tap those holes in the hot-melt glue. I suspected all along that the soft glue wouldn’t hold screw threads, but I figured it was worth a try. But, after the experience with the drill, I wasn’t about to insert my fancy expensive tap into the hot-melt glue. Before giving up and ripping all of the hot-melt glue off of the switch, I decided to try inserting screws without tapping.
It worked! Okay, I admit it doesn’t look very professional.
The machine screws are held securely in the hot-melt glue. In fact, you can see in the picture above that the mere act of screwing in the screws seems to have melted or otherwise adhered hot-melt glue into the screw threads. No nuts are necessary. Actually, I can’t seem to get any nuts to go onto the screws because the hot-glue has gummed up the threads.
The switch is now firmly in place and Sweet is ready to roll again!
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