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!
Text extracted from: http://www.robotroom.com/Repairs.html
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