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The Monkey Mints line-following robot is nearly complete. The final steps are to machine some circuit-board standoffs and press-fit hubs for Lego tires.
The underside of the printed circuit board (PCB) is not flat. It contains various surface-mount electronic components, such as capacitors and resistors. In addition, there are the pointy tips of the DIP socket and connectors. This unevenness makes it difficult to attach the PCB to the container with straight screws.
Even if the circuit board was completely flat, it can’t be allowed to contact the container. The sheet metal would short-circuit electrical paths on the printed circuit board, causing temporary malfunctions or permanent damage. Since this was a candy container, there is a thin sanitary film coating the sheet metal, but we don’t want to rely on that for electrical insulation.
Standoffs prevent the circuit board from shorting against case. (Wow. This is a really crisp picture.)
The solution is to use washers or spacers to raise the circuit board away from the casing. When employed in this manner, the spacers are called “circuit-board standoffs”.
The black standoffs (pointed to by the pink arrows in the above picture) are 1/4-inch in height. Notice that there is very little room between the standoffs and the motors.
It would be difficult to locate washers or spacers that are only 0.11 inches around, 0.25 inches tall, and have 0.067-inch diameter holes in the middle for #0-80 fasteners. Therefore, I decided to make my own.
Unfortunately, under a certain size, it becomes difficult for an inexpensive mill and an impatient operator to create parts. Narrow diameter drills tend to walk (bend away), and table looseness and spindle runout in the drill machine become significant at such small scales.
Computer-generated layout for #0-80 screw homemade spacers.
To make things easier, I’m going to rely on my digital read out (DRO) rather than counting turns of the hand wheels. Also, I’m going to make many more spacers than I need, with the assumption that some will turn out better than others will.
To ensure the tops and bottoms are completely flat and that the heights are consistent for all of the spacers, I am starting with a 1/4-inch thick plastic sheet made from ABS.
Making square spacers on a mill/drill: Drill, mill slots, and mill free.
In the photograph above, from left-to-right, the steps are:
Tab residue needs to be trimmed off homemade spacers.
As you can see above, the leading edge of the round mill slices the spacer free before the entire side can be cut. This leaves a long thin plastic tab on a corner (left side of photo). The middle spacer was cut with two passes (first pass at 90% depth), leaving only a little bit of tab.
The right side of the photo shows the finished circuit-board standoff after clipping away the tab and sanding the residue. The hole is roughly centered, but overall it is slightly crude. Yet, the standoff didn’t take very long to make and is perfectly functional.
Most of my robot projects use Lego wheels, since they are high quality, come in a variety of sizes, and are plentiful. The Monkey Mints robot needs the smallest Lego tires, in terms of diameter and width.
If my childhood memories serve me, the selected wheels (pictured below) became available in the 1980s. I remember building little Lego cars with them.
The red Lego hubs are molded onto a 2 mm shaft. The Gizmoszone’s 6 mm gearmotor has a tiny 1.5 mm shaft. So, even if I could manage to extract the Lego shaft without shattering the red hub, the hole diameter would be too large.
Instead, I machined new hubs out of black Delrin plastic on a lathe. I didn’t try to reproduce the fancy Lego beveling -- I just stuck to the basics.
Believe it or not, the wheel friction-fits onto the gearmotor shaft. That is, the hub doesn’t need glue or a setscrew to stay in place.
This feat is made possible by the fact that plastic compresses away from the tool during machining, and springs back when the tool is removed. Even though we used a 1.5 mm drill, the final hole is slightly under that size. When the motor shaft is pressed into the plastic hub, the undersized hole grips the shaft. Neat trick, huh?
Press fitting is not appropriate for larger wheels or more significant lateral forces. Also, the hole would not be undersized if the hub was made out of metal or if the drill were repeatedly inserted and removed.
Monkey mints candy container.
That’s all there is. I hope you’ve enjoyed reading about the making of the Monkey Mints robot. To celebrate this milestone, I think I'll have a mint.