When we build our robots at beatty-robotics.com, my daughters and I sand our aluminum parts so that they have a smooth surface finish, but it’s hard work. We’ve tried different types of power sanders, but none have done the job. It always comes down to hand sanding. I thought that maybe a bench top belt sander would do the trick, but most belt sanders are primarily for REMOVING material rather than achieving a fine finish. They have big, rough, fast motors.
For a long time, I had pretty much resigned myself to hand sanding. You can see in the picture at the end of this post how we rubbed the small aluminum robot parts on 320-grit sandpaper. But whenever we sanded, I kept thinking, “If we can do this sanding by hand, then why can’t a machine do it? What is it about hand sanding that makes it special?” The key is the slow speed and careful precision of a human hand’s movement. A normal belt sander vibrates too much and runs way too fast. It rips off millimeters of material almost instantly. You can slow the big AC motor down by throttling the voltage, but then it loses torque. I looked around for a high-precision variable-speed bench top belt sander, but I couldn’t find one. Did we need to build a new tool for our machine shop?
One day, I was walking through a woodworking store and saw that they had some old, heavily used equipment for sale, including some 75-year-old rusty junk that they had pulled out of an old-timer’s garage. A cool vintage sander caught my eye, mostly because I love the old-style machines that run on visible belts, like in Edison’s steam-driven machine shop. This one wasn’t that old, of course, but it was still pretty neat. Then I had an idea.
I bought the sander for $20 with the thought that maybe we could hack into it (sorry antique aficionados), reconfigure it, and turn it into the machine we needed. First, we cleaned it up, removed the rust, and got the rollers and other mechanical parts running smoothly. The wiring on the original AC motor was exposed and dangerous and the motor ran too rough, so we pulled it all out. I found a used Bodine precision DC gear motor on eBay (1.8 Amp, 1/4 HP, 500 rpm, 5:1 gear ratio). We mounted the motor into the frame of the vintage machine, installed a 5/8” bore v-belt pulley onto the motor shaft, and hooked it up to the drive belt. The motor’s 5:1 gear ratio meant that it would operate roughly five times slower, but provide five times as much torque. Then we wired the motor into a 0-130 volt, DC, filtered PWM, variable motor controller. The potentiometer on the controller would allow us to control the speed of the sandpaper. Once we got all that working, we installed a roll of 4” x 36” 320-grit and gave it a try. It worked beautifully. We sand aluminum parts by pressing them flat against the moving sandpaper. We have replicated the precision and motion of finish sanding, but its faster, smoother, and easier. The keys to its success were the slow speed, the high torque, the precision motor (no vibration), and the ability to finely tune the speed.
So, we’re happy to report that the latest tool we’ve added to our robotics workshop isn’t a high-tech new gizmo like our CNC, but a repurposed vintage machine, which we think is pretty cool.