Keith's Electronics Blog

Last fall when I was looking for ways to drive a reciprocating fabric cutter, Ed Nisley suggested that I look at hobby brushless DC motors as possible sources of enough of both speed and torque. Not knowing anything about their care and feeding (and after a significant delay), I did my homework and learned how to lash one together well enough to make it spin.

After which I inevitably put together a completely unusable cutterhead prototype.

My intent all along was to use the same tattoo gun brass cam as on the tiny motors, but last weekend I couldn’t find a tiny hex key for the set screw to remove it from the previous prototype and affix it to the BLDC motor. It being a very short drive from Missingtherighttoolville to neighboring Badideatown, I printed a plastic cam and new connecting rod to fit a standard skate bearing. Let me tell you, turning on that cutter was like holding a powerful vibrating thing with a razor-sharp blade in your hand.

It didn’t cut the fabric particularly well, either. It had plenty of torque but lacked either sufficient speed or travel to cut unclamped fabric; it just shoved it out of the way. (It cut just fine when the fabric was held in tension, but that’s not the objective.) The eccentric mass of that heavy skate bearing did not motivate me to turn it up faster, particularly when I already had the other cam in mind.

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In a previous installment, I used a 900-rpm gearmotor in my reciprocating-blade prototype and it didn’t have the speed needed to cut fabric sitting on a styrofoam spoilboard. The only place I could find to order a higher-speed version was AliExpress, and it took a while to arrive.

Quickly swapped into the same prototype cutterhead, with predictable results: It has a higher no-load speed but bogs down in the cut. This style of gearmotor won’t be my solution.

I had not worked with hobby BLDC motors before (I keep saying “hobby” here because my cordless drill and driver have BLDC motors); but as you can see, they’re cute as the dickens. Much cuter than chickens. About walnut-sized, and very cheerful. More cheerful than chickens?

YouTuber How To Mechatronics has a stellar video How Brushless Motor and ESC Work and How To Control them using Arduino explaining all the theoretical and operational details of BLDC motors and their electronic speed controllers (ESCs). If you want to know (pretty much) everything about them, go watch! Or for a quick overview, keep reading.

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The previous cutterhead prototype from about a year ago that used a tattoo gun motor reciprocated the blade at a high speed and cut the fabric well, but stalled easily in the foam spoilboard. I needed more torque and I was hoping I could get by with less speed.

And now I remember how I stumbled across the idea to use a tattoo gun motor — Nick Poole’s SparkFun post on building a tattoo gun from one of their cute widdle gearmotors, which are available in a wide range of gear ratios and output speeds. So when the tattoo motor didn’t have enough torque, I ordered two different speeds of the micro gearmotors from SparkFun, following Nick’s path of innovation.

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