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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If I’m going to try to develop a CNC fabric cutter, it’s going to be called the CutterRouter.

There, got that out of the way.

If I’m going to try to develop a CNC fabric cutter, at least the prototype and ideally the final version will use all commodity components plus easily-fabbed parts (3D printing, lasercutting, easy woodworking with common shop tools and not requiring a high degree of accuracy). The self-imposed choice of commodity components makes me want to use a readily-available X-ACTO® blade as the knife.

If I’m going to try to develop a CNC fabric cutter, I want to develop a working prototype cutterhead first. It’s the only part of a CNC fabric cutter that’s significantly different than a 3D printer or mill. So if a cutterhead can be made to work, the rest is easy; if it can’t be made to work, the rest is moot.

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Surely by now the sewn-product industry has CNC fabric cutting?

Yes, but I don’t know much about it and my acquaintances haven’t worked at places that use it. What I’ve been able to find appears to start around $30-40K for a fairly short table, with extensions available. That entry pricing only makes sense for a fairly high volume of product.

Could we do better, and is there a market for it if we could?

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In the sewn-product industry, product development can have a rapid-iteration phase that would be familiar to anyone in hardware or software development. Whether it’s testing the fit and strength of a new backpack or checking the fit and appearance of a garment on the fit model, a single prototype at a time may be made and revised weekly, daily, or even more often.

In the garment industry, “made to measure” refers to clothing whose pattern is made from measurements of the individual wearer and then either drafted algorithmically or pieced together from library parts for each possible value of each collected measurement.

In product development and in made-to-measure production, a number of factors of fabric cutting are different than in mass production:

• The manufacturer or producer may be making a single quantity, certainly only a small quantity.
• The manufacturer or producer is unlikely to be making multiple sizes at the same time.
• The above factors rule out the long markers used in mass production.
• The above factors rule out lay-ups of many plies of fabric.
• The manufacturer or producer may not even have a marker — pattern pieces might be hand-drafted and on separate pieces of paper or oaktag (card stock).

I can’t speak definitely about every possible cutting scenario in product development and made-to-measure production, but I can cover common cases.

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