Theory, Practice, And Ducted Fans

About a year ago, [Wyman’s Workshop] needed a fan. But not just a regular-old fan, no sir. A ducted fan. You know, those fancy fan designs where the stationary shroud is so close to the moving fan blades that there’s essentially no gap, and a huge gain in aerodynamic efficiency? At least in theory?

Well, in practice, you can watch how it turned out in this video. (Also embedded below.) If you’re more of a “how-to-build-it” type, you’ll want to check out his build video — there’s lots of gluing 3D prints and woodworking. But we’re just in it for the ducted fan data!

And that’s why we’re writing it up! [Wyman] made a nice thrust-testing rig that the fan can pull on to figure out how much force it put out. And the theory aimed at 652 g of thrust, which was roughly confirmed. And then you get to power: with a 500 watt motor, he ended up producing 47 watts. Spoiler: he’s overloading the motor, even though he used a fairly beefy bench grinder motor.

So he re-did the fan design, from scratch, to better match the motor. And it performed better than the theory said it would. A pleasant surprise, but it meant re-doing the theory, including the full volume of the fan blade, which finally brought theory and practice together. Which then lead him design a whole slew of fan blades and test them out against each other.

He ends the video with a teaser that he’ll show us the results from various inlet profiles and fan cones and such. But the video is a year old, so we’re not holding our breath. Still, if you’re at all interested in fan design, and aren’t afraid of high-school physics, it’s worth your time.

Don’t care about the advantages of ducted fans, but simply want to make your quad look totally awesome?  Have we got the hack for you!

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Stopping A Bench Grinder Quickly

In every workshop ever, there’s a power tool that goes unnoticed. It’s the bench grinder. It’s useful when you need it, and completely invisible when you don’t. We take the bench grinder for granted, in part because we keep it over there with that box of oily rags, and partly because it’s so unassuming.

But you can really mess your hands up on a bench grinder. Words like ‘degloving’ are thrown around, and that doesn’t involve actual gloves. For his Hackaday Prize entry, [Scott] is adding safety to the ubiquitous bench grinder. It’s called the Grinder Minder, and it aims to make the humble bench grinder a lot safer.

There are a few goals to the Grinder Minder, most importantly is DC injection braking. This stops the grinder from spinning, and if you’ve ever turned off a bench grinder and waited for it to spin down, you know there’s either a lot of energy in a grinder wheel. Grinder Minder also adds accidental restart protection and an actual ANSI-compliant emergency stop. All of this is designed so that’s it’s a direct drop-in electronics package for a standard off-the-shelf grinder.

The early prototypes for the Grinder Minder have the requisite MOSFETs and gigantic wire-wound resistors , but the team has recently hit an impasse. The current market research tells them the best way forward is designing a product for bigger, more powerful tools that use three-phase power. The team is currently researching what this means for their project, and we’re looking forward to seeing where that research lands them.

Using An Angle Grinder As A Bench Grinder


[Saeid Momtahan] made a couple of attachments that let him use his angle grinder as a bench grinder. It may be better to refer to it as a bench motor, as he uses both a grinding wheel and a wire brush while showing off his project.

The attachments come in two parts. The first is a piece of square tube that runs parallel to the body of the grinder on the side opposite the handle. This doubles as a larger gripping area when using it as an angle grinder, as well as giving him something to clamp in his bench vise. The second attachment serves as a rest for the work piece. Above you can see him brushing some rusty stock clean with the wire attachment.

It’s nice to have the option of doubling up a tool’s tasks rather than buying yet another item that may not get used all that much. We also love the idea of building your own tools. If you don’t have a welding rig to fabricate these add-ons here’s a li-ion battery based system to get you thinking.

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