Specialized Knife Sharpener from Old Airplane

“Surely sharpening a knife can’t be that hard” one might think, as they destroy the edge on their pocket knife by flailing it wildly against a whetstone of indeterminate grain. In reality, knife sharpening is as nuanced a practice as virtually any other field, and getting a quality finish is much harder than it seems. It also gets increasingly complex with different blades, as [Turbo Conquering Mega Eagle] shows with is customized knife sharpening jig.

The hardest part in any blade sharpening is getting the proper bevel angle. A heavy angle is good for heavy-duty tools like axes, but for fine work like shaving a more sharp angle is required. Usually, a table-mounted jig is required but due to production constraints, a handheld one was used. It’s made with push rods and a cam follower from an airplane engine (parts are plentiful since this particular engine breaks all the time) and can impart very specific bevel angles on blades. For example, machetes have a heavy angle near the handle but a finer point towards the tip, and this tool helps streamline sharpening many knives quickly.

If you want to try your hand at another project that’s not as straightforward as it might seem, you might want to build a knife from scratch before you make an attempt at a sharpening tool. It’s just as nuanced a process, but with a little practice can be done with only a few tools.

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A Polar Coordinate CNC Plotter Even Descartes Could Love

Take apart a few old DVD drives, stitch them together with cable ties, add a pen and paper, and you’ve got a simple CNC plotter. They’re quick and easy projects that are fun, but they do tend to be a little on the “plug and chug” side. But a CNC plotter that uses polar coordinates? That takes a little more effort.

The vast majority of CNC projects, from simple two-axis plotters to big CNC routers, all tend to use Cartesian coordinate systems, where points on a plane are described by their distances from an origin point on two perpendicular axes. Everything is nice and square, measurements are straightforward, and the math is easy. [davidatfsg] decided to level up his CNC plotter a bit by choosing a polar coordinate system, with points described as a vector extending a certain distance from the origin at a specified angle. Most of the plotter is built from FischerTechnik parts, with a single linear axis intersecting the center point of a rotary drawing platform. Standard G-code is translated to polar coordinates by a Java applet before being sent to a custom Arduino controller to execute the moves. Check out the video below; it’s pretty mesmerizing to watch, and we can’t help but wonder how a polar 3D-printer would work out.

Have polar coordinates got you stumped? It can be a bit of an adjustment from Cartesian space for sure. It can be worth it, though, showing up in everything from cable plotters to POV fidget spinners and even to color space models.

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Mechanisms: The Screw Thread

They hold together everything from the most delicate watch to the largest bridge. The world is literally kept from coming apart by screws and bolts, and yet we don’t often give a thought to these mechanisms. Part of that is probably because we’ve gotten so good at making them that they’re seen as cheap commodities, but the physics and engineering behind the screw thread is interesting stuff.

We all likely remember an early science lesson wherein the basic building blocks of all mechanisms laid out. The simple machines are mechanisms that use an applied force to do work, such as the inclined plane, the lever, and the pulley. For instance, an inclined plane, in the form of a splitting wedge, directs the force of blows against its flat face into a chunk of wood, forcing the wood apart.

Screw threads are another simple machine, and can be thought of as a long, gently sloped inclined plane wrapped around a cylinder. Cut a long right triangle out of paper, wrap it around a pencil starting at the big end, and the hypotenuse forms a helical ramp that looks just like a thread. Of course, for a screw thread to do any work, it has to project out more than the thickness of a piece of paper, and the shape of the projection determines the mechanical properties of the screw.

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Hacked Car Axle Yields Custom 90° Gearbox

Need a sturdy angle gearbox to handle power transmission for your next big project? Why not harvest a rear axle from a car and make one yourself?

When you think about it, the axle of a rear-wheel drive vehicle is really just a couple of 90° gearboxes linked together internally, and a pretty sturdy assembly that’s readily available for free or on the cheap. [Donn DIY]’s need for a gearbox to run a mower lead him to a boneyard for the raw material. The video below shows some truly impressive work with that indispensable tool of hardware hackers, the angle grinder. Not only does he amputate one of the half axles with it, he actually creates almost perfect splines on the remaining shortened shaft. Such work is usually done on a milling machine with a dividing head and an end mill, but [DonnDIY]’s junkyard approach worked great. Just goes to show how much you can accomplish with what you’ve got when you have no choice.

We’re surprised to not see any of [DonnDIY]’s projects featured here before, as he seems to have quite a body of hacks built up. We hope to feature some more of his stuff soon, but in the meantime, you can always check out some of the perils and pitfalls of automotive differentials.

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A CNC You Could Pop-Rivet Together

You have to be careful with CNC; it’s a slippery slope. You start off one day just trying out a 3D printer, and it’s not six months before you’re elbow deep in a discarded Xerox looking for stepper motors and precision rods. This is evident from [Dan] and his brother’s angle aluminum CNC build.

Five or six years ago they teamed up to build one of those MDF CNC routers. It was okay, but really only cut foam. So they moved on to a Rostock 3D printer. This worked much better, and for a few years it sated them. However, recently, they just weren’t getting what they needed from it. The 3D printer had taught them a lot of new things, 3D modeling, the ins of running a CNC, and a whole slew of making skills. They decided to tackle the CNC again.

The new design is simple and cheap. The frame is angle aluminum held together with screws. The motion components are all 3D printed. The spindle is just an import rotary tool. It’s a simple design, and it should serve them well for light, low precision cuts. We suspect that it’s not the last machine the pair will build. You can see it in action in the video after the break.

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