Used EDM Electrodes Repurposed As Air Bearings For Precision Machine Tools

If you’ve ever played air hockey, you know how the tiny jets of air shooting up from the pinholes in the playing surface reduce friction with the puck. But what if you turned that upside down? What if the puck had holes that shot the air downward? We’re not sure how the gameplay would be on such an inverse air hockey table, but [Dave Preiss] has made DIY air bearings from such a setup, and they’re pretty impressive.

Air bearings are often found in ultra-precision machine tools where nanometer-scale positioning is needed. Such gear is often breathtakingly expensive, but [Dave]’s version of the bearings used in these machines are surprisingly cheap. The working surfaces are made from slugs of porous graphite, originally used as electrodes for electrical discharge machining (EDM). The material is easily flattened with abrasives against a reference granite plate, after which it’s pressed into a 3D-printed plastic plenum. The plenum accepts a fitting for compressed air, which wends its way out the micron-sized pores in the graphite and supports the load on a thin cushion of air. In addition to puck-style planar bearings, [Dave] tried his hand at a rotary bearing, arguably more useful to precision machine tool builds. That proved to be a bit more challenging, but the video below shows that he was able to get it working pretty well.

We really enjoyed learning about air bearings from [Dave]’s experiments, and we look forward to seeing them put to use. Perhaps it will be in something like the micron-precision lathe we featured recently.

14 thoughts on “Used EDM Electrodes Repurposed As Air Bearings For Precision Machine Tools

      1. They can easily be made stiff enough for steel milling (just takes more air and/or surface area), but keeping them CLEAN enough is another matter. Even small amounts of abrasive dust and viscous/sticky fluids (eg, oils) can ruin porous media bearings, and keeping linear rails sealed against all the junk that mills fling around so vigorously would be a pain.

      2. Porous media bearings are not the best choice for that application. You wan’t a lot of stiffness in the bearing, so a strong preload force. Either by adding mass or a vacuum system. Your bearing can then simply consists of small holes around the bottom of the bearing surface.

    1. Air bearings would be a waste on a 3D printer, because unless the printer is absolute trash, the overwhelming majority of its inaccuracy comes from places other than the positioning system.

  1. Don’t forget to fit a limescale remover in your line if you’ve got hard air in your area…. I kid, but ample driers and oil traps probably not a bad plan, don’t wanna clog those up.

    1. Riffing on a [Tech Ingredients] project, I’d bubble the air through a tall column of water, bubble it again through an airstone in a tall column of water, then finally bubble it through an airstone in a tall column of liquid desiccant to remove any humidity. Overkill, to be sure, but it sure would make for ultra-clean, ultra-dry air!

    1. WAY too coarse, and you wouldn’t have as easy a time lapping it perfectly flat, the porous media needs to be very fine to work right. The media itself needs to act as a restriction just before the air reaches the bearing surface, in the case of non-porous air bearings this is achieved through the use of a small restrictive orifice. Basically the flow needs to be low enough to not cause pneumatic hammer while high enough to not contact the surface.

      Think of a garden hose, as the water leaves the house it’s not very high pressure but gradually push something (your thumb, a board, a wall) against the opening and you’ll feel that as you obateuct the rate of flow more and more the pressure increases and increases more and more. This is similar to a resistor in electronics, for a given flow rate a higher resistance gives a greater voltage (electrical version of pressure) across the resistor. An air bearing can be considered as a voltage divider, one resistor being the orifice or graphite and the other resistor being the restriction caused by squeezing the two bearing surfaces together like blocking off a garden hose. The closer the surfaces the higher the resistance, right? Well if you have a voltage divider with a small resistance by the incoming electricity you’ll get nearly the full incoming voltage across a wide range of resistances, similarly if there’s no resistance there’s nearly full air pressure across a wide range of distances between the two bearings. However, if you increase the resistance of that first resistor then there needs to be a much higher resistance to cause the voltage to rise across that second resistor, similarly if we increase the restriction on the incoming air with a small hole or porous media to slow it down, then the distances between the bearings needs to be much smaller to build up the same pressure which means a narrower range in which the bearing will exert any force. Without the restriction that force is exerted over too wide of a range causing that pneumatic hammer effect like a buzzing sound as it bounces around. You might ask why not just restrict the flow as much as physically possible but the answer is that there are no perfect surfaces and as you restrict the flow more and more you get higher stiffness and greater stability but you also get more and more likely for those surface imperfections to “bump” into each other and causing damage to your bearing. Calculating the right flow rate is a balancing act between the loads you need, the pressure you’re allowed, and the level of precision in your bearing surfaces. It’s always a compromise between stiffness and the physical limits of the materials you work with. Another benefit of the graphite is that if you don’t get your bearing perfect or if something goes wrong and your machine crashes, an air stone will scratch up your precision machine surface to where it’s completely unusable yet graphite will simply wear off a thin smear of graphite and usually keep working again once the problem is fixed.

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