Electric Compass for a Plasma Cutter

If you are a Maker space or individual lucky enough to own a Plasma Cutter, this electric protractor compass could be handy. The folks over at [MakeItExtreme] built this circle cutting tool to help cut circles and rings in thick metal sheets using their plasma cutter.

The whole thing is built around an electro-magnet, so the jig will only work with magnetic metals. There are not a lot of design details, but it’s possible to infer how to build one looking at the video and the photos on their blog. There’s a couple of nice hacks along the way. Since the electro-magnet is stationary while the rest of the jig rotates, the main mounting bolt had a hole drilled through it to help route the cable. The rotating protractor arm is made from a slab of aluminium and holds all the other parts together – the drive motor, the central hub and the plasma head. The motor used appears to be a 60rpm AC synchro motor. These types usually have an RC phase shifting network between the two coils to allow direction reversal. Friction drive is used to rotate the jig, with the friction coming from a pair of rubber tube bands attached to the electro-magnet and the motor drive hub. The plasma head holder has a rod-end with a roller bearing attached, acting as a caster wheel, ensuring the arc gap is maintained as the jig rotates. A few switches to activate the electro-magnet, motor forward / reverse and plasma enable complete the setup.

Their blog, and YouTube channel has a lot of other interesting projects that they keep building. Check it out.

26 thoughts on “Electric Compass for a Plasma Cutter

  1. great work, very useful thing to have around if one doesn’t have a cnc jig for ones plasma cutter, it should also make better circles.
    couldn’t one use a magnetic backplate to fix it to non magnetic materials?

  2. This is a really neat project!

    I think it could be the basis for a really neat item that would be able to cut (mostly) arbitrary shapes by changing the existing motor drive out with something that can do real motion control (servo on a belt drive?) and adding another motor on the arm to move the cutting head in and out. Then, you have an R-theta system that could cut shapes as determined in software!

        1. You could only cut shapes that are able to be cut around a single rotary axis and are limited by the diameter of the “cutting beam” length. The longer that axis gets, the stiffer it must be. You could try to use a wheel to support the cutting beam but moving back over already cut lines will mean moving over dross and slag, which will dramatically affect cut quality unless you can suspend the head completely. Which is fine for short runs like this but problematic for longer diameter cuts.

          I can see a niche for it and I could see the utility for certain applications. I just think it would be a good bit harder to implement and still have a number of tradeoffs compared to just doing a proper stiff, dedicated XY table. You could make a foldable XY table that can break down too. There are plenty of 3D printers that do that already. Not sure how well they would play with a plasma cutter by default though.

          Maybe combine a https://handibot.com/ with a plasma head? Plasma isn’t very happy with constant starts and stops though.

    1. Sort of. Not all magnetic metals are ferrous!

      Of the pure elements, iron (ferrum) is the best-known magnetic metal, of course. It’s so prototypical that ‘ferromagnetism’, the best-known kind of magnetism, is named after it.

      Ferrous metals also are sometimes considered to include nickel and cobalt in addition to iron. All of them are highly ferromagnetic and also have similar chemical and metallurgical properties. However, they are not iron and there are many alloys that use Ni and Co which are magnetic.

      There are also other metals that are ferromagnetic but not similar to iron. Some of the lanthanides (rare earth) have this property, notably gadolinium, dysprosium, and holmium (though the latter two are not ferromagnetic at room temperature). Rare-earth magnets are the strongest permanent magnets, but these are alloys that contain iron, nickel, or cobalt too (in order to make them magnetic at room temperature).

      When it comes to alloys, there are indeed ferromagnetic materials that do not contain any ferrous element. Chromium oxide is ferromagnetic but has no ferrous metal in it – if you are old enough you may remember that magnetic cassette tapes used this substance. Gallium manganese arsenide (Ga,Mn)As is both a semiconductor and ferromagnetic.

      1. That is certainly very interesting but the term ‘magnetic metal’ to me is a metal that produces a magnetic field of its own which is not really necessary in this case.

    1. Plasma cut quality is impacted to a fairly large degree by both the speed and the height of the cutting head. Other items, such as amp settings, shield gas, quality of the electrode, the correct electrode, etc also play a role.

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