Rob
It depends a bit (or a lot) on what you want to use it for. Are you just wanting this to hand cut or to put it on a cnc. There are different types of the arc starting like scratch start, pilot arc and blow back types.

Here is a bit of reading http://linuxcnc.org/docs/devel/html/...nc-primer.html I know this is linuxcnc stuff but it can be done just with a PP.

I built one at the end of last year. https://photos.app.goo.gl/J8c1sCMjmuRzkhoJ7