CNC Plasma Cutter

I took a CNC plasma cutting class tonight at TechShop.  It was really well run by this guy that does custom tools for working on old muscle cars named John.


He had us cut out a frame support bracket that welds onto some muscle car.  We cut it out of 12ga mild steel (.110" thick).

The cutter is mounted to an x,y gantry that's computer controlled.  You improt a .dxf file of whatever 2D shape you wanna make and then convert it to G code which controls two steppers.  The up and down movement of the nozzle is controlled with a different program that lives on the machine and you don't control.  The machine can easily hold a 4'x8' sheet.

CNC Plasma cutter setup at TechShop

There's a water bath below (actually touching) the sheet steel that absorbs any noxious fumes generated when the plasma arc obliterates the metal its cutting.  Apparently there's a plasma arc that's created with high voltage and current (we used 60A at 144V!) that jumps from the nozzle to the metal you're cutting.  You can't cut anything that doesn't conduct electricity btw.  But it basically melts the metal immediately and then a swirling jet of air comes through the plasma arc and blows the metal out of the way.  Essentially it acts very similar to an end mill except that instead of a spinning sharp tool edge, you have a super-hot spinning arc.  They even refer to climb cutting and conventional cutting in the class to demonstrate that the quality of the cut with a plasma cutter depends on which side of the arc you cut based on your direction of travel.  Basically, if you're going in a straight line, you get a better cut on the right side of the arc than the left.

There's some software that you use to import your geometry called partworks.  It lets you convert to G-code which gets loaded on the machine.

Software program

Then you run the thing.

Wear glasses when looking at this picture

Close-up of nozzle

Nozzle pieces.  The one with the fins is the electrode.

There are basically two electrodes/nozzles that are used (at least by John).  You can use a 40A or 60A nozzle.  40A would be for thicknesses less than .100.  You can cut stainless steel up to 1/2" thick with this thing!

There's a pretty decent bezel angle created when you cut and it varies greatly depending on feed rate but is generally around 3degrees.  It seems like the slower you cut, the more bezel angle you get, but I can't remember why that sticks in my mind.  If you cut too slow, the arc acts erratically because the metal gets blown away where you're cutting and the nozzle is essentially sitting over free space so it looks for the nearest metal to jump to.  You get some pretty jagged cuts this way.  This isn't precision cutting by any means, but we cut out some decent sized pieces of mild steel in a matter of minutes.

Slag on bottom of part

As you can see from the photo, the slag created by the plasma arc is significant.  This ends up on the bottom side of the part.  The top looks pretty clean.  You can break off this slag with your finger, so it's not much of a problem, but takes some post-processing to finish.

Gonna try and make some moped chassis using this thing.