How does plasma cutting work? – And why isn’t there a puddle of copper there… Part 2

Hi all.  Last time I wrote about the basics of what happens inside a standard plasma torch and how fluid dynamics is key in the cutting process.  I’d like to continue my last post by discussing high definition plasma cutting, in particular about the fluid dynamics inside the hi-def torch.

All of the major plasma power supply/torch manufacturers have what the industry terms ‘high-definition’ product lines.  Technically what separates standard plasma cutting and high definition plasma cutting are two things: constriction of the arc and more precise plasma gas flow.  This constricted arc leaves a more square edge with less dross, and enables higher cut speeds.  The kerf size with a high definition torch is quite a bit smaller also due to the smaller arc and more precise flows.

Let’s take a look at a cross-section view of a high definition torch.


Compared to the standard torch in my first post, this is somewhat more complex.  The first main difference is the path directly outside the nozzle for the plasma gas to be vented.  This is done so the ‘excess’ plasma gas that is not in the exact center of the torch can be vented to the atmosphere so the plasma arc itself can be even smaller.  The high definition torch also has a separate shield gas line so a different type gas can be used for the shield.

Another feature of high definition systems is the accurate concentricity of the plasma consumables.  OK – I admit the term ‘concentricity’  is a strange one, but just say it a couple times and tell me it’s not fun to say!  This means that the torch is designed in such a way that the center of the electrode, nozzle and all of the other parts are very accurately aligned.  This alignment aids fluid dynamics by keeping the gas flow swirl very precise in the center of the torch.

As discussed in the previous post, the gas flow dynamics play a central role in plasma cutting.  The high definition process brings this to a new level.  In high definition systems, there is a device (called the metering console) that is close to the torch, usually about 6 feet.  The plasma and shield gases are delivered to the metering console at full pressure (about 120 psi) and the metering console dynamically adjusts the pressure to the desired level, which may be as low as 25 psi..  This eliminates pressure errors due to line drop and varying incoming pressure.  The metering console also provides pressure ramp-up and ramp-down to greatly enhance consumable life.  I’ll be talking about that in a post in the future.

I hope these posts explain a little bit about how plasma cutting works.  So long for now…. and happy cutting!