A few things regarding standoff vs drag cutting with a plasma torch.
- The two major consumable parts in a plasma torch are the electrode and the nozzle. Some users call the electrode a tip, others call the nozzle a tip.....so for less confusion I will refer to them as electrodes and nozzles.
- Electrode is inserted into the torch....where it makes an electrical connection to the Negative output from the plasma power supply. The electrode (in an air plasma torch) has a slug of hafnium bonded to its copper body, with the hafnium on the face of the electrode to be used as the electron Emitter.....it is where the 25,000 degree plasma arc is formed. The hafnium slug can reach temperatures of 3000 F., so it is air cooled in air plasma torches below 130 amps, liquid cooled in higher power plasma systems.
- The nozzle fits accurately over the electrode and is spaced by a swirl ring (or some sort of non conductive insulator) in order to maintain proper electrical and air flow spacing between the two parts. The nozzle makes an electrical connection to the pilot arc control circuitry inside the power supply. This circuit attaches the nozzle to the positive output of the power supply during arc initation and non-transferred pilot arc modes. During conventional cutting the nozzle is disconnected from positive, essentially "floating" from an electrical point of view. The primary function of the nozzle is to shape the arc into a perfectly round high speed jet of superheated gas. It's secondary function is to get the arc started by allowing electrical energy to pass from the negative electrode to the positive nozzle...creating a temperature rise in the air flow that ionizes the air making it more electrically conductive, this allows a pilot arc to initiated and exit through the nozzle orifice.
- As soon as the pilot arc is established and assuming the plasma torch is close to the material to be cut (the material is attached to the positive output of the power supply via the work cable, often (incorrectly) referred to as a "ground cable". If the pilot arc is close (generally less than 1/4") from the material, then the pilot arc will transfer some energy to the material and electrical current will flow through the work cable back to the positive side of the power supply. The power supply senses electrical current flowing, tells the pilot arc control circuit to disconnect the nozzle from the positive connection....and at this point all of the power transfers from the pilot arc path (negative electrode to positive nozzle) to the transferred arc path (negative electrode to positive material (or workpiece)). The power supply now ramps up the power from low pilot arc amperage to high cutting amperage....the plasma cutting process has begun!
- Early plasma torches always had an exposed nozzle....in other words the nozzle protruded out the front of the torch. With any plasma torch over 30 amps during transferred arc cutting (the nozzle is floating electrically)....if the nozzle touches the workpiece (connected via the work cable to positive).....expect that the nozzle will change its potential from "floating" to positive (same as the workpiece). So dragging any exposed nozzle plasma torch on the material causes the nozzle to switch back to positive, allowing the negative potential from the electrode to jump from electrode to nozzle (inside the torch), then through the copper body of the nozzle down to the positive workpiece. This is a phenomenon known as "double arcing". Expect that when any plasma torch operating at 30 amps or above with an exposed nozzle touches the plate that double arc will occur. This causes "current splitting" (some energy goes to the cutting arc, some splits off and shorts to the material), lowering your cutting power. The double arc to the material also cause "stiction", which is an effect of the copper trying to weld itself to the material...typically roughening the smooth motion of the torch and providing a very rough cut edge. Last, but probably the worst effect: the nozzle wears out very rapidly...you will see an out of round, cratered nozzle orifice after just minutes of drag cutting.....the orifice is supposed to shape the arc....so imagine what the cut will look like with an out of round orifice. (widely varying angularity, rough edges, slower cut speeds).
- Shielded torch technology. The shield is another component that attaches to the front of the torch. While the original shielded plasma torches were introduced by Hypertherm in the mid 1980's...most of the patented technology has expired (there are some exceptions!) and anyone can use shield technology to eliminate double arcing and allow drag cutting without stiction and longer nozzle life. The shield attaches on the torch to non conductive threads that allow the shield to electrically float, the front of the shield has an orifice and often some bleed holes as some cooling air flow passes between the shield and the nozzle. Shields are designed for hand cutting with the correct arc length when dragging directly on the surface of the workpiece, and there are mechanized or "standoff" shields that are designed for keeping the torch at the correct standoff using some sort of automated torch height control system for mechanized cutting.
-There are a lot of other things the shield is used for in modern torches today that improve height control, pierce thickness, arc energy density as well as allowing for different shield gasses for better metallurgy on certain materials.
Bottom line: There are some exposed nozzle's available for certain torches that are advertised as "drag" tips, often these nozzles have a castleation machined into the face to allow dragging...instead of the smooth face noticed on other nozzle designs. If the nozzle is exposed (as described above), regardless of the shape on the front end....expect double arcing, stiction and shorter life. If you can get shielded technology for your torch...this is always the best bet for drag cutting...best quality, best life.
Under 30 amps you can drag cut with an exposed nozzle with most torches with acceptable results...the double arcing still occurs but at low enough power so the side effects are not as noticeable. Higher amperage dragging with an exposed nozzle will produce poor results!
Hope this helps with understanding of the continuous drag vs standoff debate with Plasma cutting!
Jim Colt Hypertherm