Plasma Torch Problems
Angularity (taper) in Holes When Plasma Cutting

By Jim Colt

With an air plasma system....the arc lags at about a 15 degree angle when you have the cutting speed optimized to minimize dross (resolidified metal that forms on the bottom of the cut). Oxygen or high definition plasma systems minimize this effect with less lag due to higher energy density and a better exothermic reaction due to the higher percentage of oxygen.

When cutting holes...or circles......think about what a lagging arc> does as the torch continuously has to turn to the left or the right to make the circle or hole. It lags, and because it lags...the bottom of the hole becomes smaller than the top. This is much more noticeable on smaller diameters than on larger diameters.

More sophisticated cutting machines and software have the ability to program the speed differently on different parts of the cut....an M code that changes the speed is programmed for holes....and actually slows the cutting speed by 25 to 50% depending on the hole size....this slowdown minimizes taper....but produces a byproduct of low speed dross on the bottom of the holes.

If the angularity or taper is consistent around the perimeter of a hole.....then the above technique may improve the quality....however, if the taper changes dramatically around the part...all or some of the following may be the cause:

a. Peirce height / cut height not being set correctly. The manufacturers recommended peirce height is critical to making the torch consumable last a long time...and the manufacturers recommended cut height is critical to provide good cut quality. So....you must peirce the plate at the peirce height....then the z axis (THC or height control) must lower the torch to the cut height before the torch is out of the programmed lead-in....so that the torch is at the correct cut height when on the circle or hole cut path. If this isn't done properly...expect the hole to be out of round....and have a taper that varies with the cut height variation.
b. The most common cause of variable angularity or taper can be blamed on damaged consumables, contaminated gas...or the incorrect combination of consumable parts/gas pressure/current. The plasma arc is a balancing act of physics.....utilizing some tricky gas flow characteristics to constrict an arc that can approach 50,000 degrees F......that is being squeezed through a copper nozzle that melts at maybe 1300 degrees F. If the gas (air) is contaminated with oil, water or particulate....it is no longer air, and has different temperature and electrical resistance characteristics.....with throw the physics balancing act off! When the balance is off...the first thing to go is the nozzle.....the bore becomes out of tolerance and can not properly shape and control the arc.....hence varying angularity and dross.

A nozzle can also be damaged on the very first peirce...if the peirce height is too close.....and can also be damaged very quickly if the torch fires with no plate underneath it...as in firing the pilot arc in the air.

 
c. Varying angularity can also be blamed on a torch that is not perpendicular to the plate....or a torch and/or lead set with gas restrictions or leaks
 
d. Plasma torch technology has dramatically improved over the years. If you have a plasma system of older torch technology...or of poor design....all bets on angularity are off!

These are a few things to think about and check.

Best regards,

Email: Jim Colt


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Created: 01/02/2006
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