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Pierce Height, Cut Height, Height Control Explained

Important information to prevent common issues faced by many getting started in cnc plasma cutting.
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Pierce Height, Cut Height, Height Control Explained

Post by jimcolt » Mon Feb 04, 2013 11:31 am

Every different brand of torch height control uses some different terms for the critical functions of height control. What is important to consumable parts life is the Pierce Height and the Pierce Delay time....the proper settings for both of these is listed in the mechanized cut charts in your Powermax85 operators manual. With your height control there is another function called "ignition height"....which is for some plasma systems that cannot start the arc at the pierce it is too far from the plate. With your Powermax85 you should set ignition height and pierce height the same.

Cut height occurs after the torch starts will index down closer to the cut height....which again is listed in the cut charts in your Powermax85 operators manual. On most materials it will be .06" with thye 85. Arc voltage control takes over a short distance into the can set the arc voltage to the voltage setting in the Powermax85 cut chart as a starting point, then, if you notice the torch is running higher or lower than the cut height (.06")...adjust the voltage setting to a higher voltage for a higher height...or lower voltage for a lower height.

The following is an explanation of how a height control system works.

There are 3 cut height settings on a good quality THC. Pierce height, cut height, and arc voltage control.

1. Pierce height is preset by the operator to the plasma torch manufacturers recomended pierce height for a given material and thickness. The THC indexes down to the plate before the torch fires, locates the plate (there are a few different methods for sensing the top surface of the plate), then retracts to the pierce height. The torch then fires.....and should stay in place, no x y or z movement, until the pierce delay (recommended by the torch manufacturer for each thickness) times out.

2. Cut height is preset by the operator according to torch manufacturers recommended cut height. As soon as the pierce delay times out the torch rapidly indexes down to the cut height. At the same time x and y motion begin driving the torch through the part program.

3. Arc voltage control uses feedback from the plasma system to measure the voltage between the electrode (negative) and the plate (positive). (some call this "tip volts", which is incorrect as the torch nozzle is known as the "tip", yet it is the electrode and plate that the voltage is referenced from) At a given cut speed and fixed torch to work distance this voltage remains constant. If the plate is warped so that it moves away from the torch during steady state cutting...the arc between the electrode and the plate gets longer, a longer arc means the voltage gets higher. The torch height control see's an increase in arc voltage and signals the z axis drive to move the torch closer to the plate. Arc voltage monitoring takes over torch height functions as soon as the machine is within a certain percentage of the programmed cut as soon as acceleration gets the machine to a certain speed...the arc voltage control takes over. Consequently when the x and y motion decelerates below this threshold percentage...the avc is disabled so the the torch does not dive during corner or small feature slowdown events .

If your torch height noticeably changes during the transition between cut height and avc, then you should adjust your arc voltage setting so that there is no change during this transition. So, if the torch indexes to cut height, starts moving, then moves further away from the plate....reduce the arc voltage setting. A rule of thumb is that 5 volts will equal approximately .020" torch reducing arc voltage by 5 volts will move the torch closer to the plate by roughly .020", increasing voltage will do the opposite.

Things that affect torch height when operating in avc (arc voltage control)

1. Gas pressure fluctuation. Changing gas pressure changes the resistance of the plasma arc, which changes the arc voltage.

2. Speed change. Slower cut speed makes the kerf wider, which means the arc is longer, which means the voltage is the THC moves the torch closer to the plate.

3. Worn electrode. Plasma electrodes wear by forming a pit in the hafnium emitter. If the pit is .020" deep, the torch will run closer to the plate by .020" the arc is .020" longer and the height control is trying to maintain a constant voltage.

Bottom line....the proper physical height is more important than setting the exact arc voltage that is listed in the plasma torch manufacturers manual. Adjust the voltage so the physical height is correct...don't worry about the voltage reading!

Jim Colt Hypertherm

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