why's plasma tables using z axe ?

[a_nabil00]

hi guys ,
i'm started to build a new plasma table , but i'm still in the collecting information stage .
and the first thing i'm wondering about is what's the benefit of z axe ?
why not i just make it simple by using x and y ?
hope u could help me guys .
regards ,

[packrat]

So the torch to work distance can be adjusted.

[a_nabil00]

thnx packrat , i didn't expect such an immediate reply
so , if i'm going to use it in a fixed thickness , i dont have to make a z axes
i pretend to use it in 1-2 mm thickness

[jimcolt]

Here is a lengthy explanation of the importance of a good z axis torch height control for plasma cutting. Many people build machines with just x and y motion control....and these machine will cut with plasma. However, to make the machine automated (isn't that what a cnc machines is supposed to be?) so you can walk away while it is cutting.....a torch height control on the z axis is needed.

I thought I'd post this as it is an often misunderstood part of the CNC plasma cutting process. There are a lot of things that go on in the background of THC (Torch Height Control) with an automated plasma system....and it is a very important part of the plasma cutting process. A cutting machine without automatic torch height control will exhibit poor plasma consumable life as well as poor cut quality. Unfortunately, even with a good THC, improper settings or lack of a basic understanding of how the system works can lead to the same results as having no THC! Here's a simplified explanation of what has to happen for every cut cycle with a proper THC:

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 mottion 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 speed.....so 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 noticably changes during the transition between cut height and avc, then you should adust 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 thunb is that 5 volts will equal approximately .020" torch movement......so 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 higher...so 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"....as 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

[Ubatoid]

You sure can go ahead without using a Z axis, I have been playing with a system that only has x and y. Problems you will encounter though as I have found especially cutting thin sheet is warping. This throws the gap between torch and material all over the place giving a rather erratic cut and kerf width and tip touch downs. If you where cutting thicker a material it's not so much of a problem. This is why you will find a lot of builders talking about THC or torch hight control systems that measure the voltage of the arc and move the torch up and down to maintain a set distance. Definitely worth considering.

[a_nabil00]

Thnx guys , i really appreciate ur effort .
i understand now what's the THC and it's benefits , but the real probelm now that i can't find any automated plasma torch here in egypt .
so, i think to take a risk and try a handle torch in my system .
what do u think about it ? have anyone tried it ?

[jimcolt]

There are many machines using a hand torch for cnc cutting.....with a height control system as well.

[a_nabil00]

Originally Posted by jimcolt There are many machines using a hand torch for cnc cutting.....with a height control system as well.

sounds good , so i'll go ahead
is there any advice before starting