Set Z-axis

The z-axis has an important function, namely to set the torch height. The  height  of the torch in relation to the workpiece is of great importance for a good cutting result. And at the right height  the mouthpiece last longer.

At the wrong height there is a chance that the torch will hit the workpiece. On this page we will discuss what tasks the z-axis performs and how to set it up.

First, an overview of the tasks that the z-axis performs in the correct order. Then we will discuss these tasks individually. All values ​​of the settings can be found in this overview. An example of a g-code can be seen in the image below. Here  the z-axis machine commands are displayed. Read on the page "What is a g-code" more about this.

  1. Probe
  2. Pierce height
  3. Pierce delay
  4. Cutting height
  5. Torch Height Controller (THC)


(sample g-code z-axis commands created with Fusion360)

1. Probe

The Probe (code G31 in the g-code image) is the first step the z-axis will make. Here the z-axis is given the command to go to -100 mm.  Behind the letter F the speed at which it moves is stated (Feedrate). During this movement the z-axis will be stopped the moment the torch touches the workpiece by means of a switch incorporated in the z-axis drive.

NB. Before the switch gives a signal, the torch has probably already touched the workpiece because there is a free play in the switch. This is different for each machine and must therefore be determined for each machine. We will come back to this later in this article because this has an influence on the setting of the cutting height.  View on this page how to determine the clearance.

2. Pierce height

After the Probe action the z-axis will go to its safe travel height (rule N35 on the picture) and then to the piercing height. The piercing height is there so that the torch can pierce at a good height above the workpiece. This is because liquid material splashes up when the torch starts burning. You don't want this in  the nozzle of the torch. Generally this is at a height of 4 mm. This is 2 mm higher than the cutting height (in the picture this is 2.5 mm at rule N40We will discuss this in more detail in point 4. Cutting height in this article.

3. Pierce delay

After the z-axis has reached its piercing height, the CNC machine switches on the plasma cutter (code M3 in the g-code image). You can imagine that it takes longer to pierce through a thicker plate than a thinner plate. That's what the pierce delay is for. This is command G4 in the image and is shown with a number after the letter P. In this case number 1This means that there is a waiting period of 1 second to give the plasma cutter the chance to burn through (pierce) the plate.

4. Cutting height

The last action the z-axis will perform before the x and y axes start moving is to go to its cutting height. This is generally a height of 2 mm. These settings can be found in the previously mentioned overview at the top of this page. But here comes the thing.Because what we haven't discussed yet is rule N30 in the g-code example. This code setting is important in combination with rule N55.

Bee rule N55 four things come together.

  1. play in the switch.
  2. the cutting height value.
  3. the zero position.
  4. the cutting speed

How is this put together?

There is an additional action that happens during the Probe. At the moment that the torch touches the plate (Probe function), we discussed earlier that there is a free play on the switch. This is 1.5 mm in this case. The cutting height is 2 mm.

But why is there a - sign in front of it? You would think that it would have to go to +3.5 mm in height from its zero point,  instead of further down. This is because the z-axis is fooled at the moment the switch indicates that the torch has touched the plate. The z-axis is then fooled at rule N30 not set to 0 but to -3.5 mm. The g-code that goes with this is G92, which stands for: compensation coordinates. This is not a displacement that he is going to perform at that moment, but a value input. At rule N55 where the z-axis goes to its cutting height, the actual command is go to your origin. Hence the Pierce height is not 4 (in this case 4.5) but 2.5 mm. This is 2.5 mm above its origin.

Ultimately, all actions are performed from a zero coordinate or a coordinate is adjusted. All these settings can be adjusted in the CAM software. Where you can adjust these and how this is processed in the g-code differs per CAM program.

In Fusion360 these settings are adjustable as variables in the last step when you save the g-code. This when you have opened the Pritec Mach3 Post Processor.

At SheetCam  These settings are processed in the Tools.

NB! To process the play in the Probe switch, you can enter the deviation in the Post processor.
Here's how you do it:
At the top of the screen go to Options -> Machine -> Post Processor
Then click on Edit Post and enter the clearance (mm) in SwitchOffset.
This play is then incorporated into each g-code file.
5. Torch height controller
In the ideal situation, after all the z-axis settings, the torch will move at a cutting height of 2 mm above the plate. However, when the plate is curved, the torch will move closer or further away from the plate. The torch height controller (THC) can adjust this when necessary. It takes over the z-axis height control when it receives a voltage from the plasma cutter. It then works as a kind of multimeter that measures the circulating consumption of the plasma cutter. You can set a nominal value here that corresponds to the ideal cutting height, speed and ampere. It will want to maintain this nominal value and therefore constantly adjusts the z-axis in terms of height. More information about the THC controller can be found at this page.