Setting the Z-axis

The Z-axis has an important function, namely adjusting the torch height. The height of the torch relative to the workpiece is of great importance for a clean cutting result. Furthermore, the nozzle will last longer at the correct height.

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

First, an overview of the tasks performed by the z-axis in the correct order. Then, we will discuss these tasks individually. You can find all the setting values ​​in this overview. The image below shows an example of g-code. 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)

 


(example 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 is going to take. Here, the z-axis receives the command to go to -100 mm. Behind the letter F the speed at which he moves is stated (FeedrateDuring this movement, the z-axis will be stopped the moment the torch touches the workpiece by means of a switch integrated into the z-axis drive. 

NB. Before the switch emits a signal, the torch has likely already touched the workpiece because there is free play in the switch. This varies by machine and must therefore be determined for each machine. We will return to this later in this article, as it affects the cutting height setting. View at this page how you can determine the clearance.

2. Pierce height

After the Probe operation, the z-axis will move to its safe displacement height (rule N35 in the image) and then to the piercing height. The piercing height ensures that the torch can pierce at a good height above the workpiece. This is because liquid material splashes up the moment the torch starts burning. You would rather not have 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 image this is 2.5 mm at line N40). We will go into this in more detail under 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 one. That is why the pierce delay exists. This is command G4 in the image and is displayed with a number after the letter P. In this case number 1. This means that there is a waiting time of 1 second to give the plasma cutter a chance to burn through the plate (pierce).  

4. Cutting height

The final action the z-axis will perform before the x and y axes start moving is to move 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 interesting part. Because what we haven't discussed yet is line 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 actually put together? 

There is an extra action that takes place during the Probe. The moment the torch touches the plate (Probe function), we discussed earlier that there is free play on the switch. In this case, this is 1.5 mm. The cutting height is 2 mm.

But why is there a minus sign in front of it? You would say that it then needs to move to +3.5 mm in height from its zero point, instead of further down. This is because the z-axis is tricked the moment the switch indicates that the torch has touched the plate. The z-axis is then line N30 not set to 0 but to -3.5 mm. The corresponding g-code is G92, which stands for: coordinate compensation. This is therefore not a movement that it is going to perform at that moment, but a value input. At rule N55 Where the z-axis approaches its cutting height, the actual command is to go to your zero point. That is why the Pierce height is not 4 (in this case 4.5) but 2.5 mm. This is 2.5 mm above its zero point. 

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

In Fusion360, these settings can be configured as a variable in the final step when saving the g-code. This is done when you have opened the Pritec Mach3 Post Processor.. 

 

In SheetCam, these settings are incorporated into the Tools. 

 

NB! To process the backlash in the Probe switch, you can enter the deviation into the Post processor.
Here's how you do it:
Go to Options at the top of the screen -> Machine -> Post Processor 
Then click on Edit Post and enter the clearance (mm) in SwitchOffset.
This slack is then processed in every g-code file.
5. Torch height controller
In the ideal situation, after all Z-axis settings have been made, 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 to or further away from the plate. The torch height controller (THC) can adjust this when necessary. It takes over Z-axis height control the moment it receives a voltage from the plasma cutter. It then acts as a kind of multimeter that measures the circulation power consumption of the plasma cutter. You can set a nominal value here that corresponds to the ideal cutting height, speed, and amperage. It will want to maintain this nominal value and therefore constantly adjusts the Z-axis height. You can find more information about the THC controller at this page.