Floating head auto-zero set-up confusion. Please, somebody,

Hey everyone, I really hope that there is some of you guys out there who are able to help a struggling builder.

I have built a CNC Plasma table and it works very well in its basic form ie, CAD (Visio), CAM (Sheetcam) and Control (Mach3). It cuts out what I want but irregularities in the material can cause poor cut quality and/or the arc to go out if the gap gets too big.

I decided to start with (ie, before I get THC) I would build a floating head so that the machine will “auto-zero” before each time it pierces the material.

So far so good, physically the head works and I can move it so it actuates the micro switch and illuminates the LED on the diagnostics page.

I did some research into auto zeroing the Z axis and found several posts on the subject. I have got my own head around the fact that I need to insert some lines of G-code into the program so that the head will wind down to the limit switch and reset the “Z”, then wind back up a pre-set amount and then move to pierce height.

I copied the code that I found on this forum…

N0120 G28.1 Z0.50
N0130 G92 Z0.0
N0140 G00 Z0.1370
N0150 G92 Z0.0
N0160 G00 Z0.2000

…and adapted it to my own machines needs ie,…

N0120 G28.1 Z0.50
N0130 G92 Z0.0
N0140 G00 Z13.25
N0150 G92 Z0.0
N0160 G00 Z5

this I added to the “run code snippet” bit in Sheetcam and selected it in the “operations” window, posted the code and ran it with Mach3.

The problem

The code seems to run right at the start of the cycle and not on the pierce as I require.

on further reading I picked up that I should add the code to an edited “Post processor”. I looked up the post processor that I’m using (Mach3 Plasma) and opened it with (windows) notebook.

That is where the trouble starts! I am out of my depth here and could do with a few pointers in the right direction.

  1. Am I using the correct post processor for a machine with a floating head?

  2. If I were to edit this file, where in hecks name do I insert the few lines of code?

  3. If I should use the “run a code snippet” thing in Sheetcam then how do I get it to run in the right place?


Please can someone help with this as I had hair when I started and I’m pretty thin on top now!

Thanks for your time. Any more info required I’ll get it posted ASAP.

You need to use a THC post processor. I would recommend the MP1000-THC-scriber post. Select the post then click on the ‘edit post button’. Near the start of the file you will see a bunch of parameters. The only ones you need to worry about are:

refHome = true
switchOffset = 13.25

You may also want to play with refDistance. If you have a bunch of cuts close togather the post won’t bother to re-reference for each cut. This value controls when it references. If you want to reference every time, set it to 0.

  1. If I should use the “run a code snippet” thing in Sheetcam then how do I get it to run in the right place?

Code snippets aren’t really meant for referencing. You could use them this way by putting the code snippet in the start point. In start point mode, right click on the start point and select properties. In the peroperties is an option to use a snippet. It would howver become very tedious as you would have to do this for every reference. The post is pretty much set and forget.

Hooray it works!

Thanks very much for your help with this matter. I used a different post processor (MP1000-THC - scriber) and edited it as directed. The floating head only bloody works! SWEET!!


Would you also recommend the MP1000-THC-scriber post when using LinuxCNC? I am trying to setup my floating head. I have added my G38.2/G92 find and set code in the OnPenDown section of the EMC Plasma post, but no matter where I place my addition in this section, pierce height occurs before my code in the final output.

Thanks for any guidance!

Never mind. I came across another post with the same basic question.


Hi That is great, That is awesome information, i was also looking for this.