Post Processors


When making g-code for any CNC machine, there will come a time where you’ll click the final button to “Generate G-code”. Doing so, how will you know that the g-code file is going to be properly suited to your particular CNC? Though many CNCs are able to interpret what’s broadly known as ‘g-code’, the reality is that different manufacturers have their own quirks that their machines expect to see; you can think of this as g-code being the primary language while different CNCs speak with dialects or accents. For the AltMill, LongMill and many other consumer-level CNCs, this dialect is known as “GRBL”.

This is where post-processing comes in. A ‘post-processor’ is simply a set of rules that can be followed to add tweaks to the main g-code and ensure it’ll work for particular machines. If the post-processor isn’t selected or set up correctly this will usually result in errors, stalls, or unexpected behaviour while running your jobs.

To help out, we’ve put together a simple table which shows popular CAM programs and their AltMill-compatible post-processors. Some CAM programs may not have post-processor options since they’re already designed for hobby CNC use:

CAM SoftwarePost-Processor
Vectric Cut2D, Vectric VCarve, Vectric AspireGRBL (mm)
Carveco Maker, Carveco Maker+UGS GRBL
Fusion 360GRBL**
Easel, Easel ProNo selection required
Carbide CreateGRBL
CAMLabNo selection required
ESTL CamNo selection required
FreeCADGRBL (mm)
Inventor CAMGRBL (mm)

**To avoid potential issues with Fusion 360, we also recommend you make the following checks to your post-processor:

  • Change the ‘Safe Retracts‘ to “Clearance Height” and NOT “G28”. It will be obvious that you have this on if you don’t have limit switches or forget to home your machine and a the start of the job your bit plunges suddenly really deep into your material.
  • Output M6 set as “No” (unless you plan to set up tool changing)
  • Output Tool Number set as “No” (unless you plan to set up tool changing)

If you are using Vectric VCarve, you can create a custom machine configuration file for your AltMill, which will specify the post-processor as well as the machine working area dimensions.

Creating AltMill machine configuration on Vectric with correct settings

The following video guides you through the process, but make sure to change the width and height both to 1245mm for the AltMill.