There is a fine line between editing and engineering. You should stop editing and hire a professional post developer (like In-House Solutions or Postability) if:
Professional posts cost between $500 and $3,000. Crashing a spindle costs $15,000. Do the math.
Problem: Your post outputs M8 (flood), but you need M88 (high-pressure).
Solution: Find the scoolant string definitions. Change:
scoolant : "M8" → scoolant : "M88"
Then ensure the pcan function calls scoolant.
Your post processor is not a static file. It is a living reflection of your machine shop’s standards. Every time you buy a new machine, a new probe (Renishaw), or a new bar feeder, your post will need editing.
Master the basics: understand psof$, ptlchg$, and modal logic. Use a proper editor. Test ruthlessly. And remember—the best post processor is invisible. You click "Post," perfect G-code appears, and the machine runs. No drama. No manual edits. That is the art of post processor editing.
Mastercam Post Processor Editing Guide Mastercam post processor editing is a vital skill that bridges the gap between your CAM toolpaths and the specific physical requirements of your CNC machine. By customizing post processors, you can eliminate manual "hand-editing" of G-code, which reduces errors and optimizes machine performance. Understanding the Post Processing System
In Mastercam, "posting" is governed by three primary interconnected files: Machine Definition (.mcam-mmd):
Defines the physical geometry and kinematic layout of the machine (e.g., axes, rotary limits). Control Definition (.mcam-control): mastercam post processor editing
Determines how the control handles specific code types, such as arcs, drilling cycles, and subprograms. Post Processor (.pst):
The scripting file that formats the actual NC output according to the machine's syntax requirements. Key Tools for Editing
Successful editing requires using the right software environment to avoid syntax errors:
Editing Mastercam post processors (the files) is a powerful way to tailor G-code output to your specific CNC machine. While there isn't one "perfect" article, several high-quality resources and expert tips provide a comprehensive guide to the process. The Best Tool for Editing: Mastercam Code Expert Most pros agree that the Mastercam Code Expert is the best environment for editing posts. eMastercam.com Why use it:
It includes built-in recognition for predefined variables and operators, color-coding them to help you avoid typos. How to access: File > Edit/Open external within Mastercam and select your eMastercam.com Essential Editing Tips from the Pros Always Backup First:
Never edit your "live" post without a safety copy. Rename your working file (e.g., Post_v1_Edited.pst ) so you can always revert if the code breaks. Use the Post Debugger:
To see exactly which line of the post processor is generating a specific line of G-code, enable the Post Debugger . In Mastercam, go to File > Configuration > Post Dialog Defaults There is a fine line between editing and engineering
and check "Enable post debugger". When you post code, click the ladybug icon to step through the logic. Check "Post Switches" First:
Many common changes (like turning off tool pre-calls or changing G-code formats) are already built-in. Look at the top of the file for a "switches" table where you can simply change a to toggle features. Note Your Changes: Use a unique character (like a pound symbol
or your initials) to mark every change you make. This makes it easy to search for your modifications later. Top Resource Links adding / editing tool comments - eMastercam.com
Register now to participate in the forums, access the download area, buy Mastercam training materials, post processors and more. * eMastercam.com Opening a post for editing - eMastercam.com
Mastercam post processors serve as the vital bridge between your CAM software and your CNC machine's controller. While the default posts provided with the software cover a wide range of machines, high-level manufacturing often requires fine-tuning to optimize cycle times, ensure safety, and leverage specific machine features. Mastering the art of post processor editing allows you to automate manual G-code edits, reducing the risk of human error on the shop floor. The Architecture of a Post Processor
Before diving into edits, it is essential to understand that most Mastercam posts use the MP language. A standard post consists of several key sections:
Variable Definitions: This is where you initialize the data holders for coordinates, feed rates, and logic switches.Formatting Statements: These lines dictate how numbers appear in the G-code, such as the number of decimal places or the inclusion of leading/trailing zeros.Predefined Blocks: These are the "events" in the code, such as the start of a file, a tool change, or the end of a program.Lookup Tables: These maps correlate Mastercam internal values to specific machine G and M codes. Setting Up Your Environment Professional posts cost between $500 and $3,000
To edit a post safely, never work on your primary file. Create a backup copy of the .PST and .MCPOST files. Use a dedicated editor like Code Expert, which comes bundled with Mastercam. It offers syntax highlighting and "jump to" features that make navigating thousands of lines of code significantly easier. Common Post Modifications
Customizing Tool Change SequencesOne of the most frequent requests is modifying how a machine behaves during a tool change. You can edit the "ptlchg$" block to include specific safety moves, such as sending the Z-axis to a home position before the tool carousel rotates. You can also add logic to suppress redundant tool calls if the same tool is used in consecutive operations.
Adding Manual Entry and CommentsCommunication between the programmer and the operator is critical. By editing the post to automatically output tool descriptions, programmed feed rates, or custom operator notes at the beginning of each tool section, you create a more transparent workflow. This is typically handled in the "pheader " sections.
Managing Coolant CodesModern machines often have multiple coolant options: flood, mist, through-spindle, or high-pressure air. If your post only outputs M08, you can modify the coolant logic to recognize Mastercam's X-style coolant commands, ensuring the right M-code is paired with the right operation every time.
Adjusting High-Speed Machining ModesFor mold and die work, specific G-codes like G05.1 (Fanuc) or COMPCAD (Siemens) are required for high-speed look-ahead. You can hard-code these into the start-of-operation blocks so they activate automatically when a high-speed toolpath is detected, ensuring the machine doesn't "shudder" during complex 3D moves. Debugging and Testing
Once you have made your changes, the testing phase must be rigorous. Use the "Debug" mode in Mastercam to step through the post line-by-line. This tool shows you exactly which line of the post processor generated a specific line of G-code.
Always run the resulting code through a simulation software or a "dry run" on the machine with the feed rate turned down and no part in the vise. Verify that every tool change, spindle start, and rapid move matches your intent. Conclusion
Post processor editing is a transition from being a software user to a process architect. By investing time into the MP language, you eliminate the need for "hand-coding" at the machine, create safer programs, and ultimately get the most out of your CNC investment. Small logic changes today can save hundreds of hours of manual editing over the course of a production year.
This document presents a comprehensive technical deep dive into the architecture, logic, and methodology of editing Mastercam Post Processors. It is structured for CNC programmers, manufacturing engineers, and post processor developers seeking to move beyond basic modifications into structural customization.