Demystifying the Machine: A Guide to Mastercam Post Processor Editing

If you’ve been in CNC machining for more than a week, you’ve likely heard the phrase: “It’s just a post thing.”

In the world of Mastercam, the Post Processor is the silent gatekeeper between your CAM toolpath and your machine’s motion. When it works, you never think about it. When it fails, it crashes tools, scraps parts, or stops the spindle cold.

Editing a Mastercam Post Processor (.pst and .psb files) isn’t just a technical skill—it’s a survival skill for any shop that wants to move beyond generic G-code.

Here is everything you need to know about why, when, and how to edit your Mastercam Post.

8.1 The .log and .ncb Files

When running a post, Mastercam generates debug files if configured.

• Editor Action: Add verbose$ = 1 (depending on post version) to output step-by-step variable states to the comments in the NC file.

3. The "M00" Trick

If you don't know why a post is skipping a line, force it to stop. Insert this debug code into the line you are testing: "M00 (DEBUG: Coolant turned on here)", e$ When your machine runs, it will stop, and you can see if the code appeared before or after your motion.

3. Tool Change Routine – ptlchg$ and ptlchg0$

• ptlchg$: Called when a new tool is used.
• ptlchg0$: Called when the same tool is used for a new operation (null tool change).

Common edit: Force a tool length measure on every tool change.

ptlchg$          # Tool change
  pbld, n$, "M06", "T", t$, e$
  pbld, n$, "G43", "H", t$, "Z0.1", "M08", e$   # Add H offset and coolant

Key Concepts

• Post language: Mastercam posts are written in Mastercam Post Builder (MPB) syntax (text-based with placeholders) or in older Post Processor language; some firms use edited template files (.pst/.psb).
• Tokens/placeholders: Variables (e.g., tool number, feedrate, axis positions) inserted into templates are replaced at post time.
• Modes & flags: Tracks modal states (units, feed/spindle modes) to avoid redundant output and produce concise code.
• Subroutines/macros: Reusable blocks for repeated sequences (toolchange, homing, probing).
• Axis mapping & kinematics: Maps Mastercam axes to machine axes; multi-axis requires correct transformation and rotary axis handling.
• Safety & simulation: Always simulate edited posts in CAM proofing and, if available, controller simulation or dry-run on machine with low feed overrides.

Introduction: The Final Link in the CAM Chain

In the world of CNC machining, Mastercam is the brains—the place where toolpaths are born from solid models. But the voice that speaks to your machine tool is not Mastercam itself; it is the Post Processor. The post processor is a translator. It converts the generic, neutral toolpath data (NCI - Numerical Control Interface) into the specific, dialect-heavy G-code that your Haas, DMG MORI, Mazak, or Fanuc control understands.

Editing a post processor is not a luxury; it is a necessity for any shop that demands efficiency, safety, and customized output. A poorly edited post can crash a spindle. A masterfully edited post can shave seconds off every cycle and eliminate manual editing at the control.