Modern CNC manufacturing demands precision, speed, and repeatability. Mastercam, as one of the most widely used CAD/CAM platforms, provides robust tools for creating toolpaths, but translating those toolpaths into machine-specific G-code is a critical step that often introduces bottlenecks. Automating post-processing in Mastercam eliminates manual file adjustments, reduces the risk of syntax errors, and enables seamless transitions between different CNC machines. This article provides a comprehensive guide to setting up, customizing, and automating post-processors in Mastercam for diverse machine configurations—ranging from 3-axis mills and lathes to multi-axis machining centers and wire EDMs.

Understanding Post-Processing in Mastercam

Post-processing is the bridge between Mastercam’s internal toolpath data and the specific dialect of G-code that a CNC controller understands. Each machine manufacturer—Haas, Mazak, DMG Mori, Okuma, Fanuc, Siemens, etc.—uses variations in code format, canned cycles, and safety blocks. Mastercam ships with hundreds of ready-to-use post-processors, but production environments often require customizations to handle non-standard tool changers, coolant commands, or probing routines. Without proper post-processing, the machine may stall, cut incorrectly, or even crash.

Mastercam post-processors are text files with a .pst extension (plus associated .psb binary files) written in Mastercam’s proprietary Post Language. These files contain variables, logic, and formatting instructions that define exactly how NC code is generated. A well-configured post-processor ensures:

  • Correct axis mapping: Rotational axes (A, B, C), spindle orientation, and tool datum points.
  • Optimal feed and speed output: Inheriting values from Mastercam tool libraries while respecting machine limits.
  • Safety block generation: Home return, coolant on/off, and spindle start/stop in the proper sequence.
  • Subprogram and macro compliance: Using local subroutines or parametric programming where applicable.

Automation takes this one step further by scripting the selection, modification, and execution of these post-processors, allowing operators to process dozens of part programs with minimal manual intervention.

Step 1: Mapping Machine Specifications and Controller Requirements

Before customizing or automating any post-processor, you must document the exact requirements of each CNC machine in your shop. Create a spreadsheet or database with the following fields:

  • Controller type and version (e.g., Fanuc 31i-B5, Siemens 840D sl)
  • Maximum rapid traverse rates and acceleration limits
  • Tool changer style (umbrella, carousel, chain, or direct) and tool change position
  • Coolant configuration (flood, mist, through-spindle, air blast)
  • Preferred G-code format (absolute/incremental, decimal places, leading zeros)
  • Block delete, optional stop, and program number rules
  • Special cycles (rigid tapping, peck drilling, helical interpolation, etc.)

Accurate specifications ensure that your automated post-processing does not produce code that violates the machine’s safe operating envelope. For instance, a machine with a low maximum feed rate for tapping will need a post that clamps feed rates accordingly.

Step 2: Selecting and Customizing Post-Processors

Using Mastercam’s Built-in Post Library

Mastercam includes a library of post-processors in the Posts folder. To access it, open Mastercam, go to Machine Type > Machine Definition, and select the appropriate machine. The control definition will reference a specific post-processor. Often, a generic Fanuc post covers many machines, but you may need to tweak it for your specific controller.

Customization begins by editing the .pst file with a text editor (Notepad++, VS Code, or Mastercam’s Post Editor). Key areas to modify include:

  • machine_definition: Setting the machine type (Mill, Lathe, Router)
  • tool_change: Defining the tool change sequence, including M06 timing, spindle orientation, and coolant commands
  • feed_rate: Format and output of F-word (e.g., F200.0 vs F200)
  • spindle: Direction (M03/M04) and dwell after start
  • misc_real and misc_int: Custom M-codes or parameters exposed in Mastercam operations

Creating a Custom Post from an Existing Template

If your machine uses a non-standard controller (e.g., a Heidenhain TNC 640), start from the closest generic template. Mastercam provides templates for common protocols:

  • Generic Fanuc 3X Mill
  • Generic Haas 3X Mill
  • Generic Siemens 840D Mill
  • Generic Heidenhain TNC 640 Mill
  • Generic Mazak Matrix Mill

Copy the template into your custom folder, rename it, and then adjust parameters. Always test the new post with a simple geometry (e.g., a 100mm cube with one contour and one pocket) to verify that G-code matches the expected format.

Step 3: Automating Post-Processing with Scripts and Custom Workflows

Manual post-processing of each file is time-consuming and error-prone. Automation can be implemented at several levels:

3.1 Using Mastercam’s Batch Post-Processing Tool

Mastercam includes a built-in batch post-processing utility. Access it via File > Batch. Here you can add multiple MCAM files, assign a post-processor to each (or use the machine definition from the file), and output all NC programs at once. This is ideal for job shops that run several parts on the same machine. However, if you need to use different posts per operation, the batch tool requires manual per-file assignment.

3.2 Automating with VB Script (VBScript) or C-Hooks

For advanced automation, Mastercam exposes an object model via VBScript (and .NET via the Mastercam .NET API). You can write a script that:

  • Opens a Mastercam file
  • Reads the operation list and machine group
  • Selects a post-processor based on the machine type embedded in the operation
  • Post-processes and saves the NC file to a predefined network folder
  • Logs completion and errors to a text file or database

A simple VBScript example (pseudo-code):

Set app = CreateObject("Mastercam.Application")
app.Open "C:\Jobs\Part1.mcam"
Set ops = app.ActiveDocument.Operations
postPath = "C:\Posts\Haas_3X.pst"
app.PostProcess postPath, "C:\NC\Part1.nc"
app.CloseDocument

This script can be triggered from a Windows batch file, scheduled task, or called from a DNC system. For manufacturing cells with multiple machines, a script can loop through a directory of .mcam files and assign posts based on file naming conventions (e.g., files ending in _HAAS get the Haas post).

3.3 Integration with Manufacturing Execution Systems (MES)

In a fully automated environment, the MES sends a job order to Mastercam (or a scheduler like Mastercam’s Job Setup). The MES can call a script that:

  • Retrieves the correct post-processor from a database
  • Triggers Mastercam to regen toolpaths if needed (using the regeneration script)
  • Posts the NC file
  • Transfers the program directly to the machine via DNC or network

This eliminates any human decision-making about which post to use, drastically reducing setup errors.

Step 4: Automated Post-Processing for Multi-Axis and Special Machines

Automating post-processing for 5-axis machines, mill-turn centers, and Swiss-type lathes requires additional logic. For example, a 5-axis post must handle:

  • Tool axis control (table/table, head/head, or mixed)
  • Collision avoidance rotations
  • Output of A, B, C rotary positions with appropriate retract sequences
  • RTCP (Rotary Tool Center Point) enable/disable codes

Mastercam’s post language includes pre-defined variables like rot_on_x and pri_axis_type that configure how rotary axes are output. Automation can select the correct post based on the machine type defined in the Mastercam file’s machine group—for instance, a file with a 5X machine group automatically links to a Hermle_C40U.pst.

Similarly, mill-turn posts must manage both milling and turning operations in a single program, coordinating dual spindles and Y-axis movements. Automating this ensures that the post sees a turning toolpath and outputs G33 (threading) or G71 (roughing) as appropriate.

Step 5: Best Practices for Reliable Automation

  • Version-control your posts: Store .pst files in Git or a shared network drive with version history. Tag releases when you modify post logic.
  • Create a validation suite: Develop a set of test Mastercam files that cover common operations—facing, contouring, drilling, tapping, 3D surface, and multi-axis. After each post customization, run the suite through your automation script and compare the output to expected NC files.
  • Use conditional logic in posts: Embed if/else statements in the .pst to handle different tool types (e.g., if tool type = tap, output fixed cycle). This reduces the need for multiple posts.
  • Implement error trapping in scripts: Your automation VBScript or C# application should catch exceptions (file not found, post missing, Mastercam crash) and write to an error log. Alert operators via email or dashboard.
  • Benchmark post time: Some complex posts (especially for 5-axis with many retracts) can take minutes. Monitor post-processing time in your automation, and consider offloading posts to a dedicated server with Mastercam Runtime installed if throughput is critical.

Step 6: Leverage Machine Simulation to Validate Automatically

Mastercam includes machine simulation (via ModuleWorks) that can verify the NC code against a solid model of the machine. You can automate simulation as well—after posting, your script can launch Mastercam’s simulation engine (using the .sim file from the machine definition) and check for collisions or gouges. If a violation is found, the script can halt the process and flag the job for review. This step is invaluable when applying automated posts to new parts without manual verification.

For a fully lights-out manufacturing cell, combine automated post-processing, simulation, and DNC file transfer into a single pipeline. For example, when a job is released from ERP, a server-side script:

  • Pulls the Mastercam file from a shared project folder
  • Regenerates toolpaths if needed (using Mastercam’s update operation)
  • Posts using the appropriate post (determined by machine ID in the file)
  • Simulates the resulting NC code
  • Copies the NC file to the machine’s FTP folder
  • Sends a text notification to the operator

This process can run overnight, allowing the shop to begin machining immediately the next morning.

Common Pitfalls and How to Avoid Them

  • Hard-coded machine limits: Avoid putting absolute values (like max feed = 5000) directly in the post if the machine has varying limits. Instead, read real-time parameters from a configuration file that can be updated without modifying the .pst.
  • Unmatched axis assignments: When automating multiple machines, double-check that a post designed for a table/table 5-axis is not accidentally applied to a head/head machine. Use Mastercam’s machine definition to store metadata that your script can parse.
  • Neglecting format differences: Some controllers require decimal points (X10.0) while others accept integer-only with implied decimal (X100). Non-integer positions can produce disastrous moves if the post format is wrong. Always include a format verification step in your automation script.
  • Ignoring post version compatibility: Mastercam updates can change the internal post language structure. After updating Mastercam, test all automated posts with the validation suite before going live.

Additionally, consider the lifecycle of post-processors. As you add new machines, clone and modify existing posts rather than building from scratch. This maintains consistency in formatting and reduces debugging time.

External Resources and Advanced Customization

For deep customization, Mastercam provides a Post Language Reference Guide and a Post Processor Support page with sample code. Third-party resources like CNC Training’s Mastercam Post Processor course can accelerate learning. For shops using Fanuc controls, the Fanuc CNC Custom Macro B programming manual is essential for writing posts that output proper subroutines.

If you find editing raw .pst files daunting, consider using Mastercam’s Post Manager (on the Machine Definition page in Mastercam 2024+), which provides a more visual interface for customizing outputs. However, for full automation, scripting is still required.

Conclusion

Automating post-processing tasks in Mastercam is not a one-size-fits-all solution, but with careful planning, it yields dramatic improvements in throughput and error reduction. By thoroughly documenting machine specifications, customizing post-processors with conditional logic, and implementing scripts for batch or triggered processing, manufacturers can eliminate manual file generation. Adding machine simulation and integration with MES creates a closed-loop system that runs reliably even with minimal human oversight. As CNC machines become more complex, mastering post-automation in Mastercam is a competitive necessity for any advanced manufacturing facility.

Start small: automate one machine’s post-processor for a repeat job. Validate the output, then scale to your entire shop floor. With each iteration, your automation pipeline will become more robust, freeing skilled operators to focus on setup and optimization rather than file typing.