Most machines are conventional enough that stock CAM and a stock post-processor work fine. This is about the ones that are not — special-purpose machines, unusual kinematics, and controllers with their own way of doing things.
First: is it the post or the toolpath?
The distinction determines whether you need a modest project or a large one.
| Symptom | Likely cause | Fix |
|---|---|---|
| Motion is right, syntax is wrong | Post-processor | Custom post — modest |
| Cycles come out as long-hand moves | Post-processor | Custom post |
| Rotary direction or sign is wrong | Post-processor | Custom post |
| CAM cannot express the motion at all | Toolpath generation | Custom CAM — large |
| Kinematics unsupported by CAM | Toolpath generation | Custom CAM or plugin |
| Process is not standard machining | Toolpath generation | Custom CAM |
The diagnostic question: if you hand-wrote the G-code, would the machine do the right thing? If yes, the toolpath logic is fine and you need a post-processor. If you could not express the motion in G-code at all, the problem is upstream in toolpath generation — a much larger project.
What makes a machine "niche"
- Unusual axis arrangement — rotaries in configurations CAM does not model.
- Additional axes beyond the standard set — a secondary head, a positioner, a tailstock with motion.
- Uncommon controller with its own dialect and cycle set.
- Process-specific commands — laser power ramping, waterjet pierce cycles, deposition rate control.
- Interlocked auxiliary motion that must be coordinated with cutting.
What a proper post encodes
A post-processor is a translation layer, and its quality depends on how completely it models your machine:
- Kinematic configuration — which axes exist, how they are arranged, their limits and directions.
- Controller dialect — supported G and M codes, cycle syntax, modal behaviour.
- Offset scheme — how work and tool offsets are applied, and in what order.
- Tool change sequence — the exact required order, including orientation and coolant.
- Safe start and end blocks — the state the machine must be in at program boundaries.
- Undocumented quirks — everything the operators know and nobody wrote down.
That last item is why a post cannot be written from a manual. The manual describes the controller; the operators know the machine.
The verification requirement
A post-processor is not finished when the G-code looks correct. It is finished when it has been proven on the machine. Software verification catches syntax and obvious motion errors; it cannot catch a controller interpreting an offset differently than expected — and that failure mode moves the machine somewhere unexpected at rapid feed.
The commissioning sequence that keeps this safe:
- Compare against known-good programs that already run correctly, line by line.
- Dry run with no tool, generous clearance, single block, hand on feed hold.
- Cut air at working position with the tool fitted.
- Cut material on a test part and inspect the result dimensionally.
- Exercise the awkward paths — tool changes, rotary moves, cycle cancellation, feed hold and resume.
For machine builders specifically
If you build and sell machines, post-processors are part of your product whether you treat them that way or not. A customer who cannot generate correct programs will conclude the machine is difficult, not that their CAM is misconfigured.
- Supply verified posts for the CAM systems your buyers actually use.
- Version them alongside machine revisions, since a mechanical change can alter the post.
- Document the machine configuration so a customer's own CAM engineer can adapt it.
- Provide sample programs proven on the machine, as a reference for whoever maintains the post later.
This reduces support load substantially, and it removes a common source of early dissatisfaction with an otherwise good machine.
When custom CAM genuinely is required
- The toolpath itself cannot be generated — the process is not standard machining.
- Kinematics are outside anything the CAM system can model.
- You need toolpath generation embedded in your own product.
- The strategies your process needs do not exist in commercial CAM.
Outside those cases, a custom post over commercial CAM gets you a correct program at a fraction of the cost — and keeps you on toolpath logic that has been proven across thousands of shops.
Have an unusual machine that generic CAM will not drive properly? Tell us the kinematics and controller — the post-versus-CAM diagnosis is usually quick. See our CAM service, post-processors explained, and custom vs commercial CAM.