The Weakest Link

Any process is most likely to fail at its weakest point. The post processor is often the weakest link in 5-axis machining even though it might not get high visibility during a machine tool purchase. Here are some reasons why it is reckless to invest capital in machinery and software without having a clear plan for post processor implementation.

Computer-Aided Manufacturing (CAM) is a workflow process commonly used in modern manufacturing to produce components on numerically control machine tools. Looking in more detail, it can be reduced to three primary steps:

 

 

The first may be called the “pre-processor” –  importing digital data models, whether from inside design groups or outside customers and vendors. Then there is the tool path planning and calculation process, the “processor”. Finally is the communication from the CAM software to the machine tool language, the “post processor”. The manufacturing engineer does much more than part-programming tasks, namely fixture design, tooling selections, and material procurements.

Any process is most likely to fail at its weakest point.  Ask any manufacturing engineer about their post processor experiences (or look in the mirror).  The post processor is often the weakest link.  This article discusses the many approaches and processes used to address the post processor topic.

HISTORY
The APT (Automated Programming Tool) language was developed in the 1950s with U.S. Air Force funding. APT includes the pre-processor or geometry commands, the processor for tool path planning, and the post processor defining machine tool commands and controls.

Some CAM software products still have heritage to the APT language today, but with a graphical user interface mask. Many existing post processors use APT as a basis for post processor translations between the CAM software and the machine tool. Though APT is rigidly defined and there is little ambiguity between different APT “flavors” prepared by different CAM software vendors, APT is too limiting today in many cases.

THE 3-AXIS DEMO
CAM software broadened in use by small and medium machine shops in the 1980s and 1990s. At this time, the CAM software sales process often included a customer visit and the programming and machining of a customer part. This required a post processor to be in inventory or developed on the fly to a level sufficient to drive the customer’s machine tool, which the customer found very impressive. The shop owner could see the software system in action and reduce ultimate implementation risk at the same time.

This was possible due in part to the basic requirements of a 3-axis post processor. 3-axis machine kinematics are fairly basic and the syntax for the machine’s controller unit was fairly consistent across machines due to a predominance of Fanuc or Fanuc-style control language. Though the control manages many functions for the machine tool, its most basic role is the interface from the CAM software to the machine tool for language and syntax commands. This fairly common language of the 3-axis world surely enabled the “3-axis Demo”.

MULTI-AXIS AND MULTI-TASK MACHINES 
The manufacturing world has since progressed to more complex machine tools and applications. Multi-axis milling machines enable fewer setups, simplified fixturing, and shorter cutters in producing complex geometries. The intention is to perform multiple, even disparate process steps, such as milling and turning, on the same machine tool. They also simplify handling and reduce floor space.

Higher capital investment costs for these machine tools also provide clear return from improved results and shop productivity. Such machine types also increase the burden on CAM software for both programming tasks and post processor capabilities.

Multi-task machining is often a series of simple operations – face milling, drilling, and turning. Post processing for each of these subtasks is often well known; the challenge is in combining the post processor sub-functions in a rational manner. The mathematical foundation of multi-axis post processors is much more intensive. These solutions generally cannot be prototyped by “hand programming” and reading output NC instruction files is not always good advice for reverse engineering post processor functions.

THE 5-AXIS DEMO
Generally, the CAM software industry does not perform the 5-Axis Demo in the same way as the preceding 3-axis Demo. Many software vendors (not all) foresee challenges to develop good 5-axis machining instructions during a live demo. They also have concerns about using unchecked post processors. And, in many systems, 5-axis calculation times can be much longer than 3-axis programming.

But it does not have to be this way. At its core, 5-axis tool path calculations and 5-axis postprocessing are a series of basic mathematic equations. What is nice about mathematics (and one reason I gravitated toward a career in technology) is that I found great satisfaction in the simple statement “math works”. Define the correct mathematical formulas and conditions to define a process or phenomenon, and any valid inputs assuredly lead to valid outputs.

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Alan Levine

Alan Levine is the managing director of OPEN MIND Technologies USA, Inc., 1492 Highland Avenue, Unit 3, Needham MA 02492, 339-225-4557, Fax: 270-912-5822, alan.levine@openmind-tech.com, www.openmind-tech.com.

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