The Weakest Link
- November 8, 2011
by Alan Levine
Computer-Aided Manufacturing (CAM) is a workflow process commonly used in modern manufacturing to produce components on numerically controlled machine tools.
Looking in more detail, the CAM process can be reduced to three primary steps. The first is importing digital data models, whether from inside design groups or outside customers and vendors. This may be called the "pre-processor.” Then there is the tool path planning and calculation process, or the "processor,” and finally the communication from the CAM software to the machine tool language called the "post-processor.” The manufacturing engineer does much more than part programming tasks–including fixture design, tooling selections, and material procurements.
Any process is most likely to fail at its weakest link. sk any manufacturing engineer about their postprocessor 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.
The APT (Automated Programming Tool) language was developed in the 1950s with United States Air Force funding. The APT language includes the pre-processor or geometry commands, the processor for tool path planning, and the post-processor to define commands and controls for the machine tool.
There are still some CAM software products today having heritage to the APT language, but with a graphical user interface mask, and there are still many postprocessors using APT as a basis for postprocessor translations between the CAM software and the machine tool. Though APT is rigidly defined and there is little ambiguity between different "flavors” of APT that are prepared by different CAM software vendors, APT is too limiting today in many cases.
The three axis demo
CAM software became broadly utilized by small and medium machine shops in the 1980s and 1990s. During this time, the CAM software selling process often included a customer visit with the programming and machining of a customer part. This required a postprocessor to be in inventory or developed on-the-fly to a sufficient level to drive the customer's machine tool. The result was very impressive to the customer. The shop owner could see the software system in action as well as reducing ultimate implementation risk.
This process was possible in part due to the basic requirements of a three axis postprocessor. Three axis machine kinematics is 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 and obvious role is the interface from the CAM software to the machine tool for language and syntax commands. But the fairly common language of the three axis world surely enabled the "three axis demo”.
Multi-axis and multi-task machines
The manufacturing world has since progressed to have more complex machine tools and applications. Multi-axis milling machines enable fewer set-ups, simplified fixturing, use of shorter cutters, as well as the production of complex geometry. The intention of multi-task machines is to perform multiple process steps, even disparate steps such as milling and turning on the same machine tool. Both machine tool types simplify handling and reduce the utilized floor space on the shop floor. Higher capital investment costs for these machine tools also provide clear return from improved results and shop productivity.
But these machine types place increased 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. The postprocessing for each of these subtasks is often well known, and the challenge is in combining the postprocessor sub-functions in a rational manner.
Multi-axis postprocessors have a much more intensive mathematical foundation. These solutions generally cannot be prototyped by "hand programming” and reading output NC instruction files is not always a good technique for reverse engineering the postprocessor functions.
The five axis demo
Generally, the CAM software industry does not perform the five axis demo in the same way as its predecessor three axis Demo. Many software vendors (not all) foresee challenges to develop good five axis machining instructions during a live demo; and they also have concerns about using unchecked post-processors. Further, in many systems, five axis calculation times can be much longer than with three axis programming.
But it does not have to be this way. The five axis tool path calculations and five axis postprocessor, at its core, are a series of basic mathematic equations. What is nice about mathematics, and one reason that I gravitated toward studies and a career in technology, is that I found great satisfaction in the simple statement "math works.” If you define the correct mathematical formulas and conditions to define a process or phenomenon, then any valid inputs assuredly lead to valid outputs.
The 5-axis post-processor provides controller syntax and language commands to the machine tool. Fanuc, Siemens and Heidenhain control the large predominance of 5-axis machine tools. Many others still emulate Fanuc language. Though the devil is in the details and most post-processors have unique requirements, five axis postprocessors can be developed with high confidence.
So the five axis demo remains a possible procurement option for the purchaser, but depends on the confidence and support of the software vendor. As with the three axis demo of days yore, a strong five axis demo, emphasized by live cutting is very impressive to the shop owner. any software vendors back-pedal quickly at the thought of a live five axis demo, without the ability to prepare every step to reduce their risk. Many software vendors also back-pedal when offered with the chance to actually cut a part on the customer's five axis machine.
The customer should pursue these demos, even if a consulting fee is required for this task. Marketing departments all make colorful brochures, and good engineers can guide a demo to produce the desired results. But a live test cut tells a story – interaction with the machine tool, cutting times and surface finish–that is clear to all.
Five axis post-processor sourcing
When purchasing five axis milling software, the prospect should understand the impact of the source of their post-processor. Interestingly, there are many ways to obtain a five axis post-processor. Machine shops should carefully consider these options as it may be a strong determinant of their success not only with the CAM software, but also the fundamental performance of the machine tool.
A basic way to source a five axis post-processor is through a technical chat-room or user forum. Users in different locations, often using nicknames (rather than real names) come together to share information that is used to connect 5-axis CAM software to expensive five axis machines. Consider the risks in this process compared to the five axis demo where the prospective software and post-processor are used and proven before they are purchased. Some end-users believe that post-processors should be free and that a chat room connects the user to a community of like-minded peers. Are these post-processors documented? Is there technical support with such a post-processor? How wise is it to obtain a free post-processor if the typical five axis postprocessor sells for approximately one per cent of the cost of the milling machine?
Other users want a toolkit so that they can build and adapt their post-processors? The customer gains control with this process. The counter-argument is that machine shops and their employees should be primarily focused on making parts for their customers, and not being a software developer. Today's machines have many options and complex logic – such as tool center point programming, tilted work plane, multiple coordinate systems, and invoking axis brakes. Professional software development companies should have experience and available libraries (from the same or similar machine types) to quickly implement a stable post-processor, and the customer can focus on programming, metal cutting and productivity.
Some machine vendors offer post-processors with their machine tools. They present these solutions as proven. True, the machine vendor has intimate knowledge of their machine, so the output commands from the post-processor can be confirmed to be compatible with a machine. But the input to a post-processor comes from the CAM software tool path processor. There are many CAM software products and each has unique intermediate file formats, which are rarely published for outside post-processor companies. Further, the intermediate file format specifications frequently change with new software versions, to enable new CAM software capabilities. Accordingly, post-processor utilities supplied by a machine tool vendor (or a third party through a machine tool vendor), may not be compatible with every CAM software product or each new product release. Too often, a customer may stumble with this approach after new software releases, until the "formatter” is reverse-engineered to meet the standards of the new software version.
The remaining and commonly used option is to obtain a post-processor from the CAM software company. In this approach, it is easy for the CAM software vendor to assure compatibility between the CAM software release and the post-processor. The potential open-end in this approach is for the CAM software to ensure compatibility to the machine tool. Here, the CAM software vendor can rely on similarities between machines that share a common controller, or can use a standard requirements document to obtain the specific commands for key functions on a machine. And of course, the syntax requirements for a machine control do not change year-by-year.
Not only should there be a defined protocol to obtain post-processor requirements, there should also be a standard process to test a post-processor. In only rare cases should a post-processor be considered plug-and-play. Even with the same software (post-processor input) and machine tool / controller (post-processor output), there may be many controller options or user preferences that require adaptation or configuration to the post-processor.
Multi-axis post-processors often do not get high visibility during the selling process, but should. Customers that overlook details about post-processors such as sourcing, service, and testing, often find out after the purchase that they should have focused more on the post-processor during the sales process. In today's high efficiency world, it is reckless to make capital purchases of machine tools and software, without having defined a clear plan for post-processor implementation.
Alan Levine is managing director, Open MindTechnologies USA Inc., Needham, MA.