Search for the perfect post processor: is it CAM's soul--or its Achilles' heel? (Software Solutions).
Despite its vital role in communication between programmer and the factory floor, many post-processors are not very good; this is due to the technical complexities they deal with. The number of features in a CNC ranges from several hundred to several thousand. Each feature accesses some capability, function or operating peculiarity in a machine tool. To fully use the machine tool's flexibility and power, the post must address every one of those CNC features.
The search for the perfect post is complicated by the fact that no two identical machine tool-CNC configurations exist. The number of possible combinations is nearly incalculable. It's not surprising, therefore, that there is a high frustration level with posts.
The users' edge
At Pathtrace Systems Inc. (and its parent company Pathtrace plc, Reading, England), we've given much thought to these difficulties. We believe the people who use CAM systems and program machine tools create the best posts. But until now they have not been given the proper tools. We believe these tools should be built into the CAM system. At Pathtrace, this is done with the EdgeCAM Code Wizard.
CAM vendors supplied the first posts. They were standardized for the most popular machine tool and control combinations. CAM and CNC vendors still offer these posts in libraries but only the basic functions are covered. When users bought high-function machine tools and advanced CNCs, they had three choices:
* Pay thousands of dollars for a unique custom post and wait a year. During that time, key capabilities--often the ones for which the machine tool and control were bought--would effectively be out of reach.
* Comb CAM vendors' libraries of existing custom posts for a match that would work well enough. Paid for by other users, these posts now number in the thousands. Even so, some of the newest CNC and machine tool features might be left out.
* Tailor a vendor's custom post to get a very close match. This approach lets users add support for new features to a proven post. Sometimes called "configurable" posts, these offer the user a choice of a question-and-answer format or script editing. The Q&A has proved somewhat inflexible in configuring the post to exact requirements. The script approach was handy, powerful and flexible. "Power users" welcomed it, but most CAM users found the scripting languages difficult to learn and prone to errors.
A new approach
There is a better way to access all the power of a machine tool and its CNC: Start with a generalized and thoroughly tested template. We believe that the best approach is a true Microsoft Windows-style wizard, a software "applet" that gathers data into a predetermined format. If implemented correctly, users can generate usable posts right out of the box.
Using a true wizard means that a post can be created with a few dozen menu picks. These are fleshed out with machine tool and CNC data on axis travel limits, control of dogleg rapid traverses, tool change positions and the locations of Clear and Home. All are found in the vendors' reference materials. Much of the user's work is drag-and-drop. And since typing is minimal, most of the tedium of debugging a post is eliminated.
Many production-machining operations need only two posts: one for 2- or 4-axis turning and one for 3- or 5-axis milling. Differences between machines in operating details can be handled with the template.
The generalized template approach solves many post-processor problems:
LOW COST--The template capability can be built into the CAM package. Technical details can be presented in easily understandable form; once training is completed, outside assistance may not be needed.
SPEED--Most users soon learn to create a complex post in two hours and a simple one in five minutes. On average, we find that a wizard-like post-processor template holds about 70 percent of the functions of a custom post for a given machine tool and CNC configuration. This includes the basics such controlling the X-, Y- and Z-axes; linear, circular and helical interpolation; spindle speeds; tool changing; and coolant sequences.
POWER--The wizard template can accommodate differing control parameters and multiple levels of "firmware" built into the CNC. These can result in very different posts for seemingly identical machine tool and control configurations. The template also has the formatting flexibility for company-specific or shop-specific preferences.
EASE OF USE--Wizards avoid hard-to-learn proprietary scripting tools. Users rarely create more than a few posts, and those infrequently, so scripting skills are easily lost. Ditto the reference documentation, if any. This means users have to renegotiate the learning curve each time a new post was needed.
OWNERSHIP--If the templates are built into the CAM package, the posts are created on the user's system. They can't be resold without the user's permission. "Recirculated" posts, paid for by users, are the bulk of some CAM vendors' libraries of offerings. Needless to day, no royalties or commissions are paid.
SUPPORTABILITY--The wizard approach to templates enforces consistency, greatly simplifying support and upgrades. Each upgrade and service pack contains new templates. An on-screen update button alerts users, and the new templates can be imported automatically. This helps keep post-processors current with the latest productivity boosters.
This approach also allows selecting the post when programming begins--not as an afterthought. That allows the machining environment to be configured to maximize the machine tool as the toolpath is generated. The up-front selection of a post also simplifies moving a job between machine tools.
The search is on
The most important thing in a postprocessor is getting a good fit so the search for the perfect post is ongoing. It's no different than the search for the perfect fit in any technology or, for that matter, any tool.
Today's highly intelligent CNCs have dozens of automatic subroutines or "canned cycles," all of which are addressed very well with wizards. These innovations make the NC programs shorter, easier to understand and quicker to simulate and verify. These programs also run more efficiently in the CNC-meaning fewer unexpected halts in machining. Templates can even generate posts that can see into subroutines, helping programmers and machinists keep track of the G- and M-codes invoked.
Machine tool builders have added many innovations in recent years such as mills with a rotary axis, lathes with driven tooling and similar advances. Though complex, posts for both can be handled within the wizard environment.
Basically, the post is a tool to map these capabilities and features to the CAM system. From the limited viewpoint of information technology (IT), it is true that the post really does post-process the CAM system's output, the tool path. But it does so much more. The post is the key to the interoperability of the CAM system and the machine tools it programs.
Users should be especially wary of any post-processor vendor who says: "If we don't have it, you don't need it."
Pathtrace Systems Inc., Southfield, MI, www.rsleads.com/202tp-181 or circle 181
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|Publication:||Tooling & Production|
|Date:||Feb 1, 2002|
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