5/1/2002 | 9 MINUTE READ

From Art to Part With CNC Machining

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If you're a manufacturing company with numerically controlled (NC) machines, you've probably immersed yourself in the whole CNC programming - CAD/CAM thing. Remember when all your machinist had to do was click the spindle on and off, watch the cycle times and feedrates, change the tool and activate the clamp - manually. Well, today's NC machines are no longer islands unto themselves - they are driven by sophisticated software and hardware systems that have helped industry jump into the 21st century.


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Building these automated machining structures has been complex at best because there always has been a mismatch between software system lifetimes and the information they handle. This problem can get even more complicated with engineering software because CAD and the various analysis systems can change their programs every few years. But blueprints - like those for automobiles and aerospace - must be archived for decades. Storing this type of information with software system "languages" that may not be here tomorrow clearly isn't a solution, and can be very expensive to manufacturers.

To help solve the problem of translating a company's data language and speed along the product development process, the STEP software-integrated process was created (See STEP-ping Up to the Plate sidebar). Within today's art-to-part environment, product data can be managed through many different CAD/CAM/CAE systems with little integration. All or part of original CAD data is recreated from system to system in order to maintain design intent, with CAD systems continuing to have difficulty recognizing non-native CAD formats. But STEP has begun changing all of that.

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Picture this - a job shop downloads a 3-D model in the STEP file format from a website. The STEP file provides direct input for the machine tool production information such as geometry, features, machining steps and toolpaths. This results in a reduction in setup, prep and machining times.

"Today's machining tools are driven by NC programming based on codes developed a half century ago for paper tape readers," says Martin Hardwick, president of STEP Tools, Inc. (Troy, NY) - a software integration company. "As a result, CAM programmers go through a lengthy process to translate part geometry, generate toolpaths, determine feeds and speeds, as well as select tool sizes. In effect, they have to think for the machine tool - a high-tech product driven by low-tech codes. STEP has changed the programmer's world."


CNC Toolpaths

Creating toolpaths for CNC machines can be quite an undertaking for a company. For programmers, creating toolpaths that are safe and produce production parts in a cost-effective way has always been the goal. As with everything complex, however, there is always room for error. Granted, software has made toolpath creation easier, but the need still exists to test and verify programs before production machining begins. After creating the toolpath, it is converted into commands that are specific to a particular NC machine.

But a number of mistakes can occur at any time during the tooling process:

  • The programmer can inaccurately input data or cause an error through incorrect use of the CAM system.
  • The CAM can produce errors in the toolpath.
  • The post-processor can introduce errors or output code unsuitable for the machine's control.
To combat this, several toolpath verification techniques have surfaced to help machine parts safely and correctly. The old way was to manually scan the NC data, which was time-consuming and prone to errors. Prototype machining - using wood, plastic and other materials - tends to be costly and can lead to machine tool collision. CNC simulation technology, which can verify part accuracy on the computer at the programming stage, has become the most popular technique that manufacturers take advantage of when doing parts.

While many manufacturers still perform production testing, software can greatly reduce the many steps of toolpath testing. With this in mind, different levels of sophistication for toolpath verification have been created to help the programmer and manufacturer do their jobs more effectively.

3-D simulation enables users to actually see what will occur on the machine and how the part will take shape, rather than imagining it. The next level is verification, which enables users to detect errors in programming techniques. Fast feeds and potential collisions can be detected and eliminated during the early programming phase. Detailed analysis like this enables the user to compare original design with the finished machined part to reveal gouges or undercuts. Verification software also enables the user to send CNC programs to the shop floor that are not only proven, but also are optimized with the best possible feedrate information.

The software does this by automatically analyzing the machining conditions and the amount of material removed by each cut, and then giving the best feedrate for that particular cut. The result is reduced machining time, enhanced part quality, longer machine tool life, and less wear and tear on the machines. "The machinist can be confident that he can turn his back and know the CNC will not crash on him," says Jeff Werner, marketing communications manager at CGTech, Inc. (Irvine, CA) - a company specializing in software solutions for NC manufacturing. "Verification software can make sure that the toolpath is accurate and efficient before any machining is done."


CNC Is People

Starting CNC at your company does not necessarily mean replacing a machinist with a computer, because you can't do anything with CNC that you can't do manually as well. One of the most unappreciated factors in good CNC parts production is the people. Industry leaders may be lead to believe that as long as they have a computer model and a CNC machine, good finished parts are just a given. But when you don't always have a 3-D model - like in automaking or airplane manufacturing - what are you supposed to produce?

The only reference may be vintage blueprints from those customers. If it's a linear part made up of mostly flat sides and holes, a CNC programmer will type the code in manually. There are software solutions, but depending on the part's complexity, it's not always practical to redraw the item. Though there is the chance for human error, it does allow the programmer to develop a familiarity with the part. You still need the machinist to come up with a good setup, select the best cutting tools based on the setup and/or material and trouble shoot any problems that may arise.

"A good machinist is necessary for tool setups, especially when CNC is involved," says Ron Gurr, plant supervisor for Fabris PG (Stony Creek, Ontario) - a division of Fabris, Inc. "Any competent person can touch-off tools (measure cutting tool heights) and load a CNC program into a CNC machine tool (lathe or mill) with a lead hand overseeing the project. But when a problem arises, be it a program, tooling or machine, it is imperative that properly trained people are on hand to solve the challenges. Experienced machinists will set up and work out the 'bugs' on a job, so that a junior worker can take over production and produce quality pieces. CNC doesn't replace the machinists - it only raises the level of the technology with which they work."