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Thursday, August 28, 2014

Machining Curved Surfaces with Accuracy and Speed

okuma_001When it comes to curved surfaces, smooth finishes are critical. Super-NURBS is a programming functionality for CNC machining that meets high-accuracy finish requirements while speeding up cycle times. The result is drastically reduced hand-finishing time to achieve high-quality parts quickly. Users can deliver parts out the door faster, with better finish quality and accuracy. Super-NURBS is suited for parts with high-accuracy requirements, such as those for mold & die and aerospace/defense.

The term “NURBS” is an acronym for Non-Uniform Rational Basis Spline. This was developed in the 1950s as a mathematical way to regenerate freeform curves and shapes such as car bodies and ship hulls. It became commonly used in most CAD software packages, along with CAM systems, and is part of industry standards like IGES and STEP files.

The “Super” denotes a higher degree of accuracy when the surface being machined has complex shapes or large changes of diameter within a short linear distance. With Super-NURBS, it comes down to controlling — and shortening — the feedback loop.

Super-NURBS is a proprietary technology for achieving NURBS functionality within a CNC machining environment. With Super-NURBS, the CNC control projects a predetermined tolerance band that creates a spline considered to be the best fit for the job. This eliminates redundancies and allows for a faster and smoother tool path (see diagram). With less data to crunch, movements are sped up — while maintaining a high level of accuracy to generate high-quality surface finishes.

super_nurbs_diagram

Click to enlarge.

Super-NURBS is able to deliver high speeds and quality finishes. This is due in part to its foundation in the mechatronics approach, the blending of mechanical and electrical development to assure optimized communication between the machine and its control. Super-NURBS also delivers the benefits:

  • Faster Control Loops Speed: The feedback loop determines how quickly the CNC machine can “talk” to the encoder and servo units. With Super-NURBS, the machine is faster in telling the servo where to move, and in receiving confirmation of getting information, allowing for more ultra-high feedrates to be used. At times, this can involve “talking” to as many as five servos simultaneously.
  • Digital-to-Digital: While controls are digital, many machine tools in the industry still use analog. This requires translation activity that slows down the CNC machining process. With machines outfitted with Super-NURBS, the control loops feed is digital-to-digital, which speeds up the communications process and therefore allows for faster machining. There is no need for a look-ahead within a Super-NURBS system because it’s inherently fast without it.
  • Machine Build Quality: The complete package of machine and control, including the machine’s build quality, enhances overall functionality. Super-NURBS benefits from machines with ribbed castings for strength, large base casting with three-point leveling, integrated ballscrew mounts to ensure rigidity, cantilevered column to reduce weight and resist bending, handscraped cast mating surfaces for superior flatness and contact, and a work area enclosed in sheet metal to eliminate hot chips and coolant from contacting castings, which minimizes thermal influences.

 

Impacts for the CNC Machining Environment

With complex curvature shapes, a CAM system will output a series of X, Y, and Z points (in a 3-axis example). It will create the curve using piece-wise linear movements. Therefore, in order to generate a curve, it will utilize many small linear movements. When left in that form, as part of the feedback processing loop, the CNC control will attempt to drive to each individual point and ask for confirmation if it made it to its intended destination.

The CNC machine may never reach it’s programmed speed because it can only crunch so much data before it has to start slowing down to maintain the feedback loop and keep the programmed accuracy. This is something machinists and programmers have no control over, as the machine control makes the decision on how fast it can go while maintaining the point-to-point accuracy of the shape.

With Super-NURBS, the control projects a tolerance band that is predetermined by the programmer through the series of points. Based on this, it will create a spline that is a “best fit” and may eliminate some of the points that are redundant to the spline path. This allows for a smoother tool path, and because points have been eliminated, the positioning feedback does not need to crunch as much data and can speed up the movements while still maintaining accuracy.

The result is controlled high-speed, high-quality CNC machining. With faster roughing speeds and better surface finishes, hand-finishing is potentially eliminated. It also adds a higher degree of accuracy for surfaces with complex shapes or large changes of diameter within a short linear distance.

As redundancies are eliminated, this allows for a smoother and swifter tool path. There are increases in throughput, since raw stock is processed in shorter time frames. The technology grants a greater level of control and effectively shortens the feedback loop. It potentially eliminates the long and costly step of hand-finishing, polishing, or grinding.

 

Jim Endsley is machining center product specialist for Okuma America Corp. Okuma is a worldwide provider of CNC machine tools. Headquartered in Nagoya, Japan, Okuma manufactures its own drives, motors, encoders, spindles, and CNC controls. Its product line includes vertical machining centers, horizontal machining centers, double-column machining centers, 5-axis machines, grinders, and wheel machines. For more, visit www.okuma.com/americas

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