Full Speed Ahead With Nurbs

The common analogy used to explain how NURBS (Non-Uniform Rational B Spline) interpolation works goes something like this: In linear interpolation, a curve is approximated by a series of straight lines, and the cutting head must follow directions for a series of very short cuts. To exaggerate, imagine approximating a circle by using an octagon. To perform multiple short cuts requires immense amounts of programming code, and may require the cutting head to operate at less than maximum speed as it waits for the controller to process the programming.

In NURBS interpolation, the control can interpret curves as a smooth curve, so less data is needed to represent the curve and cutting speed is not hindered by the processing speed of the control. In addition, the resulting curved surface is more accurate because it is not made up of numerous straight lines with necessary deviation from the curve.

A Siemens press release title “Precision Mold and Die Making” may sum it up best: “Approximation by linear blocks is too time consuming for high speed profiling. Using NURBS-optimized part programs reduces processing time and provides for smoother contours.”

Scouting the recent CMTS show in Toronto revealed that Siemens Sinumerik 840D control is one of the few CNC machine controls available in North America capable of NURBS interpolation. GE Fanuc’s 15-B and 16iMA, and the Meldas 600 Series CNC from Mitsubishi Electric Automation also have this capability.

NURBS controls have only been on the market for a few years, but some Canadian moldmakers are taking advantage of the new technology. Dan Hannigan, area manager for Hascotech Inc., estimates there are at least five Okada machining centres in the Windsor area with NURBS-capable controls.

Not every mold shop needs this technology, but it is especially beneficial to any application where the control features are limiting machine speed or the accuracy of the part.

NEW APPLICATION OF AN OLD IDEA

NURBS is not a new term. It is basically an accurate way to define a free-form curve that CAD/CAM systems have used for many years.

Most CAD systems use NURBS as the default to define a free-form curve. The data is then passed to the CAM process (post-processing) to generate a G code formatted linear part program. The resulting part program sent to the CNC is generally several megabytes in length.

When a CNC has a NURBS interpolation function, post-processing to generate the linear part program is not required. The CAM process simply needs to format the NURBS data for the CNC. The resulting part programs are 1/10th to 1/100th the size of a linear part program, according to GE Fanuc.

In short, the advantages of NURBS interpolation, says Bill Griffith, CNC product manager for GE Fanuc Automation, are that part programs are much smaller, more accurate parts require less hand finishing, machines are not limited by block processing time, and post-processing is simplified.

In a paper presented to SME, Griffith explains: “By eliminating the process of approximating the curve with linear segments based on the tolerance value, and substituting the new CAM process of maintaining NURBS data, the accuracy of the part will improve. In addition, improved performance will be realized by the curve being a smooth curve versus a curve made up of many small linear segments. This performance improvement is due mainly to the improvement of acceleration/deceleration time.”

GET NURBS WITHOUT BUYING A NEW MACHINE

There are options other than buying a new high-speed milling machine to get the benefits of NURBS-capable controls. The controls can be retrofit to existing machines, or software add-ons for existing controls can provide similar benefits.

If you choose to retrofit the control, be aware that you may have to modify your CAD/CAM processes as well.

CAM processes and software add-ons fall into one of two categories, says Griffith. One type takes the linear segmented part program and converts it back to a NURBS formatted part program. With this type you will see many improvements, but the improvement in accuracy will not be as great, he notes.

The second type retains the original CAD-generated NURBS data and achieves all of the same advantages as NURBS interpolation, within the limits of your current CNC.CPL