CNC Tip:

Q & A

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I work at Boeing as a Manufacturing Analyst in R&D. Could you give me a detailed explanation of a new word going around called a "nurb" in the machining world? Thanks, Keith D. Hanson

Response:

I'm not sure how detailed I can get, since I've never used nurbs interpolation. There was a great article on nurbs interpolation in a previous issue of Modern Machine Shop (check your back issues or log onto their site to see if it's still available). Frankly speaking, I feel that this feature is more of a band-aid for some of the problems related to machining sculptured surfaces than a long term solution. As you probably know, programs for five axis shapes can be very long. Some CAM systems (as well as add-on software) have the ability to massage the CAM system output, modifying the numerous tiny G01 motions into a series of (fewer) G02 and G03 commands. While this has nothing to do with nurbs, the goal with nurbs interpolation is also to minimize the length of extremely long programs.

Though I'm not familiar with the actual techniques being used, nurbs interpolation is a method of creating a series of movements for a three dimensional shape from a limited amount of input. I've heard mixed reviews when it comes to how accurately the shape being machined will be cut. And again, as true high speed machining controls become more and more popular (eliminating the problems associated with lengthy programs), nurbs interpolation will not be required.

Comments:

From Mike Koch of Euromach Precision Mfg. Inc.

O.K. fist of all I have never used nurbs interpolation so as to how well it works I can't comment. Basically when you create a model in a cad/cam system the data is stored as nurbs data ( Non Uniform Rational Basis Spline). This a mathematical way of storing your model (drawing), shapes are represented as lines surrounded by control points which have a certain assigned weight which controls the 3D shape of the lines.

Now when you create tool paths from your model the cam system converts nurb spline data, tooling data and so on into an "intermediate numeric code" ( using Surfcam lingo) which is then used by the post processor to create a g code program for your cnc machine. Typically doing 3d surface cutting you will get a program that is comprised of many very small linear movements (g01) that the machine control processes into arc moves. These programs can be huge, you can filter these moves into arc and plane moves (g02 g03 g17 g18 g19) however this supposedly reduces accuracy. What I have read on nurbs interpolation is that instead of posting g code you post nurbs code (which i have done just to see what it looks like) if your machine control supports it. The theory is that the nurbs data is more efficient in terms of information per line and the amount of processing power and time required by the machine control to do 3d surface cutting, resulting in greater accuracy and higher constant cutting speeds.( I read about this in a release from Sandvik on high speed machining.)