Aerospace Expertise


Understanding the Basics CNCs do exactly what they’re told to do. Like most com-


puters, there isn’t a magic feature that turns a computerized device into something that knows what’s in the user’s head. Te CNC will only do what the programmer instructs it to do. CAD/CAM systems generate a series of points based on a


tolerance defined by the part programmer. Tese points typi- cally are not exactly on the surface of the part as designed, but they are within the defined tolerance band from the


CAD/CAM systems, and the existing approximated series of points can work with limited to no modification depending on what CNC features are activated. Modern CNCs have advanced features that interpret the


series of CAM points differently. As described above, with no CNC advanced features used, the result is a linearized approximation. However, if advanced lookahead algorithms are used, the CNC can interpret the series of points as a definition of a


surface of the part. Te majority of part programs are simply an ordered series of points defined by the CAM system refor- matted in G-code.


Connecting the Dots With no additional information or advanced CNC features


being implemented, the only thing a CNC knows to do is con- nect the dots defined by the CAM system. Without advanced features, a CNC’s function would be similar to a child with a crayon connecting the dots in a simple dot-to-dot drawing. Te higher resolution dot-to-dot drawings have closer spac- ing which can result in a more accurate representation of the object, but the end result is still lots of straight lines between points, as in the top diagram here. Te advanced technology needed to be more accurate than


a child’s simple dot-to-dot drawing has been available for a de- cade or more. When using advanced CNC capabilities, there is no additional investment in soſtware systems or existing


120 Aerospace & Defense Manufacturing 2014


smooth curve. Te result is an optimal, more accurate tool- path, as shown in the second diagram above. Because the features use the abilities of the CNC and servo


system to interpolate a single curve rather than numerous point-to-point moves, the time to produce the high-quality part will typically be significantly less and the surface will be smoother and more accurate. As an example, on a FANUC CNC the advanced lookahead feature is called “High Speed Smooth TCP”. A racecar example is an easy way to visualize the speed


difference: Run the same car on the same track without going off the road or planned route. In the first lap, the car drives in straight lines with sharp changes of direction whenever a line nears the edge of the road. Tis is an example of a CNC with- out advanced features. Ten, in the second lap, the car follows the smooth curve. Te smooth curve lap takes significantly less time and the car can achieve higher speeds, similar to a CNC using advanced features.


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