As CPU speeds have increased and CNC manufacturers have incorporated this speed into highly integrated CNC systems, there have been phenomenal changes that increase CNC performance.

The faster, more responsive systems do more than just process program blocks faster. In fact, a CNC system that can process part program blocks at a very high rate may perform as well as a system that processes data at a slower rate, because there are other potential bottlenecks downstream that the overall feature content of the CNC system also has to address.

  Most mold shops today intuitively understand that high speed machining requires more than just block processing time (BPT). In many ways, the analogy of a race car illustrates why this is so. Should the fastest car win the race? Even a casual observer of racing knows there is more to it than this.

 First, the driver's knowledge of the race track is important. He has to know a sharp curve is coming so he can slow down just enough to take the curve safely and efficiently. CNC look-ahead performs a similar role in high-feed-rate mold machining, giving the CNC advanced knowledge of the sharp curves coming up.

  Similarly, how quickly the driver reacts to what other drivers do, and other unpredictable effects, can be compared to the CNC's servo loop times—including position loop, velocity loop and current loop.

  Consider also the smoothness of the driver's execution as he goes around the track. Skillful braking and accelerating have a significant impact on performance. Bell-type acc/dec in the CNC system gives similar smoothness to machine tool acceleration. Look-ahead helps here as well, because it allows many small acc/dec adjustments to replace an abrupt acc/dec change.

  The analogy also applies in other ways. The power of the engine can be compared to the drives and motors. The weight of the car can be compared to the mass of the moving elements of the machine tool. The strength and rigidity of the car can be compared to the strength and rigidity of the machine. And the CNC's ability to maintain a specified path error can be related to how well the driver keeps the car on the track.

  One other way the analogy relates to the state of CNC today is this: A car that isn't one of the very fastest may not need the most skilled driver. In the past, it was only high-end CNCs that could maintain high accuracy at high speeds. Today, mid-level and low-end CNCs are so powerful that they may also do an acceptable job.

  The high-end CNC still offers the best available performance, but perhaps for the machine you have, the lower-level CNC will permit the same performance as a CNC at the top of the line. It used to be that the CNC was the limiting factor determining the maximum feed rate in mold machining, but today the limiting factor is the mechanics of the machine. A better CNC won't deliver more performance if the machine itself is already operating at its performance limit.

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