CNC's fast moves

Manufacturing Engineering, May 1995 by Noaker, Paula M

Rarely is the control the limiting factor in a high-speed machining application. More common productivity roadblocks are machine tool dynamics, long NC programs, and the wrong speeds and feeds. o optimize metal removal rates, you can often use the CNC to navigate around the machining system's limitations.

The CNC functionality you require may not cost as much as you think. One reason, says Steve Manolis at control manufacturer Heidenhain (Schaumburg, IL), is that advances in functionality often offer lean-frog levels of productivity that far outpace the cost of a new control.

Additional savings come from using aids, such as distributed numerical control to feed programs to controls and real-time process monitoring technology. These help controls react fast to changes in the manufacturing process to reduce scrap and improve part quality. There also may be an advantage to rethinking the way you machine parts, for example, by using fewer, more versatile tools to hog out complex contours in aluminum blocks.

TIME TRIALS

Heidenhain's latest high-speed control is the TNC 426. Besides digital servo control, it has a block processing time of 4 msec and look-ahead capability of 126 NC blocks for feed adjustment. This is a substantial gain in functionality over the company's TNC 407, which offers 30-block look ahead and 25-msec block processing time. The specifications are impressive, but they aren't worth much if the machine tool isn't dynamically capable of moving at high speeds and feeds--or you don't know how to use them to improve processing efficiency.

For example, most control builders recommend examining several characteristics when specifying a control for high-speed work. Block processing speed is a good place to begin, but it shouldn't be the only variable used to compare controls. (Manolis defines a basic block in an NC program as the bytes of information required to control all the functions for one machining position or operation. A single program will contain a variety of block sizes.)

According to Bill Griffith at GE Fanuc Automation North America (Charlottesville, VA), control builders may provide two different block processing times, with the first measured from when the control accesses a part program from its resident memory and the second measured from when the CNC accesses a part program from an external device. "The first value is easier to determine than the second," says Griffith. "You must also understand how block processing time will vary based on how many simultaneous axes will be programmed in a block and other programmed conditions, such as cutter radius compensation."

Griffith suggests that end users also ask control vendors about other specifications that can bottleneck processing, such as the following:

* Servo sample time. The sample time of the servo velocity feedback and the position feedback must be faster or equal to the system's maximum block processing time.

* Maximum position feedback pulse rate. The maximum feed rate at which the axis can be programmed depends on both feedback resolution and the maximum position feedback pulse rate.

* Interpolation rate. This is the cyclical rate at which the main CNC processor passes commands to axis processors. If block processing time is 2 msec and the interpolation rate is 10 msec, there is a bottleneck.

Digital servo control also promotes high-speed control. "Some controls still provide analog output to the motors and servodrives," says Manolis. "In analog systems, there are two loops between the control and machine interface. The controller monitors the position loop. The velocity loop is monitored by tach feedback between the servoamplifier and the servomotor. The result can be slow response times in telling the servomotor to speed up or slow down."

With digital control, NC monitors both the position and the velocity loop. Manolis reports response is faster and resolution finer, especially in contouring. Heidenhain's TNC 426, for example, can maintain resolution of 0.1 micron, 10X better than analog-controlled NC.

When specifying a control for a new machine tool or for a retrofit, you should understand the two types of acceleration and deceleration provided by controls. According to Griffith, high-speed and high-precision machining applications require a combination of the two.

"To get the best path accuracy with Fanuc's Series 15 M," says Griffith, "the control must calculate acceleration and deceleration before doing any interpolation. Preprocessing this data is necessary to look ahead to detect a part's comers and curvatures.

"Acceleration and deceleration after interpolation are important to eliminate shock on the machine. Think of turning a corner. At the transition point, you need acceleration and deceleration to smooth the comer. Older machine tools require more of this acceleration and deceleration because they really aren't designed to accelerate at the rates required for high-speed machining."

Parallel processing also is important. On Fanuc's Series 15 MB, most control functions, such as graphics, communications, individual axis control, and PLC functions, have their own processor. These work in parallel with the 32-bit main processor and communicate over a common 32-bit bus. The reason is simple. Too many serial functions on a processor would slow it down.