Fast CNCs bring new face to machining.
What is needed, according to H Victor Mendes, CNC systems program manager at GE Fanuc Automation North America Inc, Charlottesville, VA, is a "fast" computer numerical control (CNC) to machine parts quickly, accurately, and smoothly. Here's how Mr Mendes says it works:
Machine-tool builders and users generally evaluate CNC speed through the time measurement known as block-processing time (BPT). This is the time required for a CNC to read each movement block of a part program and process it for use by the servo system. The shorter the BPT, the faster the CNC. A short BPT is a plus, even when the actual machining speed is relatively slow. Complex sculptured Surfaces on aircraft and automobile components and molds, for example, benefit from a short BPT. The programs that generate sculptured parts are created on CAD/CAM systems or digitizers. These programs may have hundreds of thousands of blocks, with each block corresponding to a very small movement of the machine tool's axes. As the movement commanded by a particular block is executed, the CNC already is reading the next block and preparing it for pulse distribution. The most rapid block execution will occur with the shortest movement command and the highest feedrate.
There may be times when execution of a current block is completed before the next block is ready for pulse distribution. In these cases, movement of all axes stops while the servosystem waits for the CNC to distribute the pulses for the next move. This is known as a dwell. Because the machining process is not continuous at this point, the cutting tool spins around aimlessly, machining the same spot. This is not good for the surface, and speed is slower than the commanded speed. To achieve uninterrupted machining, the BPT must be shorter than the execution time of each block. Consider a scenario where each block corresponds to a movement of 1 mm and the specified feedrate is 3 m/min or 50 mm/sec. This means that 50 blocks must be processed every second; that is, 20 msec per block. Therefore, a BPT of 20 msec or shorter is required for this application. If the BPT of the CNC is 4 msec, for instance, then the machining speed could be increased to 15 m/min and still result in continuous execution. The fastest CNC currently on the market can reach a processing speed of 0.25 msec per block, equivalent to 240 m/min for 1 mm incremental moves. This speed can be used only as a reference, however, because today's machine tools can't move that fast! To take advantage of a fast CNC in this case, the movement increments must be broken down into smaller segments. If 240 m/min can be achieved for 1 mm moves, then 24 m/min can be realized for 0.1 mm moves, or 3 m/min for 0.0125 mm moves. A reduced length of movement in each block will enable a more accurate approximation of the ideal curved surfaces that a user wants to machine.
When comparing the performance of different CNCs, it's important to remember that the BPT is not a constant value for a given CNC, but rather depends on several points:
* Number of axes. The greater the number of simultaneous axes, the longer it will take for a CNC to calculate the distribution pulses.
* Cutter compensation. When cutter compensation is active, the CNC must perform more calculation, usually increasing BPT.
* Look-ahead mode. The look-ahead acceleration/deceleration mode also requires a great deal of calculation, thus lengthening BPT.
In general, part programs that are likely to require a short BPT are quite long-often equivalent to several kilometers of paper-tape length. Programs this long normally are stored in a host computer and continually transferred to the CNC through a high-speed direct numerical control (DNC) interface.
It is critical that the transfer rate be compatible with the required BPT. If, for example, blocks with 20 characters are expected to be processed in 4 msec, then the data-transfer rate should be at least 50,000 baud. A fast CNC should be able to handle such high transfer rates.
For more information from GE Fanuc Automation, circle 372.
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|Title Annotation:||Manufacturing Solutions|
|Publication:||Tooling & Production|
|Date:||Dec 1, 1991|
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