Fulfilling the for need for speed

Aug. 1, 2001
Options keep VMCs at top performance.

Options keep VMCs at top performance.

Adding an automatic toolchanger, such as one from Fadal, can provide tool-to-tool change times as fast as 1.9 sec.

Variable-speed chip conveyors, such as those offered by Fadal, are an easy add-on to optimize VMC productivity.

With the pressure on most shops to reduce costs and increase productivity, every time saving feature on a machine tool counts. With the knowledge

gained from over 27,000 VMCs in the field, the engineering department at Fadal Machining Centers is continually developing time-saving options and features for its VMCs. Those options include rotary tables, coolant systems, high-speed spindles, toolchangers, and chip conveyors.

Rotary tables decrease cycle times by automatically positioning a workpiece so multiple dies of a part can be machined in one setup. They also eliminate mounting multiple workpieces one by one and than having to perform toolchanges for each individual workpiece. The result is increased unattended operation, so workers can perform other tasks.

Another aid in unattended machining is Fadal's auto-aim coolant system, in which an automatic coolant nozzle continuously directs coolant toward the tool center or tool edge. Or, the nozzle can aim coolant over the length of the tool to prevent premature wear caused by heat and chip loads.

For shops, especially those surviving on JIT delivery work, that need higher spindle rpms, Fadal offers a high-speed spindle. This spindle uses ceramic bearings and an air-over-oil metered lubrication system to enhance surface finish and quality, and boost cutting speeds.

Adding toolchangers and chip conveyors to VMCs in the field can increase machine throughput. Fadal's new 40-taper, 24-pocket dual-arm automatic toolchanger, for example, has a 1.9-sec tool-to-tool change time. The device uses a swing motion to pick up a new tool and an old tool simultaneously and makes the exchange in one smooth, continuous action.

"The speed of this dual-arm ATC increases throughput on large production runs, as well as on short runs with many toolchanges," says Barry Kasarda, Fadal's vice president of engineering. "In a production operation, every second of time saved on a tool change adds directly to the bottom line."

The ATC accommodates 3-in.-diameter tools or 4.5-in.-diameter tools if there are no adjacent tools. It handles tool lengths up to 15-in. and tools weighing up to 15 lb. An enclosed carriage protects tool shanks from debris and stops runout caused by chips and coolant. In addition, a locking drawbar packs 2,000 lb of force to hold a tool in place under severe cutting conditions.

"In a production environment, the last thing you want to do is shut down the machine to remove chips," says Kasarda. "A variable-speed chip conveyor puts an more spent every day Shoveling chips."

With variable speeds from 31 to 130 in./min, Fadal's conveyor, for example, keeps up with any size chip-making operation. The conveyor moves chips out of the machine and to a level where they can be caught in a 55 gallon barrel. In addition, the device features perforated conveyor hinges that let coolant fall back into the machine, minimizing coolant buildup in the chip container. A 1 /8-hp direct drive gives the conveyor adjustable forward and reverse motion, independent of the CNC control.

A matter of control
While certain options are effective, there is no area of greater impact on VMC speed than software and controls. It's a given that fast CNC processing speeds equal high productivity in machine operations. The faster the control processes a program, the faster a part is machined. But just what is processing speed? Different machine tool manufacturers define processing speed in different ways. Typically, processing speed is defined in blocks-per-second or linesper-second, which are not necessarily the same thing. Single blocks can be defined as one CNC word, such as X2 or Y2, or as many CNC words that will fit on a line of code up to the carriage return — depending on how one defines a "block." Fadal defines a block as one line of code with as many CNC words that will fit up to the carriage return.

A range of factors, including processor speed, CNC architecture, and the number of processors, influences how fast a control performs. For example, Fadal's 88HS control features up to nine processors with one processor dedicated to each axis. In addition, the processors use parallel processing, which lets the multiple processors perform multiple tasks simultaneously. The result is a 3,000-block/sec processing speed.

The key issue with block-processing speed is for the CNC to perform tasks fast enough to keep pace with the programmed feedrate of the VMC. And that becomes harder when a CNC's central processor is asked to carry out several tasks at once.

However, block speed isn't everything. Just as important is the time it takes the controller to direct the move. Toolpaths with tighter tolerances and higher densities have more points in the program. For a control to keep up with the feedrate under such conditions, it must have a high sampling rate, which is defined as the time it takes for the control to receive position feedback from the servo loops.

The control uses this data to determine the position target and velocity of each new servo command — the more complex a part the higher the point density and the greater the number of axis moves.

It isn't difficult to see how a processor could become overloaded with information. When too many tasks are required of a processor, it moves from one task to another, and some tasks have to wait. This, in turn, can lead to data starvation, which means the CNC cannot process a program fast enough to keep pace with the complexity of multi-axis moves. The result is a poor surface finish, long cycle times, and program shutdowns.

Another add-on that keeps cycle times short and maintains performance is tool-load-compensation software. Fadal's, for instance, recognizes various cutting issues, such as hard spots in the metal, and optimizes feedrates accordingly. The result is better quality parts with minimal monitoring and less tool breakage.

Mr. deCaussin works internationally for Fadal Machining Centers of Chatsworth, Calif., and is part of the company's founding family.

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