Cutting speeds of 500 inches per minute are a reality with the new 15,000 rpm wye/delta VECTOR spindle system for high speed machining from Fadal Machining Centers (Chatsworth, California). An incremental design change, which includes a cogged belt, delivers power more efficiently to the 15,000 rpm spindle.
Both high speed material removal for aluminum and lower speed steel cutting are enhanced with the application of a 22.5 HP wye/delta spindle motor. To achieve better performance in high and low speed ranges, the spindle-to-drive ratio was changed from 1:3 to 1:2. Dynamometer readings at customer beta test sites indicate output of 34 horsepower at 15,000 rpm. Incorporating a cogged belt into the original design, which ensures positive delivery of power, eliminates slippage during either high or low range metal removal.
At high speed, friction caused by the spindle’s rotating elements transfers heat to the head casting and causes thermal growth and accuracy problems. In this new spindle design, spindle speed and accuracy are maintained over time by combining hybrid ceramic bearings with an air/oil injection system that helps stabilize spindle temperature.
Ceramic (silicon nitride) bearings in steel races feature less mass and higher stiffness, which enables them to maintain shape and require less inertia. Ceramic bearings have a heat expansion rate of about one-third that of steel, which prevents heat build-up at higher speeds so bearing preload is more constant.
A high-precision air/oil injection system lubricates the spindle bearings without excessive oil buildup for cool running at high speeds. The air/oil injection system injects a precise amount of steam turbine oil droplets into the air stream. The mixture is filtered through a 5-micron filter. As a result, less than 0.025cc of oil is used per 25 minutes. Oil droplets are injected into the air stream at a precise time determined by sensors. Excess oil is removed before it can load bearings and cause heat buildup and noisy operation.
The air/oil system prevents moisture from entering the spindle cavity with a constant flushing of bearings with new oil, which forces any extant moisture out. This extends the life expectancy of the spindle substantially. It also extends the useful duty cycle times because the spindle requires less warm-up or cool-down to achieve optimum operating temperature.
The horsepower available at high rotational speeds is doubled with Fadal’s application of a 22.5 horsepower wye/delta spindle drive motor. Productivity, accuracy and workpiece material capabilities are expanded with a wider range of torque, speed and horsepower, which enables low-speed, high torque steel cutting operations and high-material removal rates during high-speed, high-horsepower aluminum cutting operations.
A wider performance range is achieved by utilizing both the conventional wye and delta motor windings. The wye winding supplies high torque at low spindle speeds. The advanced delta winding delivers double the horsepower at high speeds, which eliminates upper speed range drag and facilitates high-speed machining. The result is a gearless electronic transmission, which automatically shifts from wye to delta windings to provide consistent torque at a given spindle speed setting and higher horsepower across a machine tool’s high speed range. A design characteristic inherent in these drives is the capability of maintaining consistent torque at a given speed under uneven load conditions, whether operating at 150 or 15,000 rpm.
Power loss at high speed, caused by back EMF (electro-magnetic frequency) generated in a standard vector-drive spindle motor at higher rotating speeds, is eliminated with wye/delta. Back EMF actually robs horsepower from the spindle, which causes high-speed tools to stall before optimum performance is reached. By utilizing Delta motor windings, back EMF at high speed is reduced, so more horsepower is delivered to the cutting tool.
The capability of the Fadal 15,000 rpm spindle to cut at 500 inches per minute will substantially reduce cycle time for parts cut on slower spindles. In tests, the new 15,000 rpm spindle can represent as much as a 55 percent reduction in overall cycle time. This is good news for the average CNC shop.