Machines turbocharge fuel efficiency

Sept. 11, 2005
Five-axis machining supplies turbocharger impellers at a cost 90% less than typical.

When machining impellers from solid aluminum, Moore FSP 300x HMCs at Taps remove over 50% of part weight, complete impellers in about one hour, and hold volumetric tolerances in the 0.001-in. range.

Table-on-table rotary designs of Moore FSP 300x HMCs eliminate offset weights of trunions to boost toolpath response and accuracy during complex contouring cuts at Taps.

Moore tunes its FSP 300x HMCs and their controls to minimize harmonic effect, vibration and jerk.

If it weren't for Turbocam Automated Production Systems (Taps), most new cars and trucks wouldn't pass efficiency and emission standards. That's because the shop, in Barrington, N. H., is one of few that supplies the high-precision turbocharger impellers that automakers say are key to meeting today's stringent national standards. Through advanced fiveaxis machining, Taps cranks out hundreds of impellers each day, cut from solid aluminum at prices competitive with old-style cast impellers yet to much tighter tolerances. In addition, Taps impellers cost 90% less than other typical machined versions, which wins the shop a lion's share of this first-tier market.

Like most turbomachinery parts, the impeller is a 100% five-axis workpiece with virtually no straight cuts or pure radii. Roughly conical, it has numerous airfoil-shaped vanes radiating helically from a hub and tapering axially. Every split second during impeller machining, cutting paths change along all machine axes, and thus, each axis is constantly in an acceleration/ deceleration mode, which results in substantial G-forces.

At the heart of Taps' grueling, five-axis, impeller-machining operations are four FSP model 300x HMCs from Moore Tool Co. of Bridgeport, Conn. The machines boast fixed-position horizontal spindles that move along X and Y axes only. Restricting spindle motion to horizontal movements only, without changing the spindle's extension, increases rigidity and improves vibration dampening under the high inertial forces that are typical in impeller machining.

With table-on-table rotary designs, the machines eliminate offset weights of trunions to boost toolpath response and accuracy during complex contouring cuts at Taps. Having horizontal spindles also means the machines quickly clear away chips, so they aren't double cut.

Large, cast-iron, box-in-box bases with three-point leveling further enhance stability and vibration absorption. The machines contour at feedrates up to 787 ipm, rapid as fast as 1,181 ipm, and accelerate in five-axis modes at 653 in./sec2, or 1.7 G.

The FSP 300s also incorporate highgain, closed-loop digital servo systems for fast positioning, high feedrates, and quick accelerations in all axes. Moore achieves such gains through the nature of the machine's design and by tuning the machine and controls to minimize harmonic effect, vibration, and jerk to levels "below the radar" of the machine's Siemens Sinumerik 840D control system. According to Rob Smith, a manager at Moore, "with a less-rigid machine running at the speeds Taps does, the control system would almost certainly become dynamically unstable at such high gains."

He says the Siemens control delivers high machining and processing speeds and true five-axis operation. And it does so without any staking of axes, which slows down complex contouring work.

The 840D control directs 10 channels, 10 mode groups, and 31 axes. To meet high demands on axes and channels, NCU computing power and storage areas are expandable and can be combined. This lets the 840D join up to 248 axes and spindles in one CNC.

The first FSP 300 that made its way to Taps currently does prototype and production machining for one of the company's highest-volume parts, a 4-in. impeller cut from 2618 aluminum. With a Moore-designed shuttle feeder, the machine runs unattended, completes impellers in about 1 hr, removes over 50% of part weight, and holds volumetric tolerances approaching 0.001 in.

Taps qualified the machine by test cutting a 10-in. impeller that required seven contour cuts and seven toolchanges to machine it from solid. The FSP 300 completed the part in under 3 hr., and all surfaces were well within dimensional and finish specifications. Depending on the cut, feedrates ran from 190 to 282 sfpm, while spindle speeds ranged from 24,000 to 40,000 rpm.

"This is lightning fast five-axis contouring by any measure," says Andrew Hussey, Taps engineering manager. "FSP 300x performance matches that of the best European five-axis CNCs and is typically higher than any U.S.-made CNC we've seen."

Based on the success of the first FSP 300, the shop acquired three more to run the same impellers. Machining results are so good, it takes just two employees to hand finish the output of all the machines running around the clock.

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