Advanced Manufacturing: Rapid Roughing of Tough Materials

March 3, 2009
A nontraditional “hybrid” machining method with roots in the aircraft industry speedily rough-cuts tough materials such as Inconel and titanium

Traditionally, aircraft turbine engines used complex rotors made up of individual blades or airfoils bolted or friction welded to a slotted hub. Some aircraft OEMs replaced this assembly with a one-piece “bladed integrated disk,” shortened to “blisk,” to reduce engine weight, aerodynamic losses, and engine complexity. Blisks are usually roughed-out by CNCs, electrochemical milling (ECM), or electrical-discharge machining (EDM), and then hand polished. A more recent and faster roughing approach comes from so-called “blisk” machines that combine processes similar to ECM and EDM into a proprietary technology called BlueArc, (generically known as ECDM).

It all started when GE Aviation in Evandale, Ohio, asked KRC Machine Tool Services in Independence, Ky., to build two machines for ECDMing blisks out of solid Inconel billets. Recall that EDM removes metal by placing a wire, or an electrode — a shaped graphite tool (anode) — near, but not touching, the workpiece (cathode) which is connected to a power supply. This action produces a rapid series of repetitive sparks that gradually erode the metal workpiece to the correct shape. A nonconducting solution flushes away debris. ECM, on the other hand, is another non-contact method that “machines” the workpiece using a chemical reaction similar to reverse electroplating.

GE Aviation provided the proprietary software and a variable-power supply that outputs 0 to 70 V and 0 to
500 A. A well-known machine-tool manufacturer provided the machine base, ball screws, and toolchanger. The vertical machining center also came with linear ways on all five axes and direct-coupled servomotors, giving it the needed rapid accel/decel in feed rates. KRC modified the 30-unit toolchanger so it could accommodate the
7-mm-diameter, 30-in.- long copper rods. The intent was that fully loading the toolchanger would let the blisk machine run for hours, nonstop, until all the tools were consumed.

The shop also built the spindle, electrically isolating it from the rest of the machine. This was needed because a positive power supply travels through the spindle and the rod (here, the part is negative). Machining the complex blisks necessitated tilting the spindle, so the shop mounted it to a rotary table on the Z axis. This arrangement can accommodate blisks that weigh up to 800 lb.

Because the tools are so long and would whip when spun in the spindle, the shop also had to devise a bottom support — a custom air bearing — for the rod to feed through. The bushing on the bottom side is a coolant ring as well, with coolant nozzles all the way around. It creates a “halo blast” in the heat-affected zone, right where the tool weaves around the part. The coolant system includes a 4 × 4 × 4-ft stainless-steel tank. Coolant also pumps at 100 psi through a hole in the center of the rod.

The upshot is that at about 40 ipm, the blisk machine runs twice as fast as ECM and five times faster than a conventional roughing mill. GE granted KRC a license to make and sell the technology worldwide.

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