Equipment

There are two standard types of machines used in REP: short bar and long bar.

Short-Bar Apparatus. The short-bar machine (Fig. 3) accepts consumable anodes up to 89 mm (3 2 in.) in diameter by 250 mm (10 in.) long. The anode is held in a collet in a precision spindle, the head of which projects into the tank through a rotating seal mechanism.

Fig. 3 Short-bar PREP

Usually at least 80% of the length of a short bar is converted to powder. Electrode stub removal and the introduction of new electrodes to the collet is performed manually through a glove port that is located in the front of the machine adjacent to the cathode or plasma torch. Short-bar methodology is appropriate for converting experimental quantities of material, alloys that are inherently brittle, and materials that have a low specific stiffness where an electrode of long aspect ratio is not practical.

Long-Bar Apparatus. Increased productivity and conversion efficiency are realized in the long-bar operation (Fig. 4), which is designed to consume 63.5 mm (2 2 in.) diam electrodes that are up to 1830 mm (72 in.) long. Typically the assembly includes a precision spindle similar to that used in the short-bar machine. Instead of being fixed in position relative to the tank, the spindle is mounted on a table that also carries the drive motor and the electrical transfer brush mechanism. The table moves toward the tank from an extended starting position, feeding the electrode through the special seal-and-bearing assembly.

Fig. 4 Long-bar PREP

When the head of the spindle approaches the seal housing, the process is interrupted while the stub of the consumed electrode, now typically 230 to 250 mm (9 to 10 in.) long, is pushed through the seal by another long bar that is mounted in the spindle head when it is retracted to the starting position. Stubs produced in long-bar machines may be joined to new bars to obtain effectively 100% conversion to powder.

The rotational speed used is determined by the desired particle size. Standard speeds range between 314 rad/s (3000 rpm) and 1570 rad/s (15,000 rpm). Smaller diameter short-bars have been rotated up to 2620 rad/s (25,000 rpm). Because highspeed rotation rates are employed, electrodes must have precise dimensions; they must also be straight to keep mechanical out-of-balance forces to a minimum.

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