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Finishing 2

Similar But Different

Centrifugal barrel finishing machines, or 1-larperizers, are similar in purpose to conventional barrel tumbling machines. Many of the same products can be deburred, radiused, or polished in either type of machine. But the operating principle of the Harperizer (made by Harper Buffing Machine Co. in East Hartford, Connecticut) differs significantly from that of conventional tumbling units.

In a Harperizer, two or more drums, or barrels, are mounted in a rotating turret (see photo and accompanying sketch). The turret rotates in one direction while the drums rotate in the opposite direction. The rotation of the drums causes the parts (the reeds) and the finishing media to interact; the retrograde motion of the drums produces a very smooth action within the drums.

The high centrifugal forces (up to 300s in certain applications) cause a small abrasive particle to do a relatively large amount of work. It was this high-energy potential that led Delco engineers to believe that Harperizing might be the solution to the problems encountered in radiusing the center hole in the R-4 reed.

After a series of tests at Harper's plant, Delco took delivery of an 18" Harperizer (two drums, each 18" long x 18" diameter). A series of tests showed the optimum cycle finishing to be 5 hours at 10 gs to radius the reed and 30 mm at 3 gs to polish it. That's quite a reduction from 90 to 96 hours!

Typical Finishing Cycles

For the R-4 reed and products similar to it, a typical load in an 18" Harperizer required fused aluminum oxide media; an abrasive, such as Novaculite; an additive, such as calcined alumina or glass shot beads, to prevent capillary attraction if the parts are thin and flat; a compound to prevent corrosion and to keep the media from glazing or loading with soils generated during deburring; water for lubricity and heat conduction; and the parts to be deburred or radiused.

For simple deburring and radiusing, Delco has found the following procedures most effective:

  • Run the machine for a predetermined time at the maximum permissible speed; then remove and inspect sample parts. If the deburring and radiusing requirements have been met, remove the parts immediately.
  • If the parts are to be given a final polishing cycle, leave them in the machine. But, for best results, dump the dirty water and rinse residual soils from the media and the parts. Then refill the drum(s) with all the products used for the original load, except for the abrasive. Minimize the time that the parts are exposed to flushing water to prevent corrosion.
  • When the drum has been properly reloaded for the cleaning or polishing cycle, run the machine at 5 0s for approximately 10% of whatever the cycle time was for the deburring or radiusing operation.
  • At the end of the cleaning or polishing cycle, remove the entire load from the drum by dumping it onto the removable screen deck that is attached to a portable hoist pan. With a water hose, wash away the waste products of the polishing cycle: this will also speed passage of the media through the separation screen.
  • When the polished parts have been separated from the media, dry them immediately or place them in a corrosion-inhibiting bath. In either case, handle them quickly to minimize spotting or corrosion.
  • Corncob media in the #8 to #12 size range is a very effective drying material.

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