April 19, 2009

Rough or Defective Surface

Rough or defective surface are smoothed by:
  1. mechanical polishing on rotating wheels or continuous, abrasive coated belts
  2. spin finishing
  3. vibratory finishing

Fissures, skin blisters, and other defects with a depth of 25 to 50 μm can usually be erased with these metal removal methods, when conditions are adopted to reach the bottom of the defects. Deeper defects are infrequent.

Rough or defective surface are often polished on wheels or belts, while parting lines are polished by the same operator. If polishing is mechanized to advance die castings attached to a conveyor through successive belts or wheels to polish different areas, a manual operation might be required later to complete the smoothing of parting lines if they are too curvy. The finish range from 0.2 to 0.4 μm, depending on the abrasive and the pressure.

Smoothing by spinning in abrasive is accomplished by attaching die castings to spindles or drums rotated with a peripheral speed of about 600 m/min in a slurry of abrasive material such as ground corn cobs or nut shells mixed with a small amount of grease or other lubricant. Time periods usually range from 5 to 10 min and the finish from 0.1 to 0.2 μm, depending on the abrasive.

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