Home>ASTM Standards>ASTM B252-92(R2020) pdf free download

ASTM B252-92(R2020) pdf free download

ASTM B252-92(R2020) pdf free download.Standard Guide for Preparation of Zinc Alloy Die Castings for Electroplating and Conversion Coatings
6. Smoothing of Parting Lines
6.1 Parting lines are smoothed by (1) mechanical polishing with abrasive-coated wheels or belts, (2) tumbling with abra- sive media, or (3) vibration with abrasives. 6.2 Abrasives with a size range of220 to 300 mesh glued on cloth wheels or continuous cloth belts that run over flexible back-up wheels are usually used for mechanical polishing of parting lines. Wheel diameters range from 5 to 40 cm, depending on the complexity of the shape. Wheels are rotated with a minimum peripheral speed of 2500 m/min. A peripheral speed of2100 m/min should not be exceeded with belts. Lower speeds of the order of 1100 to 1400 m/min are fairly common for small die castings polished on small diameter wheels. Abrasive belts should not be used dry but should be lubricated with a small amount ofgrease. Die castings usually are handled individually to polish parting lines smooth. This may require 30 s or less for small castings, and sometimes 5 or 6 min for larger ones. 6.3 Tumbling in horizontal barrels, loaded with abrasive stones such as limestone, preformed and fused aluminum oxide, ceramic shapes or abrasive-loaded plastic chips, and a lubricant such as soap or detergent solution, removes parting- line burrs from die castings in 4 to 12 h. The barrels may be rotated at 4 r/min. Higher speeds reduce the time cycles and costs, but also increase the danger of impingement of parts against zinc surfaces. A hexagonal barrel with a capacity of 0.5 m 3 can be loaded with 450 kg of abrasive stones or chips and 90 kg of zinc die castings.6.4 Vibration in a bed of resin-bonded abrasive chips removes parting-line burrs, typically in 1 to 4 h. Frequencies range from 700 to 2100 cpm and amplitudes from 0.8 to 6.4 mm. A vibrating tub with a capacity of 0.5 m 3 can be loaded with about 900 kg ofabrasive media and 180 kg ofzinc die castings. A dilute solution of detergent or soap is continu- ously metered through the bed of media and parts to keep their surfaces clean and maximize surface smoothing. Parting lines may be mechanically polished before vibratory processing when a large amount of flash must be removed.
7. Smoothing of Rough or Defective Surfaces
7.1 Rough or defective surfaces are smoothed by (1) me- chanical polishing on rotating wheels or continuous, abrasive- coated belts, (2) spin finishing, (3) vibratory finishing, or (4) controlled shot peening. Fissures, skin blisters, and other defects with a depth of 25 to 50 µm can usually be erased with these metal-removal methods. Deeper defects are infrequent. 7.2 Mechanical polishing for smoothing rough or defective surfaces is similar to mechanical polishing for smoothing parting line areas (see 6.2). Parting lines and rough or defective surfaces are frequently 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 may be required later to complete the smoothing of parting lines if they are too curved. The finish ranges from 0.2 to 0.6 µm, depending on the abrasive and the pressure. 7.3 Smoothing by spinning in abrasives 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. Times usually range from 5 to 10 min and the finish from 0.1 to 0.2 µm, depending on the abrasive. 7.4 Vibrating tubs loaded with plastic chips (such as poly- urethane) impregnated with an abrasive (such as aluminum oxide) smooth the surfaces of die castings in 2 to 4 h when frequencies are in the range of 1700 to 2100 cpm and amplitudes are adjusted to 3.2 to 6.4 mm. Vibratory machines produce a finish of 0.15 to 0.25 µm, with a cutting rate of 5 µm ⁄h. A smoother finish of 0.075 to 0.125 µm can be obtained with plastic media containing finer abrasive, which removes metal at a slower rate. Media and zinc parts are usually loaded with a ratio of 5:1 or 6:1. Surface gouges may occur with a smaller ratio.

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