In the production of manufactured goods made of metal, surface-finishing operations may be necessary at various stages. Original surfaces may need to be prepared for further processing—an example would be the descaling of a forging before it is machined. An intermediate surface may need to be cleaned or polished before final treatment, and a final surface often must be protected from corrosion—perhaps by painting or electroplating, which also serve as decorative effects. Surface finishing may be divided into preparatory treatments, which are mostly metal-cleaning operations, and final treatments, which offer protection, decoration, or an extremely hard or smooth surface to prevent failure in service—for example, the heat treatment and grinding to precise size of bearing surfaces on machine parts.
Fettling and descaling
After forging or casting, metal objects must often be treated to remove casting sand, metal scale, and rough edges before they can be cold worked by machine tools, otherwise dimensional accuracy may be difficult to achieve.
Grinding and wire brushing are undertaken with hand-held tools that are driven pneumatically or electrically. In barrel tumbling, the articles are tumbled with small, star-shaped pieces of cast iron in a barrel rotating at about 10 or 15 rpm, creating a burnished surface.
Various forms of blasting techniques have been developed to clean and smooth components at some stage during manufacture. The different techniques used are commonly referred to as sandblasting and shot blasting, although sand is no longer used as an abrasive—silica sand, while inexpensive, generates toxic dust that gives rise to a risk of the lung disease silicosis. Generally, a stream of abrasive particles is directed over and against the part at high speed so that the abrasive impact cleans the surface thoroughly.
A wide range of abrasive media is available to suit different cleaning and finishing applications. Iron and steel grit are used for rust and scale removal and for deburring, while alumina abrasives offer fast cutting for surface cleaning. Steel shot gives a fine surface finish and is used for surface peening; glass beads have a similar but more gentle action. Plastic parts are deflashed with soft blast abrasives such as nylon and crushed fruit stones, which are also used for fine cleaning and paint removal.
Typical applications include the removal of sand cores and scale from castings before machining, surface preparation for painting, cleaning of used components for servicing or refurbishing, and the deflashing of plastics moldings.
Shot blasting consists of directing a steady high-velocity stream of shot (round or angular pieces of chilled cast iron) at the article to be cleaned. The stream is directed at the object through a flexible tube. Small objects are placed in a chamber that may have a window through which the operator can monitor the cleaning on a one-time-only basis, or the process may be automated with fixed nozzles in mass production.
The wet process of cleaning castings uses water jets at a pressure of 80 times the atmospheric pressure and a velocity of 186 mph (300 km/h). The electrochemical method of cleaning employs an alkali liquor in an electrolytic tank.
Descaling is also accomplished by pickling, which consists of immersion of the metal stock in a solution of 5 to 10 percent sulfuric acid at 140 to 176°F (60–80°C). Because of its relative slowness, pickling has declined in favor of shot blasting for cleaning forgings. Pickling is also used, however, to clean hot-rolled stock in the steel mill. Whenever tight dimensional tolerance and smooth surface finish are required from metal stock, cold rolling is necessary, because it gives a smooth, dense surface. Pickled hot-rolled stock is often finished to size by cold rolling. To prepare hot-rolled product for cold rolling, to remove an oxide layer after hardening, or to roughen the surface of strip metal for special applications, a reel-to-reel strip grinder is sometimes used.
