Final polishing of part surfaces is carried out mechanically through the use of abrasive wheels and tapes with polishing pastes or through the use of round type vibratory machines. Polishing paste application makes it possible to reach roughness value equal to Rа=0,05–0,12 microns. At the same time polishing process usually involves manual labour.
For polishing such special equipment as grinding machines is used. These machines are equipped with elastic felt and cloth wheels, which in turn are to be treated with polishing pastes. The last ones consist of micro powders, binding and surface-active agents and provide an activating effect on the workpiece metal surface. The typical pasta includes 60-70% of abrasive and 30-40% of binding agents. The content of the activating additives does not exceed 2%.
Sometimes polishing in round type machines is used without liquid medium. In this case leather pieces, felt, chamois, dry oak, ash or hornbeam sawdust are used as polishing materials. The relation between workpieces and filling material is 1:2–1:6 and between workpieces and leather pieces is 1:30–1:50.
The glossy surface of steel parts is achieved by using an alkalescent solution with abrasive in the form of steel polishing balls 9–18 mm in diameter. It's possible to reduce the roughness value to Rа=0,32–0,63 microns.
The polishing intensity at round type machines can be exceeded by combining mechanical and chemical exposure to polishing material. In this case an acid medium makes it easier to remove scales off metal surface, while an alkaline medium reduces skiving action.
At present the so called underwater polishing is widely used. It is carried out in round type machines with liquid medium, represented by a solution of common soap. Prior parts defatting is necessary operation for achievement of a good processing quality. To enhance the process intensity, the solution is heated to 40–50оC.
Mechanical polishing shortcomings::
- Limitations on surface processing complexity
- Abrasives can penetrate into the metal structure.
- multiplicity of stages
- usage of several plants
- a huge investment of time
- process control complexity
Chemical polishing can serve as an effective substitution for mechanical processing method, especially in decorative finish of products made from aluminium, copper and alloys based on copper, and in decorative finish of articles made of precious metals.