Mechanical polishing

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%.

 

The polishing process can be intensified by application of the pastes containing 2-8% of synthetic diamond micro powder. In this case polishing includes several stages. In the first stage the rigid cloth wheels are treated with fatty pastes containing more coarse abrasive. The wheels revolve with enhanced rotation frequency.
 
 
In the stage of final polishing high quality polish is achieved by using soft wheels treated with dry polishing pastes. The wheels revolve with low rotation frequency. In addition to that the material consumption for polishing ofwork surface with area equal to 1 m2 totals 0,02–0,3 of felt wheels and 100–120 g of GOI polishing paste. While polishing intricate-profile parts, elastic tapes in the form of strips or bundles along with polishing pastes are used.
 
 
Round type vibratory machines are distinguished by considerably greater performance. Workpieces are loaded in bulk in large quantities. Such polishing method makes it possible to achieve reducing of the roughness value from Rа = 1,6–0,8 microns to Rа = 0,20–20 microns. Using of vibration and planetary rotation allows to improve performance by 8-10 times.

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.

 
An operating life of a solution does not exceed 24 hours, after which the solution is to be substituted by a new one. With all its diversity mechanical polishing is rather labour-consuming and environmentally harmful, and it requires a considerable quantity of expendables.

Mechanical polishing shortcomings::

  1. Limitations on surface processing complexity
  2. Abrasives can penetrate into the metal structure.
  3. Harmfulness
  4. Resource-demanding technology:
    1. multiplicity of stages
    2. usage of several plants
    3. a huge investment of time
    4. 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.