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MODERN ENGINEERING PRACTICE - 18


Electro-plating plant




































FEW owners of bicycles are probably aware that the plating of a bicycle’s bright parts may have involved the use of machines perhaps 100 feet in length and having elaborate automatic handling mechanisms. Were it not for the ingenuity of the engineer in designing such apparatus the plating of the parts, and of countless articles of domestic use, would be extremely costly.


The photograph above shows a chromium-plating machine of comparatively small size. The lower portion of the machine consists of a series of tanks through which the parts are successively passed; the mechanism at the top is for automatically traversing them and for lifting them from one tank and lowering them into the next.


The process of electro-deposition is, briefly, as follows. The part to be plated is hung in a solution of the metal which is to be deposited and is connected with the negative pole of the current supply, the part itself forming the cathode. The positive pole may be of various materials and is called the anode.


When the current is switched on, the circuit is completed through the solution, the “flow” being from anode to cathode. The metal in the solution is separated out and is deposited on the part.


The row of tanks comprising a nickel-plating machine begins with one containing a hot cleaning solution which is agitated by jets of compressed air, an electric current being passed through it at the same time. The dirt rises to the top of the tank in the form of a scum which is blown away by a jet of compressed air and drained off. The parts are hung on frames which are automatically traversed through the tanks and in and out of them.


The next tank is a lead-lined one containing clean cold water, also air-agitated, and having a waste overflow. A spray of clean water at the surface completes the swilling operation as the parts leave the tank. Next comes a cyanide tank in which the parts are kept for a short time to remove any oxides which may have formed on them in the first tank.


A second cold water swill follows and after that comes a dip into a weak acid solution to neutralize any cyanide which may have escaped the swill. Another cold swill follows the acid bath and then the parts are transferred to the nickel-plating bath. This bath is lead-lined and the anodes are, nowadays, of depolarized nickel.


The parts are then washed and loaded on to the suspender frames of the chromium-plating machine. These frames are coupled to the negative pole of the current supply and are first taken round the plating vat, which is of iron lined with a special hard lead and, inside that, with reinforced glass sheets. This tank is heated by a water jacket and the fumes from it are drawn away by a fan.


After the chromium plating has been finished the parts are given a cold swill to remove the chromic acid, then passed to another tank in which any remaining acid is neutralized. A cold and a hot swill complete the process.



You can read more on “How Bicycles are Built”, “Modern Engineering Practice” and “Romance of Motor Car Making” on this website.

Electro-Plating Plant