Many engineering components which are made of sheet metal are given with protective coating to prevent it from corrosion. Protective coating can be in the form of oil, grease, paint, metal spraying, etc. which is to be removed before repair. Some metals form scales of oxide during their storage. For removal of oil, grease, scale etc. various plants and machine are used such as sand blasting plant, degreasing plant. Degreasing plant is used for removing oil and grease from the surface of the job. Sand blasting plant is used for removing scale from the surface of hard metal. Some parts are to be heat treated to maintain the physical and mechanical properties of the metal before and after the completion of work. Salt bath is used for heat treatment of Aluminium and its alloys.
Sand Blasting Plant
Sand blasting plant is used for removing scale from the surface of the hard metal to expose the original metal, which is undergoing any protective treatment. There are different types and sizes of sand blasting plant. A specimen of Jack Man Sand Blasting Cabinet for aircraft sparking plug is given below.
The sand blast cleaner is of the cabinet type, mounted on two detachable side frames for ease of transport. The bottom of the cabinet is formed into a hopper terminating in a self- sealing valve, and access to the interior is through the door in the front panel of the cabinet, the door being provided with runner sleeved arm-holes. Immediately above the door is a transparent window, and a suitably protected lamp is mounted in the roof for illuminating the interior of the cabinet. A pedal operated valve is provided to control the air supply, The air being led by means of a flexible pipe through the right hand side of the cabinet to mixing chamber mounted on a flexible pipe inside the cabinet.
On operating the air valve a mixture of cleaning compound and air is drawn from the self-sealing valve by the injector action of the air nozzle inside the mixing chamber. The mixture is conducted to the mixing chamber by means of a flexible pipe and is finally blown through the nozzle into the sparking plug (components being cleaned).
An electrically driven extractor fan, mounted at the back of the cabinet, maintains a slight negative pressure difference between the interior of the cabinet and the outside atmosphere. This small pressure difference ensures clear visibility in the cabinet and prevents leakage of the fine dust into the servicing bay. A duct is attached to the back of the cabinet to conduct the exhaust from the fan to the outside of the building. Two circular rubber faced stands and one baffle plate is provided for holding the components of sparking plug having detachable centre assemblies.
Salt bath is used for heat treatment of Aluminium and its alloys .The equipment consist of a metal cabinet containing a mild steel bath and a pair of kerosene heaters which can be used alternatively for continuous supply of heat. The bath containing a mixture of potassium nitrate and sodium nitrate is centrally mounted on the cabinet and the access is through the hinged door in the upper part of cabinet. A pyrometer is provided to indicate the temperature of the bath. Fumes rising from the bath, when it is heated go out into atmosphere through the pipe at the rear side of the cabinet. The heater which can be adjusted for height is supported on the side of the cabinet.
Splashing of the salt should be avoided. The heat should be gradually applied. When the salt becomes hot the top hard surface should be carefully broken in order to release an air lock that may exist below the crust .Personnel should be careful whilst using salt bath as its appearance gives no indication of the high temperature. The maximum temperature employed is 505 C.
Metal parts frequently require cleaning prior to inspection, assembly, heat treatment or the application of a protective coating. Cleaning necessitates the removal of oil, grease, dirt, etc. One approved method for this is the use of a chemical solvent. Trichloroethylene (TCL) is frequently selected for this purpose. Trichloroethylene is a colourless spirituous liquid, which boils at 87C. The vapour given of is much heavier than air; the liquid may therefore be boiled in the lower part, of a deep container and condensed in the upper part without any loss of fluid by diffusion. Articles may be cleaned by suspending them in the vapour rising from boiling liquid or immersing them in boiling liquid.
A plant employing boiling liquid or Trichloroethylene vapour consist of one or more deep vessel with means for heating the liquid in the lower part of them and provided with cooling pipes below the mouth of the vessel for condensing the vapour. Heating may be done by one of the following means:
(a) Steam pipes
(b) High pressure hot water pipes
(c) A gas or oil burner
(d) Electric heater
Temperature above 120C is to be avoided, otherwise the Trichloroethylene may decompose and produce a detrimental acid. Cold water is passed through the cooling pipes, and thermostatic or mechanical controls are provided to shut down the plant when the sump becomes excessive hot.
In view of the toxic nature of Trichloroethylene (TCL), good ventilation is to be provided to remove any vapour escaping from the plant. An efficient exhaust duct, few feet above the plant rim to roof height is recommended. Personnel deployed on degreasing plant should not inhale the trichloroethylene fumes as it’s very injurious to health.