Vacuum science and technology

The meaning of the word, ‘vacuum’ is complete emptiness. Thus we can conclude that vacuum means space in which air and other gases are absent. Technically vacuum is significant only when the gas pressure becomes less than 760 torr. The most common example of creating vacuum is the suction in an ordinary drinking straw. Many devices require vacuum conditions for their operations. In dewar flask, television tube, electric bulb, vacuum cleaner etc. we use vacuum conditions. The boiling temperature of a liquid is reduced in vacuum. Transmission of sound is difficult through a region having high degree of vacuum. The advance in technology is also due to vacuum technology. X-ray technology, microelectronic technology, computer technology, metallurgical advances etc. are mainly due to vacuum technology.

Production of vacuum

A pump is required to produce vacuum. In mechanical pumps gas is initially trapped and compressed. Later gas is removed from the low pressure to the high pressure side of the pump, where it is expelled to the open air either directly or through a second mechanical ‘backing pump’. In diffusion pump, gas molecules are forced to interact with heavy molecules of pump fluid. In chemical pumps, gas molecules are allowed to combine chemically with highly reactive substances like titanium. Sorption pumps use charcoal or zeolites. They absorb gas molecules. Cryo pumps are vessels with chemically nonreactive internal parts. Another method is ionization of the gas where accelerating the ions in a high voltage field, the gas molecules are forced to move into a wall or target electrode.

Measurement of vacuum

In Barometric type of measurement, we use mercury or oil. In mechanical type, diaphragm gauge is used in which the pressure causes mechanical movement of a septum forming one wall of an evacuated enclosure. This is similar to that of an aneroid barometer. In Mcleod gauge, assumption followed is that Boyle’s law is valid. Manometric liquid used in Mcleod gauge is mercury. Thermodynamic principles are utilized in thermal conductivity gauges. Hot-cathode ionization gauges and cold-cathode ionization gauges are other examples. Knudsen gauges depends on the recoil force on a warm surface when a molecule rebounds from it. Pirani gauge is based on the property of variation of thermal conductivity with pressure.

MANUFACTURING PROCESSES

In casting process, the metal in the molten state is poured into a mould and then allowed to solidify into a fine desired shape. In deformation process, by force, material is deformed. Forging, rolling, extrusion, drawing, bending, deep drawing etc. are examples. Machining processes are also called metal cutting or chip forming. This process includes turning, milling, drilling, broaching, shaving, grinding, polishing, lapping, honing, buffing, sawing etc. In plastic/polymer processing, shape casting method is employed. Powder metallurgy, joining processes, heat treatment, surface treatment, assembly processes are the other manufacturing methods.

PRODUCTION MECHANISMS

In piece or job-lot production, the parts are produced in small entities where periodical repetition is restricted. Standard cutting tools and universal measuring facilities are used here. The skill of laborers is very important here. In medium production, principles of interchangeability are to be strictly compiled with. An example is printing of books and some other examples are machine tools, engines etc. In mass production, parts are produced in large quantities. The most advanced and effective form of mass production is ‘continuous-flow production’ where time required for each operation of the production line is equal to or a multiple of the set standard time all along the line. Here either semi-skilled or skilled workers can be employed.

Computers in manufacturing

In the production of a part, now computers are employed in designing and production. Computer aided design (CAD) and computer aided manufacturing (CAM) are now common terms in industry. For controlling and production, numerical control (NC), Computer numerical control (CNC) and direct numerical control (DNC) are used. Some industry use Computer-aided process planning (CAPP). Material management planning (MRP), computer aided quality control (CAQC), flexible manufacturing systems (FMS), group technology (GT), Computer integrated manufacturing system (CIMS) etc. Productivity improvement, quality, better equipment utilization, reduced inventory, faster delivery etc. are the revolutionary things happened by the usage of computers

Foundry furnaces

Cupola is the common type of foundry furnace. This is used for melting cast iron. In Cupola, control over close temperature is very difficult. A Cupola is a shaft furnace of cylindrical shape erected on legs or columns. The Cupola shell is made of steel plate where the interior is lined with refractory bricks to protect the shell from getting over-heated. Here fuel used is coke. This coke and molten metal contact will lead to escape elements like Si and foreign element accumulation may occur. This is really a disadvantage. In crucible or pot furnaces, we can melt ferrous and non-ferrous metals. A disadvantage of these furnaces is the high fuel consumption.

Extrusion

Here usually block of material or metal is enclosed in a container and forced to flow through the opening of a die. During extrusion, metal is forcefully subjected to plastic deformation and as result both elongation and reduction may happen. Here the metal flowing out of the die is the product while the slug which remains in the die is not used. In direct hot extrusion, heated billet is placed in the container. In indirect extrusion, the die is fixed on the end of a hollow ram and enters container. The extrusion ratio is an important term which is defined as the cross-sectional area of the billet to the cross-sectional area of the product. Mechanical and hydraulic presses are used for extrusion.

Diamond turning and boring

Diamond tools are used whenever we have to turn or bore below 0.0125mm. Materials in the abrasive powder form sharply erode cutting edge leading to uncertain and expensive tool. Hence the machines used here for turning and boring must be in an ideal condition with all ways and guides straight, erect and true and all bearings and spindles running in perfect truth with ultralow clearance and practically no vibration. We can use this tool to turn or bore light alloys, bronzes and tin alloys, bearing metal etc.

Grinding

This is a common method employed for the finishing work of steel. This requires very skilled workers who can repeat continuously a restricted grade of surface finish to fine geometric tolerances. Here machines and tools used should have very good condition with all the ways and guides straight, erect and true and attention should be given for the balancing of the wheels. The most important thing here is wheel surface preparation. Tolerable flow of chip and granule free coolant is essential here. The choice of wheel grit, speeds coolants will vary with the material being cut and the finish demanded.

Lapping

This is a surface finishing process used on flat or cylindrical surfaces but often employed externally. Here abrading of a surface by the method of lay is carried out. Lap is nothing but a material made of a material softer than the material to be lapped. Lap is charged with the fine abrasive particles. The lap and the surface to be finished are rubbed together using fine abrasive particles and these particles become embedded in the softer lap. Lapping is employed to ensure geometrical truth of the surface, to correct shape imperfections and to obtain dimensional accuracy.

Honing

This is a special type of grinding or abrading process. Here removal of material occurs in minimum quantity. Usually in common practice, this method is employed to remove the grinding marks or tool marks left on the surface after various finishing processes. Honing is performed in internal surfaces, holes etc. Example is automobile cylinders. Soft materials which cannot be lapped can be honed finely since we are utilizing bonded abrasive in this process. Honing machines are identical to vertical drilling machines but in this case spindle reciprocation is carried out by hydraulic means.

Buffing

This is a tool for finishing. In this finishing process, surface to be finished is brought in contact with a rotating cloth buffing wheel that usually has been charged with ultrafine abrasives. The polishing/finishing action is very much identical to lapping. Softer metals on buffing, leaves some indication of flow if we are not using abrasives. This flow reduces the high spots leading to a high degree of polishing. The materials used to fabricate buffing wheels are linen, cotton, broad cloth and canvass. This is an expensive process.

Barrel tumbling

This produces high luster and also removes burrs. A tumbling barrel is eight sided lined with wood. Parts are packed into the barrel or drum together with slugs. Delicate parts should not shift loosely during tumbling and in some cases the parts are fixed to racks within the barrel so that they will not touch or strike against one another. Tumbling is usually used for removing fins, flashes and scales from parts. This method is also used for cleaning of forgings, stampings and castings. For deburring and for the improvement of micrometer finish, we can use this method.