1.2Ferrous Metals and Alloys
By virtue of their wide range of mechanical, physical, and chemical properties, ferrous metals and alloys are among the most useful of all metals. Ferrous metals and alloys contain iron as their base metal: the general categories are cast irons, carbon and alloy steels, stainless steels, tool and die steels.
The term cast iron refers to a family of ferrous alloys composed of iron, carbon(ranging from 2.11% to about 4.5%),and silicon(up to about 3.5%).Cast irons are usually classified as follows:
1.Gray cast iron,or gray iron;
2.Ductile cast iron, nodular cast iron, or spherical graphite cast iron;
3.White cast iron;
http://m.wendangku.net/doc/ebfbad5f360cba1aa911da10.htmlpacted graphite iron。
The equilibrium phase diagram relevant to cast irons is shown in Fig.1.1,in which the right boundary is 100% carbon,that is pure graphite.The eutectic temperature is 1154h C (2109h F), and so cast irons are completely liquid at temperatures lower than those required for liquid steels. Consequently, iron with high carbon content can be cast at lower temperatures than can steels.
Carbon steels are generally classified by their proportion(by weight) of carbon content.
1.Low-carbon steel, also called mild steel, has less than 1.30% carbon. It is generally used for common industrial products, such as bolts,nuts, sheet, plate, and tubes, and for machine components that do not require high strength.
Conventional machining is the group of machining operations that use single- or multi-point tools to remove material in the form of chips.Metal cutting involves removing metal through machining operations.Machining traditionally takes place on lathes, drill presses, and milling machines with the use of various cutting tools.Most machining has very low set-up cost compared with forming, molding, and casting processes.However, machining is much more expensive for high volumes.Machining is necessary where tight tolerances on dimensions and
finishes are required.
5.2 Turning and Lathe
Turning is one of the most common of metal cutting operations. In turning, a workpiece is rotated about its axis as single-point cutting tools are fed into it, shearing away excess material and creating the desired cylindrical surface.Turning can occur on both external and internal surfaces to produce an axially-symmetrical contoured part.Parts ranging from pocket watch components to large diameter marine propeller shafts can be turned on a lathe.
Apart from turning, several other operations can also be performed on a lathe.
Boring and internal turning.Boring and internal turning are performed on the internal surfaces by a boring bar or suitable internal cutting tools. If the initial workpiece is solid, a drilling operation must be performed first. The drilling tool is held in the tailstock, and the latter is then fed against the workpiece. When boring is done in a lathe, the work usually is held in a chuck or on a face plate. Holes may be bored straight, tapered, or to irregular contours. Boring is essentially internal turning while feeding the tool parallel to the rotation axis of the workpiece.
Quality and accuracy are major considerations in making parts or structures. Interchangeable parts require a high degree of accuracy to fit together. With increasing accuracy or less variation in the dimension, the labor and machinery required to manufacture apart is more cost intensive. Any manufacturer should have a thorough knowledge of the tolerances to increase the quality and reliability of a manufactured part with the least expense.
An engineering drawing must be properly dimensioned in order to convey the designer’s intent to the end user. Dimensions of parts given on blueprints and manufactured to those dimensions should be exactly alike and fit properly. Unfortunately, it is impossible to make things to an exact or dimension. Most dimensions have a varying degree of accuracy and a means od specifying acceptable limitations in dimensional variance so that a manufactured part will be accepted and still function. It is necessary that the dimensions, shapes and mutual position of surfaces of individual parts are kept within a certain accuracy to achieve their correct and reliable functioning. Routine processes do not allow maintenance(or measurement) of the given geometrical properties
with absolute accuracy. Actual surfaces of the produced parts therefore differ from ideal surfaces prescribed in drawings. Deviations of actual surfaces are divided into four groups to enable assessment, prescription and checking of the permitted inaccuracy during production:
1:Dimensional deviations; 2:Shape deviations;
3:Position deviations; 4:Surface roughness deviations.