The quality of a machined surface is becoming important to satisfy the increasing demands of component performance and reliability. Machined parts used in military, aerospace, and automotive industries are subjected to high stresses, temperatures, and hostile environments.
The dynamic loading and design capabilities of machined components are limited by the fatigue strength of the material, which is commonly linked to the fatigue fractures that always nucleate on or near the surface of the machined components. Stress corrosion resistance is another important material property that can be directly linked to the machined surface characteristics.
When machining any component, it is necessary to satisfy the surface technological requirements in terms of high product accuracy, good surface finish, and a minimum of drawbacks that may arise as a result of possible surface alterations by the machining process. The nature of the surface layer has a strong influence on the mechanical properties of the part.
Any machined surface has two main aspects – the first aspect is concerned with the surface texture or the geometric irregularities of the surface, and the second one is concerned with the surface integrity, which includes the metallurgical alterations of the surface and surface layer. Surface texture and surface integrity must be defined, measured, and controlled within specific limits during any machining operation.
Surface texture is concerned with the geometric irregularities of the surface of a solid material, which is defined in terms of surface roughness, waviness, lay, and flaws:
- Surface roughness consists of the fine irregularities of the surface texture, including feed marks generated by the machining process.
- Waviness consists of the more widely spaced components of surface texture that may occur due to the machine or part deflection, vibration, or chatter.
- Lay is the direction of the predominant surface pattern.
- Flaws are surface interruptions such as cracks, scratches, and ridges.
Stylus contact type instruments are widely used to provide numerical values of surface roughness in terms of the arithmetic average (Ra).
The quality of surface finish affects the functional properties of the machined parts as follows:
- Wear resistance. Larger macro irregularities result in nonuniform wear of different sections of the surface where the projected areas of the surface are worn first. With surface waviness, surface crests are worn out first. Similarly, surface ridges and micro irregularities are subjected to elastic deformation and may be crushed or sheared by the forces between the sliding parts.
- Fatigue strength. Metal fatigue takes place in the areas of the deepest scratches and undercuts caused by the machining operation. The valleys between the ridges of the machined surface may become the focus of concentration of internal stresses. Cracks and microcracks may also enhance the failure of the machined parts.
- Corrosion resistance. The resistance of the machined surface to the corrosive action of liquid, gas, water, and acid depends on the machined surface finish. The higher the quality of surface finish, the smaller the area of contact with the corrosive medium, and the better the corrosion resistance. The corrosive action acts more intensively on the surface valleys between the ridges of micro irregularities. The deeper the valleys, the more destructive will be the corrosive action that will be directed toward the depth of the metal.
- Strength of interference. The strength of an interference fit between two mating parts depends on the height of micro irregularities left after the machining process.
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