Surface Texture is defined as a degree of finish conveyed to the machinist by a system of symbols devised by a Standards Association, eg. ASA – American Standards Association, BS – British Standards
Modern technology has demanded improved surface finishes to ensure proper functioning and long life of machine parts. Pistons, bearings, and gears depend to a great extent on a good surface finish for proper functioning and therefore, require little or no break-in period. Finer finishes often require additional operation, such as lapping or honing. The higher finishes are not always required on parts and only result in higher production costs. To prevent overfinishing a part, the desired finish is indicated on the shop drawing. Information specifying the degree of finish is conveyed to the machinist by a system of symbols devised by Standards Associations, eg. ASA American Standards Association and BS British Standards. These symbols provide a standard system of determining and indicating surface finish. The inch unit for surface finish measurement is microinch (µin), while the metric unit is micrometer (µm)
SURFACE TEXTURE TERMS AND DEFINITIONS
Standard terminology and symbols to describe surface finish
FLAWS
Flaws or defects, are random irregularities, such as scratches, cracks, holes, depression, seams, tears or inclusions. These defects can be caused during the machining or production process such as moulding,
drawing, forging, machining, eg, holes cause by air bubbles during casting, crack and tears by forging and drawing process.
LAY
Lay or directionality, is the direction of the predominant surface pattern caused by the machining process and it is usually visible to the naked eye.
ROUGHNESS
Roughness is defined as closely spaced, irregular deviation on a scale smaller than that of waviness. It is caused by the cutting tool or the abrasive grain action and the machine feed. Roughness may be superimposed on waviness.
Roughness height
Roughness height, Ra is the deviation to the centre line in micro inches or micrometers
Roughness width
Roughness Width is the distance between successive roughness peaks parallel to the nominal surface in inches or millimeters.
Waviness
Waviness is a recurrent deviation from a flat surface, much like waves on the surface of water. It is measured and described in terms of the surface between adjacent crests of the waves (waviness width) and height between the crests and valleys of the waves (waviness height). Waviness can be caused by:
Lay or directionality, is the direction of the predominant surface pattern caused by the machining process and it is usually visible to the naked eye.
ROUGHNESS
Roughness is defined as closely spaced, irregular deviation on a scale smaller than that of waviness. It is caused by the cutting tool or the abrasive grain action and the machine feed. Roughness may be superimposed on waviness.
Roughness height
Roughness height, Ra is the deviation to the centre line in micro inches or micrometers
Roughness width
Roughness Width is the distance between successive roughness peaks parallel to the nominal surface in inches or millimeters.
Waviness
Waviness is a recurrent deviation from a flat surface, much like waves on the surface of water. It is measured and described in terms of the surface between adjacent crests of the waves (waviness width) and height between the crests and valleys of the waves (waviness height). Waviness can be caused by:
a) deflection of tools, dies or the work piece
b) force or temperature sufficient to cause warping
c) uneven lubrication
d) vibration
e) any periodic mechanical or thermal variations on the system during manufacturing operations.
