Product designers constantly strive to design machinery that can run faster, last longer, and operate more precisely than ever. Modern development of high speed machines has resulted in higher loading and increased speeds of moving parts. Bearings, seals, shafts, machine ways, and gears, for example must be accurate - both dimensionally and geometrically. Unfortunately, most manufacturing processes produce parts with surfaces that are either unsatisfactory from the standpoint of geometrical perfection or quality of surface texture. As industry tries harder to approach perfection, interest has focussed more closely than ever before on the microfinishing processes like -- honing, lapping, and superfinishing. Each process was designed to generate a particular geometrical surface and to correct specific irregularities and so must be applied carefully to a given production sequence. Also, each process is a final operation in the machining sequence for a precision part and is usually preceded by conventional grinding. This primer begins by explaining how industry controls and measures the precise degree of smoothness and roughness of a finished surface.
In most cases, surface finish control starts in the drafting room. The designer has the responsibility of specifying a surface that will give the maximum performance and surface life at the lowest cost. In selecting a required surface finish for a particular part, the designer must base his/her decision on past experience with similar parts, on field service data, or on engineering tests. There are two principal reasons for surface control:
1. To reduce friction
When a film of lubricant must be maintained between two moving parts, the surface irregularities must be small enough so they will not penetrate the oil film under the most severe operating conditions. Bearings, journals, cylinder bores, piston pins, bushings, pad bearings, helical and worm gears, seal surfaces, and machine ways are examples where this condition must be fulfilled.
2. To control wear
Surface finish is also important to the wear service of certain parts that are subject to dry friction, such as machine-tools bits, threading dies, stamping dies, rolls, clutch plates, and brake drums.
Often, surface finish must be controlled for the purpose of increasing the fatigue strength of highly stressed members which are subjected to load reversals. A smooth surface eliminates the sharp irregularities which are the greatest potential source of fatigue cracks.
For parts such as gears, surface finish control may be necessary to ensure quiet operations. In other cases, however, where a boundary lubrication condition exists or where surfaces may not be compatible, as in two extremely hard surfaces running together, a slightly roughened surface will usually assist in lubrication.
A specific degree of surface roughness is also required in order to accommodate wear-in of certain parts. Most new moving parts do not attain a condition of complete lubrication as a result of imperfect geometry, running clearances, and thermal distortions. Therefore, the surfaces must wear in by a process of actual removal of metal. The surface finish must be a compromise between sufficient roughness for proper wear-in and sufficient smoothness for expected service life.
It is used to designate the characteristics of surface texture on a drawing of a production part. The symbol is always placed in the standard upright position, as shown in figure below, never at an angle or upside down. The symbol is generally omitted on views of parts when the finish quality of a surface is not important. Generally speaking, the ideal finish is the roughest one that will do the job satisfactorily.
There are three general methods by which the surface texture and the surface geometry may be explored and evaluated: electronic, optical, and visual or tactual.
There are two types of electronic instruments which measure actual surface texture: averaging (or velocity type) and profiling (or displacement type). Averaging or tracer-type instruments employ a stylus that is drawn across the surface to be measured. The vertical motion of the tracer is amplified electrically and is impressed on a recorder to draw the profile of the surface or is fed into an averaging meter to give a number (AA) representing the roughness value of the surface. Profiling equipment is used principally in laboratories for research and development applications. Considerable skill is required to operate the equipment and analyze and interpret the data.
Optical or area systems use optical methods for surface evaluation. Equipment ranges from exploration of the surface with simple microscopes or three-dimensional microtopography to highly sophisticated techniques such as inferometry.
Area systems inspect all the surface, not simply one line across it. The surface texture in this process is clearly distinguished from the surface geometry. Because there is no stylus, the surface is not mechanically contacted, and thus there can be no damage to the workpiece surface. Another important advantage of optical inspection methods is that the biasing effect of the stylus radius is eliminated.
3. Visual ot Tactual
The visual or tactual is the simplest and most straight forward method
of surface measurement. It is also the least accurate. Figure below shows
a commercial set of master precision reference specimens with 15 replicated
surfaces, ranging in roughness from 2 to 125 in. in height. Comparators
of this type are readily available with various surface finish from 2 to
1000 in. is available. The scales, used with or without a magnifier, are
placed adjacent to the workpiece under examination and the surfaces are
compared visibly or tactually by drawing the tip of the fingernail across
each at right angles to the tool marks. The fingernail touch or "feel"
will be the same when both finishes are identical.