tolerances - surface finish

Mostly in design we are talking about fulfilling a component's specification. Whether it's about dimensional tolerance, shape, or maybe the desired surface finish. As we know before, surface finish and tolerance are closely linked. A component that are manufactured precisely, would have a good surface finish, on the other hand, rough surface finish usually hold a less precise dimension

The ability of a manufacturing operation to produce a specific surface roughness depends on many factors. For example, in end mill cutting, the final surface depends on the rotational speed of the end mill cutter, the velocity of the traverse, the rate of feed, the amount and type of lubrication at the point of cutting, and the mechanical properties of the piece being machined. A small change in any of the above factors can have a significant effect on the surface produced. There are 3 components defining the surface texture, that is : lay, surface roughness, & waviness

LAY
Lay is a measure of the direction of the predominant machining pattern. A lay pattern is a repetitive impression created on the surface of a part. It is often representative of a specific manufacturing operation. The lay may be specified when it has an effect on the function of the part. Unless otherwise specified, roughness is measured perpendicular to the lay.

SURFACE ROUGHNESS

Surface roughness, more commonly shortened to roughness, is a measure of the finely spaced surface irregularities. Surface finish may be measured in two ways: contact and non-contact methods. Contact methods involve dragging a measurement stylus across the surface; these instruments are called profilometers. Non-contact methods include: interferometry, confocal microscopy, focus variation,structured light, electrical capacitance, electron microscopy, and photogrametry.

The most common method is to use a diamond stylus profilometer. The stylus is run perpendicular to the lay of the surface. The probe usually traces along a straight line on a flat surface or in a circular arc around a cylindrical surface. The length of the path that it traces is called the measurement length. The wavelength of the lowest frequency filter that will be used to analyze the data is usually defined as the sampling length. Most standards recommend that the measurement length should be at least seven times longer than the sampling length. The assessment length or evaluation length is the length of data that will be used for analysis.

The disadvantage of a profilometer is that it is not accurate when the size of the features of the surface are close to the same size as the stylus. Another disadvantage is that profilometers have difficulty detecting flaws of the same general size as the roughness of the surface.There are also limitations for non-contact instruments. For example, instruments that rely on optical interference cannot resolve features that are less than some fraction of the frequency of their operating wavelength. This limitation can make it difficult to accurately measure roughness even on common objects, since the interesting features may be well below the wavelength of light. The wavelength of red light is about 650 nm, while the Ra of a ground shaft might be 2000 nm.

The first step of analysis is to filter the raw data to remove very high frequency data since it can often be attributed to vibrations or debris on the surface. Next, the data is separated into roughness, waviness and form. This can be accomplished using reference lines, envelope methods, digital filters, fractals or other techniques. Finally, the data is summarized using one or more roughness parameters, or a graph. In the past, surface finish was usually analyzed by hand. The roughness trace would be plotted on graph paper, and an experienced machinist decided what data to ignore and where to place the mean line. Today, the measured data is stored on a computer, and analyzed using methods from signal analysis and statistics

WAVINESS
Waviness is the measure of surface irregularities with a spacing greater than that of surface roughness. These usually occur due to warping,vibrations, or defection during machining.

Many factors contribute to the surface finish in manufacturing. In forming processes, such as molding or metal forming, surface finish of the die determines the surface finish of the workpiece. In machining the interaction of the cutting edges and the microstructure of the material being cut both contribute to the final surface finish. In general, the cost of manufacturing a surface increases as the surface finish improves.
Just as different manufacturing processes produce parts at various tolerances, they are also capable of different roughnesses.

Here's a chart showing roughness and each manufacturing method's ability :