Surface finish is not only about appearance, but may also affect performance. Some parts genuinely require a certain degree of roughness to attach stickers, liquids, gaskets, etc. This is the reason to never leave surface requirements open-ended; if surface finish matters, know how to specify it.
This guide will take you through what surface finish/roughness is, its importance, how to measure it, and how to use symbols to accurately specify requirements.
From what surface finish is, its significance in engineering, and the basics of measurement, right through to the surface roughness chart and common values, our resource walks you through every step. There’s a clear surface roughness chart included for good measure.
What is Surface Finish?
This nature of the surface is known as surface finish, surface texture, or surface topography. It is the small, local deviation of a surface from the perfectly flat ideal (a true plane).
Surface texture is one of the few important factors that govern friction and transfer layer formation during sliding.
Great labors have been expended studying the effect of surface texture on friction and wear in sliding conditions. Surface textures may be isotropic or anisotropic. Sometimes stick-slip friction phenomena are seen during sliding, depending on the texture.
Each of the many kinds of (of course, by now the reader should understand, we mean too many rather than too few manufacturing operations, such as machining, etc. produces a surface texture.
The operation is optimized so that the texture produced is usable. If not dead, some further operation will be performed to change the original texture.
That possibility may be grinding (abrasive cutting), polishing, lapping, abrasive blasting, honing, erosive (EDM), milling machining, lithography, industrial etching, chemical milling, laser-texturing, or other.
What to Expect with Surface Finishes?
Surface finishes differ widely in appearance depending on the type of machining or other process used. Machining stock may be done by cutting, polishing, or casting, and in each case, the process must “interact” with the part surface.
The particular way the process interacts with the part surface yields a unique final result, with unique properties, roughness, and appearance or appearance. This is just an indication that an understanding of surface finish often involves a firm knowledge of manufacturing processes.
Before a manufacturing process can be “nicely” selected for a project, the related surface finish should be carefully analyzed. Cutting the stock material by “parting off” will produce a different surface than might be produced by subsequently milling it or even by finely grinding or electro-polishing it.
Certain general characteristics of surface finish help to characterize or to describe a surface finish. These characteristics are: Roughness, Waviness, and Lay.
Roughness
Roughness is often confused with surface finish. However, it is only one of the characteristics of surface finish; it is the actual roughness of the surface of a material, expressed and quantified.
Waviness
Waviness is the term actually used to designate the larger variations in a surface. In other words, if roughness has an apparently short wavelength, waviness has a longer wavelength, i.e., more distance between the peaks and troughs.
It may be due to tool marks, vibration of the machine, or even actual variation in the material itself.
Lay
Lay describes the general direction, alignment, and pattern of the surface texture. It refers to the dominant direction of the markings on the surface, whether this will be found to be the result of tool marks (from machining) or alignment of polishing strokes.
Lay is important in many areas, as it can have a significant effect on certain applications of sealing or aspects relating to friction.
Helpful insight may be found in our materials pages for what to expect for surface finishes when making use of the services we provide; do reference this information so that you are aware of what colors, textures, or finishes will be encountered.
Surface Finish vs. Surface Roughness
Surface finish is sometimes confused with surface roughness, but refers to the general quality of a surface, while surface roughness concentrates on the actual deviations of a surface.
Roughness is defined as a measure of the small-scale variation in height between peak and valley on the surface. It gives an indication of the texture and small-scale smoothness of a surface.
However, although surface roughness is a part of surface finish, the latter is much more extensive in its scope.
Symbols for Indicating Surface Finish
The surface finish quality on the surface of the metal, produced by some production method other than machining, is indicated on the drawing by a tick symbol.
This basic symbol is made up of two legs of unequal length, which are inclined to one another at an angle of approximately 60 degrees and meet on a line produced through them, representing the surface of the part to be machined.
If it is specified that it is to be produced by the removal of material by some form of machining, a horizontal bar is to be added across the bar of the basic symbol.
If the surface is to be produced by no removal of material or if a warranty that the surface shall be left in the very state resulting from the other manufacturing process, happily (for ex., the removal of the material) or otherwise, is needed, a circle is written in the basic symbol as shown in fig-C.
If the general manufacturing processes as contemplated by the drawing are, in and of themselves, sufficient to produce the desired surface finish, no indication of it will be necessary on the drawing.
How to Measure Surface Roughness
Surface roughness is, put simply, a measure of the smoothness of a surface and is usually expressed as a numeric value, denoted Ra.
The Ra surface roughness chart is basically a measurement of the average height of the peaks and the depths of the valleys over the total length of the surface being measured.
As previously noted, three basic components constitute a surface: roughness, waviness, and lay. Hence, there are many influences on the basic geometrical features of a surface.
Of course, there are many techniques for measuring surface roughness. These measuring systems include:
- Direct measurement methods
- Non-contact methods
- Comparison methods
- In-process methods
Direct measurement methods use a stylus to measure surface roughness. The stylus is drawn across a surface perpendicular to its pattern. The machinist uses a registered profile (generated by the stylus measuring system) to obtain the parameters of roughness.
Non-contact methods use light or sound instead of the stylus. Optical instruments, such as white light and confocal, take the place of the stylus. These instruments work on different principles for measurement. Physical probes can be replaced with optical sensors or microscopes.
“The instrument used will first send an ultrasonic pulse against the surface; the sound is then altered and reflected to the instrument. The reflected wave is then examined, and the roughness parameters can be found.
Comparison techniques use surface roughness samples generated by the equipment or process in question. The manufacturer then uses his sense of touch and sight to compare these results with the surface of known roughness parameters.
An example of an in-process technique is inductance. This method is used for determining surface roughness using magnetic materials. The inductance pickup uses electromagnetic energy to determine the distance from the pickup to the surface.
This parametric value can then be compared with, and used to derive various values for, representative values of roughness parameters.