Surface Roughness Measurement and Applications
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A question sometimes asked is "At what point does surface roughness become waviness?" This is almost impossible to answer.
The change from the concept of roughness to that of waviness often depends on the size of the workpiece.
For example, the irregular spacing which would be regarded as roughness on a machine spindle would be regarded as waviness on a watch staff. The number of waves in the functional length also has some influence on how we classify the irregularities. One wave on a watch staff might be considered as curvature, but a larger number of waves on a longer shaft may be accepted as waviness.
It is better to separate roughness, waviness and form according to their cause, as this also relates to the performance factors. So, we can define surface roughness, waviness and form as follows.
What is Surface Roughness or Roughness?
Surface roughness or roughness is defined as the irregularities which are inherent in the production process (e.g. cutting tool or abrasive grit). Surface roughness is quantified by the deviations in the direction of the normal vector of a real surface from its ideal form. If these deviations are large, the surface is rough; if they are small, the surface is smooth.
What is Waviness?
Waviness is part of the texture on which surface roughness is superimposed. It may result from vibrations, chatter or work deflections and strains in the material. It is also impossible to specify precisely where waviness stops, and the shape becomes part of the general form of the part.
What is Form?
Form is the general shape of the surface, ignoring variations due to roughness and waviness.
Roughness, Waviness and Form
These distinctions are therefore qualitative not quantitative yet are of considerable importance as defining them this way is well established and functionally sound. Surface roughness is produced only by the method of manufacture resulting from the process rather than the machine. Marks can be left by the tool or grit itself: these will be of a periodic nature for some processes and more random in others.
There is also a finer structure formed by tearing of the part during machining, the build-up of debris at the edge and small blemishes in the tool tip. Waviness, however, is attributed to the individual machine, imbalance in the grinding wheel, lead screw inaccuracy and lack of rigidity.
Form errors are often caused by the part not being held firmly enough or a slideway not being straight, or heat generated during the process that can cause a surface to bend.
It should be emphasized that these three characteristics are never found in isolation. Most surfaces are the result of a combination of the effects of surface roughness, waviness and form.
How to Measure Surface Roughness?
Since the individual surface roughness irregularities are too small to see with the naked eye, a Surface Roughness Measurement Tester is required. A small stylus is drawn across the surface at a constant speed for a set distance. An electrical signal is obtained and amplified to produce a much-enlarged vertical magnification.
This signal result is displayed in a graphical output, together with numerical values that characterize the surface texture or surface roughness. Watch the video below of the Form Talysurf® PGI NOVUS Surface Profilometer.
The ISO standard for surface roughness measurements is a 60° or 90° conical stylus with a spherical tip of 2μm radius. However, this is quite a delicate stylus, and needs a surface roughness measuring instrument with excellent mechanical properties to achieve this.
The Surtronic® Duo II Surface Roughness Measurement Tester is designed to measure surface roughness, and can be utilised in conjunction with the HSE Slips assessment tool software to check flooring surface roughness.
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In addition, the Surtronic® S-100 Series Surface Roughness Measurement Tester offers a versatile solution for all your surface finish measurement requirements.
What are the Surface Roughness Measurement Parameters?
In order to predict the behaviour of a component during use or to control the manufacturing process, it is necessary to quantify these surface characteristics by using surface texture parameters.
Surface texture parameters can be separated into three basic types; Amplitude Parameters (vertical characteristics), Spacing Parameters (horizontal characteristics) and Hybrid Parameters (combination of spacing and amplitude parameters).
Examples of typical surface roughness measurement parameters can be seen below:
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What are the Applications of Surface Roughness?
In many applications surface roughness is closely allied to function, for instance where two surfaces are in close moving contact with each other their roughness will affect their sealing or wear properties. This might suggest that it is a case of "the smoother the better", but this is not always true as other factors may be involved.
Where lubrication is involved it has been found that roughness valleys are required to hold oil. Also, the financial aspect must be considered: it costs a lot of money to produce very smooth surfaces and the expense of this exercise can add to the bill considerably without gaining a great deal of performance.
However, when two surfaces in relative motion (e.g. a shaft and its bearing) are lubricated, some wear will occur. If the surfaces are rough, they will soon become smoother as the peaks wear away. Since this removes metal there will be a quicker change in the fit of the two parts than if the finish was at the optimum from the start. On the other hand, some parts such as clamping devices or a pin with an 'interference fit' depend on friction for their functionality.
Another application where surface roughness can have an influence on performance is the use of lip seals to prevent the escape of hydraulic fluids. If the finish is too smooth it is difficult to maintain a fluid film between the shaft and the seal. If the finish is too rough it can cause abrasion and consequent breakdown, leading to failure. Inspection of the texture left on a component after machining will often reveal tool defects, incorrect tool settings or wrong tool speeds and feeds.
The appearance of a surface can be of some importance. For instance, sheet steel used for motor car bodies must have a finish which will allow paint to bond to the surface without any "orange peel" effect and with an even appearance. Anybody who has tried to paint onto a glass surface will appreciate the difficulty in getting a firm bonded finish. Metallic parts are not the only components to require control; both paper and plastic parts need the same degree of repeatability.
Contact-type surface roughness testers provide reliable measurements because they directly touch the sample. However, direct contact to a sample often has many disadvantages as outlined below.
On contact-type surface roughness testers, the stylus must be polished because the tip will wear down over time. The mode of wear varies, so the stylus tip may become flat or rounded depending on the material and shape of the measurement target object. Different stylus shapes will naturally generate different wave profiles.
One method for determining stylus wear is to use a commercially available wear-inspection test piece. Wear is determined by comparing the data profile (groove width) of the test piece before and after the wear of the stylus.
Since styluses are made of sapphire or diamond, such hard materials can scratch the surface of the sample surface. Especially when repeating parallel adjustments, it is easy for the stylus to scratch the sample during rapid feed.
Because the tip of the stylus is spherical, the stylus cannot trace the shape properly if the width of the groove (scratch, etc.) is narrower than the radius of the stylus tip.
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