The Layman's Guide to Surface Texture

How do we measure surface texture?

In the past surface texture has been assessed by the judgement of the inspector either by eye or even fingernail. In order to put a number to the surface texture, we need to use a more accurate means of measurement. A typical surface measuring instrument will consist of a stylus with a small tip (fingernail), a gauge or transducer, a traverse datum and a processor. The surface is measured by moving the stylus across the surface. As the stylus moves up and down along the surface, the transducer converts this movement into a signal which is then exported to a processor which converts this into a number and usually a visual profile.

For correct data collection, the gauge needs to pass over the surface in a straight line such that only the stylus tip follows the surface under test. This is done using a straightness datum. This can consist of some form of datum bar that is usually lapped or precision ground to a high straightness tolerance. On small portable instruments this is not always a good option and can add to the expense of the instrument. In these cases, it is possible to use an alternate means of datum.

This is a skid.

How do we separate Roughness, Waviness and Form?

In order to separate the three elements, we use filters. On most surface texture measuring instruments we can select either Roughness or Waviness Filters.

Selecting a Roughness Filter will remove waviness elements, leaving the roughness profile for evaluation. Selecting a Waviness Filter will remove roughness elements, leaving the waviness profile for evaluation. Separating the roughness and waviness is achieved by using filter cut-offs.

What are filters?

Electronic or mathematical methods or algorithms which separate out different wavelengths and allow us to see only the wavelengths we are interested in.

What is a cut-off?

In basic terms, a cut-off is a filter and is used as a means of separating or filtering the wavelengths of a component. Cut-offs have a numerical value that when selected will reduce or remove the unwanted wavelengths on the surface. For example, a roughness filter cut-off with a numeric value of 0.8mm will allow wavelengths below 0.8mm to be assessed with wavelengths above 0.8mm being reduced in amplitude: the greater the wavelength, the more severe the reduction. For a waviness filter cut-off with a numeric value of 0.8mm, wavelengths above 0.8mm will be asssessed with wavelengths below 0.8mm being reduced in amplitude.

What is a sample length?

After the data has been filtered with a cut ? off, we then sample it. Sampling is done by breaking the data into equal sample lengths. The sample lengths have the same numeric value as the cut-off. In other words, if you use a 0.8mm cut off, then the filtered data will be broken down into 0.8mm sample lengths, These sample lengths are chosen in such a way that a good statistical analysis can be made of the surface. In most cases, five sample lengths are used for analysis.

What is a skid?

A skid is a part of the gauge that has a radius large enough to prevent movement in and out the roughness characteristics of the surface. The stylus and the skid are usually independent in their height (Z) movement but move together in the measurement direction. Surface deviations are recorded as the difference between the stylus and the skid movement in the Z direction. In other words, the skid acts as the straightness datum ? it? skids? over the top of the surface.

What is an assessment length?

An assessment length is the amount of data left after filtering that is then used analysis.

What is surface texture?

Every surface has some form of texture that takes the form of a series of peaks and valleys. these peaks and valleys vary in height and spacing and have properties that are a result of the way the surface was produced. For example, surfaces produced by cutting tools tend to have uniform spacing with defined cutting directions whilst those produced by grinding have random spacing.

In surface texture there are many factors that, when combined, characterise a surface?s profile. For example:

* the microstructure of the material

* the action of the cutting tool

* the instability of the cutting tool on the material

* errors in the machine tool guideways

* errors in the machine tool guideways

* deformations due to stress patterns in the component

What is the difference between roughness, waviness and form?

We analyze three main elements of surface texture - roughness, waviness and form.

Roughness ? this is usually the process marks or witness marks produced by the action of the cutting tool or machining process, but may include other factors such as the structure of the material.

Waviness ? this is usually produced by instabilities in the machining process, such as an imbalance in a grinding wheel, or by deliberate actions in the machining process. Waviness has a longer wavelength than roughness which is superimposed on the waviness.

Form ? this is the general shape of the surface, ignoring variations die to roughness and waviness. Deviations from the desired form can be caused by many factors. For example:

     * the part being held too firmly or not firmly enough

     * inaccuracies of slides or guideways of machines

     * stress patterns in the component

Surface, Waviness and Form are rarely found in isolation. Most surfaces are a combination of all three and it is usual to assess them separately.

Please note: there is no set point at which roughness becomes waviness or vice versa as this depends on the size and nature of the application. For example, the waviness element on an optical lens may be considered as roughness on an automotive part.

What measurement length should I make?

The measurement length is dictated by the numerical value of the cut-off, which itself is dictated by the type of surface inspection.

Typically, a measurement may consist of a traverse of 6-7 times the cut-off selected. For example, 7 cut-offs at 0.8mm = 5.6mm. One of two cut-offs will then be removed according to the filter type and the remaining cut-offs used for assessment. This only applies when measuring roughness.

For measuring waviness or primary profiles, the data length is chosen according to application and the nature of the surface. In general, that data length needs to be sufficient to give a true representation of the texture of the surface.

What standard cut-offs are available?

There are internationally recognized cut-offs of varying lengths. These are 0.08mm, 0.25mm, 0.8mm, 2.5mm and 8mm.

What types of filters are there?

Early measuring instruments used analogue (electronic) filters. These types of filters are also known as 2CR filters. The 2CR stands for two capacitors and two resistors. These electronic filters, although still accepted and recognized by international standards, do suffer from phase distortion caused by the nature of their electronic components. To remove this effect, we have another type of filter called a 2CR PC filter. The PC in this case stands for phase corrected. This type of filter suffers from less distortion than the 2CR but is still an electronic filter and, as such, still suffers from some distortion.

Modern instruments use phase correct filters, such as the Gaussian filter. These types of filters drastically reduce filter distortion, although they can only be implemented where filtering is done by mathematical algorithms: computer based processing.

On most modern computer-based instruments, analogue filters are digitally simulated so that correlation between new an old instruments can be made.

Which cut-off should I use?

In general, you select a roughness cut-off in order to assess the characteristics of the surface you require. These are usually the process marks or witness marks produced by the machining process. To produce a good statistical analysis of these process marks, you would normally select a cut-off in the order of 10 times the wavelengths under consideration. These wavelengths may be the turning marks on the  component. Warning; cut-offs should be determined by the nature of the component and not by the length of the component. Choosing the wrong cut-off will in some cases severely effect the outcome of the result.

Why do we measure surface texture?

We measure surface texture for two main reasons.

     * To try to predict the performance of the component.

     * To try to control the manufacturing process as the manufacturing process leaves its signature in the surface texture.

Why have I lost cut-offs during analysis?

Because the nature of the filtering process causes distortions at one or both end of the data or profile. These distortions are removed to ensure only genuine detail is included in the analysis.

For 2CR filters, data equivalent to 2 sample lengths are discarded from the beginning of the filtered data. For 2CR PC (phase connected) filters. 1 sample length from each of the filtered data is discarded.

Because the Gaussian filter has less distorted effect, only half a sample length is lost at each end of the filtered data.