Surface Measurement

Surface topography is of great importance in specifying the function of a surface. A significant proportion of component failure starts at the surface due to either an isolated manufacturing discontinuity or gradual deterioration of the surface quality. Typical of the former is the laps and folds which cause fatigue failures and of the latter is the grinding damage due to the use of a worn wheel resulting in stress corrosion and fatigue failure. The most important parameter describing surface integrity is surface roughness. In the manufacturing industry, surface must be within certain limits of roughness. Therefore, measuring surface roughness is vital to quality control of machining work piece. Below are the definition of surface roughness and its main measurement methods.

Description of Surface Roughness

The roughness of a surface can be measured in different ways which are classified into three basic categories:
• Statistical descriptors that give average behavior of the surface height. For example, average roughness Ra; the root mean square roughness Rq; the skewness Sk and the kurtosis K.
• Extreme value descriptors that depend on isolated events. Examples are the maximum peak height Rp, the maximum valley height Rv, and the maximum peak to valley height Rmax.
• Texture descriptors that describe variations of the surface based on multiple events. An example for this descriptor is the correlation length.
Among these descriptors, the Ra measure is one of the most effective surface roughness measures commonly adopted in general engineering practice. It gives a good general description of the height variations in the surface. The following figure shows a cross section through the surface , a mean line is first found that is parallel to the general surface direction and divides the surface in such a way that the sum of the areas formed above the line is equal to the sum of the areas formed below the line. The surface roughness Ra is now given by the sum of the absolute values of all the areas above and below the mean line divided by the sampling length. Therefore, the surface roughness value is given by

Ra=(|area abc|+|area cde|)/f.

where f is the feed.

Main Measurement Methods of Surface Roughness

Inspection and assessment of surface roughness of machined workpieces can be carried out by means of different measurement techniques. These methods can be ranked into the following classes:

Direct Measurement Methods

Direct methods assess surface finish by means of stylus type devices. Measurements are obtained using a stylus drawn along the surface to be measured: the stylus motion perpendicular to the surface is registered. This registered profile is then used to calculate the roughness parameters. This method requires interruption of the machine process, and the sharp diamond stylus may make micro-scratches on surfaces.

Comparison Based Techniques

Comparison techniques use specimens of surface roughness produced by the same process, material and machining parameters as the surface to be compared. Visual and tactile senses are used to compare a specimen with a surface of known surface finish. Because of the subjective judgment involved, this method is useful for surface roughness Rq>1.6 micron.

Non Contact Methods

There have been some work done to attempt to measure surface roughness using non contact technique. Here is an electronic speckle correlation method given as an example.

When coherent light illuminates a rough surface, the diffracted waves from each point of the surface mutually interfere to form a pattern which appears as a grain pattern of bright and dark regions. The spatial statistical properties of this speckle image can be related to the surface characteristics. The degree of correlation of two speckle patterns produced from the same surface by two different illumination beams can be used as a roughness parameter.

The following figure shows the measure principle. A rough surface is illuminated by a monochromatic plane wave with an angle of incidence with respect to the normal to the surface, multiscatterring and shadowing effects are neglected.  The photosensor of a CCD camera placed in the focal plane of a Fourier lens is used for recording speckle patterns. Assuming Cartesian coordinates x,y,z, a rough surface can be represented by its ordinates Z(x,y) with respect to an arbitrary datum plane having transverse coordinates (x,y). Then the rms surface roughness can be defined and calculated.

On-process measurement

Many methods have been used to measure surface roughness in process. For example:

Machine vision. In this technique, a light source is used to illuminate the surface with a digital system to viewing the surface and the data being sent to a computer to be analyzed. The digitized data is then used with a correlation chart to get actual roughness values.

Inductance method.  An inductance pickup is used to measure the distance between the surface and the pickup. This measurement gives a parametric value that may be used to give a comparative roughness. However, this method is limited to measuring magnetic materials.

Ultrasound. A spherically focused ultrasonic sensor is positioned with a non normal incidence angle above the surface. The sensor sends out an ultrasonic pulse to the a personal computer for analysis and calculation of roughness parameters.