Roughness Testers

Roughness

Roughness is a measure of surface texture. It is calculated from the vertical deviations of the real surface from its ideal form. If these deviations are large, the surface is rough. If they are small, the surface is smooth. Roughness plays an important role in determining how a real object will interact with its environment.

Irregular surfaces usually wear faster and have a higher coefficient of friction than smooth surfaces (see tribology). Roughness is often a good indication of the performance of a mechanical component, since surface irregularities may be initiation points for cracks or corrosion.

The standards that describe how the roughness of a surface is calculated, and with which roughness testers comply, are ISO 4287:1997, JIS B0601:2001, DIN, and ISO 12085:1998.

Operating principle of the roughness tester.

When measuring the roughness of a surface, the gauge is placed on the surface and then its mechanical part slides uniformly along the surface (Figure 1). The gauge detects surface roughness via the sharp built-in sensor (diamond). Surface roughness causes displacements of the sensor, resulting in an induced voltage in the coil located inside the gauge. The signal is then fed to a rectifier for phase correction. The above process converts the mechanical motion into an electrical analog signal that is proportional to the surface roughness. Finally, the device’s DSP processor performs the calculations for the selected measurement parameters, and the measurement result is then displayed on the LCD screen.

The stylus, mounted on the hinge with the appropriately fitted diamond tip, slides parallel to the surface of the object. Following the surface profile, the stylus moves vertically and its vertical displacements are recorded as the measured profile.

Basic definitions for measuring the roughness of a surface.

Roughness profile

This is the profile curve after a high-pass filter (cutoff wavelength λc) is applied to the primary curve and the long wavelengths are excluded. The parameters calculated from the roughness curve are called "roughness parameters" and the symbol for these parameters is "R". Below are the definitions of the most common ones (Ra, Rq, Rt, Rz, Rp, Rv, R3z,) encountered in surface roughness testers. (Figure 3)

Waviness profile

This is the curve after applying a band-pass cutoff filter (cutoff value λf / λc) to the primary curve. The cutoff value λf excludes the long-wavelength portion, while the cutoff value λc excludes the short-wavelength portion. The parameters calculated from the waviness curve are called "waviness parameters". (Figure 3)

Cutoff wavelength

The cutoff wavelength is the branching point between the roughness components and the waviness components, as shown in the image below.

The magnitude of the transfer function as a function of wavelength.

Sampling length

The fixed length sampled from the surface profile curve in order to calculate the surface characteristics. This is called the sampling length. The sampling lengths of the roughness profile and the waviness profile are the same as the corresponding cutoff values.

Primary profile

This is the profile curve after applying a low-pass filter (cutoff value λs) to the measured surface profile curve and the short-wavelength portion is excluded. The parameters calculated from the primary curve are called "primary profile parameters". (Figure 3)

Evaluation length

The characteristics of the profile curve are calculated over a fixed length that includes at least one sampling length. This length is called the evaluation length. As a standard, the evaluation length is defined as five times the sampling length.

The table below provides the appropriate cutoff lengths depending on the application and the average roughness Ra.

Table 1: Cutoff length depending on the average roughness value and the corresponding application.

Types of curves of a surface profile.

Roughness measurement parameters (peaks and valleys).

**Average roughness, Ra,** the arithmetic mean of the absolute values of the profile heights over the evaluation length.

**Root mean square roughness, Rq,** is the square root of the mean of the squares of the profile height over the evaluation length.

**Maximum profile height, Rt,** the vertical distance between the highest and the lowest point of the profile within the evaluation length.

**Mean maximum profile height, Rz,** is the mean of the successive Rti values calculated over the evaluation length. This parameter is the same as Rz (DIN) when there are five sampling lengths within the evaluation length.

**Maximum profile peak height, Rp,** the distance between the highest point of the profile and the mean line within the evaluation length.

**Maximum profile valley depth, Rv,** is the distance between the deepest valley of the profile and the mean line within the evaluation length.

**Third maximum peak-to-valley height, R3z,** is the mean of the third maximum peak-to-valley height over the evaluation length.