Surface Finish Instrumentation
The functional performance and mechanical properties of materials are all affected by surface finish. A total of 90% of all engineering component failures are believed to be initiated through mechanisms such as fatigue cracking, abrasive wear, or stress corrosion cracking. The functional performance requirements for surfaces vary hugely; those of vehicle brake disks are clearly different to those of bearing surfaces.
The surface texture characteristics are generated by a combination of the machining process, feed rate, tool speed, tool geometry, and also the environment conditions. It is very important to gain an understanding of how the manufacturing process modifies the surface of the material in use when planning any measurement. Specific processes such as the direction of lay (machining marks), and the level of control (e.g. a regular repeatable cylindrical grinding process vs a hand polishing process) should be considered.
The surface texture may be assessed comparative or direct measurement. Comparative methods assess the surface texture by either observation or the feel of the surface. The technique involves dragging the finger nail over the surface to be measured and then comparing the referencing standards. Although this was one of the first methods for surface texture measurement, it is still a popular method today.
Direct measurement of the surface is usually carried out with a tactile measuring instrument that is fitted with a diamond tipped stylus. This form of instrumentation can measure vertical movements down to nanometre values and due to the effects of vibration and shock, these are generally only used in a laboratory environment. The contact pressure applied at the tip of the stylus is very small in order to ensure it does not plough the surface which would result in false readings.
There are two types of stylus instrument in general use, those that have a skid to support the stylus, and those that do not. In the skidded instrument, the stylus is supported by a curved, metal skid which rests on the work piece surface and acts as a reference. The current ISO standards do not recommend skidded instrumentation, although these are still used in workshops as they are more robust than skidless instruments, and also provide additional filtering to the data.
There are many other non-contact or optical measuring instruments which can measure surface texture quickly. However, these non-contact methods depend upon reflections from the surface and these reflections can vary with different materials, depth, and characteristic of the surface. As a result, the only measurements traceable to either National or International Standards are currently made by tactile stylus instruments.
The surfaces to be measured can incorporate waviness or curvature which if not removed from the measurement will go on to affect the final result. Filtering can remove these effects, either automatically by modern digital equipment, or by using a skidded or larger styli. A stylus with a larger 5μm tip radius will filter out troughs that it cannot reach, and a skidded probe will filter out long wavelength waviness as the skid acts as the reference level.
Analysis of the surface requires magnifying the vertical displacement of the styli compared with the horizontal travel. This is known as the aspect ratio which is typically 400:1. The most common sampling length is 0.8mm (cut off length) with the evaluation length being the average 5 such lengths over 4.0mm.
Surface finish is typically measured using Ra (Roughness Average, also known as Centre Line Average [CLA], Arithmetic Average [AA]). This is defined as the average height from a mean line of all coordinates of the surface over the length of the assessment. This is the oldest and most common texture parameter, though today many new parameters have been derived with similar functionality.
A 2D line profile may be used to define the characteristics of a surface. On its own, Ra does not fully define the properties of a surface as can been seen in the illustrations below, where different machining processes have the same Ra, but the shape of the surfaces differ significantly.
The most commonly used 2D measurement parameters are as follows:
- Ra: Arithmetic mean of the absolute peak height and valley depth values.
- Rq: Root Mean Square (RMS)
- Rv: Maximum valley depth.
- Rp: Maximum peak height.
- Rt: Maximum height of the profile (Rt = Rp – Rv) within evaluation length
- Rz: Ten point height parameter.
- Rsk: Skewness – a measure of whether the bulk of the material is above or below the mean line.
- Rku: Kurtosis – measure of the sharpness of the surface.
Surface texture measurement takes place at the sub-micron level and therefore extra care should be taken in order to:
- Control the environment. Temperature variation, draughts and vibration will have a more noticeable effect on results taken at the sub-micron level.
- Keep the part, probe, fixture, and other parts of the measurement system clean. Dust, dirt, coolant and other contaminates will have a big impact on the measurement capability.
- To protect the styli from damage. Damaged styli will cause measurement errors as shown below.
- It is important to note that if comparative methods are used, it must be with surfaces produced by similar techniques.
- Dedicated fixtures will improve measurement repeatability. When carrying out repeatability measurements, it is important to measure the same surface as close to the original position as possible. This is because the surface will vary over the part and 2D measurement is only over a very small length. Fixturing will also help prevent misalignment of the stylus.
- The measurement device should be placed level to the measurement surface. Some measurement devices can be affected when both the device and measurement surface is tilted. If such a situation cannot be avoided, the effects of measuring at the required angle will need to then be assessed.
- Care must be taken when selecting instruments to measure curved surfaces, such as aerofoil surfaces. With portable skidded devices the supported skids have been found to prevent full contact of the styli with the curved surface. With skidless instruments, the range of movement in the vertical direction can also limit the curvature of the surface that can be measured.
- Inexpensive hand-held gauges are generally not suitable for measurement of complex free form surfaces such as curved aerofoils.
- Any stylus/probe can reach the required surface without causing damage to the probe or to the surface.
- The instrument calibration should be checked regularly using the glass surface roughness standards and the instrument itself should be serviced and calibrated by the instrument OEM.
- The instrument should first be checked on a surface roughness standard similar to the measurement roughness to be evaluated. The Ra value measured should correspond with the roughness standard. If the desired value is not as expected, then the instrument should be quarantined and the problem investigated.
- After turning the machine on, time should be given for the machine to stabilise. The minimum time for this is usually documented in the manufacturer’s instructions.
- The stylus shape will be a cone with either a 60˚ or a 90˚ inclusive angle. Tip radii should be 2μm or 5μm.
- The 16% rule on acceptance of surfaces is as follows when a surface roughness value is specified in the product definition:- A surface is considered acceptable if less than 16% of the measured values (over the measured length) are over/under the limit specified.
- Max Rule: when a maximum value is specified, none of the measured values (over the measured length) can exceed the specified value.
- The measurement direction should traverse perpendicular to the direction of the lay unless otherwise indicated. For aerofoil surfaces the measurement should be taken in the direction of the gas flow.
- Smaller tip sizes tend to report larger values of Ra, so care should be taken when choosing tip size.
- In the case of a skidless probe, the stylus tip should be the ONLY part of the probe to touch the part.
- Although surface roughness measuring instruments apply very little pressure to the surface, a 0.002mm stylus radius can create a small track in the material if the surface is not too hard. This will result in false readings.
- The use of the correct filter and correct sample length is very important as misapplication can lead to significantly incorrect interpretation of surface parameters.
- Results of the surface roughness measurements using Stylus instruments are the only true traceable devices to the national standards.