The importance of measuring surface roughness

In the earlier days of the automotive industry, many things were different to our modern standards. It’s not just the lack of seatbelts that is a world apart from our modern understanding of the industry’s needs: surface roughness is a specific element that has been important throughout the industry’s development yet has changed as much as the safety standards.

Cars that we now consider vintage used to have a period where it was necessary to ‘run-in’ the engine when they were brand new. During the running-in (for the first thousand miles or so of driving) the speed of the car was limited. Why? Essentially, with the tools available at the time, it was not possible to make the surfaces of engine components such as the cylinder bores as smooth as with modern production methods. Because of this, the moving parts of engines were specifically made to be very tight, which meant that the engine would essentially sand down these moving parts through friction, until eventually it would run more smoothly because of, quite literally, smoother surfaces. It’s an early example of how manufacturers recognised the value of surface characteristics. Mike John, technical director at The Sempre Group, explains the importance of surface roughness and how to get the most out of this data.

Key highlights:

• Surface roughness technologies have advanced greatly
• It is vital for any machined product that uses moving parts, such as engines
• Capturing data allows not only analysis of trends, but helps in preventative adjustments

“Surface roughness often dictates how one part interacts with another. For example, if a shaft is rotating inside a bearing, a rough surface is undesirable because it causes excess friction. Meanwhile, a smooth and round surface ensures optimal performance by minimising resistance. If turned parts have the wrong surface characterisations, they could wear out, get bigger, smaller or rattle around.

In the automotive sector, surface texture is vital for anything that rotates in an engine. For example, camshafts will sit on a white metal bearing, a smooth object with a coating, and oil will produce a frictionless surface. However, if the surface roughness is poor, this will cause metal-to-metal contact. Consequently, the part will wear quicker, and irregularities in smoothness can produce nucleation sites where breaks and corrosion occur.”

Measuring roughness

Roughness average (Ra) is used to measure and indicate micro- and macro-level geometric irregularities. Ra is used to show when a tool starts wearing out, resulting in it producing different surface characteristics. This information can then be used to decide when a tool change is necessary to remain within spec.

But Ra only gives an average: a value that is the deviation away from a median height. It does not show which direction this deviation is happening in (peaks vs. troughs). “Knowing what parameters will provide the required functionality of the surface is the first step. For instance, manufacturers can use plateau honing to create a metallurgically stable microstructure on the wall of a cylinder bore. Then, they can characterise the peaks and troughs as a number using skewness (RSK) and determine whether this is positive or negative. Automotive manufacturers will often use three or four characteristics, including Rz and response surface methodology (RSM).

There are two methods of finish measurement — skidded and skidless. Skidded stylus systems are ideal for simple measurement of high-frequency surface roughness, while skidless technologies are better for low-frequencies ripples, waviness and surface profiles. For example, the Jenoptik Waveline W5 features a skid situated by a stylus. The skid drags along the surface to remove the need for a straightness reference, removing added costs. The system can measure up to 28 parameters and includes a changeable probe and guide system for adaptability.”

Collecting data

Compliance is usually determined by comparing the outcome of measurements against designs, which specify the required dimensions and surface texture of completed parts. Although this is recognised as an important step in the manufacturing process, most manufacturers either don’t gather the surface roughness data, or they don’t store it effectively for studying trends. If components are in spec, that’s usually good enough. With the help of automated solutions, however, data can be easily gathered and exported. Sempre gives the example of High QA Inspection Manager, which is able to scan drawings, or multiple pages, in one click and automatically extract surface roughness data. With a fully automated process, this makes enables total traceability. Using this data not only results in a more efficient part and improved functionality, but can also greatly assist in preventative tool adjustments or replacements.

We may no longer need a ‘run-in’ period for a brand new car, but that’s only because surface roughness technology has improved to the point where components can now be manufactured and measured up to a much higher standard.