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Areal Surface Metrology

Surface metrology, referred to as "Areal" surface metrology plays an important part in understanding precision machining processes. It is sometimes referred to as 2 ½ D as it is technically 3 dimensional but the vertical component is very small, this the ½ D. Cranfield Precision has a comprehensive understanding of what features make up a surface and how they pertain to machine performance and subsequent machine design.

What is a surface?

This seems like a simple and perhaps obvious question, but when looked at in detail it is slightly more complex. In the most broad terms a surface is simply the boundary between the environment and a component. But when you consider all of the features of a surface and the fact that 90% of component failures start at the surface it becomes more complex.

How are surfaces specified?

This is another seemingly simple area that has a much more complex and expansive answer. Even today, many people discuss surface texture in terms of Ra, however Ra or Roughness Average does not give very much information about a surface. It is a quantifiable parameter but it does not tell you what type of surface you have and further more, it only gives an average value of the area measured, not an accurate picture of the type of features on a surface. Further more, Ra tells you nothing of whether you have a surface with lots peaks or troughs. Therefore two surfaces with the same Ra value could have quite significantly different performance characteristics and this is ultimately what we are concerned with.

Very often, numbers are quoted for surfaces that seem to be extremely good and in some cases they seem to defy belief. This is because they are quoting a parameter that is perhaps not relevant or properly reflective of what type of surface texture you have. For example, you may have a high quality diamond turned surface that is extremely smooth but has a spike in the centre of it. This surface would provide very poor optical characteristics yet would still provide a seemingly good "optical quality" surface finish in terms of Sa. Understanding the different types of surface characteristics and how they are characterised is vitally important.

What parameters are important?

It is very important to understand which parameters are important to your process or product and what they mean. A great example of this is in the production of photo voltaic cells where the process is altered and Ra is monitored. The Ra value remains fairly constant but the performance of the cells significantly changes. This is due to the shape of the features on the surface and their ability to channel light. By monitoring the correct parameters it is possible to identify which features effect the performance of a surface.

Cranfield Precision has conducted research in collaboration with the NPL (National Physical Laboratory) into precision ground surfaces, varying a number of parameters and determining the effect of machine configuration and process variables in great detail.

What can we learn from surfaces?

Every machining process leaves a trace similar to a "finger print" that has specific characteristics. There are inherent surface characteristics that relate to the type of machining process and there are also machine specific characteristics. Both of these characteristics can be measured and controlled with a good understanding of surface metrology.

Machine and process performance can therefore be optimised through effective application and understanding of areal surface metrology.


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