Before reading the chart below there are a couple of points that may help
Q – What is Ra vs Rz?
One of the most straight forward explanations I could find is from an article in Modern Machine Shop written by George Schuetz, director of precision gauges at Mahr Federal
“Ra is calculated by an algorithm that measures the average length between the peaks and valleys and the deviation from the mean line on the entire surface within the sampling length. Ra averages all peaks and valleys of the roughness profile and then neutralizes the few outlying points so that the extreme points have no significant impact on the final results.
“Rz is calculated by measuring the vertical distance from the highest peak to the lowest valley within five sampling lengths, then averaging these distances. Rz averages only the five highest peaks and the five deepest valleys—therefore, extremes have a much greater influence on the final value.”
Q – What about Rt, Rmax, RMS…
There are many important considerations when looking into surface roughness – not limited to but including the direction of measurement
If you want to know more a good place to start is the following Mitutoyo Guide – Bulletin No 1894
This also includes VDI, N scale and Conventional finishing mark
VDI – is a scale often used by German machine manufacturers and quite widely used in Spark and Wire Erosion (Verein Deutscher Ingenieure, the Society of German Engineers)
N – New ISO (grade) scale numbers ISO 1302
It is important to note that using conversion factors to calculate between Ra and Rz is not a good idea – it is best to establish what figure for surface roughness the designer (customer) has specified and measure this. Surface measuring equipment (surface profilers) are although quite expensive readily available and they should be able to measure several features to suit a variety of designers requirements. We are unable to accept liability for information contained in the chart although a great deal of care has gone into its creation.