The ABCs of Rz
Recently an American-based German manufacturer was awarded a contract to produce some parts for a Japanese company. According to the contract, critical surface areas of the parts were to meet roughness criteria as defined by the Rz parameter. This appeared to be no problem, as Rz-mean roughness depth-is the most commonly specified surface finish parameter in Germany and several other European countries, instead of the U.S. preferred choice, Ra.
However, it turned out that the Rz specified was not the current ISO or U.S. standard version they were used to, but a much older Japanese domestic standard dating from 1982. Measurements based on this standard would not be fully compatible with current standards. None of the German manufacturer’s instruments were set up to measure to it, and it took a virtual search party to even find a copy of the standard document.
This situation is not unusual and as manufacturing becomes increasingly internationalized, instances like this occur with increasing frequency and with potential costly results. In Germany, as in several other European countries, as well as the United States, it is expected that manufacturers will switch to new standards as soon as they are published. However, in Japan and some other countries, introduction of a new standard does not prevent the use of the old edition, as long as there is a reference to it. If a company has been successfully making parts for a number of years using an existing standard, there is no requirement for them to change just because a new standard has been issued.
A Profusion of Rz ParametersIn the case of Rz, the situation is particularly complex because it is one of the oldest surface finish parameters. Unlike Ra, or average roughness, which considers all the peaks and valleys in a sample, Rz typically looks only at the five highest and five lowest points in a sampling length and averages peak to valley distances. Thus, many users consider it more sensitive to actual changes in surface finish than Ra. What made Rz so useful when it originated back in the 1920s and ‘30s is that it can be evaluated manually from a graph, or through the eyepiece of an optical instrument and does not require electronics.
But as technology has evolved around the world, a number of different ways to evaluate mean roughness depth have become codified in various national and international standards. Since the Rz parameter was first standardized, five different definitions have evolved, based on different algorithms or formulas, different sampling or evaluation lengths, and by using different filters. These five definitions can be summarized as follows:
- Definition 1. 10 point average distance between the five highest peaks and the five deepest valleys within an evaluation (reference) length, from an unfiltered profile.
- Definition 2. Distance between two lines, parallel to the mean line, that touch the unfiltered profile at the third highest peak and the third deepest valley, within the sampling length. This is an approximation of the 10-point evaluation, but the parameter was named Rz in several old JIS standards.
- Definition 3. 10 point average distance between the five highest peaks and the five deepest valleys, within a sampling length, from a roughness profile.
- Definition 4. Average maximum peak to valley distance within five sampling lengths, from a roughness profile.
- Definition 5. Sum of the largest peak height and the largest valley depth, within a sampling length, from a roughness profile.
Thus, when manufacturing engineers, machinists or parts inspectors encounter Rz on a part print, they need to know which Rz is referred to. Because there are significant differences between standards from different countries, and also between different editions or versions of a standard within a country, they need to know where the standard was issued and when.
One of the differences in Rz evaluation is in the type of filter. The first electrical filters used to separate roughness from waviness data were 2RC high-pass analog filters. These were used until new Phase Correct Gaussian filters (50% G) were introduced in ASME B46.1 in 1995 and in ISO standards in 1996.
Thus, if a part specification is based on a standard written before 50% Gaussian filters were introduced, evaluation should be based on 2RC filtering. In the real world today, many instruments are offering only 50% Gaussian filters for evaluation of all parameters, whenever they were standardized.
Comparing ResultsThe table, “Comparison of Rz Definitions,” illustrates the various definitions of the Rz parameter and lists the different national and international standards associated with each method. As might be expected, using different definitions of the parameter yields different results.
For example, compare results of measurements performed in accordance with the latest editions of these standards:
- • Rz JIS-94, based on definition 3
• Rz ASME-02, based on definition 4
• Rz ISO-97, based on definition 5
Accurate ResultsComparing apples to oranges is rarely helpful even when making fruit salad. When applied to Rz measurements, it can lead to untold frustration and considerable cost. Clearly, ensuring the part designer, manufacturer and customer all understand which Rz method is indicated is the best way to ensure accurate results. Conversion factors should be used only when necessary and even then with an abundance of caution.
Of the five different methods of Rz evaluation discussed, the most commonly specified and used are Rz ASME-02 and Rz JIS-94. However, some surface finish instruments today are capable of accommodating any Rz parameter or filtering method, even if they are not already set up to do so. The only real stumbling block is recognizing when there may be a difference in the first place. Q