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Stainless Steel Finish: 20 micro inch?

For one of our ongoing job, I need 3 MM thk Stainless Steel plates, 304 grade with a surface finish of 20 micro inches. But the market people knows the standards like B1 finish, No.1 finish etc And in all Mill Test Certificates, the finish is given same like. So can anybody please help me to find out "What is this 20 micro inch" stands for And the equivalent standard in BS or American Code.

Thanks and Regards,

ZUBAIR
Design Engineer - DUBAI, UAE
2004

Micro per inch is the unit of roughness which has some way to be interchanged to BI or else, please look up the suppliers for roughness tester and they might help.

Daryl Yeung
- Hong Kong
2004

First of all, you need to know from your customer, 20 micro inch WHAT? The micro inch (or micron in metric) is just the unit of measurement, not the surface roughness parameter. There is a whole zoo of surface roughness parameters, but by far the most common and universal is Ra, which is the arithmetic average deviation of the surface profile from the mean or center line. But there are many other surface roughness parameters also, like RMS, Rq, Rz and many more, and many of those have different definitions in different specification systems such as ISO, DIN, JAL, BS, ASME, etc.! So, it is important to insist from your customer what surface roughness parameter it is that must meet the 20 micro inch specification. Also, is that 20 micro inch finish a maximum or a minimum? You need to know that also. Finally, if we assume it is 20 micro inch Ra MAX (most common), your people must make that very clear to suppliers when ordering the steel.

Ray B Anderson
- Kent, Ohio
2004