Rz to Ra; Surface texture conversion

A. Hi Mohosin. The best advice is don't! Just MEASURE the surface's Rz instead. Although you can convert meters to inches or pounds to kilograms, because they are both units of length, you can't convert inches to pounds or meters to kilograms. The best you can hope for is some degree of correlation. If I tell you that my sister is 62 inches tall, you cannot tell me her weight; the best you can say is something like "It's reasonably safe to believe that she weighs more than 70 pounds and not more than 200 pounds."

The problem here is that going from Ra to Rz or vice-versa is a 'rough correlation' not a 'conversion' because Ra and Rz don't measure the same thing. For correlations to be relatively safe depends on which direction you want to go...
If you know the Rz, you can divide by somewhere between 4 and 7 to estimate the Ra that you will be able to pass with reasonable safety.
But if you know the Ra, you should multiply by probably 15 to 20 to estimate the Rz value you will be able to pass.

In other words, you can estimate that the Rz of your part is probably about 30 or 40, but even with such a conservative estimate you may not be able to promise it :-(

Regards,

A. Dear sir

As per the discussion of all that was found very good.
We can convert any value Like Ra to Rz which was already measured.
But the condition of measuring is very important.

Ex.: Ra means average of maximum and minimum.
Rz means points to be taken only peak values (maximum )
Measuring conditions are standardised and that should be followed.

Before measuring check whether given Ra / Rz value is in MM or Inches. Most of the confusion starts here only.

A. Hi cousin Shanath. When you use the word 'convert', in spite what is already written on this page -- that a very rough correlation rather than a conversion is the best you can hope for -- I get concerned that you may not have studied and understood what is already explained here. Rz involves taking five measurements of maximum peak-to-valley, and Rz1max is the largest of those five measurements. This classic picture from
https://www.productionmachining.com/blog/post/the-difference-between-ra-and-rz
shows two surfaces with identical Ra but ludicrously different Rz (and Rz1max):

The same page warns: "Gambling on conversion factors that you found on the internet is not professional. It is an example of poor engineering practice, and it fails to serve and protect your customer."

You have not introduced yourself yet, but because this site focuses on metal finishing, I will note that some metal coatings like nickel plating will produce a beautiful mirror finish from the profile on the left, but will make the peak and valley on the right-hand profile "stick out like a sore thumb", which is an example of why different measuring standards are used. If you need to monitor Rz1max please get a measuring device which measures it. Best of luck!

Regards,