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topic 6753

Rz to Ra; Surface texture conversion

A discussion started in 2000 but continuing through 2019


Q. Could someone please help me out with a conversion factor. I need to convert Ra to Rz. If my print states a machined surface to be controlled to a maximum of Rz125, what would be the Ra equal? My measuring equipment has parameter settings of Rz.J, Rz.I and R3z. Which setting would I want if the characteristic tolerance is Rz125? Help! Please.

Todd D [last name deleted for privacy by Editor]
- Toledo, Ohio

April 8, 2008

Rz (din) = Ra*7.2
Rz(ISO) = Ra*7.6


san [last name deleted for privacy by Editor]
- shanghai china

January 18, 2009

Q. Convert Rz value to Ra.


March 10, 2018

Q. Please guide me how to convert Ra 2 to Rz?

B Mohosin
Automotive - GURGAON, Haryana, India

March 2018

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 :-(


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

October 5, 2018

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.

suresh kumar.c
- chennai,Tamilnadu, India

July 18, 2019

Q. How to convert Ra to RZ1max?

shanath PANDIAN
- Chennai, Tamilnadu, INDIA

July 2019

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
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!


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

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