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Surface Finish Cross Reference Charts

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Q. When machining or grinding parts, surface finish is a controlling dimension. When working with German prints, Rz is used to describe surface specifications. DIN standards. Can anyone recommend a cross reference sheet showing Rz vs. Ra vs. RMS ? Ra & RMS are easy to find.

Thanks in advance.

Gerald Piotrowski
machinery co. - Warrenville, Illinois

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+++ A. Seems like a popular question. We get similar questions about once a month. First Rz and RMS are the same thing; however, they are measured differently. Rz is a result of ISO 9000 standards and specifically is measured over 5 peaks and valleys at 10 points on the part. RMS does not specify these factors. Ra to RMS is a variable ratio that depends on the manufacturing process and can vary from 1.11 to 2.10. AF Kenton Huntingdon Valley, Pennsylvania +++++++ A. We recently purchased a surface finish tester and I was informed that RMS & Rq are the same rather than RMS & Rz? Tony Potts mfg - Warren, Michigan A. Tony P., While my knowledge of mechanical finishing does not rival Mr. Kenton's, I believe that he has made an error, and that you are correct that it is Rq rather than Rz which is essentially the "same thing" as RMS. One of the quickest but clearest descriptions of the differences between Ra, RMS, Rmax, and Rz is at www.ptonline.com/articles/200403fa3.html Then you may want to jump to letter 9524, which explains the mathematical relationship between RMS and Ra pretty exhaustively. They can't be converted because they don't measure quite the same thing, but they do track each other such that pretty good approximate conversions are possible, as Mr. Kenton has said. Good luck. Regards, Ted Mooney, P.E. finishing.com Brick, New Jersey

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August 7, 2008

Q. How to convert 1.86 Ra into Rz value?

Milind D Shopeshwarkar
Manager - India

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Q. What exactly is Rmax and RMS value? Is Ra and Rz are same in meaning ie former represents ISO std. and latter one for DIN std.? How these values are calculated theoretically which is implied to the checking instrument?

Sachin Rohatgi
Automobiles - Sahibabad, Uttar Pradesh, India

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A. Here is a handy cross reference for you to use. It shows the difference between the standards.

www.clearlakefiltration.com/pdf/surface_finish_chart.pdf#search='surface%20finish%20RMS%20chart' Craig Jackson
- Greenville, South Carolina

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A.

Sachin,
Rmax is a bit different and you should not attempt to approximate it in terms of the other measurements because it is meant for a different purpose. It is essentially the deepest of five adjacent peaks and valleys, and tends to be used in electroplating and buffing, not as a measure of overall roughness, but as an indication of whether there are visible scratches on the surface.

Miland,
Rz is the most complicated of all because it has had three different definitions over the years :-(  

The approximate conversion between Ra and Rz is well explained at http://www.mmsonline.com/articles/1102gage.html and you will probably find it a bit disheartening that the "conversion factor" will depend upon in which direction you are converting.

If you are controlling via Rz but your customer will be measuring in Ra, you would divide your Rz by between 4 and 7.6 to guess at the Ra he will be reading.

 

If you are controlling via Ra but your customer will be measuring in Rz, you would multiply your Ra by more than 7.6 and as much as 20 to guess at the Rz value he will read.

The point of all this is that it's fine to "convert" one parameter to another to give yourself an overall feel for what another roughness scale is like, but they measure different things so there is no such thing as a real conversion. If your customer is going to measure a different parameter than you, you risk failing his inspection unless you apply a very big safety factor in making your "conversions".

Regards,

Ted Mooney, P.E. finishing.com Brick, New Jersey

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June 20, 2009 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread Is there any correlation between Ra & Rmax while measuring surface roughness? Chandrashekhar P process engg. - dewas (M.P.) INDIA June 22, 2009 Hi, Chandrashekhar. We have appended your inquiry to a thread which hopefully answers it. There is some small degree of correlation, similar to the fact that tall people will tend to weigh more than short people. But there is no exact relationship. If I tell you that one of my sisters-in-law is 5' 8" tall and the other is 5' 2", you might assume that the taller is the heavier, and the odds would be in your favor, but you might be wrong :-) Regards, Ted Mooney, P.E. finishing.com Brick, New Jersey

Profilometer

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August 3, 2009 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

The material we purchase comes in with an inspection sheet measuring Ry. We only have the ability to measure Ra and cannot afford to purchase a machine to measure Ry. Is Ry the same as Rmax, or Rz, the max. peak to valley height? Is there an equation to approx. convert from Ry to Ra? Thanks.

Ron Homolak
tubing finisher - Marion, South Carolina

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August 4, 2009

Hi, Ron. As you can see, we appended your inquiry to a thread which partially answers it. Yes, it is my understanding that Ry is the same thing as Rmax.

But that you cannot convert Ry to Ra and vice versa. Let's look at it from a plater's perspective: Nickel plating will level out a rough polished surface and generate a nice, smooth, mirror surface, but is incapable of filling in a severe scratch -- in fact, it will only highlight it. So a plater would like to reject any parts with a deep scratch rather than wasting money processing parts that will be defective. The Ry value tells him how deep the worst scratches are.

Ra, the average roughness, may be important for a coefficient of friction application, for lubrication situations, for general appearance without finishing, and other reasons, but it doesn't tell the plater what he needs to know, and he cannot calculate from the average roughness what the deepest scratch is.

Regards,

Ted Mooney, P.E. finishing.com Brick, New Jersey

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November 11, 2009

An example comparator:

Ra 116 (inches) = Ra 2.95 (metric) = Rmax 374 (inches) = Ry 9.5 (metric)

per Mitutoyo precision reference specimen.

Max Turnmire
- Beloit, Wisconsin

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May 3, 2011

Have a slider under a rocker arm in an engine. One side has a sphere machined into the top and a flat surface on the bottom. Is is made currently by a German company and we are shopping for a U.S. supplier and all have a problem with the Rz6.3 finish in the sphere and the flat bottom. We sent new parts for them to analyze to convert to Ra but they came back with Ra to Ry which a reply stated its used by platers. I don't think we want a glass smooth surface rather a little rough to hold an oil film. I'm surprised that profilometers won't make the conversion.

Brad Bee
engineer - Livonia, Michigan

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May 4, 2011

Hi, Brad.

Of all of the confusing and almost countless surface finish roughness specifications, Rz is the worst because there are 3 different Rz scales and meanings. As noted in Alex Tabenkin's excellent article "To Each His Own Parameter" in the June 01 issue of Quality Digest (www.qualitydigest.com/june01/html/surface.html): "If your print calls out an Rz value with no further explanation, warning bells should go off. A time bomb may be ticking."

As a starting point you need to find out what standard it is that these parts are to be measured by. It's probably an ISO/ASME standard, but that's just guessing.

Regards,

Ted Mooney, P.E. finishing.com Brick, New Jersey