Letter 20490

Surface Finish Cross Reference Chart

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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

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 Nova Finishing Systems Inc. Huntingdon Valley, PA

+++ 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 ++++++ I am also in sight for any detailed information on the difference between Ra & Rz surface finishes.Can any one help what is the standard to be referred. Yogesh Pandita - Gurgaon, Haryana, India

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

http://www.clearlakefiltration.com/pdf/surface_finish_chart.pdf#search='surface%20finish%20RMS%20chart'

Craig Jackson
- Greenville, South Carolina

We recently purchased a surface finish tester and I was informed that RMS and Rq are the same not RMS and Rz?

Tony Potts
MFG - Warren, Michigan

How to convert 1.86 Ra into Rz value?

Milind DShopeshwarkar
Manager - India

Tony P.,

While my knowledge of mechanical finishing does not rival Mr. Kenton's, I believe that he has made an error. I believe you are correct that it is Rq rather than Rz which is essentially the "same thing" as RMS.

One of the 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 cannot be exactly 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.

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

In the print surface roughness noted as Rz140, what does 140 defines?, Is it a machined or ground finish?, what is the equivalent number in Ra?.

KESAVAN SIVASAMY
Quality Assurance - Coimbatore, Tamil Nadu & India

June 20, 2009

Is there any correlation between Ra & Rmax while measuring surface roughness

Chandrashekhar P
Process engg. - dewas (M.P.) INDIA

Hi, Chandrashekhar. We have appended your inquiry to a thread which hopefully answers it. There is some degree of correlation, similar to the fact that tall people tend to weigh more than short people. But there is no exact relationship.

Regards,

Ted Mooney finishing.com Brick, New Jersey

August 3, 2009

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

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 you cannot convert Ry to Ra and vice versa. To understand why, look at it from a plater's perspective. Plating processes such as nickel plating will level out a roughly polished surface and generate a nice, smooth, mirror surface -- but they are completely incapable of filling in a severe scratch -- in fact, they will highlight it. Rather than wasting money processing parts that will certainly be defective, the plater wants to reject anything with a scratch too deep. 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 there is no way he can calculate from the average roughness what the deepest scratch is. Regards, Ted Mooney finishing.com Brick, New Jersey October 6, 2009 hey guys, i am from China, and i get a drawing from my American client. the surface requirement is Rms10-16; but i just know about Ra Rz Ry, not knowing Rms. according to your experience, should we grind or machine to reach Rms10-16, is it very rough or is it very smooth? thanks for your help! Frankie Fu - Wuhu, Anhui, China November 11, 2009 An example comparator: Ra 116 (inches) = Ra 2.95 (metric) = Rmax 374 (inches) = Ry 9.5 (metric) per Mitutoyo presision reference specimen. Max Turnmire - Beloit, Wisconsin