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topic 5396 p2

Ra surface finish -- unit-less, or what are the units, p.2



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A discussion started in 2000 but continuing through 2019

February 20, 2014

Q. Profilometers have various "cut-off Lengths" (Lc), such as .003, .01, .03 etc... Should this be changed when different 'RA's' are called for, such as a 16RA, 8RA or a 2RA? Again thank you for your time.

Paul Desko [returning]
Quality - Torrington, Connecticut, USA


February 21, 2014

A. Never looked at this question the way you are doing. OK. #1 A profilometer measures the surface roughness of any part. It is an average of the surface the stylus travels. #2 It can not do anything about the number because that is the roughness of the part or the section that was read with the profilometer. The number only indicates to the end user how smooth or how much more work has to be done to the part to get it into an acceptable surface finish to either paint, plate, or leave it. In the medical industry, the smoother finish or lower number is desirable to reduce the possibility of bacteria adhering to the part when in use.

tony kenton
AF Kenton
Hatboro, Pennsylvania


February 2014

A. Hi Paul. The advances in surface topography over the course of my career make my head spin, with terms that were unheard of or at least completely ignored years ago now becoming critical as we continue towards miniaturization. So I needed a 15-minute refresher on this Lc and Ls issue. I'm talking myself through this, and explaining it to readers, so please don't interpret these examples as talking down to you, since I'm sure you know much more about the topic than I do --

In brief, in addition to the local roughness of a surface, it also may have a long wavelength waviness. For example, we can picture a football field where the Astroturf may have a particular roughness that we measure and are careful about so cleats grab but players' skin doesn't -- but the field may have a bit of ponding at the left hashmark on the 20-yard line that we may not notice and which doesn't matter and which may even inject a serendipity that fans demand. So we don't study every square inch of the whole field, just a little patch, and our long wave cutoff (Lc) may be 6 inches rather than 100 yards. This Lc we choose has more to do with functionality than with with whether Brand A artificial turf is rougher than Brand B.

Regarding Ls, the short wave cutoff, I once read that a shoreline looks generally the same regardless of magnification. So you magnify the picture a hundred times and you still see the same level of graininess. Magnify the picture of that short section a hundred times and you still see it; do it again and you still see it. So no profilometer or other instrument that is following that shoreline is really following it, it is following it at one scale, but is jumping over the graininess at some smaller scale. The roughness/graininess below some magnification level is ignored; that's Ls.

I think the answer to your question is that Lc must be specified on the basis of functionality -- what really matters. You are specifying an Ra in the first place because it impacts the functionality of the part. The instrument can't tell the function of the part and why Lc matters, but the designer can. As a practical matter, I'd assume that the larger the machined area, usually the higher the Lc.

Ls is conventionally specified as 1/100 of Lc. Good luck.

Regards,

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E.
finishing.com - Pine Beach, New Jersey
Striving to live Aloha


February 24, 2014

Q. So then, how does 'Lt' or length of travel, of the profilometer factor into this equation? The cylinders that we grind are as small as 3" in dia. x 30" long up to 59" in dia" and up to 400" long! Our customers do not specify any Lc or Lt, so it is left up to us to decide what is best for them. Thanks again!

Paul Desko [returning]
Quality - Torrington, Connecticut, USA


February 2014

A. As I say, this whole idea of Lc and Ls is a bit foreign to my career experience, and it may be the same with others. But actually ISO 1302-2002 says that the drawings are supposed to specify Lc and Ls.

But if the designer doesn't specify them, no, it is NOT up to you to decide what is best for them -- because you are not in a position to hazard guesses about what a designer is contending against. So I guess you say that in the absence of contrary info, mid range readings are usually best, so that's how you pick Lc on your profilometer and that's that :-)

As for Lt, I keep seeing different definitions for that, some claiming Lt = Lc, some saying the profile is evaluated 5x, so Lt = 5 x Lc plus pretravel and post travel. I think you have to refer to the operating instructions for your profilometer to determine exactly what it means to the instrument you are using, but that you should pick the middle of the range for Lc. Sorry, I can't take it any further because going further would imply an expertise I don't have :-(

Regards,

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E.
finishing.com - Pine Beach, New Jersey
Striving to live Aloha


February 25, 2014

A. Just some additional thoughts. If you did not talk with your customer about this issue, make sure you document what you did do and show support information.

tony kenton
AF Kenton
Hatboro, Pennsylvania



February 25, 2015

Q. I am reviewing a drawing that calls out a surface roughness of Ra 16 Min. How would you interpret this callout? I have seen "MAX" call outs, but never "Min". Also is there a specification that defines how to use "Max" and "Min"?

Jose Supan
- Conshohocken, Pennsylvania, USA


March 3, 2015

A. It is a little strange that they should use terminology min; however,I think you should check with your customer. As good customer relations, I'd inform him that technically you do not have to do anything to the part for you to conform to his requirement except bill him for nothing unless the parts are way under his referenced specs. in which case you have to make them rougher.

tony kenton
AF Kenton
Hatboro, Pennsylvania


March 4, 2015

A. Hello, regarding the Ra min, I have seen it scarcely but it is quite common on bore with press fit operations; per say I have a bore where we press a seal and we need the Ra to range from 0.8 min to 2.5 max; smoother surface than specified would cause the seal to move from position, higher than specified would cause damage to the seal for the friction during the press in operation.

We honestly make it reader friendly by stating: Ra 0.8 - 2.5.

And that is that... so yes, you have to be careful sometimes with smoother than expected surfaces.

Luis Munoz
- El Paso Texas USA


March 17, 2015

Q. Can you help me understand this call out:
8Ra, Rmr (bearing ratio) 50-70%, Rsk negative skew ratio

Tony Ryland
- Watertown South Dakota USA



August 20, 2016

Dear sir,
my question is how to measure Ra value and what is exact unit.
What is difference between Ra value 0.3 or Ra value 8

parmeshwar pawar
- pune , maharashtra india


August 2016

adv.
Profilometer

A. Hi parmeshwar. As I'm sure that you can appreciate, surface roughness can be a rather involved issue. Consider: is sandpaper smoother or rougher than a splintered wooden boardwalk? Looking from a good distance the wood boardwalk, with its divots and missing chunks of wood and long splinters is much rougher than the sandpaper, whose roughness you probably can't even see from 10 feet away. But up close, the wooden boardwalk will have slick spots of smooth polished wood that you can slip on, whereas the sandpaper will look and feel very rough. So there really isn't an answer to the question of which is rougher until you carefully define the roughness that you want to consider and measure.

Ra is one of many ways of looking at and ranking roughness, and you need to apply your mind to understand what it means, but there are pictures and explanations at www.wisetool.com/surfacefinish.htm and many other websites.

Ra is measured with an instrument called a profilometer which has a tiny needle or laser which tracks the surface and does the math to tell you the Ra of the surface.

Ra is measured in millimeters (or in microinches in the US system). People can't tell you the difference between Ra of 0.3 and Ra of 8 until you tell them whether the readings are in millimeters or microinches ... but Ra 0.3 millimeters would equal about Ra 12 microinches.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com - Pine Beach, New Jersey
Striving to live Aloha



Dwg. says Ra6.3; we did Ra1.6 -- is it okay?

October 21, 2016

Q. Dear All: I wanted to understand a bit on machining surface finish. Sketch calls for Ra6.3. Actual achieved is 1.6 on CNC. Should we interpret Ra6.3 as max or min. There is no range specified. Kindly advise

Dinesh Bajaj
- Delhi, India


A. Hi Dinesh. Brian Terry answered that question above. Good luck.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com - Pine Beach, New Jersey
Striving to live Aloha



September 10, 2018

Q. I have just purchased a surface finishing tester and need to understand what the Ra reading is telling me in microns. For instance if I have a 0.6Ra finish that requires taking down to 0.4Ra, How many microns of metal will I be removing and what will the finish micron be ?
Is there a chart I can use as I seem to see loads of varying information?

Kevin Brown
Engineer - UK


September 2018

A. Hi Kevin. Surface roughness is a topic that is sufficiently complicated that quick answers seem to muddy the water instead of clarifying it. Speaking for myself I always have to spend 20 minutes refreshing my memory before I can say anything; so let's try this ...

Ra is the most widely used measure of surface roughness. The needle or laser on your instrument tracks the profile rather like a hiker, let's call him "our a", traversing the peaks and valleys of a trail. He can tell us that trail 0.6Ra is less rough than 0.8Ra, but rougher than trail 0.4Ra. But if you ask "our a" hiker to explain precisely what he means, he can't do it without using definitions & vocabulary. Thus Ra = arithmetical mean deviation or 5396RaFormula.

To hiker "our a", a trail with three 200-foot peaks is exactly as rough as a trail with one 400-foot peak and two 100-foot peaks. But if you now set your tester to a different scale, say Rq, the new hiker "our q" may sort of agree with "our a" about the relative difficulty of various trails, but he won't agree exactly. The tallest peaks and deepest troughs of the trail concern "our q" far more than their median height (perhaps he is less skillful at rock climbing than "our a"). He rates the difficulty of the trails not by the average elevation changes from troughs to peaks but by the square of that elevation change. To "our q", the trail with the 400-foot peak and two 100-foot peaks is much rougher than the one with three 200-foot peaks. And Ra & Rq are not the only surface roughness scales.

I think your question about how much material removal is necessary to get from 0.6Ra to 0.4Ra is unanswerable. If your hikers start shoveling away the highest peaks, throwing the fill into the troughs, they'll get to a smoother path quicker than if you tell them to put the fill in a dump truck for carting away, leaving the troughs unfilled. Similarly, burnishing (squashing the peaks down into the troughs) might get you to 0.4Ra without removing any material, whereas turning might require removing quite a lot. Good luck.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com - Pine Beach, New Jersey
Striving to live Aloha



Is this surface finish acceptable?

March 5, 2019

Q. Hi, we have a product which, after nickel plating, the surface requirement is :

MAXIMUM MACHINED SURFACE - 125µin Ra
MAXIMUM SURFACE ROUGHNESS - 250µin Ra

Please find the actual surface condition in below pics:

5396-1b

5396-1c

5396-1a

Someone told me this is acceptable, but I don't think so. Could you provide your ideas?

The imperfections were measured with calibrated measuring & test equipment and were determined to be a maximum .062" x .062"

BTW, the nickel plating seems fine after the copper sulfate test.

Thanks for your help.

Jing Zhang
buyer - Newington, New Hampshire (USA)


March 2019

adv.
Profilometer: Pocket-Surf

A. Hi Jing. I recently read that an artificial intelligence program demonstrated greater skill than expert dermatologists/oncologists at reading whether skin markings were melanoma or not, so I'm certainly not laughing at the idea of trying to quantify surface finishes from a photograph. But I do think we're nowhere near that point, yet, so measuring the finish with a profilometer is your only way of saying whether it complies with the spec or not.

You haven't yet said what this item is and is used for, and with no tape measure or familiar object in the picture I can't judge the scale; still I personally would find it very hard to believe that this finish, with its scratches -- and especially with its deep pits -- will prove satisfactory for its purpose.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com - Pine Beach, New Jersey
Striving to live Aloha


April 6, 2019

A. FYI The Ra's given by you for the product is considered a rough finish because normal rolled steel sheet is around 30 RMS or 120 Ra. Unless the end product is for the food or medical industry your finish is acceptable.

tony kenton
AF Kenton
Hatboro, Pennsylvania


April 2019

A. Hi again. If your profilometer needle falls into any of the large potholes in the first picture, I seriously doubt that it's going to read 125 microinch or better :-)

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com - Pine Beach, New Jersey
Striving to live Aloha


September 4, 2019

A. I have always measured RA by running the stylus across the entire surface of the feature (as by definition, RA is an "average" ) If imperfections are present they are included in the travel path of the profilometers' stylus. Another general rule I apply is if micro meter or micro inch is not specified, by the customer on their print, I would use micro inch if the print dimensions are in inches, and micro meters if the print dimensions are in millimeters.

Rick Papineau
Greystone QA CQT 28 yrs. - Lincoln Rhode Island

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