Ra surface finish -- unit-less, microinch or micrometer

A. David:

Surface roughness values, including Ra need a dimension to be interpreted. The common unit for machining applications in the US is microinch (millionths of an inch). Metric values would be microns (millionths of a meter) or nanometer (billionths of a meter) for polished surfaces.

A. All of the following have units of length ---
Ra: The arithmetic mean
Rq: The root mean square
Ry: Maximum peak-to-valley height

Hope this helps.

A. The average roughness, Ra, is expressed in units of height. In the Imperial (English) system, Ra typically expressed in "millionths" of an inch. This is also referred to as "microinches" or sometimes just as "micro" (however the latter is just slang). In the metric system, Ra is typically expressed as "millionths of a meter" also called "micrometers" or "microns".

A. Hi Michael. A mirror finish on metal is about an Ra of 4 microinches or 0.1 microns. I don't know what would happen if you ran a profilometer over a smooth glass finish and am curious myself :-)

Regards,

Profilometer: Mahr Pocket-Surf

on eBay or

Amazon

(affil links)

A. Hi Zaki. You would use a profilometer.

Regards,

A. No, Lia. That is incorrect information, it works the opposite way. Ra 125 is rougher than Ra 32. However, for grit size it works the opposite direction: the higher the grit number, the finer the grit and the finer finish it can produce.

Regards,

A. Gary Spring,

I would imagine as your part was a moulding the designer would spec a min and max Ra either for aesthetic purposes or to reduce manufacturing costs. The lower the Ra the more processing will be required; i.e., a mirror finish will be more expensive than a good commercial polish.

Steve.

A. Hi, Tare.

A "micron" is a micrometer, one millionth of a meter.

Regards,

A. For measuring Ra value, you could use Surface Roughness Tester; it's available in the market place.

About 125 in measuring and part has 155, it does not matter.

125 to 250 is common tolerance

A. I can't say what the drawing intent for your specific customer is, only my own personal experiences. If a drawing calls up a surface finish requirement it is considered to be a maximum value as far as I am concerned. I have worked on and with many of the Aerospace platforms and all drawings that I have come across have stated surface finishes as maximums (although there was one drawing that actually did the reverse and stated a minimum roughness! It was clearly marked on the drawing though).

In the end, if you are not certain what the requirement is go back to your customer and ask the question. 2 minutes on the phone may save you a lot of pain.

A. You might have something that could help others. The length could be taken into consideration as part of the overall reading. Meaning if the part was say 3 inches and got a reading of 6Ra and you were only able to measure half that length for whatever reason, you could indicate that as Ra6@50. I'm not sure that would make a lot of difference in the overall reading but it would give the observer more reliability of the quality.

A. The English language is a SOB sometimes. OK let me see if I understand what you are looking for. The length of the part is one thing. the length of the profilometer stroke is another, the reading of the surface finish is the 3rd thing. So if I understand this right, you are requesting info on the difference between the RA's in which case, the lower the number the smoother the surface finish. I don't think you are talking about different measurement scales, are you?

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.

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 -- 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,

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,

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.

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.

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.

Profilometer: Mahr Pocket-Surf

on eBay or

Amazon

(affil links)

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,

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

Regards,

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 .

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,

Profilometer: Mahr Pocket-Surf

on eBay or

Amazon

(affil links)

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,

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.

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,

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.