Made of optical quality lens, the lens measures 2' in diameter with 3X magnification. A single bright LED light provides ample light for better viewing.

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But the “32” is not arbitrary: it was determined from real average roughness measurements, with real units. So, what are the units of a “Ra 32” surface? An Ra of 32 µ-in would be typical of a machined surface, such as a brake rotor. An Ra of 32 microns, however, would be more like the surface of brick!

The “angle” is the symbol for surface roughness, and this symbol indicates that the surface should be no rougher than “32.” Prior to the release of the 2018 standards the symbol was written a little differently:

A stylus instrument measures in two dimensions, recording the height at every point along a line. Other instruments, such as optical profilers, can measure in three dimensions, acquiring an entire area of data in just a few seconds. For 3D measurements we use “Sa” as the areal/3D equivalent to Ra (two-dimensional) average roughness.

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The light switch is easy to access on the handle. It uses a single  AAA battery for easy battery replacement.  An LED magnifier provides some of the best light for magnifiers. LEDs (Light Emitting Diodes) are small, solid light bulbs which are extremely energy-efficient. They provide a very bright light for increased visibility and are known for their longevity and reliability.

There is a great deal more to this topic and to specifying, measuring and analyzing surface texture. We invite you to read the rest of this blog to learn more about it. You may also want to learn more about our in-person classes. The Surface Texture and Tribology Short Course is also available on udemy.com if you are looking to take a deeper dive into surface texture analysis.

Ra measures absolute deviations from the mean height, treating peaks and valleys equivalently. As the image below shows, many very different surfaces—some uniform roughness, some with deep valleys or sharp peaks—may all have the exact same Ra value. Many of the quality and warranty issues that we see stem from this shortcoming of Ra: the parts all meet the Ra spec, yet some squeak, or leak, or wear prematurely. How can that be, when the Ra is within tolerance? The answer is often in those peaks and valleys…

Although his several pocket magnifiers come in different sizes and shapes they all had one thing in common - illumination. As ones macular degeneration progresses the need for more light becomes more pronounced. The more light there is, the less need for a higher power magnifier.

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Sa is the areal (3D) equivalent of two-dimensional Ra. Sa is the “areal average roughness,” the average height of all measured points in the areal measurement. The “S” parameters refer to measurements of a “surface,” as opposed to the “R” parameters which are calculated from the roughness profile.

Stylus based measurement instruments are the most common tools for measuring surface texture. In these instruments a small tip is dragged across a surface while its deflection is recorded, providing a profile like the one we see in the image above.

There are high quality acrylic lenses and glass lenses. The cheap acrylic lenses often have some distortion and are not real clear, so you will need to pay more for a high quality aspheric lens.

Here is a handheld magnifier from Amazon that has some great reviews, with a high quality, distortion free lens and offers 3x magnification:

This hand held magnifying glass comes with with two interchangeable optical grade lenses - the larger is 5x magnification and the smaller lens 10x magnification.

If not otherwise specified, the number in a callout such as the one shown at the beginning of this post is assumed to be stated in micro-inches. However, without specifying the units, the specification is open for potential misinterpretation.

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The difficulty with Ra is that it gives no indication of the spatial wavelengths that comprise the texture. In the image below, the engine lifter at the top ran smoothly and quietly. The lifter at the bottom, however, created engine noise because of the waviness in its surface. Both parts met the average roughness spec. If we only measure Ra, we cannot tell the difference!

Ra (average roughness) measures the deviation of a surface from a mean height. The horizontal line through the profile represents the arithmetic mean height. The blue areas represent the deviations from that line. Ra, then, is the total blue area divided by the length of the profile.There is, of course, a bit more to it, but this is the basic gist.

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Recently, while visiting my father-in-law who has wet macular degeneration I was pleased to see that he had several handheld magnifiers placed in different rooms of the house. Without having to search for a magnifier he can quickly grab one to read a label, check a bill, or identify a prescription bottle.

Ra cannot distinguish the location of surface features across a profile. A surface with randomly scattered peaks will have the same Ra as a surface with those same peaks clustered at one end of the part. In a mating surface, a concentration of high peaks could lead to uneven loading, scratching, gouging, premature wear, etc. What’s more, a single profile measurement with a stylus instrument is quite likely to not even cross that peak material, leaving it entirely undetected.

An electronic portable magnifier can go with you wherever you go. Keep one in your pocket or purse to use when you are at restaurants, shopping or traveling.

Ra cannot discern short wavelength “roughness” from longer spatial wavelength “waviness.” These areal/3D measurements show two engine lifters with the same Ra. Yet, one ran smoothly while the other was noisy. The longer spatial wavelength texture was the issue…but Ra alone cannot tell you that.

It is one of the largest round magnifiers for a wide viewing area. The crystal clear optical grade lens goes all the way to the edge so there is no blurriness.

Always get the lowest power magnifier that allows you to read comfortably. One that is lighted often makes it possible to use a lower power. Here is 3X power folding magnifier that uses AAA batteries from Amazon:

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Two LED lights provide illumination to help you see in any lighting conditions. The easy on/off switch is on the handle.

Average Roughness, or Ra, is the most commonly specified surface texture parameter. It provides a general measure of the height of the texture across a surface. More exactly, Ra is the average of how far each point on the surface deviates in height from the mean height. Consider this profile, which is a cross-section through a typical surface:

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Key fact: Ra has units. You will hear people refer to a surface as a “32” or “64” as if it is a unitless standard. This practice dates from the use of “tactile gages” which were widely used to compare surface texture. A machine operator would drag a fingernail across the sample surface and then compare the “feel” to the samples on the tactile gage. Inexact, but quick and easy. The gages often listed the values without units, and thus several generation of machinists learned to feel a “32” surface, and to understand “roughness” as a number rather than a texture.

As we stressed earlier, a surface is a texture, not a number. A very fundamental aspect of surface texture (one which we discuss frequently in this blog and in our classes) is that surface texture consists of “spatial wavelengths,” ranging from short-wavelength roughness to longer wavelength waviness. The longest spatial wavelengths are called “form” and represent the general shape of the part.

Average roughness is a good first-pass indicator of the overall height of the surface texture. For decades it has proven useful for tracking manufacturing processes in industries from automotive to medical devices. It can be measured quickly using only inexpensive gauges, making it an effective tool for shop floor quality control.

Average roughness is typically measured in either microns (µm) or micro-inches (µ-in, µ”). One micron equals roughly 40 micro-inches (µ”). Also note that “micron” and “micrometer” are equivalent, and both terms are commonly used.

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The application dictates which spatial wavelengths are important to measure and control. If a part needs to seal, say, we will need to be concerned about its roughness (too rough and its valleys may join up to form leak paths), but we will also need to be concerned about its waviness (too much overall curvature and a gasket may not be able to comply with its shape).

These very different surfaces all have similar Ra values. Which will leak? Which will wear well? Ra cannot distinguish between them. Image courtesy Digital Metrology Solutions.

The handle easily folds up so that it is compact and easy to carry with you. What my father-in-law liked best about this lighted magnifier is that it uses a AAA battery. The battery is in the handle of the magnifier. Not only are these batteries easy to find, the battery is easy for him to replace.

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So, what does that mean, how should you measure it, what will this information tell us about the surface, and maybe most compelling: what won’t it tell us?

Hand held magnifiers can be used for spot reading such as reading instructions, labels, price tags, a bill or a menu.  Not intended for long term reading, keep one of these handy in different rooms of the house.

Another way to always have a lighted magnifier handy is to wear one around your neck.  This magnifying glass necklace by Mighty Bright works great for dark corners, dark restaurants or dark drawers.

Average roughness (Ra) is the tip of the deep and complex world of surface texture analysis. As you’ll see throughout this blog and many excellent books, we have many tools today that we use to measure surface texture, and hundreds of ways to analyze the data. This information can be incredibly useful for solving production and quality problems.

A hand held magnifier is good for spot reading - like checking the scores on the sports page, reading an electric bill, or following the instructions of a recipe. These magnifiers come in different shapes - round or rectangular and  lighted or not lighted.

Ra, however, is blind to several critical aspects of surface texture, and relying solely on Ra to control surfaces can lead to quality issues. This happens all the time, in fact, as you’ll read in examples throughout this blog.

Despite this level of complexity, though, by far the most typical encounter you will have with surface texture will be a callout on a print such as this one:

For those who need higher magnification, then this hand held electronic magnifier will help those with macular degeneration keep their independence.

Just like I keep inexpensive reading glasses in different rooms of the house, have several hand held magnifiers conveniently available in different rooms - like the kitchen, the garage, the bathroom, and the bedroom. Keep a lighted magnifying glass close at hand to help reduce frustration and to make it easier to see and read.

And his favorite light for a magnifier is an LED light. Amazon carries an assortment of pocket magnifiers. But my father-in-law likes the one that folds up easy and uses AAA batteries. Some lighted magnifiers use the tiny button batteries that he finds too hard to remove and replace.  You can find the same LED magnifier here at Amazon:

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