Measuring Surface Roughness and Texture

We usually think of "smooth" as "slick" or "slippery." But did you know that a surface that is too smooth can actually be 'sticky'? Racing slicks on dragsters increase traction with the pavement, and a racing sailboat hull that is too smooth can actually increase drag through the water. In fact, the quality or finish of a surface can affect a lot of things, from how a part will fit, wear and reflect light, to how it will transmit heat, distribute lubrication and accept coatings.

The nature of a part's surface should be determined by the part's function: you want a surface that fulfills the engineering requirements of the application, without wasting time and effort on a higher quality finish than is necessary. So when an engineer specifies surface finish on a print, the intent is usually not just to make the part look good.

Years ago, an experienced machinist would use a scratch pad and an educated fingernail to determine the surface quality of a part. Today, while scratch pads are still in occasional use, we have a whole plethora of surface finish standards, a wide array of parameters to measure surface, and a robust variety of gages with which to measure it.

Every machining operation leaves its marks on a part surface. As shown in Figure 1, surface finish – also known as profile – is composed of two components: waviness and roughness. Waviness, or longer-wavelength variation, is caused by macro-type influences, like worn spindle bearings or vibration from other equipment on the shop floor. Roughness – the short-wavelength pattern of tool marks from grinding, milling or other machining processes – is influenced by the condition and quality of the tooling. Both can be influenced by the operator's choice of feed rate and depth of cut.

As dimensional tolerances have grown steadily tighter over the years and the need for documentation and traceability ever greater, the role of surface finish measurement in the manufacturing process has grown dramatically. In the 1940s the proportion of the tolerance band taken up by surface irregularities was roughly 15 percent. Today that proportion is frequently 50 percent or more.

For example, say the maximum peak-to-valley height on a surface is usually four or five times greater than the average surface finish, as measured by the Ra method. A part with an Ra (average roughness) value of 16 µ in, therefore, probably has a peak-to-valley height of 64 µ in or greater. If you're trying to meet a dimensional spec of .0001 in, the 16 µ in finish represents over half the allowable tolerance.

There are two basic types of surface finish gage, as shown in Figure 2: skid-type, or averaging systems, and skidless, or profiling systems. Skidded gages have a hinged probe assembly, with the probe riding next to a relatively broad skid that also contacts the workpiece. The skid tends to filter out waviness, so the probe measures only short-wavelength variations. A skidded gage usually has a dial or LCD readout to display the measurement as a single numerical value.

Skidless gages use a precision internal surface as a reference, so the probe can respond to waviness as well as roughness. In order to allow separate analysis of long- and short-wavelength variations, profiling gages usually generate a chart (on paper or on a computer screen) rather than a single numerical result.

Every application reacts differently to different combinations of roughness and waviness, and industry has responded by creating more than 100 different formulae with which to calculate surface finish parameters from the same measurement data. Each of the parameters has its own advantages and limitations, and many are very application-specific.

Often one parameter is incapable of defining a surface adequately, and two, three or even more parameters are required to complete the definition of a surface, and in some cases, the relationship or ratio of one parameter to another is required.

However, most shops are able to confine their measurements to a half-dozen parameters or so. In almost all cases, measurements are presented in microinch or micron units.

Ra is the most widely used parameter, because it provides an arithmetic average of surface irregularities measured from a mean line that lies somewhere between the highest and lowest points on a given cut-off length. A slightly more sophisticated variant, Rq, uses a root mean square calculation to find geometric average roughness – an averaged average, if you will.

Both of these, however, tend to minimize the influence of surface anomalies like burrs or scratches. To differentiate between "spiky" and "scratched" surfaces having the same Ra, additional parameters should be specified, such as Rp (Maximum Peak Height), Rv (Maximum Valley Depth) and Ry (Maximum Peak-to-Valley Roughness Height).

If surface finish is called out on a drawing but not otherwise specified, it is standard practice to assume Ra. But no single parameter is best for all types of parts, and many applications are best served by using two or more parameters: for example, Ra in combination with Rmax (maximum roughness) may provide a good general idea of the part's performance.

In Europe, the more common parameter for roughness is Rz, or Mean Roughness Depth. This is the average distance between the highest peak and the deepest valley in five sampling lengths, or cutoffs. Rz is more sensitive than Ra to the changes in surface finish, because maximum profile heights, and not the averages, are being examined. There is no firm conversion between Ra and Rz parameters, as the actual ratio depends upon the shape of the profile, but the approximate ratio of Rz = 4 x Ra can be used.

Maintaining good control over surface finish is not just a challenge, but an opportunity. In some cases, good surface control may allow you to safely reduce precision in other areas. Surface finish measurement procedures, general terminology, definitions of most parameters, filtering information, etc., can be found in American Standard ASME B46.1 – 2002, Surface Texture; and in International Standards, ISO 4287, ISO 4288, and several others.

George Schuetz is the Director of Precision Gages for Mahr Federal Inc.,
1144 Eddy Street, Providence, RI 02905,
401-784-3100, Fax: 401-784-3246,
george.schuetz@mahr.com,
www.mahr.com.