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What's rough about surface finish?

What's rough about surface finish?

In answer to the above question, everything! Demand for products with better quality and performance is forcing US manufacturers to rethink traditional designs and methods, and is leading directly to the ascension of surface finish as a critical consideration during product development. For any product with moving parts, performance and efficiency are positively correlated to the functional compatibility of contacting surfaces. It's the texture of these surfaces that controls lubrication, friction, wear, load-bearing, and sealing properties.

Over the last five years, research has found that the traditional surface parameter --RA (roughness average)-- doesn't adequately quantify surfaces on a functional basis. As a result, several other parameters, such as bearing ratio (tp), peak count (PC), roughness height (RT), skewness (RSK), and peak height (RP), to name just a few, are being specified to more accurately address the functional aspects of critical surfaces.

While this is real progress, RA is still by far the most commonly specified surface-finish parameter. It's estimated that it is used on 90 percent of all partprints in this country that include a surface-finish spec. Unfortunately, there is much confusion about what RA is intended to quantify, how it should be properly specified, and how it should be measured.

Understanding basics

To fully understand what RA is intended to quantify, it's necessary to look at what happens to a part surface during metalcutting. In machining a shaft, for example, the resultant tool mark is a helix, Figure 1. Under magnification, this is the first surface component seen on the part.

A second group of components is the result of vibration from within and without the machine tool. Bearing wear, grinding-wheel imbalance, and gearbox problems are examples of sources of internal vibrations. Vibrations from an external source may be the result of ambient noise fed into the machine tool. These vibrations are "waviness' components, and always are undesirable because they can't be directly controlled by the machining process.

A third surface component is caused by machine-tool misalignment, which creates a long wavelength condition called "error of form'. Since each component is superimposed on the other during machining, the resultant surface is complex; it is known as the surface's total profile, Figure 2.

RA for roughness

A designer must assume that the effects of the waviness and error-of-form components are negligible; to do otherwise would immediately assume an out-of-control condition.

To correctly evaluate RA, the roughness component must be extracted from the total profile. This is accomplished by selecting a cutoff (sometimes referred to as cutoff width), which is simply the length of surface within which the evaluation of RA is made.

An instrument that measures RA initially calculates the least squares mean line of the profile within the cutoff length selected. It then measures the perpendicular distances from the mean line to the profile peaks and valleys and finds their average (i.e., RA), Figure 3.

In the ANSI B46.1-1978 surface-texture standard, you're given the following choices for cutoff lengths: 0.003, 0.010, 0.030, 0.100, 0.300, 1.000. The ratio between successive lengths is approximately 3:1.

There are two general rules that indicate correct cutoff selection in nearly every case. The first is that the cutoff length must be small enough to eliminate any waviness component from the evaluation of RA. The second rule is that the cutoff length must be long enough to include at least five irregularities (tool marks) caused by the manufacturing process.

Looking at the choice of cutoff lengths clarifies the latter rule. The range of different machining processes is vast-- grinding, turning, milling, shaping, planing, boring, broaching, honing, polishing, lapping, EDM, ECM, to name just a few. The tool mark produced on the surface by each method differs in amplitude, wavelength, and lie.

Obviously, if too broad a cutoff was used for a surface generated by a process that produces a relatively closely spaced roughness pattern (tool marks), such as grinding, erroneous readings would result because a waviness component was included, Figure 4. Conversely, if too narrow a cutoff was used for a surface produced by a process that generates a relatively widely spaced roughness pattern, such as single-point boring, the readings would indicate a much smoother surface than actually was present.

In spite of the evidence, a major misconception still persists in the area of cutoff selection. The ANSI B46.1-1978 standard states, "The 0.030 value is preferred for most surfaces, and is to be used unless otherwise specified.' The 0.030 cutoff has mistakenly become the universal rule in the metalworking industry rather than the very general guideline it was intended to be. It should be clear that selection of cutoff be governed by the manufacturing process that generates the surface, and specified accordingly.

Roughness average is the most widely used parameter today, and will be so for many years. It is imperative, therefore, that it be understood and specified correctly so it can be complemented with other, more functionally oriented parameters to better control the way a machined surface will perform.

Photo: 1. The tool mark is the roughness component of the surface.

Its amplitude and wavelength is controlled by factors such as feed, speed, and depth of cut. It's important to note that this is the only surface-finish component that can be controlled by machining.

Photo: 2. Components in a typical surface finish.

Photo: 3. RA displayed by an average roughness instrument can be erroneous when a waviness component is mixed into the reading by using the wrong cutoff (shown by new cutoff section). Since most average roughness instruments present only numerical readings, and don't produce a profile trace such as in the illustration, you wouldn't know that a waviness component was introduced and so would be unable to determine which RA reading is correct.

Photo: 4. Various machining processes produce different surface-finish amplitudes and wavelengths. Vertical marks indicate desired cutoff to encompass a minimum of five roughness irregularities (tool marks).
COPYRIGHT 1985 Nelson Publishing
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Copyright 1985 Gale, Cengage Learning. All rights reserved.

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Author:Ackroyd, Peter I.
Publication:Tooling & Production
Date:Jun 1, 1985
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