Quality 101: Get a Handle on Waviness
April 29, 2009
Today, the majority of manufacturers have come to understand and accept the roughness profile, Ra. But how much does industry understand the importance of the related waviness profile, Wt? As the waviness profile is becoming more common in process control, operators should learn how to understand this parameter.
Waviness Vs. RoughnessThe relationship between roughness and waviness can be illustrated by picturing a landscape of rolling hills covered in grass. The hills are analogous to the waveform, while the grass covering the hills is analogous to roughness.
To provide a sense of the scale for these features, in about 75% of measurement routines undertaken, surface profiles with a wavelength of 0.03 inch or shorter are classified as roughness, while surface features with wavelengths greater than 0.03 inch generally fall into the category of waviness.
Roughness is characteristic of tool marks left in the wake of a machining pass caused by both the cutting action and surface nonuniformity. Waviness is most often the result of small fluctuations in process conditions such as changing distances between the cutting tool and the surface of the workpiece. These fluctuations may be caused by cutting tool wear or worn machine bearings, both of which generate unbalanced conditions, chatter, vibration and instability in the machining setup.
Because in most cases roughness and waviness are caused by different aspects of the manufacturing process, evaluating each individually is useful for process control and diagnostics wherein the overall objective is to hold the machining process toward the upper limit of the spec for improved economics.
Today’s stylus-based surface measurement instruments are capable of capturing both roughness and waviness information; the trick is to separate one from the other.
FilteringFiltering surface profiles involves running the primary data-that which describes the real surface-through a smoothing filter. The degree of smoothing is determined by selection of a filter cutoff wavelength. The filter cutoff specifies the wavelength that a given analysis will use to separate roughness from waviness. The choice of cutoff is based on the particular process characteristics that are to be monitored.
Shorter wavelengths, with a greater frequency and smaller amplitude, will appear in the roughness profile, while longer wavelengths, with a lower frequency but larger amplitude, will appear in the waviness profile. Both ASME B46.1-2002 and ISO 4288-1996 include tables providing standard cutoff values that enable designers, engineers, production and quality people to be on the same page when specifying waviness parameters.
The filtering process can be likened to a sieve wherein the coarseness of the screen selected is analogous to the cutoff value, per ASME or ISO standards. In this analogy, the primary data expressing the real surface profile is made up of all sizes of large and small stones and dust. All this material is run through the screen. Stones too large to fit through the screen remain on top; they represent the waviness data. The small stones and dust pass through the screen for collection below; they represent roughness data.
Productivity WaveWaviness measurements are usually part of the inspection regimen implemented after major metal removal operations are complete. Waviness parameters most often are used to predict how effectively one surface will seal against another surface, either when in direct contact or with the use of intermediate gaskets.
In simplest terms, waviness is checked to prevent leaks, critically important in automotive, powertrain, aerospace, medical manufacturing and hydraulic/pneumatic applications in which cases, chambers and similar components must mate and seal. A waviness inspection routine for machined parts helps keep processes within parameters.