Long-wavelength waviness and short-wavelength roughness require multiple instruments and parameters to be measured correctly-the smaller the number, the better. #qualitygagingtips

We usually think of "smooth" as "smooth" or "smooth". But did you know that a surface that is too smooth can be sticky? For example, racing oil on a trailer will increase the traction on the road, and similarly, a too smooth sailboat hull will cause a greater degree of water resistance. The quality or finish of the surface affects parameters such as how the parts are assembled and worn, reflected light, heat transfer, distribution of lubrication, and acceptance of coatings.

The intended use of the finished part should determine the nature of the surface. The goal is to meet the engineering requirements of the application without wasting time and effort on surface treatments higher than necessary. Therefore, when the engineer specifies the surface finish on the print, the purpose is not only to make the part look beautiful, but also functional. 

Years ago, experienced mechanics used scrapers and educated nails to determine the surface quality of parts. Today, although the squeegee is still used occasionally, there are a large number of surface finish standards, various parameters for measuring the surface, and various powerful measuring tools for measuring the surface.

Each machining operation will leave marks on the surface of the part. As shown in Figure 1, surface finish (also called profile) consists of two parts: waviness and roughness. Ripple or longer wavelength changes are caused by macro-type effects, such as worn spindle bearings or vibrations of other equipment in the workshop. Roughness—the short-wavelength pattern of tool marks during grinding, milling, or other machining—is affected by the condition and quality of the tool. The operator's choice of feed rate and depth of cut also affects these two components.

As dimensional tolerances have become stricter over the years, and the need for documentation and traceability has increased, the role of surface finish measurement in the manufacturing process has increased dramatically. In the 1940s, surface irregularities accounted for approximately 15% of the tolerance zone. Today, this ratio is often 50% or more.

As shown in Figure 2, two basic types of surface finish meters reveal these measurement results. These are slipping (average) systems and non-slip (profiling) systems.

The sliding gauge has a hinged probe assembly. The probe is mounted next to a relatively wide slider, which also contacts the workpiece. The slider tends to filter out ripples, so the probe only measures short wavelength changes. The sliding gauge has a dial or LCD reading, which can display the measurement result as a single value.

The non-slip gauge uses a precise inner surface as a reference, so the probe can respond to waviness and roughness. In order to allow separate analysis of long-wavelength and short-wavelength changes, profile measuring instruments usually generate graphs (on paper or computer screens) rather than individual numerical results.

Each application responds differently to various combinations of roughness and waviness, so the industry has created more than 100 formulas to respond to the same measurement data to calculate surface finish parameters. Each parameter has its own advantages and limitations, and many are application-specific.

For most applications, there is no single parameter that can provide all the information the shop floor needs to define the surface. This means that a complete definition requires a combination of two or three parameters. In some cases, the relationship or ratio of one parameter to another parameter can be used as a parameter.

However, most shops are able to limit their measurements to about six parameters, using microinch or micrometer units for measurement.

Ra is the most widely used parameter because it provides the arithmetic mean of surface irregularities measured from an average line located somewhere between the highest point and the lowest point on a given cut-off length.

In order to distinguish between "spike" and "scratched" surfaces with the same Ra, the store should use other parameters such as Rp (maximum peak height), Rv (maximum valley depth), and Ry (maximum peak valley roughness height).

If the surface finish is marked on the drawing but not otherwise specified, the standard practice assumes Ra. But no single parameter is suitable for all types of parts, and many applications are best to use two or more parameters: for example, the combination of Ra and Rmax (maximum roughness) can provide a good overall concept of part performance.

Therefore, maintaining good control over surface finish is not only a challenge, but also an opportunity. In some cases, good surface control allows you to safely reduce the accuracy in other areas.

Almost every machine tool manufacturer lists part of the machine specifications, accuracy, and repeatability data. What is usually not given is the method used to derive the numbers. Although these methods are defined in the linear positioning standard, not all builders use the same standard.

The functional gear test, also known as the total radial compound deviation, is a way of looking at the total effect of gear errors. This test method simulates the conditions under which a set of gears may operate due to gears meshing together.

Few manufacturing companies rely on ballbar testing to maintain machine tool accuracy as much as Silfex. Now, advanced training and the switch to Renishaw’s QC20-W wireless system allows the company to take the advantages of ballbar testing to a higher level.