Using Hybrid Parameters

Computer-aided design has dramatically facilitated the process of getting ideas into the marketplace (and also sent a lot of drafting boards and T-squares to the junk heap). These days, products ranging from children’s toys to passenger jets are completely designed in a virtual environment before a single part is manufactured.

However, real engineering is still required when parts need to interact with other parts or when they need to perform a particular function. No single process can provide a finish on a workpiece that both seals and lubricates. Engineers must specify a combination of processes such as boring, grinding and honing to produce a surface that is not only dimensionally correct but is also functionally correct.

These functional machined surfaces are becoming more and more complex as parts and processes are pushed to their limits. Likewise, more and more sophisticated equipment is needed to understand the function and do the surface analysis. In addition, users are now faced with more than 100 different ways of analyzing the results. Let’s walk through the cylinder example, component by component.

Roughness is the first basic component. This is a profile of the fine irregularities on the surface, composed essentially of tool marks. Selecting different speeds and feeds during manufacturing can usually change these marks. The most common parameter for monitoring roughness is Ra, or roughness average, where a larger number indicates a rougher surface.

The other component needed to understand the functional characteristics of a surface is waviness. While roughness looks at a relatively short sampling length (usually less than 0.030 inch), waviness looks at wider intervals of surface texture. Unless otherwise specified, it includes all the irregularities in a space greater than that of the roughness sampling length and less than the waviness sampling length.

In order to fully understand the functional characteristics of a surface, you have to analyze the relationship between roughness and waviness. This is done using one or more hybrid parameters. Amplitude parameters are related to profile height and are primarily used for basic roughness characteristics such as roughness average, peak to valley and profile height. Spacing parameters, on the other hand, are sensitive to variations in the profile height as well as wavelength, such as peak count and mean spacing. Hybrid parameters include wavelength, bearing length ratio and bearing area curve, and they are designed to track surface friction, hardness or cosmetic appearance.

The hybrid parameters for our cylinder would be Tp (Rmr) evaluation and the use of a Bearing Analysis Curve (BAC). Tp is the ASME symbol for bearing length ratio, while Rmr is the DIN/ISO symbol for material ratio. This parameter is the ratio, expressed as a percent, of the material filled length to the evaluation length at a profile section level. In Figure 1, the chart shows how each of the contact and non-contact areas are totaled up over the entire evaluation length. By changing the cutting depth (C), the percentage of the area making contact increases.

In order to begin the evaluation of this surface, it is first measured for Rz, a roughness parameter for mean roughness depth, which is the arithmetic mean value of individual roughness depths of consecutive sampling lengths.

So from the maximum peak, 3 percent of the contact surface is eliminated and then the Tp (Rmr) analysis is begun. Again, by moving the level of C, or the cutting depth, the ratio of material to evaluation length is changed. From this BAC evaluation, the honing process can be adjusted to achieve the cutting depth that provides the proper surface area for sealing and lubrication.