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Gaging Surface Finish

Most gages that measure surface finish fall into either of two main families.

by Drew Koppelmann

A part's surface finish can determine how well it fits, wears, transmits heat, distributes lubrication and accepts coatings, as well as how it looks and feels. Engineers often write manufacturing specifications for surface finish with these considerations in mind. Quality control must therefore be prepared to inspect surface finish conditions.

The total profile of a part's surface has two major components: roughness and waviness. Waviness is a long-wavelength condition, which is influenced by such factors as the condition of the machine tool's spindle bearings and vibration from other sources on the shop floor. Roughness -- a shorter-wavelength pattern of irregularities that is overlaid on top of the waviness pattern -- is the pattern of tool marks on the part. Both components may be influenced by the machine operator's choice of feeds and speeds.

Both components are also subject to measurement and analysis using a variety of parameters. Roughness, for example, may be measured as a function of the highest "peaks"  or the deepest "valleys"  on the part surface, or as a function of the total distance between the highest peaks and lowest valleys, or as the average of all the peaks and valleys. These represent just four possibilities out of literally dozens of different parameters that may be chosen to measure roughness. (In almost all cases, results are presented in either microinches or micrometers. For example, average roughness according to the Ra parameter may be displayed as Ra = 50 µ" .)

It is the engineer's responsibility to select parameters that satisfy the requirements of the application. For example, a bearing's ability to support a rotating shaft may require a certain average roughness in order to maintain an oil film of a particular thickness. Excessive peak heights might cause scoring of the shaft, while maximum valley depth might be relatively unimportant. In another application, the importance of the parameters might be reversed or other parameters might be of significance.

The large number of parameters available is a direct reflection of the diversity of surface finish requirements in different applications. There are significant differences in functional requirements between applications as distinct as a medical implant and an automotive engine bearing, for example. Even if the two have the same average roughness, other parameters may be very different.

Most gages that measure surface finish fall into either of two main families. These differ substantially in cost, capabilities and complexity, each having a role to play in surface finish inspection.

Skidded, or averaging, systems are low in cost and complexity. The sensitive stylus, which traces the surface of the workpiece, is supported by a hinged probe assembly that also rides on a skid along the surface. This tends to filter out waviness, so the gage measures only roughness. Skidded gages produce a single numerical value as the measurement result, although some offer a choice of more than one roughness parameter. They can be small enough to fit into a worker's pocket, and they are well-suited to shop floor use. A worker can take quick measurements with a skidded gage to check that roughness is within tolerance and the manufacturing process is under control.

Skidless systems offer greater technical capabilities. The probe in a skidless system rides against a smooth, flat internal reference surface, enabling the stylus to respond to a part's total profile: waviness as well as roughness. The computer controllers common on skidless pages can break out roughness and waviness as separate components, and analyze each according to numerous parameters.

If a particular parameter is out of tolerance, for example, it is  often possible to "zoom in"  on a particular segment of the measurement for more detailed analysis. As sometimes occurs, a single anomaly may put a surface out of tolerance, but closer analysis might show that the part surface is otherwise good -- indicating that no change in production settings is required.

Where quick, simple roughness measurements are sufficient, skidded systems are often adequate. If more in-depth analysis or access to a greater range of parameters is required, the more capable skidless systems are appropriate.


About the author

As applications manager, gaging products, at Federal Products Co. in Providence, Rhode Island, Drew Koppelmann provides dimensional gaging applications assistance to companies in a wide range of industries, including automotive, aerospace, packaging and electronics. He may be reached by fax at (401) 784-3246.

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