Setting a Roughness Goal and Meeting It

Subjective surface finishes have no place in today’s high-precision manufacturing environment.

Historically, surface finish has been a controversial subject in tool rooms, machine shops and production departments. For years, surface quality was the sole responsibility of the machine operator. Craftsmen were in constant consultation, and often heated argument, deciding whether a finish was "good" or "bad."

Even with the emergence and widespread implementation of quality control programs throughout the industry, human judgment has remained a strong factor in evaluating surface quality. Today, the high precision of most machined and ground parts as well as the high cost of manufacturing them rule out the subjective approach to surface measurement.

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Nearly every manufacturer should be concerned about the surface quality of the parts or products they produce whether they are talking about the finish produced by a casting process, a rolling mill, EDM, surface grinding or super-finishing operations such as honing or lapping. However, many shops are not using surface finish measurement as profitably as they could be.

In many cases, it is important to take a new look at surface finishing methods and surface quality standards, and then focus on the degree of smoothness that is really required. In today's severe economic climate, it pays to establish a specific goal in every surface finishing operation and to stop when you reach it.

At one time, it was not uncommon for a drawing specification to state “finish to a high luster” if a part’s shiny finish was necessary purely for cosmetic or appearance reasons. This leaves room for speculation as to just what “high luster” implies. It is a subjective term, and today objectivity is required. Personal ideals of surface finish are no longer commercially practical or profitable. In the case of the cosmetic finish, the manufacturer must ask specifically for a surface finish parameter and value.

With the portable surface gages available, a part can be measured at the point of manufacture and analyzed. If necessary, machining operations can be cut back until the surface meets the surface finish specification. In this way, a manufac­turer can avoid over-finishing a part, which, in high-volume pro­duction, can add up to significant cost savings. Therefore, it pays to follow the specs on finish exactly. If the draw­ing calls for 20 microinches, do not shoot for 10. In continuous, long-run, high volume production, an extra-fine, smooth surface may not be worth the time, materials and man-hours expended to achieve it.

At times, an ultra-smooth finish may defeat subsequent processing steps and have to be “roughed up” for the application of a protective coating or paint. Mechanically, it may cause slippage of moving parts. In this case, the cost of reworking is added to the cost of over-finishing. Over-finishing affects output as well. In striving for a better surface, there is more downtime for dressing wheels or sharpening tools. The rate of table feed or carriage speed is slowed by changing from a coarse-grit to a finer-grit grinding wheel. The tradeoffs are less metal removal and lower productivity.

Often, having a specified finish can eliminate a secondary operation. You do not want to hone if broaching, boring, reaming or drilling will do the job. Additionally, there is no need for lapping if milling or surface grinding will suffice. Regardless of the job, if you machine or grind a workpiece even an additional, unnecessary five minutes in high production, for example, you are talking about a great deal of money in manufacturing costs.

Certainly, if a machinist is working with dimensional tolerances of 0.010 inch or more with non-critical clearances it is possible the visual inspection process would be adequate. However, high-precision metalworking operations demand a far more sensitive and quantitative evaluation of surface roughness for which the technology is readily available. Today's instrumentation permits highly accurate measurement of surface finishes right at the point of manufacture. For a manufacturer doing fine finish work, it is far more effective to invest in a tracer-type surface measurement instrument, which will provide a much more accurate picture of the surface texture irregularities they will be producing.

Furthermore, all surface irregularities on mating parts have important and prolonged effects on wear and performance. Some effects are more surreptitious than others. For example, with a shaft revolving in a bearing, or a reciprocating piston rod, a "rough" surface can cause unnecessary wear over a period of time—with the shaft acting as a reamer and the piston rod as a broach.

But unless one can quantify the requirements and then measure them, there is no guarantee that the product will meets its design requirements and performance.