> First in a series
Chemically accelerated/assisted superfinishing, also known as isotropic superfinishing, was introduced into the gear industry more than 20 years ago. Since that time, it has become widely accepted as a solution for numerous gear failure modes and performance enhancements. As a technology becomes more ingrained in an industry, the new wears off and it becomes more like a commodity product. Engineering drawings become more generic, as they start to incorporate low surface roughness callouts, but little other guidance. Buyers start shopping for the lowest cost and fastest delivery not understanding that, perhaps, the technology is still evolving and improving, or that not all processes and suppliers will produce the same results. Like many “mature” technologies, isotropic superfinishing is still being improved and advanced in the gear industry, and if you are not in search of a particular solution requiring this technology, you may never know.
Isotropic superfinishing is a customizable manufacturing operation whereby tens or hundreds of gears can be simultaneously processed in the same piece of vibratory equipment, or where a small number of highly critical gears can be processed in an isolated (compartment-based) fashion. If all of the raw gears placed in the equipment are identical at the start, then they are all identically finished at the end of the process. Every gear tooth will have the same surface finish and geometry since the parts continually and randomly move through the equipment and statistically experience the same chemical and media exposure.
The stock/metal removal is facilitated by an aqueous active chemical product introduced into the vibratory equipment during the process. It works in conjunction with high-density, non-abrasive ceramic media to provide the mechanical rubbing aspect. The chemical product produces a stable, soft conversion coating on the surface of the gears being processed. The rubbing motion across the gear generated by the equipment and media effectively wipes the conversion coating off the “peaks” of the gear’s exposed surfaces but leaves the “valleys” untouched. No finishing occurs where media is unable to contact or rub the metal surfaces. The conversion coating is continually re-formed and rubbed away during the processing step, producing a surface-smoothing mechanism. This process is continued in the vibratory equipment until the surfaces of the gear are free of asperities. At this point, the active chemical product is rinsed from the machine with a neutral/alkaline solution. The conversion coating is rubbed off the gear surfaces one final time to produce the superfinished surface. In this final step, commonly referred to as burnishing, no metal is removed, just the residual conversion coating.
Most people envision an extremely reflective, mirror-like tooth flank when they see the drawing call-out an isotropic superfinish, or Ra < 4 µin. (0.1 µm). But, in fact, the gear can look very different and still meet those generic requirements. See Figure 1 for an example of a mirror-like isotropic superfinish.
In some instances, a part may appear hazy due to an incomplete burnish, but still meet the finish requirement. In other cases, the conversion coating may not be removed at all, generating anything from a grey to dark black appearance. See Figure 2 for an example of an isotropically superfinished gear that retains the black conversion coating. In some cases, shortcuts and poor process control can lead to an appearance that masks other finishing issues such as uneven finishing, or even tooth profile distortion.
The removal of the conversion coating is not necessary, or cost-effective for some end uses. The automotive industry demands fast tact times and low cost, so the burnishing step may be eliminated to meet these requirements while still delivering an isotropically superfinished gear that performs much better than it would in a ground or lapped condition. These types of gears typically do not receive secondary processes after isotropic superfinishing such as coatings, so the presence of any residual conversion coating is not an issue.
In more demanding industries, coatings are becoming more prevalent. In some cases, the webbing is coated for corrosion resistance. Additionally, tooth flank coatings are being evaluated for potential performance enhancement. When combining isotropic superfinishing and coating technologies, surfaces that retain the conversion coating can raise adhesion concerns.
In one recent project, the Gear Research Institute (GRI) at Penn State University reached out for assistance reviewing test gears that had been isotropically superfinished to remove surface damage resulting from the shot peening of the tooth flanks. These gears were destined for use to evaluate several coatings through fatigue testing. Upon receipt at GRI, the gears appeared to meet the surface finish requirements, but they retained the black conversion coating as shown in Figure 2. The team at GRI was concerned with the appearance, as they typically receive test gears sent for isotropic superfinishing that are mirror-like, as shown in Figure 1. Their main concern was that the coatings may not adhere properly thus ruining their testing and the expensive gears. These gears had been not processed by REM, but REM has been a member of the GRI Aerospace Bloc for many years and is recognized as an expert in the field of gear surface enhancement and isotropic superfinishing, so GRI reached out to us to evaluate these gears.
After a quick review of the project with the team and GRI and their customer, it was determined that these gears would be sent to REM for further evaluation. Upon receiving the gears, REM was able to confirm that these gears had been improperly or incompletely burnished. After a discussion with GRI and their customer, REM agreed to burnish the parts to the mirror-like appearance GRI was accustomed to receiving. In the next installation of this article, we will review what we found hiding under the thick conversion coating.