Why are Gear Finishing Processes Necessary?

A basic, square-cut gear consists of a combination of two simple machines: the wheel and the wedge. Even though the basic concept seems simple, the wide variety of applications that call for gears have driven relentless innovation in the way these components are made. You might not know it if your career hasn’t led you to explore the topic of how gears are fabricated, but it is a very involved and high-tech operation. Not all gears are created equal.

As with any machine component, a given gear must get built in a way that remains both economically sound and effective in its application, whether for a car, plan or heavy equipment. To achieve the strength, wear properties and other characteristics these diverse and numerous applications call for, engineers created unique finishing processes which hone and treat the surface of a gear. These processes are the crucial final step in gear fabrication.

What Is a “Finish” and Why Do Gears Need It?

Every gear in your car, whether a part of the transmission or plays a role elsewhere, subjects itself to unique loads in the course of a daily drive. This shows why fabrication techniques for gears have continued to evolve since their introduction in 50 A.D. Some gears must remain balanced and smooth so they can spin extremely quickly, some must remain strong enough to withstand high loads, and others must combine both of these qualities.

Today, gears used in high-performance applications get finished using high-tech processes which smooth their engagement surfaces and allow them to perform better for longer. The three most popular finishing processes are Turbo-Abrasive Machining, Vibratory Finishing, and Centrifugal Barrel Finishing. Let’s take a closer look at each.

Turbo-Abrasive Machining
Commonly referred to as TAM, turbo-abrasive machining first conceived as a method for finishing aerospace products. Its convenience when working with larger pieces has made it more popular as the cost of the technology has come down. Today, it can be used in less exotic applications. One of its primary benefits lies in reinforcing components against compressive stress — in other words, crushing forces.

TAM works by partially immersing a piece in a bed of media and rotating it. The duration, rotational speed and media all serve as variables that allow fabricators to dial in the proper parameters for a given project. Due to an automated process, it gains an advantage for heavy industry applications through repetition.
This production technique has become widely used thanks to the high strength and smooth finish of parts finished using TAM. The high degree of consistency that comes with it has helped it increase in popularity.

Vibratory Finishing
Vibratory finishing involves placing one or more parts into the tub of a vibratory tumbler. A wet or dry media compound gets added to the tub and vibrated against the parts in order to clean, deburr and smooth them. The process can also cut and polish larger pieces if desired.

Depending on the size of the piece, different media gets used. Compounds ranging from very fine media optimized for use with small pieces, to larger media for use on bulky pieces can easily separate and prepare using a vibrating screen machine. While some compounds may call for mixed media, most rely on a uniform media size to determine the type of finish applied to a part.

Vibratory finishing can polish small opening and channels in parts. This choice works best for small intricate pieces due to its minimal stress on parts.

Centrifugal Barrel Finishing
The third popular finishing technique in use today is centrifugal barrel finishing or CFB. In this approach, multiple pieces get mounted around a central spindle, which sits inside a rotating barrel or drum. The drum is rotated, and individual spindles counter rotate to expose parts to media inside the drum. This process is a less common, but similar application to vibratory finishing.

CFB is known for the exceptionally high pressures that can be achieved inside of the drum, which few other gear-finishing methods can match. Fabricators can take advantage of these forces to shape pieces rapidly. For example, this technique might be used to shape a large number of pinion gears that will serve the life of a car in its steering assembly, seeing use each time it hits the road.

For parts that require extremely hard surface finishes, CFB’s ability to apply media at very high pressure provides a means of shaping parts quickly. Fabricators can feel confident that small parts requiring very particular surface characteristics will meet the required specifications, which might prove more difficult to achieve using other methods.

Leave It to the Professionals
Determining an appropriate finishing process for a given gear or part can be difficult in situations. It’s a matter of engineering. Whereas before, fabricators had to guess at how a given part would stand up and what type of finish it required, today we have high-tech methods to analyze how a part behaves under stress. By testing the way previous designs behave under stress and analyzing how they fare over long periods of use, engineers can make informed decisions about which method is appropriate or better for a specific gear.

Article by

Megan Ray Nichols