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  • Kirk Abolafia

Manufacturing Gears With Pulsed Electrochemical Machining (PECM)


Advanced materials and new manufacturing processes broaden the capabilities of gears for increasingly stressful applications. This trend within gear manufacturing is similar to other industries, as gear manufacturers must adopt new machining methods in order to produce smaller, lighter parts while simultaneously lowering production costs. Integrating advanced materials into high-resolution parts creates unique manufacturing challenges, currently without a clear, singular solution.


For a broad majority of gear manufacturing purposes, though, conventional machining processes produce gears with sufficient geometries, friction coefficients and strength. The content of this article is therefore irrelevant for manufacturers of polymer gears, or non-industry related applications of gears. Rather, this article’s purpose is to showcase an alternative non-conventional method of producing certain gears that may alleviate issues regarding lowering high-volume production costs of complex gears compromised of advanced materials: Voxel's pulsed electrochemical machining (PECM).


Photo of helicopter.
Among the most crucial components in rotorcraft drive systems are bevel gears composed of advanced alloys. Photo credit: Wikimedia Commons

Research on certain nonconventional machining methods on gears has already been conducted, yielding promising results. One notable field of research is for electrochemical processes, demonstrating a promising future of gear manufacturing for a variety of reasons. For example, research on Electrochemical Honing (ECH) produced excellent surface finishing and high resolution of parts in experimental studies, compared to conventional methods such as hobbing and milling. ECH on bevel gears of AISI 1040 carbon steel produced tooth surface roughness of Ra 0.31um (12.2uin)/Rt 6.88 um (270.8 uin).


While this data supporting the usage of ECH has impressive characteristics, there are inherent drawbacks somewhat comparable to most conventional machining processes. Primarily, the tool in ECH and most other nonconventional processes still have direct contact with the workpiece, thereby causing two important issues.


  • Contact-based machining inherently causes tool wear, raising potential production costs and impacting precision capabilities over time

  • Direct contact produces heat, which may impact the machinability of thin-walled, thermally sensitive structures

Thankfully, pulsed electrochemical machining (PECM) alleviates the issues regarding tool/workpiece contact, while simultaneously providing a solution to high-volume production of gears comprised of advanced composites and complex geometries—plus improved surface finishing capabilities to other electrochemical methods.

This article will cover:

  • Current gear applications

  • Potential gear applications

To learn more about how PECM works, see our introductory article on PECM.


Current Applications for PECM

Let's review some current examples on how PECM is utilized for gear manufacturing. PECM is particularly advantageous for machining strain wave gears, also known as harmonic gearing. Within these gearing systems, an elliptically-shaped plug rotates a flexible spline along the inner teeth of another spline. Strain wave gears have unique advantageous properties, such as high contact ratio and zero backlash, all within a relatively compact and lightweight design.

Strain Wave Gear
Animation of a harmonic drive, or a strain wave gear profile. Credit: Wikimedia Commons

These properties, such as high torque capability and high reduction ratio, make strain wave gearing attractive for aerospace, defense, and particularly space equipment. For example, NASA has expressed interest in increased implementation of strain wave gears for future space travel by utilizing advanced materials such as bulk metallic glass, due to the material’s ability to operate in extremely low temperatures without lubricants.

These unique gear couplings involve complex geometries and advanced materials—posing challenges for manufacturers attempting to produce these in high volumes. In particular, the thin-walled features of the flex spline cup are difficult to machine, especially if advanced alloys like bulk metallic glasses are utilized.

PECM is an ideal machining process for this scenario. Material hardness of steel, or bulk metallic glass, is irrelevant to the electrochemical process, and the thin-walled features of the flex splines are achievable by PECM. Voxel Innovations used PECM to machine thin-walled features .075mm or .003” thick, with a 20:1 aspect ratio, although thinner walls are certainly possible via PECM.

Other electrochemical machining processes have been utilized for gear manufacturing as a direct comparison to conventional machining methods. In one such case, ECM was used comparatively to CNC milling on spur face gears. These gear systems, often used in transmission gearing, are especially sensitive to inadequate tolerances and surface roughness. Very small inadequacies can directly impact the gear’s overall functionality.

By constructing a unique cathode to the spur face gear profile, researchers were able to reduce high-volume production costs by up to 70% and reduce machining time by nearly 75%, compared to an average CNC milling operation.

Potential Gear Applications of Electrochemical Machining

As previously mentioned, PECM is ideal when tasked to machine parts with unique geometries. But before we explain what applications could be ideal for PECM, we should briefly outline what gear geometries and materials may not be ideal.


Nonideal Gears

Any gears comprised of polymers, plastics, or wood are disqualified from the PECM process, as the electrochemical reaction can only be used on conductive metals.

Worm gear
Worm gears (pictured, top) are not a good candidate for electrochemical machining.

The applications and geometries of worm drives may also not be ideal for PECM. These gear drives are often gashed and subsequently hobbed, usually achieving desirable tolerances. Worm gears would also pose difficulties to PECM, due to their cylindrical geometries. The cathode is vertically lowered onto the workpiece, and constructing a cathode for high-volume production of worm gears, while possible, would be a considerable challenge that would likely be more affordably achieved via hobbing/milling.

Large gears with a wider clearance in tooth profile are another instance where PECM may not be ideal. For example, most chain sprockets would not benefit from a high-precision machining process. Lubrication is often more important than surface quality to reducing friction coefficient on these types of gear drives.


Let's now review what manufacturing scenarios could be ideal for PECM to solve.

Tensile Strength The material characteristics of a gear are a decisive factor to determine if PECM can benefit its machining process. Among other factors, the tensile strength of a gear’s material is directly correlated with the gear’s ability to withstand high static loads. Therefore, machinability is a primary obstacle for manufacturers considering high-tensile gear materials:

  • Nickel Alloys: Gear manufacturers will integrate Nickel-based alloys to improve material hardness and strength without compromising much ductility. However, most nickel superalloys are particularly difficult to machine in unique geometries, such as turbine vanes and gear profiles. PECM is capable of machining nickel-based alloys as quickly as processing aluminum (Data from Voxel Innovations).

  • Case-hardened and Through-hardened Steel are other methods to increase strength and decrease ductility in gear manufacturing, best utilized for improving wear resistance. Case-hardened materials have been shown to prevent crack formation and neutralize bending stress for complex, high-stress gear types, such as spiral bevel gears. While most manufacturers must machine parts before case hardening, PECM can be utilized at any stage in the case hardening process. Unlike conventional machining methods, using PECM after case hardening will not compromise the tool or the part’s features. PECM can simultaneously reduce machining costs of case-hardened materials and improve the part’s quality.


PECM’s capabilities have been tested on a wide range of high-tensile strength materials machining parts with geometries more complex than most gears. Some examples:

  • PECM can create .15mm wide, .15mm deep channels in stainless steel

  • PECM was used as a secondary machining process on a nickel-alloy turbine vane, reducing minimum wall thickness and improving the part’s durability

  • PECM shows promising results on experimental machining of refractory metals

A variety of gearboxes undergo extreme tensile strength, such as those within industrial wind turbines. Due to the inherent variability of wind currents in nature, these gearboxes are subjected to varying stresses. For wind turbine manufacturers, improving tensile strength of gearboxes directly affects energy efficiency and durability—and thereby competitiveness—of the turbine.


Wind turbine drive train
Wind turbine drivetrains operate under enormous pressure, and the smallest improvements to the efficiency of these systems can drastically improve the turbine's energy output. Photo Credit: Department of Energy

Manufacturers may be able to utilize PECM to make wind turbines more competitive in the energy industry. PECM can machine gears with higher-tensile strength materials in smaller geometries, which may directly affect the turbine’s energy output and durability.

Friction Coefficients

Improving the friction coefficient of gears is another consistent challenge for gear manufacturers. Certain specialized gearboxes (such as those within harmonic drives) have effectively eliminated this issue, but for most gear drives, efficiency is directly correlated with the friction coefficient of gears.

Friction has several detrimental effects on a gear train. Primarily, friction produces wear over time, directly impacting gear functionality. Automotive manufacturers, for example, must consider the surface quality of gears to optimize overall fuel efficiency. A variety of factors are considered during the manufacturing process to reduce friction coefficient.


Gear tooth fatigue.
Inadequate surface quality can sometimes contribute to gear fatigue and, ultimately, failure. Photo credit: Wikimedia Commons

Lubricants are certainly important for reducing friction, but when designing a gear profile to minimize friction, working depth, clearance, pressure angle, and gear material are primary factors considered. PECM can both improve the part’s features but also vastly improve surface quality simultaneously.

A superior surface finishing alone can improve a gear’s friction coefficient by up to 30% compared to ground gears. Superfinished gears also require significantly less torque input, vastly improving fuel efficiency of parts.

PECM has demonstrated superior surface finishing qualities comparable to electropolishing—as low as .005-.4 um Ra on a variety of materials relevant to gear manufacturing.

A brief aside: additive manufacturing (AM) has not found popularity amongst gear manufacturers compared to other industries. However, gear manufacturers seeking to utilize metal AM for gear production should also consider PECM as a secondary machining process to improve part features and surface quality. If AM technology is further implemented for gear manufacturing, PECM may be a beneficial option, especially for high-volume part production. Surface quality is an inherent disadvantage of 3D printing (due to support structure remnants, downskin surfaces, etc) and electrochemical processes are a direct solution to improve surface quality, and thereby the durability of these parts.

High-volume Production

Due to the lack of tool wear, PECM is ideal for high-volume production of certain parts. The <10um repeatability of PECM can help manufacturers produce hundreds of thousands of identical gears without the cost burden of tool replacement.

Most manufacturers have sufficient operations for high-volume production of gears, primarily machining the gears as opposed to extrusion or powder metallurgy. Specifically, high-volume machining of gears utilizes gear generation such as hobbing and shaping.

These methods are certainly advantageous for high-volume production of gears but have critical drawbacks to consider. For example, hobbing allows stacking of multiple workpieces for higher production rates but is still largely limited to machining external gear teeth. Other machining methods such as gear shaping have a wider range of capabilities, but at the cost of inferior production rates.

After the manufacturing process, most gear operations will include a secondary operation for finishing, such as honing, grinding and shaving. While each of these finishing processes involve varying costs, capabilities and production speed, they are still ultimately a separate process from the initial manufacturing. With a single cathode movement, PECM combines the machining stage and the surface finishing into a single operation.



 

Do you have additional questions, or do you have a part that may be a good fit for PECM?

Also, be sure to check out some related content from our Education Portal:


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