There are numerous methods and processes available for post processing industrially manufactured metal components, with various advantages and drawbacks depending on the size, geometry, material and finishing requirements. Below we’ll compare several established processes for finishing gear wheels and assess them in terms of health and safety, environmental impact and cost-effectiveness.

One Gear Wheel – Many Processing Challenges

Many highly loaded steel gear wheels are produced for use in the aerospace industry and must be ground to an extremely high degree of precision and hardened to levels greater than 60 HRC. The surface of a gear wheel is solid and tough and in order for it to function safely and reliably later on, the surface structure must be flawless and extremely isotropic, meaning it must be highly uniform in all directions. As well as requiring a roughness of below Ra 0,1 µm, the involute surface cannot be wavy or show any grinding grooves. Additionally, the edges on the sides and on the tooth head must be evenly rounded.

Not All Processing Methods Hit The Mark

One option for gear polishing  is manual or mechanical robotic smoothing and rounding. With this method, the processing quality throughout the tool’s life cycle does not remain constant, as progressive wear on the grinding wheel or brush results in an increasingly imprecise surface finish over time. Both methods produce very uneven results when processing inner geometries because the tools can accommodate different components only to a very limited extent, and are difficult, if not impossible, to position in tight places. Current technology often favors trough vibrators for processing and using certain acidic process liquids can help to reduce process times. However, most of these substances are hazardous and employees must wear special protective equipment when handling them. Some of the substances used have been classed as substances of very high concern by the REACH regulation (Registration, Evaluation, Authorisation and Restriction of Chemicals) as they are carcinogenic, mutagenic, and in the case of sodium chromate, toxic to reproduction. 

While this process might tick all the boxes in terms of finishing requirements, the process liquid can generally only be used once and must be replaced once processing is finished. This results in increased purchasing, neutralization and disposal costs, meaning the process is far from cost-effective.

Dry electropolishing processes are another option for polishing metal workpieces in a single operation. They use electrolyte granulate to reach and process even inner geometries. However, not all granulate is suited to the workpiece shape, and electrolyte can get jammed in the workpieces. One of the biggest drawbacks of this method is that it cannot remove the waviness caused by the forming process of grinding as its abrasion level is simply too low. The average process time ranges from 30 minutes to several hours. The electrolyte granulate must also be disposed of as hazardous waste once it has reached the end of its given service life, and buying new granulate to replace it is an additional financial cost.

Stream Finishing – A Green Alternative

One thing is clear: Stream finishing results are on par with those of chemically accelerated mass finishing. But the key difference is that stream finishing is a purely mechanical process that does not use acids or electrolyte granulate, making post-processing much more cost-effective.

How Does Stream Finishing Achieve Such Good Results?

In stream finishing, which was developed in-house by OTEC, components are clamped in a holder and immersed in a rotating container filled with an abrasive or polishing medium. The workpiece also rotates, and the relative motion between the component and the media produces an even finish without the need for manual work. The process turns out highly precise surfaces with a roughness as low as Ra 0.01 µm on complex geometries.

The enormous machining forces used in stream finishing can remove material faster and more accurately than any other type of surface processing. The fine media used can get through to the inner geometries and easily process them.

Getting Into Gear: Wear-Resistant Gear Wheels Courtesy Of Stream Finishing

Stream finishing improves the overall surface quality of gear wheels by levelling the surface and removing roughness peaks and machining grooves to produce an isotropic surface, and a specified roughness based on the customer’s requirements. In addition to reducing friction and wear, the process also produces a sheer layer by changing the surface structure and creating compressive residual stress, which is highly effective in conditioning the gear wheel for use.

Stream Finishing Reduces Reject Rate

OTEC’s stream finishing process boasts an extremely low process variation of just 1 µm – which is five to ten times lower than chemically accelerated processes. This is good news for the production process as it means that there is greater leeway to use the tolerance range in the upstream forming process of grinding. Having to reject workpieces due to high variation during the finishing process is now a thing of the past. When gear wheels must be ground to an extremely high degree of precision, reducing the reject rate can save tens of thousands per reject.

Application Example For A Highly Loaded Gear Wheel

Before processing (left):
Ra 0.19 µm
Rz 1,358 µm
Rpk 0,418 µm

After processing (right):
Ra 0,042 µm
Rz 0,551 µm
Rpk 0,139 µm

The Benefits Of OTEC’s Stream Finishing

• Rapid and efficient deburring, edge rounding and smoothing in one single step
• Very low process variation (1 µm) compared with other methods (~5–10 µm)
• Exceptionally high process reliability obtained by mixing new and reused grinding bodies
• Extremely isotropic surface
• No roughness peaks (Rpk < 0,1 μm): less wear, no run-in necessary, longer oil service life
• Low risk of micropitting
• Cost-effective process suitable for recirculation
• No expensive disposal of abrasives
• No handling of dangerous chemicals
• Very short processing times – four times quicker than chemically accelerated processes

Stream finishing is suitable for a variety of components, including those with complex geometries, a weight of up to 200 kg and a diameter of up to 650 mm, such as turbine discs, turbine blades, blisks, servo valves and gear wheels for the aerospace Industry.

Contact us today to request sample processing.