“It’s competition,” said the American businessman John Mackey, “that forces companies to get out of their complacency.” We are seeing this in the global electric vehicle (EV) markets. Manufacturers must produce lighter and more compact EV transmissions from tough materials, like low-alloy steels — and those that rely on traditional manufacturing processes risk falling behind.
Here, Mats Wennmo, Global Automotive Transmission Manager at the metal cutting specialist Sandvik Coromant, explains why power skiving is crucial for competitive manufacturing.
Credit: Sandvik Coromant
Credit: Sandvik Coromant
Sales of electric vehicles (EVs) have taken a major leap forward in recent years. According to EV-Volumes, 541,780 new plug-in electric cars were registered globally in February 2022, twice-as-many as in February the previous year. Meanwhile, Virta reports that 1.06 million new EVs were registered in Europe in the first six months of 2021, versus 413 during the first half of previous year. That’s a rise of more than 157%, while registrations in China — the other major market driving EV sales — and the US rose by over 197 per cent and over 166 per cent, respectively.
From these figures, it’s clear that the global EV markets are growing fast — but what is driving these sales? One factor is the ambitious zero emission targets pledged by the European Union (EU), Asia and the US. The EU has committed to reduce its greenhouse gases to at least 40% below 1990’s levels, and China to reduce its levels to 60-65% below those of 2005, by 2030. The US, meanwhile, has pledged a reduction of 26-28% below 2005’s levels by 2025.
What role can EVs play in helping these countries achieve their goals? In truth, comparisons between the environmental advantages of EVs versus internal combustion engine vehicles aren’t straightforward. According to Carbon Brief, it depends on the size of the vehicles, the accuracy of the fuel-economy estimates used, how electricity emissions are calculated, what driving patterns are assumed, and even the weather in regions where the vehicles are used. There is no single estimate that applies everywhere. Nevertheless, Carbon Brief’s report concludes that, on the whole, EVs are responsible for considerably lower emissions over their lifetime than internal combustion engine vehicles.
E-mobility technologies can also support the other two major factors that will drive EV sales. First, a global move towards the use of more efficient and renewable energy sources, for instance, as outlined recently by European Commission President Ursula von der Leyen. Second, changing attitudes among consumers: half of those surveyed in PwC’s December 2021 Global Consumer Insights Pulse Survey said their perceptions had become more eco-friendly.
Planetary gears
For carmakers and original equipment manufacturers (OEMs), the above factors all underline the need to move away from traditional combustion engines, and China and Europe will take the lead in these developments. These EV markets will also form a diverse and competitive playing field as larger established companies, like Porsche, compete with smaller, globally-expanding manufacturers, like Polestar.
At the same time, the manufacturing of EVs will also present additional challenges — which we’ll explore — with a risk that manufacturers relying on traditional production processes will get left behind. To examine these challenges, let’s focus on the manufacturing of gear components.
The ability to control the revolutions per minute (rpm) with gears is essential for all kinds of vehicles, including EVs. All EV transmissions are so-called reduction transmissions, designed to reduce the vehicle’s speed so it can be controlled and driven economically. Because there are no combustion engine sounds in an EV, any noise from the transmission will be noticed. So, the main task is to avoid noises. This is where the quality of the machining setup plays a decisive role, to make the transmission as compact, light and noiseless as possible.
EV transmissions are mainly of a planetary design, with the planetary gears and the sun gear assembled inside the peripheral ring gear with a compact and light assembly. The ring gear is the most difficult component to produce, with its thin walls and high demands on roundness. Unfortunately, traditional manufacturing processes can worsen these difficulties with several time and cost disadvantages.
Traditional manufacturing
Traditional manufacturing processes typically rely on single-purpose machines. Each machine is limited to a certain area of machining, and the workpiece is passed from one to the next. This make production lines inflexible when responding to necessary changes in component design.
Moving the workpiece from machine to machine can also worsen the quality of the component by creating run-out and centre deviations. The after-heat treatment is harder to control and traditional soft machining methods, followed by grinding processes, are very expensive. These processes also require additional oil-based machining, to benefit the machining itself and also chip evacuation.
These disadvantages will only become more pronounced over the coming years, against the anticipated major manufacturing trends for EVs. We can also expect to see a demand for increased speeds in the development of new transmissions, a need for higher productivity and flexibility at the same time, and a demand for shorter return on investment (ROI). Inflexible single-purpose machines will become more disadvantageous as flexibility, productivity and profitability grow more vital for manufacturing EV parts.
This is why manufacturers should upgrade their traditional manufacturing processes. But, how should they do this? One way is by investing in multi-task machines. As mentioned, grinding equipment commonly used in traditional manufacturing can be an expensive investment. A good way to sidestep this is to divide the machining of gear components into two processes, soft- and hard machining. These methods can be implemented in a single, multi-tasking machine setup.
Multi-task machines can eliminate machining processes, their associated time and costs and also improve part quality. The machines hold benefits for the customer, too. Along with improvements in product quality, the process cycle times are better, or at least the same as, existing production solutions. In fact, Sandvik Coromant has seen minimum cost reductions of 30% for the end user.
Finally, manufacturers can achieve greater flexibility in producing future components for e-mobility. The process allows downsizing and weight reductions in transmission components. Power skiving can be applied to both internal and external gears and splines, but is especially productive when it comes to internal machining. The method works particularly well in mass production, where short lead times are decisive.
Single, continuous process
As mentioned, the ring gear is the most difficult component to produce in a planetary gear, but the concept of power skiving — which has actually been around for over a century — is emerging as the most efficient way to achieve this. Power skiving is a process that combines shaping and hobbing (a machining process for gear cutting) into a single, continuous cutting process.
This has several important advantages over traditional machining methods. Instead of relying on a single-purpose machine, with power skiving, a complete component can be machined in one multi-task machine for higher productivity and flexibility. The need for specialised machines is removed, and quality restrictions due to machine changes can be eliminated entirely. This significantly reduces total production time ― compared to processes with broaching, shaping and hobbing ― for more manageable and predictable component machining.
Power skiving is becoming more popular and, since 2014 onwards, more than 700 power skiving machine tools have been delivered. The majority of these ― over 60% ― are multi-task machines. That means the main machining processes happen within the same, single setup. This improves the quality of the component and allows for more efficient machining.
Sandvik Coromant has developed its own high-quality tools for power skiving, which are optimized to support its customers in the accurate machining of EV transmissions. They include CoroMill® 178, a solid power skiving cutter, which can be ordered as powder metallurgical high speed steel (PM-HSS) or solid carbide. There is also CoroMill® 180, an indexable insert cutter with railed insert seats that is designed for excellent and repeatable accuracy. The tools can be optimized with regards to tooling stiffness and outhang, coolant supply and maximum tool life. These combined factors offer reliable, around-the-clock production.
Reduced machining time
When a manufacturer of main gears in low-alloy steel wanted to replace its time-consuming shaping process, it turned to Sandvik Coromant. The customer replaced its existing processes with power skiving and, in addition, was able to replace the four dedicated machines it used previously just two multi-task machines.
In the end, the customer’s machining time was reduced by 90%, with considerably increased tool life. In other instances, power skiving was shown to be two-to-three times faster than traditional processes.
At each Sandvik Coromant center, each EV transmission producer or contractor is able to machine its own components with power skiving in modern multi-task machines. Skilled and experienced staff are ready to support customers’ coming investments to achieve productive, efficient and flexible gear machining for EVs ― all in one setup.
These are among the reasons why power skiving is emerging as the fastest-growing method for gear machining, with time and cost-savings. It is accessible to smaller manufacturers, helping them compete at a higher level, and to larger manufacturers to help them, in the words of John Mackey, “get out of their complacency.”
- To find out more about the advantages of power skiving when manufacturing EV transmission components, visit Sandvik Coromant’s website.
- The author, Mats Wennmo, Global Automotive Transmission Manager at Sandvik Coromant.
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