For one manufacturer of CNC grinding machines, new high-performance linear motors replaced ball-screws and improved machine performance.
It’s no secret that linear motors are highly effective motion devices. Linear motors can achieve high acceleration rates and long travel lengths with good thrust forces and extremely high positioning accuracies, while other drive mechanisms, such as belts, screws, or rack and pinions, must sacrifice at least one of these requirements in order to achieve the others. This is why linear motors are the preferred choice for highly dynamic applications such as in metrology and in semiconductor manufacturing.
In fact, based on their performance specifications, linear motors seem to be the perfect solution to address the competing requirements often found in linear motion applications. Still, linear motors are not as widely adopted as perhaps one would expect. There are a few reasons why their adoption rate lags behind other drive technologies such as belts, screws, or rack and pinion devices.
Still, despite the potential difficulties posed by heat generation, protection from contamination, high-bandwidth controls, and cost, the adoption rate of linear motors is growing. Once seen as niche solutions for semiconductor, metrology, and heavy-duty machining applications, iron core, ironless, and tubular linear motors are now used in automotive, food and packaging, and printing applications, where the moves may not be as challenging or the accuracy requirements as demanding, but where the benefits of fewer components, less downtime, and higher throughput justify the additional cost and design considerations.
Linear motors in CNC machines
One example of how linear motors are being incorporated into CNC machines comes from ANCA Machine Tools, or AMT. The company, sister company of ANCA Motion, manufactures quality CNC grinding machines. It launched several new machines during the past eight years, aiming to strengthen the value proposition they offer to CNC customers. The most significant offerings include their FX Linear and MX Linear machines, both of which are powered by the LinX Linear Motor developed by ANCA Motion.
Prior to the development of these two machine offerings, the company had observed the rise of linear motors in the industry. “AMT was aware of the benefits that linear motors can bring. Over time the ball screws can wear even on the best machines, and you have to consider backlash and the loss of preload,” said Simon Richardson, AMT MX platform product manager. “When installing and aligning a ball screw on a machine, tighter tolerances are required over the entire length of the ball screw when compared to fitting of linear motors.”
However, the company had resisted using linear motors for quite a long time. Philip Wysocki, electrical systems engineer at AMT, noted that “the traditional linear motor is flat in construction, which creates many issues when implementing these motors on machines.” Machines with flatbed linear motors typically require a separate chiller for thermal stability, and the attractive force between coil and magnet bed creates tremendous downforces on the bearings, making everything wear faster and decreasing efficiency. Plus, flatbed style linear motors used in grinding machines typically have a back-iron in their magnetic circuit, further increasing the downforces and creating cogging.
The LinX Linear Motor overcomes the problems related to flatbed linear motors and delivers superior performance due to its state-of-art cylindrical design. The motor consists of a shaft containing magnets and a forcer containing wound copper coils. The symmetric design results in zero attractive forces between the forcer and shaft, significantly reducing the loading requirement on support bearings. The thermal barrier design separates and removes heat from the motor, eradicating thermal growth for the machine. And because of its good standalone thermal stability, AMT’s machines don’t require a dedicated chiller for the motor. This provides an advantage over competitive flatbed motor-based designs by significantly reducing power usage and footprint.
The motor also features simple construction, non-critical air gap, and no physical contact between shaft and forcer, letting machine manufacturers significantly simplify installation, reduce maintenance and extend machine life. The motor’s design lets it replace ball screws in existing machines easily and makes the machine design process a lot smoother. “Due to its simple construction, the shaft ends will only require one or two supports depending on their orientation. Not only has the axis installation time been significantly reduced by more than 200% compared to ball screws, but the installation of the motors is much safer than flatbed linear motors,” notes Wysocki.
The ironless design of the motor and even force over the entire stroke lead to overall better motion performance. For instance, because of its direct- drive nature, the motor can track motion commands more accurately and repetitively to achieve a much better surface finish. So there’s no need to be concerned with cogging, backlash or reversal error. The tool’s surface finish ground by a LinX-powered machine is significantly better than the result from a best-performance ball-screw machine. In addition to the improved surface finish, cycle time is enhanced due to its higher acceleration and faster traverse speed.
Overall, the company found that the LinX Linear Motor provides improved performance at a lower cost and efficiency than ball screws and flatbed linear motors. The standalone thermal stability, high speed and acceleration, zero down forces and the ability to achieve IP67 protection make the motors a suitable solution for machine tool applications, but also in other industries such as food processing and other automation applications.