?? JEC LAB - Composites meet metalworking

COMPOSITES MEET METALWORKING - Episode 6

Applying composite materials to metalworking machine tool structures can provide significant process improvements. Carbon fiber reinforced polymer (CFRP) design delivers lightweight, stiffness and damping properties to enable more efficient machining, laser cutting and automation.

Engineering firm CompoTech has been using its automated fiber laying (AFL) and winding process to produce components for machine tool applications, where composites offer a higher stiffness-to-mass ratio and better damping properties than steel. ‘Ultra-high’ modulus pitch carbon fiber is key to replacing steel components in a machine tool. Converting the pitch fiber into a composite beam structure makes it possible to replace a same section steel beam with one of potentially up to twice the axial stiffness at a quarter of the weight.

The benefits of composites for machine tools

In machine tool applications carbon composites provide advantages in key areas such as stiffness, natural frequency, damping, lightweighting, and thermal expansion.

Super-stiff structures: To cut faster you need to accelerate the machine tool faster. To achieve this with the same accuracy of cut, the stiffness of the manipulating structure needs to increase. However, to improve the overall performance of a machining system, there is a more complex balance of natural frequency and damping to consider.

Natural frequency, a function of mass and stiffness: The low mass and high stiffness properties of pitch fiber result in high natural frequencies of a part. In machine tool design, it is often the harmonic resonance that limits the working speeds. Vibration behavior can be optimized in several ways. The dimensions, internal structure and wall thickness of the part can be adjusted to tune its natural frequencies, but the main influence in first frequencies is the fiber choice and lay-up design.

Damping during machining: One particular focus in machine tool design is the optimization of vibration damping properties. Carbon composite parts provide good vibration and vibration-damping characteristics compared with steel or aluminium alternatives. Vibration is so important in many machining applications that it requires special attention during design. Damping materials can be incorporated into the structure and internal high-density foam reinforcements may be included to improve the vibration stability of thin-walled parts.

Lightweighting machine tools: Reducing the weight of large moving components allows designers to select smaller ancillary components. When the machine tool goes into service, the reduction in energy costs or increased life span of such components can pay back any additional material costs well within the lifetime of the machine. Certain components, especially tools, can offer new functions or increase a machine's performance when machining structures that would normally be outside of its payload capacity.

Thermal expansion: As a function of the fiber thermal expansion properties, a shaft or beam can be designed to have zero thermal expansion in one direction. For high-speed milling, the design of a hybrid steel-composite spindle to reduce thermal expansion considers the combination of steel parts and specific placement and angles of the fiber to be close to zero. Testing has shown 75% less displacement on the shaft alone and a 30% decrease within the spindle motor. This difference can be significant for milling accuracy and the load on the bearings (and hence the lifespan) in spindle motors machines without the use of coolant.

Applications

Axis beams are one area where composites can provide big benefits. On any form of cutting or milling machine, the horizontal or vertical beams are key to machine performance. Their mass and stiffness can limit the working area and accuracy of a machine, as well as its acceleration. The use of composites allows weight to be reduced by up to 50%, while stiffness can allow faster acceleration for the same deflection, important for the positional accuracy required for cutting to high tolerance. The benefits of composite axis structures have been demonstrated by Eagle Laser, whose iNspire laser cutter model boasts 30% quicker cutting time.

Spindle structures, such as high-speed machining spindles and spindle covers, and tools and tool holders utilizing composites have also been shown to boost performance. Composite hybrid milling disk tools are a particular success, offering a vastly superior machining performance, and a dramatic increase in tool insert life.

These applications are discussed in more detail in the article Machine tools – Composites meet metalworking in the January/February issue of JEC Composites Magazine. Get your free copy of the magazine: ?https://www.jeccomposites.com/jec-composites-magazine/

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