Aerospace, Composites and Machining Challenges

A new long read discussing the latest developments in materials for aerospace parts

The aerospace industry often sets the standards other sectors to follow. There are numerous areas where aerospace has led the way, but so far, materials evolution has not been one of them. Aluminum has long been a core part of aerospace machining and manufacture. But things are beginning to change, with composites are becoming increasingly critical, providing structural strength at a lighter weight. As the aerospace industry relies more heavily on composites, rather than traditional metallics, I think this is a good time for some discussion on best practices for machining these increasingly complex materials.

According to the recently released ‘Growth Opportunities for Composites in Global Aerospace Market’ report, the future of the aerospace composites market looks attractive. The global aerospace composite materials market is expected to reach an estimated $3.9 billion by 2022. What’s more, the report states that North America is expected to remain the largest region during the forecast period, due to high demand for newer aircraft and the ongoing replacement of an aging fleet. The aerospace market is one of the largest, and arguably the most important, to the composites industry, so it’s reassuring to see these optimistic growth predictions.

Composites in Aerospace

The best known man-made composite materials used in aerospace, are carbon- and glass-fiber-reinforced plastic (CFRP and GFRP respectively) which consist of carbon and glass fibers. Metal matrix composites (MMC) are currently being developed and used by the aviation and aerospace industry are examples of particulate composites and consist, usually, of non-metallic particles in a metallic matrix; for instance silicon carbide particles combined with aluminum alloy. While ‘conventional’ metallics have a fundamental role in aerospace structures, there are considerable benefits offered by composites, which have yet to be fully explored or exploited. Here, we take a look at the evolving materials landscape for aerospace – and considerations for machine shops when working with new resources.

Composite Uses

A large majority of the composite material projects are related to the aerospace industry. With the recent developments of leaner and more efficient engines and new aircraft geometries, the rise in the use of composite materials has become almost exponential. This has driven an increase in the variety of machining and grinding challenges in the past years. Most typical applications involve carbon or glass reinforced composites (CFRP and GFRP). However, although less utilized, CMCs and MCMs often pose tougher challenges due to the finer tolerances and surface integrity requirements. Overall, from machinability characterization to cutting regime optimization, all machining related areas have seen a significant increase in recent years.

Composite Challenges

Composites offer significant advantages for developing new, lighter and higher-performing components. In most cases, these materials are designed with specific characteristics optimized for specific operating conditions and functions. While the mechanical and chemical characteristics of a composite material can be defined and validated through testing in the design stage, limited or no information about their machinability is generated. OPTIS engineers often encounter situations where manufacturers have to produce composite parts to high-quality standards without prior knowledge of appropriate tooling, cutting parameters, fixturing characteristics, and other considerations.

Our engineers have successfully tackled a variety of projects involving the machining and grinding of composites including ceramic composite materials (CMC), metallic composite materials (MCM), and epoxy matrix composite materials (e.g. CFRP, GFRP). These projects have focused on various areas including:

• Machinability studies on tool performance, wear and quality of the machined surface
• Optimizing cutting parameters for improving quality and productivity
• Identifying cutting parameters to minimize or eliminate surface and sub-surface defects such as material pull-outs, delamination, fiber pull-out, micro-cracking etc.
• Developing adaptive machining strategies
• Optimizing grinding wheel choice and parameters
• Development of new machining and grinding technology

Typically these, and other projects involving composite machining, gravitate around the need to identify proper tooling, fixturing and regime parameters for a successful and efficient outcome.

CAD Considerations

Composite parts are in many cases designed and formed very close to their finished specifications – especially with CFRPs and GFRPs. So it is very important that the CAD model takes into account any changes in part size and material properties during the forming or curing processes.
For near net shape parts, which need very limited additional processing once cured, engineers create a CAD model for the as-built component and another one for the finished part. Depending on the complexity of the part and the post processing needs — such as drilling, milling or sanding — curing and post processing fixtures will also have to be designed in CAD and then manufactured.

Design Gets Distorted

For CFRP and GFRP parts in particular it is not uncommon that after removing the part from the curing tool, the as-built part distorts due to the anisotropy of the material and stresses inside the material. Further distortion can occur as a result of the clamping strategy used, even if fixture tooling is built to very fine tolerances. These distortions can exceed tolerances and render the machining program incorrect, even when generated with a computer aided manufacturing (CAM) software, leading to the part being scrapped. Using software program to simulate tool motion and material removal does not help because those programs utilize CAD and cannot account for the distortion. In such cases, the actual position and shape of the workpiece inside the machine have to be determined, and the CNC program has to be adapted to those geometry characteristics to avoid under or over-cutting the part.

The OPTIS Vision

Using vision and laser scanning technology, OPTIS engineers have developed a methodology that captures the actual shape and position of a workpiece as clamped on a machining fixture, and calculates the position of the finished surface and exactly how the cutter path must be adapted.

Talk to Us

Across the aerospace supply chain, OPTIS specializes in optimizing complex or high value machined parts where tight tolerances and high quality standards are critical. By applying our expertise, Aerospace machining manufacturers and suppliers of power generation turbines can optimize their production efficiency. To find out more, talk to a machining master at OPTIS now.

About the author

Dr. Julius Schoop

Principal Engineer