How To Machine Composites, Part 1 -- Understanding Composites
Composites are replacing metal in certain applications. What does this mean for machining?
The Boeing 787 will be the world’s first large commercial airplane made mostly of carbon-fiber composite materials. Composites make up 50 percent of the structural weight of the plane and something like 80 percent of the volume. By comparison, the Boeing 777 is just 11 percent composites by weight. Boeing is making a huge commitment to replacing metals with composites, and the economics of aircraft ownership explain why.
For an airplane, the initial purchase price does not account for the majority of the ultimate total cost of the plane. However, maintaining and fueling the airplane together do account for the majority of this cost, and composites bring down both of these expenses. Aircraft composite materials are inherently more fatigue-resistant and corrosion-resistant than metals, contributing to maintenance cost savings that could be as high as $30 to 40 million over the life of a 787. The composite structures also deliver a greater strength-to-weight ratio, contributing to fuel cost savings. It has been estimated that a 787 flying the same route as a 767 (a smaller plane) would consume $5 million less per year in fuel.
The total of these and other savings from composites comes somewhere near the price of the plane. That observation has been expressed this way: If the plane is made from composites, then you get the plane for free.
Yet the dramatic shift in materials for aircraft parts entails a similarly dramatic shift in the ways those parts are made. What implications does a mostly-composite commercial airplane have for manufacturing?
More specifically, what does this mean for machining?
A composite part is a near-net-shape part. The form is laid up onto a tool that is custom-made to give the part its shape. Compared to an aluminum aerostructure component, a composite part requires very little machining.
Then again, the machining that the composite part does require can be challenging indeed. By definition, composites are not homogenous the way metal is. A “composite” is a combination of two or more materials engineered to achieve better properties than either of the component materials could achieve on their own. In a composite, one material is the matrix and at least one other is the reinforcement. Carbon fiber reinforced plastic (CFRP), the chief composite material in aircraft parts, consists of a plastic matrix with carbon fiber reinforcement. The shop that tries to machine this combination material faces a combination of challenges. The matrix could melt from too much heat, while the carbon fibers don’t cut well because they fracture instead of shearing smoothly. Meanwhile, the CFRP structures are built up from layers of material that could easily splinter or delaminate during machining.
A final source of challenge is this: By the time the composite structure is ready for machining, it has already become such a valuable part that the cost of scrapping it may be enormous.
Therefore, as more composite parts come to market, a growing number of machine shops will face this reality: They will machine composite workpieces for which the amount of machining is small compared to a metal part, but the cost, difficulty, value and impact of that machining will be considerably higher.
It is not just Boeing driving this. Practically all aircraft manufacturers are increasingly turning to composites to replace certain metal components and assemblies. Helicopters have been mostly composites for a while now. In fact, manufacturers of various high-value products are increasingly looking to composites of one form or another to take advantage of their strength, stiffness, durability, corrosion resistance, wear resistance and light weight. One estimate says that in 10 years, there may be even more CFRP going into wind turbines than into all aircraft. Meanwhile, metal matrix composites are being applied to higher-performance automotive components such as brake rotors. And because composites can also be transparent to X-rays, they are likely to find many new medical applications as well.
However, the phrasing above—that industries are increasingly looking to composites “of one form or another”—hints at an important caveat when discussing this class of materials. That is, composites are not a unified class of materials at all.
For example, CFRP is a type of polymer reinforced plastic, of which there are many varieties. Other, similarly broad varieties of composites are metal matrix composites and ceramic matrix composites. The word “composite” actually refers to a broader range of materials than the word “metal” does.
Earl Wilkerson, a CNC programming and tooling supervisor, has faced various composites machining challenges as part of his work for General Tool, a 240-employee contract manufacturer in Cincinnati, Ohio. A composite, Mr. Wilkerson says, is “any two materials that someone wants to glue together.”
How do you know how to machine a composite material that you are facing for the first time? Quite likely, you don’t. That is part of the reality of these materials. General Tool is something of a composites machining expert at this point, having developed machining experience in aircraft composites early on. Thanks to the company’s work on an early jet engine that used composites, it has now been machining aerospace composite materials for over 15 years. Even so, every new composite part is still different to the shop.
In fact, every new CFRP part is different. The term “CFRP” itself is broad. After General Tool succeeded machining its first CFRP part, the company struggled with the second one, until it discovered that the parameters needed to be slowed down and the leftover stock on some features needed to increase. The properties and composition of the second CFRP were different from the first CFRP—and this is the way it has been with CFRP ever since.
“You can’t just go to a handbook and look up ‘composites’ to get the right tools, speeds and feeds,” Mr. Wilkerson says. You can’t even look up “CFRP.” None of these materials is defined or consistent enough for that.
However, there are some lessons that this shop and other shops have learned—lessons that have allowed them to consistently succeed at machining a class of materials that continues to grow and change.
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