Research Report: Overcoming Cutting Tool Challenges in Aerospace Machining

Aerospace manufacturing has always been on the cutting edge,from materials to production techniques.

Executive Summary

From the corner office to the factory floor, everyone who works in aerospace manufacturing faces the same challenge: producing quality parts for safety-critical applications with maximal efficiency. This is acutely felt in aerospace machining with its complex, difficult-to-machine components—such as blisks— that

combine advanced materials with intricate features.

The high value of such parts, combined with long cycle times, means that engineers working in aerospace machining

can not afford to make mistakes, especially not when the quality control standards for aircraft engine components are so high.

Consistent tool life is essential to maintaining a balance between quality and productivity, so selecting the right tools for the job is key to succeeding in such a highly-competitive market. The ever-expanding universe of advanced aerospace materials has generated a parallel universe of advanced cutting tools, and with so many options it can be easy to get lost.

This report is designed to help engineers and manufacturing professionals navigate the continuously evolving landscape of aerospace machining. In the following pages, readers will learn about the latest developments in aerospace materials—specifically, heat resistant super alloys and composite materials— and how these developments impact aerospace machining. Obviously, new materials can introduce new challenges and may require new tools; but

even if you’re machining traditional alloys, speed and surface finish can see significant gains as a result of utilizing the latest cutting tool technology.

Narrowing the options when specifying cutting tools for a job can reduce costs and lead times, but only if you select the right ones. In order to do that, an engineer needs to be up-to-date on the current challenges and changes in aerospace machining. This report will teach you about:

  • New developments in proprietary and non-proprietary aerospace materials and how these changes impact cutting tools.

  • Current challenges and solutions for machining critical aerospace engine components.

  • Tips for choosing the right cutting tools that will deliver optimum performance in aerospace machining.

By the end of this report, engineers and manufacturing professionals should be better equipped to specify or select cutting tools for aerospace applications when dealing with difficult-to-machine materials under tight timelines without sacrificing quality or component integrity.

Changes in Aerospace Materials

“As technologies evolve and materials change and develop, we help customers figure out the best way to machine that material, including new tools, processes and techniques.”

Corey Schwenke, Product Manager for Solid Carbide Tools, Sandvik Coromant

Aerospace machining has become more complicated, particularly for heat resistant super alloys (HRSA) and composite materials. Both categories have advanced considerably over the past decade, and as a result, these materials are seeing increasing demand in applications that produce high-value, lightweight parts. However, this upward trend has driven the development of many more material options attractive to aerospace engineers.

HEAT RESISTANT SUPER ALLOYS (HRSAS)

In the aerospace industry, HRSAs are primarily used in hot section turbine components, such as traditional blades, blisks, combustors and associated static components, but the high temperate tolerance and creep resistance of superalloys make them useful for other components such as valves  and manifolds. As their name suggests, these materials do not easily conduct heat, including the heat from cutting forces. As a result, that heat is transferred more rapidly to the cutting tool—temperatures in the cutting zone can range from 1,100 to 1,300 C—which reduces tool life. High temperatures in the cutting zone also lead to a greater tendency towards strain and work hardening in the component being machined compared to more conventional materials, such as steel.

Diagram of  a typical gas turbine jet engine.   (Image courtesy of Jeff Dahl.)

Diagram of a typical gas turbine jet engine. (Image courtesy of Jeff Dahl.)

Heat treatments—such as annealing, solution treatment and aging—can also positively or negatively affect machinability. For example, items that are annealed are lower hardness and relatively easier to machine while aged are typically harder and up to 48 HRC. Hardness affects machinability and is a major factor in tool choice. More force is required to shear the material and form a chip.

The number of HRSAs used in aerospace manufacturing has increased considerably in recent years, though the majority of modern aircraft engine hot section parts are nickel-based, and Inconel 718 remains one of the most popular. However, as Scott Lewis, Aerospace Industry Specialist at Sandvik Coromant explained, it’s hardly the only option:

“It’s not just 718, there are many different types of nickel-based materials. Today, many of the OEMs have their own proprietary materials that are even more challenging to cutting tools than 718. Typically, your cutting data is reduced by roughly 10 percent, maybe even higher in some cases, when you’re working with these more difficult materials.”

In other words, HRSAs are challenging to machine not only because of their material properties, but also because of how varied those properties can be. Lewis states, “A small change in the hardness of Heat Resistant Super Alloys—for example, like going from 46 to 40 HRC—or compositions can significantly affect the cutting data you would choose, especially your cutting speed.”

Composites

The fact that many jet engine builders have their own proprietary nickel- based materials makes consistent aerospace machining even more difficult, and the same is true of composite materials, as Corey Schwenke, Product Manager for Solid Carbide Tools at Sandvik Coromant, explained:

“For the proprietary materials, it seems like everybody makes their own version. You may have a good experience with one manufacturer, then go to another manufacturer with the same composite drill or composite router, and not have the same results because the recipe for the material is different. I think composites are one of the biggest challenges we see within materials.”

Composites are designed to combine the best properties of dissimilar materials, typically by embedding continuous or chopped fibers (e.g., carbon, glass, aramid, etc.) into a host polymer matrix (e.g., phenolic, polyester,

epoxy, etc.) and then curing it with heat, light or a chemical catalyst. Broadly speaking, the advanced composites used in the aerospace industry can

be divided into thermosets and thermoplastics—the former may require chemical curing agents while the latter only need heat and pressure.

The availability of composite materials for aircraft production has given aerospace engineers more options, but also much more to consider.

For example, composites have relatively low through-thickness thermal conductivities, which can make the coefficients of thermal expansion very different in and out of plane. As a result, the attachment points for composites can experience higher stresses. Composites also absorb impact energy by damage modes rather than local plastic deformation. Consequently, failures in composite materials tend to be sudden and catastrophic.

Obviously, these new issues present challenges for the machinability of HRSAs and composites, as well as aircraft design in general.

Challenges in Aerospace Machining

“Just because something worked five or ten years ago doesn’t mean it’s still working at the same efficiency with different or more complex materials today.”
Scott Lewis, Aerospace Industry Specialist, Sandvik Coromant

As indicated in the preceding section, advancements in aerospace materials present several new challenges in machining. This report examines four challenges in detail:

  1. Variety of Materials
  2. Part Complexity
  3. Tolerances & Regulations
  4. Part Cost

Variety of Materials

As a highly regulated industry, aerospace engineers deal with a more in-depth certification process than other manufacturing industries. In aerospace manufacturing, certifications can be numerous and sometimes include the part, the materials, production equipment and even the process itself, right down to the tool being used. Carbide inserts are the current material of choice for finishing but the research into using exotic materials for finishing operations to increase productivity is ongoing.

Exploded view of a jet engine. (Image courtesy of Sandvik Coromant.)

Exploded view of a jet engine. (Image courtesy of Sandvik Coromant.)

When it comes to aerospace materials, nickel-based materials, composites or composite-stacked materials are the most common. “On top of that,” said Schwenke, “we see different combinations of these stacked materials— which are different types of materials that are layered together and are very difficult to machine.”

Another point to consider is that entrance and exit layers can differ from supplier to supplier. “Within the composite family, we’re seeing more thermoplastic applications coming up, and those materials tend to have lower melting points.” This means that on top of the variability that comes with using proprietary materials, the risks of melting and deformation are considerably higher when machining thermoplastics. The traditional high speed, light feed strategy for commodity thermoplastics simply won’t work with a heavily loaded composite with a filler material that’s tough, abrasive or both.

Part Complexity

Engine components such as blisks are one of the most challenging components in aerospace manufacturing, usually requiring specialized machines and software. Such components are particularly challenging for standard tooling.

Blisks are a unique challenge. With a shape totally constrained by the mass flow requirements of the engine, the traditional design trade-offs between part shape and manufacturability simply don’t apply: a blisk must be machined as designed, no matter how difficult the task. Thin walls and tight features are a given. “The complexity of some of these parts, and how they’re designed, definitely has an affect on machinability,” Lewis said. “With blisks, for example, trying to reach down in-between the blades usually means you’re extending your reach to the maximum overhang, or even beyond the max that we would typically recommend. However, special tooling options for long reach are available, such as heavy metal and carbide shanks.”

Regulations

Of all the industries that machine parts, aerospace is one of the most highly regulated. It makes sense because mistakes in this industry can come

at the cost of human life. Tolerances vary widely depending on the part.

For example, rotating components are typically more critical than static components and typically have tighter tolerances. Moreover, with extensive regulation in the aerospace industry and small part volumes, inspection is far more comprehensive than production components that are high volume and less critical.

“With certain critical parts and features, a customer may be restricted from making changes by a locked process,” Lewis explained. “When it comes to finishing operations, if they want to change a tool, it might involve a lot of paperwork or a long approval process and a destructive inspection of the part after it is machined for quality control. A destructive inspection is where they actually cut up the component to analyze the part for quality. That’s one reason shops don’t frequently change their tools.”

Part Cost

Highly complex parts made to extremely tight tolerances from proprietary or otherwise unique materials are inevitably expensive.

Quality tooling, optimal processes and toolpaths, and understanding challenging materials and complex components will help

shops avoid pitfalls that are inevitably costly. Mistakes can result in scrapped parts, or equally undesirable, underquoting the job and leaving profits on the table.

“Tools can definitely affect scrap rates,” Lewis said, “as well as accuracy and surface quality. Having the right tool for the right job is important, but so is having the right programming technique or tool path.”

TIPS FOR INCREASING PRODUCTIVITY & TOOL LIFE

“The demands of the aerospace industry mean that our customers are looking for tools that provide them with process security and surface integrity.”

Scott Lewis, Aerospace Industry Specialist, Sandvik Coromant

Lewis and Schwenke offered a number of tips for engineers looking to reduce cycle times or improve tool life. “Do some lab testing before you go out to a customer and run a tool,” Lewis said. “That gives you a better idea of how your tools are going to perform in those materials.” According to

Sandvik Coromant, one of the best ways to handle challenging components is with solid carbide tooling with a trochoidal technique. The benefits of solid carbide tooling include long reach (with a diameter-to-length ratio in excess of 10 x D), more teeth in cut and the ability to use more sophisticated tool paths, such as five-axis trochoidal techniques.

The classic dilemma for cutting tools is this: How do you balance unit cost, tool life and productivity? On the one hand, a user might prefer one tool that will last as long as possible despite its higher cost, in order to avoid as much downtime as possible. On the other hand, a different user might prefer to use multiple, lower cost and non-optimized tools that might not last as long, but which also require a lower upfront investment. The available equipment and fixturing are often a factor.

Tooling should be considered an investment and the operation should be understood holistically. Rather than looking at cost per tool, it is best to look at cost per component, as Lewis explained. “With cost per component, you’re considering the quality tool coupled with the proper techniques for optimum results,” he said. “Your tooling supplier should be able to support you in many ways. Not only helping with feeds and speeds but also providing the proper techniques to drive the tools.”

Aerospace manufacturers need more capacity for tooling because of the number of required optimized tools and sister tools. Sister tooling is a common practice for lights out operations. The benefit of using this is once you have a locked, secure process, you can have operators running multiples machines, helping with overhead costs. “Sometimes a customer wants a general-purpose tool,” Schwenke commented, “and we talk to them about ‘standard vs optimized tooling’.” Standard tools are those that are multi-purpose and can run in multiple materials and optimized tools are specific to a certain material.

“When we get into these more difficult-to-machine materials,” he continued, “you really need a tool or grade of tools that’s specifically designed for that material—you optimize your process by having optimized tools.”

According to Lewis and Schwenke, “One of the biggest mistakes that we see in aerospace manufacturing is that customers use a tool simply because it’s the one they have always used. They may not know about a newer tool or technique that can give them huge benefits,” Lewis said. “Even though they might be getting more complex materials in their shop, they’re still using that same old tool.”

CHOOSING THE RIGHT TECHNIQUES

As experienced engineers and machinists know, choosing the right cutting technique can make a significant difference to productivity. High RPM, high power production machining centers introduced the concept of lighter cuts with a faster feed rate, but complex, critical tasks in the aerospace sector require a more considered approach.

If you’re looking to achieve light engagement with a high feed, Lewis recommends using a trochoidal milling technique to lower tool engagement. “This technique is taken from the die and mold industry for hard part milling,” he explained. “It allows for lighter radial depths with high axial cuts which means the tools move more rapidly across the component, therefore reducing heat buildup which has a positive impact on tool life.

When we lower tool engagement and increase the feed rate, we can increase tool life, improve consistency and increase productivity – all things customers are looking for.”

Engineers faced with new materials—or new combinations of materials— start from their past experiences working with similar materials, noting composition and condition to determine a start value of machining parameters. From there, they read the wear of the tools and adjust accordingly until performance is optimized. This procedure is called mapping because it creates a chart of how we want the tool to perform. Chip charts and tool wear mapping help the engineer get to the best end result.

Choosing the Right Cutting Tool

“In the past, we had grades that would be used for both titanium and nickel-based materials. Now, we have solid end mills in different grades that are specifically designed and optimized for nickel-based materials and titanium.”

Scott Lewis, Aerospace Industry Specialist, Sandvik Coromant

The old cliché about choosing the right tool for the job has stayed with us for a reason. The challenges introduced by new materials in aerospace

manufacturing make selecting the optimal tool for your application all the more important. The first step, not surprisingly, is to discuss your needs with your tooling supplier.

Lewis explained that, “On customer visits, one of the first things we’ll look at is what tools are they using. Is it the right tool for the right job or for the right material? If we look at that, many times we’ll ask the question, ‘Why are you using this tool?’ and the response we get is ‘Oh, this is what we always use.’”

The pace of technological innovation in both materials and cutting tools leaves many job shops unaware of the latest options—information cutting tool suppliers are more than happy to provide. To take one example, Sandvik Coromant offers a range of solid carbide end mills designed for heavy roughing, finishing and thread milling. CoroMill Plura tools can cut difficult- to-machine aerospace materials and these tools are optimized for the material such as HRSA, titanium and composites as well as the application, including high feed side milling, large chip removal and micro machining.

The key in many cases is selecting an end mill with the right coating or geometry for your application. In response to the ever-increasing diversity of aerospace materials, some suppliers are offering cutting tools with aerospace-specific grades and coatings.

For example, Sandvik Coromant’s 1710 grade—available on CoroMill Plura HFS ISO S tools for nickel-based alloys—incorporates a hard substrate with a new coating designed to resist high working loads when machining parts

made from hardened, highly adhesive materials, such as engine cases made from Inconel 718.

BENEFITS OF SOLID CARBIDE TOOLING

As Lewis explained, “The benefits of solid carbide tools in aerospace come down to cycle time and accuracy. If you use a solid carbide round tool rather than an indexable mill, you can get down to smaller diameters with more teeth and utilizing trochoidal techniques to increase your cubic metal removal rate.”

Of course, indexable milling cutters still have a place in aerospace machining. For example, a facing operation on a large surface can be performed much more efficiently with an indexable cutter than a solid carbide tool. Nevertheless, with proper programming, solid carbide end mills can be much more efficient than indexable tools in many applications.

CoroMill Plura solid carbide tooling using trochoidal technique of tool engagement.

CoroMill Plura solid carbide tooling using trochoidal technique of tool engagement.

As indicated above, when working with HRSAs, heat is transferred from the material to the cutting tool. Proprietary Sandvik Coromant coatings such as Inveio and Zertivo were engineered to create a thermal barrier which helps protect the substrate from failure. These coatings also include adhesion reducing properties that help reduce the formation of build up edge (BUE).

Conclusion

In an industry where the competition is fierce and the production of a single component can represent a huge investment in time, money and expertise, manufacturers can’t afford to make mistakes. The combination of extensive regulations, tight tolerances and the rapid pace of innovation puts aerospace manufacturers in an even more precarious position. New materials and designs present new challenges for machining, and the only way to respond is to leverage innovations in cutting tools and techniques. It’s a maxim that holds true in any industry where productivity and time-to-market depend on constantly evolving technologies:

Keep up with the pace of change or risk falling behind.

The first airplane engine was built with the most basic cutting tools imaginable: hammers, chisels and carbon steel drill bits in a primitive drill press. Half a century later, complex gas turbine hot section blades were investment cast and machined from nickel alloys. Today, the metallurgy and manufacturing technology for aircraft engines has evolved into a complex engineering science. That kind of rapid technological development is only possible when manufacturing and production methods can keep pace with innovation. In aerospace machining, that means using cutting tools that can handle complex geometries as well as the latest materials, such as HRSAs and composites.

Choosing the right cutting tool can make the difference between being on time and on budget or making parts that are late, out of spec or (even worse) both. Selecting the right cutting tool in aerospace machining isn’t easy, but there are experts available who can help make that process much more painless.