Materials in Aerospace Part II of II: Answering the Cutting Challenge

Aircraft are becoming lighter, faster, more environmentally friendly, more fuel efficient, and more aesthetically pleasing. Aircraft designers are finding more and more ways to create next-generation aircraft by using both time-tested materials such as stainless steel and aluminum, as well as new exotic materials such as Ti5553 and advanced composites. R&D efforts are even enabling materials such as ceramics and titanium aluminides to be considered as the next candidates in the evolution of aircraft design.

With all of these improvements and technological advancements, people are excited about the aerospace industry's future. However, to make the culmination of these developments a reality, the most indispensable - and yet often overlooked - task is becoming increasingly difficult to complete: machining the components.

FACING HUGE CHALLENGES

Manufacturers and subcontractors are now forced to cut materials that they have little, if any, experience with, and the materials are proving to be extremely difficult to machine. In an industry where quality and accuracy is a matter of life-and-death, and a scrapped part could cost a company a small fortune, it is paramount that mistakes are minimized.

To aid in addressing this challenge, cutting tool manufacturers must create tools to withstand these materials and enable efficient, accurate part production. Depending on the material, tools must possess different characteristics in order to cut proficiently.

"The new strong and lightweight materials devel- oped by the aerospace industry pose a significant challenge to cutting tool manufacturers," says Jeff Eckhout, product specialist for rotating products, Valenite, LLC. "The same material properties that are attractive to aerospace manufac- turers for strength, weight and corrosion-resistance are the same properties that make the materials difficult to machine."

Titanium and nickel-based super alloys present some of the biggest challenges. These are such hard, high-strength materi- als that the force required to cut them generates a high level of heat. When high heat is generated, a failure mode from thermal cracking is likely if an incompetent cutting tool and/or process is employed. Controlling the speed is one way to avoid such high heat, but this leads to a lower level of productivity, which in turn cuts profits.

"On aluminum, it takes a very high-polished positive cutting edge to get the strong shearing action that's required," says Steve Ortiz, commercial director, ATI Stellram. "The challenge we have with titanium is that we still need the positive cutting edge, but because the material characteristics make it a little tougher, you have to combine the positive rake of the cutting action with a robust part of the geometry. So it's a combination of the positive profile with the gradual decline in the rake."

Another problem that jet engine manufacturers face is chip control, explains Don Graham, manager of turning products, Seco. When metal is shaved off a part, it comes off in long razor wire. Machine operators must periodically reach into the machine and remove the chip, which is dangerous as well as time-consuming. The machine must be shut down, and the chips are removed and thrown into a chip hopper. Between lower cutting speeds and downtime for chip removal, manufacturers face a significant challenge in maintaining adequate productivity.

The answer to this problem lies with the correct type and use of coolant. "Delivering coolant to the cutting edge before the coolant begins to disperse and mix with air will prevent the loss of heat capacity and the ability to cool the chip," Graham continues. "The coolant will also cut the chips into little segments that range in size anywhere from 1/16" long to maybe 1", depending on the cutting conditions. In addition to that, you can typically double the cutting speed, because now you're running quite a bit cooler."

It is important to apply the coolant very accurately to the chip that's being generated on the edge. "In milling, a titanium chip can be quite sticky, so as it goes around, it sticks to the edge and as it goes to cut again, it can chip the insert and also lead to a loss of tool life," adds Chris Mills, aerospace development North America, Sandvik Coromant. "So, high-pressure coolant blows the chip away, and we see that we get double tool life when we're running high-pressure coolant with dedicated nozzles."

Cutting tool manufacturers recognize that technology advancements are necessary to keep up with the quickly changing aerospace industry. The cutting tool industry is dedicated to familiarizing themselves with new processes, materials and technologies in order to assist in making the future of aerospace a reality.

"Seemingly, when people develop materials for aircraft, they don't consider machinability," Graham says. "Machinability is sort of left to the poor guy at the end of the line. So we try to pay attention to new materials coming down the road, and try to experiment with them in the lab, so that when they do come, we're prepared."

ANSWERING THE CALL

There are a number of different things to consider when attempting to develop a tool that meets the challenges of the aerospace industry. Tool geometries, substrate and coating materials, helix angles, cutting speeds, and rake angles all have a significant impact on how the part will turn out in the end.

"The geometries of the inserts probably have the greatest overall influence on any material to be machined," says Mike Gadzinski, training manager, Iscar North America. "If the correct type of carbide and coating are used on the material to be machined at a particular cutting condition, the only main variable is the insert and tool geometry. The amount of heat produced in the cut, the amount of force required, surface finish, and tolerance are all affected by the tool geometry."

Titanium materials require great strength with a high positive geometry and sharper cutting edge. The positive rake refers to the angle that's on the cutting edge. This cuts the titanium super alloy without generating a lot of cutting force.

"The insert shape that you select is extremely critical," Mills says. "For example, if you were going to go from a round insert to a zero-degree lead angle, then you can affect your material removal rate. You can have double the metal removal rate when you apply a lead angle."

In addition to geometry, the right grade and chemistry of the cutting tool also plays an important role. Physical Vapor Deposition (PVD) has proven to be the preferred process when coating substrates. However, at times this has shown to create less-than-stellar coating adhesion, and therefore cutting tool manufacturers are striving to improve this process.

"We've figured out a process where we can get better adhesion, which means we can make a thicker coating, and with a thicker coating, you've got more wear resistance," Graham says. "We've also come out with new edge preparation techniques and a new type of coating process."

The makeup of the coatings for each individual cutting tool company differs slightly, allowing manufacturers to choose based on their specific needs. The right grade of a cutting tool depends on what type of material is being machined. In terms of substrates, different materials are used for different aerospace components, giving manufacturers the best options when machining.

"For Ti5553 and Ti6-4, we think carbide, specifically micrograin carbide, is the right way to go," Graham explains. "Ti5553 is known as a beta titanium alloy, and all that means is that it's very hard, very abrasive, so cutting speeds are typically limited. Cutting it involves marrying the right geometry with the right grade, the right chemistry of the carbide tool and cutting conditions."

Micro-grain carbides are a popular choice in substrates among cutting tool companies. "We use our micro-grain ruthenium alloy substrate materials in both Ti6-4 and Ti5553 because it will withstand some of the problems in those two materials," explains Dave Watson, VP of cutting tools, ATI Stellram. "What we've tried to do is develop a substrate that covers all titanium alloys."

Composite aerospace materials also pose a new challenge for the aerospace industry. Manufacturers must often machine through carbon reinforced graphite, which is typically very abrasive, creating abrasive wear on the tool. This can be remedied by using a hard substrate tool with a diamond-like coating.

Another factor that cutting tool manufacturers must consider is when there is a high axial depth of cut, the helix angle going around the tool produces an axial cutting force, which, in effect, tries to pull the cutter out of the chuck.

The insert itself must be able to withstand these pressures, and too often, with long edge cutters, there is no axial support on the insert. The axial forces move the insert up and down within the pocket, leading to cutter breakage and the loss of an expensive tool.

Sandvik Coromant has addressed this issue by adding a shaped pocket underneath the insert, so that the insert seat holds it in place rather than just a screw. This allows manufacturers to run higher feedrates without tools or screws breaking during their processes, which is a concern when machining materials such as titanium. However, Mills stresses that the cutting tool itself is not the only factor to consider when looking to increase productivity. "We're going to make sure that, rather than trying to introduce just an insert or tool, we really try to have a complete process to handle the cutting of the different features for a typical component," he says. "We understand what sort of features are on the components and how to complete a process for that, and that includes not just the grade and geometry, but also the correct programming method, because the programming method can be so critical to success or failure."

TEAMWORK EQUALS SUCCESS

Cutting tool manufacturers recognize the need to collaborate with other industry professionals to solve the problem of machining aerospace materials efficiently and effectively. From start to finish, the process must be optimized in order to create a quality part. This necessitates a strong awareness of what machine tool builders, metallurgists, and software providers are producing, as well as a joint-effort to create a solution for the greater good.

"We work closely with ATI Aerospace, a marketing group that brings together all the resources of ATI," Ortiz says. "We have a strong, collaborative effort within the customer's organization from the standpoint that ATI is a producer and a supplier of mission critical metallics, as well as the cutting tools needed to machine these materials. All along the chain of R&D within the customer's organization, we work collaboratively to develop the optimum materials but also the best cutting tools for machining those materials."

"An operator also needs to consider adding the proper coolant selection to reduce heat and material build-up on the tool cutting edge, choose machine speed and feed parameters based on all of these factors, and take into consideration the type of machine and part fixturing being used," Eckhout adds. "One wrong choice on any of these variables can result in less than an optimal process, costing the manufacturer time and money.

Machine tool companies are building many more 5-axis and multitasking machines than ever before to answer the aerospace industry's needs. New materials show up continuously, and new processes must be developed to tackle these challenges. With the cutting tool manufacturers, machine tool builders, coolant suppliers and software producers all working together to create the optimal processes, it is up to the manufacturers in the end to educate themselves and apply these solutions to their shop floor.

Many cutting tool and machine tool companies offer training classes where their customers can learn the new technologies and apply them to their work. The first thing manufacturers need to do is to identify what they want out of their equipment, and then educate themselves on what the industry has to offer through trade publications, internet research, and ultimately, training courses and seminars.

"The person who is buying the cutting tools needs to know what they're looking for," Watson says. "There's a trade-off of tool life and metal removal rate, and there are some old paradigms from people who have been in the business for so long that they just feel that they're going to do it the same way that they've always done it."

With so many exotic materials coming into play in the aerospace industry, manufacturers have their work cut out for them in keeping up with the latest technologies. Cutting tool and machine tool companies are stepping up to address these challenges, and with a collaborative effort of all industry professionals, and a willingness to explore new processes and technologies, the aerospace industry will find itself soaring to new heights.