The aerospace industry is different from other industries because the consequences of a component failing can be horrific. No need to elaborate, but in a nutshell, everything needs to work perfectly every time. This means that every gear needs to rotate, every bracket needs to be in place, and all parts need to fit together perfectly.
Proper operation starts with the design and manufacturing process - using the right tools and the right materials. Precision fit and maintaining tight tolerances are critical because quality is more important than cost and lead time. That's why CNC machining is ideal for aerospace applications.
1 . Components manufactured using CNC machining
CNC machining is used to manufacture components for modern airplanes, from landing gear (components such as brackets and torque links) to engines (compressors and turbines). Structural parts can also be CNC machined, such as parts for fuselages, bulkheads and airframes. Gears, shafts and housings are important components in moving parts and are often CNC-machined parts as well. Finally, specifically for helicopters, piston engine housings are usually CNC machined.
2 . Materials used in aerospace components
CNC machining also works well with the materials needed for aerospace components. The extreme environments and usage conditions experienced by airplanes, helicopters and spacecraft require specialized materials. These materials include titanium, particularly grades 2 and 5, which is used in engine components due to its heat resistance and strength. However, titanium is an expensive material, so it cannot be used for every part of an airplane.
Alloy steels, particularly 4340 and 4130, are also used in the manufacture of aerospace components. 4340 steel is tough and strong, with a high potential hardness, which makes it ideal for the high loads that aircraft landing gears are subjected to. 4130 steel also has a high tensile strength, which allows it to be used in the manufacture of gears, fasteners, and external components. Steel has an advantageous strength-to-cost ratio, especially when compared to titanium, but steel is a much denser and heavier material, which limits its use in aerospace applications. Additionally, unlike titanium, steel corrodes easily and therefore must be coated if exposed to moisture.
Aluminum (7075, 2024 and 6061) is widely used because of its high strength-to-weight ratio due to its low density. It is also easy to machine - two to three times faster than steel. Aluminum 2024 has good fatigue resistance, which means it can withstand many load cycles (ideal for aircraft that have been in use for many years). Aluminum 6061 is a precipitation-hardened aluminum alloy that has good corrosion resistance and is used in aircraft wings and fuselages. Aluminum 7075 has good fatigue and corrosion resistance, which makes it widely used in aircraft structural components.
3 . Unique Challenges in Aerospace Manufacturing
CNC machining is not always a simple and easy process in any application, and this is true for aerospace components as well. Below are seven different obstacles that are often encountered when manufacturing aerospace components, along with solutions to address them.
Machining Large, Thin-Walled Parts
Some components, such as engine or compressor housings, have large internal cavities. For example, in order to manufacture the gearbox housings for helicopter blades, a CNC machine must scoop out a large chunk of material. This takes a lot of time, generates a lot of scrap and leads to residual stresses in the part. These residual stresses can lead to distortion or warpage - which can be problematic when you're working to tight tolerances and high standards.
To determine in advance whether removing large amounts of material will cause problems, you can use a pair of formulas. You need to check the IRMR (Internal Ratio of Material Removed) and ERMR (External Ratio of Material Removed).
The IRMR should be greater than 85%, which indicates that you are removing less than 15% of the internal part volume. the ERMR (comparing the final part bounding box volume to the inventory volume) should be greater than 30%. If both of these pass the test, you can proceed with prototyping and testing. However, if one or both of these values are outside of acceptable limits, your part may be difficult to manufacture within tolerance, or you may experience performance problems.
In these cases, you have several options. If the number of parts required is small, you can machine a part and test it, and continue if the first part meets specifications (and continue to test each part).
Sometimes such parts can be made using the casting process, which is better suited for making larger parts with thinner wall thicknesses. With the casting process, there is less material waste and less warpage. CNC machining may still be necessary for finishing and to meet tolerances.
Handling Complex Geometries
Due to the unique needs of aerospace components, these parts often have complex geometries in order to minimize weight or promote airflow over the part surface while maximizing strength.
However, sometimes these complex geometries are unnecessary. For example, when internal components are designed with complex organic surface geometries. Since the greater the complexity, the longer the machining time and the longer it takes to find a capable supplier, it is best to simplify the part design as much as possible.
Sometimes the only way to mitigate this is to promote Design for Manufacturability (DFM) among designers and engineers.DFM takes into account the limitations of manufacturing and considers the feasibility, time, and cost of the design from a machining perspective. This can help engineers think about where complexity is really needed and where it is less important. For example, internal components are not important for airflow, and fancy surfaces are not needed.
However, in aerospace applications, complex geometries are often unavoidable, and in these cases you can use a 5-axis (or more) machine.
Part Size: CNC Limitations The final part geometry challenge is part size. Airplanes are giant assemblies made up of millions of parts. While many of these parts are small, some large components are required. A typical machine tool bed is only a few feet long, which is not enough for structural or other large parts. This means that finding suppliers with this capability can be challenging.
To solve this problem, you'll need to find a new supplier with a large CNC machine that can handle parts of this size. Otherwise, you'll have to redesign the part to fit. This may involve breaking larger components into smaller parts. However, this may add to the overall weight because of the extra fasteners required to assemble multiple smaller parts.
Another possible solution is to change the manufacturing method. Casting can produce larger parts in a single run, but may still require CNC machining for post-processing. Casting takes longer because molds must be designed and built before any parts can be manufactured. This also makes casting more cost-effective than CNC machining for smaller parts.
4. Getting the Right Material Properties
Achieving the highly specific material properties required for aerospace applications can be difficult. Metals often require heat treatment to achieve the desired hardness and strength. Heat treating is a common process, but one that presents its own challenges.
Heat treating prior to machining will result in a significant increase in material hardness and strength, so tighter tolerances can be maintained. However, machining hardened materials is more time-consuming and tool wear is faster, making machining more expensive. If heat treatment before machining is necessary, investing in tools made from harder materials such as titanium, rather than carbide or HSS, can help ameliorate these problems.
Of course, heat treating after machining has its own problems, as it can affect the dimensions of the part. This can reduce the accuracy of the CNC process and can even cause the part to go out of specification. You can mitigate this by choosing the most effective heat treating process.
Tempering and aging involves heating the metal to a lower temperature than other processes, so the dimensions won't change much. Additionally, at the end of the heat treating process, you can use pressure quenching instead of oil quenching. Oil quenching causes the material to shrink faster, which leads to greater dimensional changes.
The easiest way to get the right performance is to accept the added cost and lead time of heat treating before machining. Again, quality is key to CNC machining, and getting that quality requires sacrificing speed and cost. In some cases, another option is to perform a small amount of final machining after the hardening process. This allows you to do most of the machining on the pre-hardened material and finish the hardened material to achieve the tolerances required for the final part.
5. Sourcing the material
Before you can handle or machine the material, however, you must find the right materials. Superalloys and specialty plastics can be difficult to source and can be costly and time-consuming to transport. These materials include titanium, nickel alloys such as nickel-silver alloys, and Ultem, which is a plastic used in aerospace applications. These materials will always be needed for aerospace components due to the specialized needs of aerospace components, so this will be a long-term challenge.
To address these limitations, aerospace companies can utilize digital manufacturing ecosystems (DME) such as Fictiv, which have access to a large number of manufacturers. Working with DME means that one of the manufacturing partners is more likely to source the necessary materials. Additionally, this leaves sourcing to the manufacturer, so engineers can focus on design.
6. Finding a Manufacturer
When it comes to sourcing, finding the right manufacturing partner is crucial. Due to the specific requirements of the aerospace industry, most aerospace companies require their manufacturers and suppliers to be certified to AS9100, which is based on ISO 9001, which regulates quality management systems and controls quality and safety that are critical in the industry.
However, not all manufacturers have this certification and it can be difficult to find those that do. Certification is expensive and time-consuming, and the volume of aerospace components is not always worth it (more on that later). But digital manufacturing companies can solve this problem. They are more likely to find suppliers with the necessary certifications among a wide range of partners.
7. Multi-variety, small-lot production
As mentioned above, the production volume of airplanes is different from that of other physical objects (such as consumer goods or electronics). This means that many aerospace components are not mass produced. An airplane requires a large number of different parts, but may only need a few hundred or fewer. This is known as multi-mix, low-volume production.
Unfortunately, multi-variety, low-volume production runs counter to the manufacturer's original intent. Manufacturers need to spend time and effort reviewing and setting up the manufacturing process to produce each part, so these programs are much less efficient and cost-effective. Some manufacturing partners simply won't accept projects that require them to spend time developing processes for complex geometries to manufacture a few parts.
In some cases, larger quantities can be ordered, thus alleviating the problem. If the parts can be post-processed, such as coated, this may allow you to increase the order quantity and store excess parts for later use. However, this option is only available for long-lasting designs that can be used for future model airplanes and will also require space to store inventory.
