Tools & Strategies for Machining ISO-S Materials in Aerospace

By Scott Causey, International Aerospace Specialist 

Jabro JHP770 and 780

Manufacture of precision components in the aerospace industry requires innovative engineering and technology, especially when machining with newer, high-performance workpiece materials. Today’s ISO-S alloys, namely nickel-, cobalt- and iron-based heat resistant superalloys (HRSA), and titaniums have many beneficial properties that make them great choices for a wide range of crucial applications. At the same time, some of their characteristics make these materials challenging to machine.

ISO-S alloys provide higher resistance to heat and wear, extreme toughness, and unwavering quality and reliability. On the flip side, they have low thermal conductivity, which reduces tool life and causes part distortion. They have tendencies to strain and precipitation harden when machined, which increases cutting forces and further degrades tool life, and the sticky behavior of these alloys creates uncontrolled built-up edge (BUE) and notch wear. Using advanced tools and application strategies can help you maximize the benefits and address the difficulties of machining these alloys. 

• Match the cutter to your desired profile. Application of ISO-S materials is common in aerospace turbine blade production. Seco offers a fir tree cutter with spiral fluting to machine the specialized profile of the blades, which has extremely tight tolerances. The Jabro® fir tree provides a smooth, easy cutting action with an advanced cutter geometry that prolongs tool life and offers unmatched accuracy.

• Limit cutting speeds when cutting titanium alloys. Structural aerospace parts, such as landing gear components, are massive and strong. When manufactured from standard materials, they are also very heavy. Today, manufacturers are using newer, lighter and stronger titanium alloys to produce lighter landing gears, but these new materials are more difficult to machine. One newer alloy is titanium 5553, which includes 5 percent aluminium, 5 percent molybdenum, 5 percent vanadium, and 3 percent chromium content. Its benefit is high tensile strength: 1160 MPa compared to 910 MPa for Ti6Al4V, but this higher tensile strength requires limiting cutting speeds to levels 50 percent of the speeds applied with Ti6Al4V.

• Apply parameters for most-difficult-to-machine material when cutting stacked alloys. Some aerospace applications involve machining components composed of stacks of differing materials. An example is an engine mount featuring a titanium 6Al4V/austentic stainless steel stack. Both materials share some properties including relatively high strength and adhesive properties that cause the cut material to stick to the endmill, and the challenge is to machine the “sandwich” or “hybrid” with adequate chip control and no vibration or burrs.

Seco’s carbide Jabro JHP 770 tool designed for machining titanium is a good solution. This tool incorporates differential flute spacing, radial relief, a specially formed chip space, and a through-coolant channel that minimizes workpiece adhesion and clears chips.

In machining the stacked materials, the key is to apply the parameters for the more difficult-to-machine material. In this example, keep in mind the titanium’s low thermal conductivity. We recommend using a moderate cutting speed of 50 m/min, with a feed of 0.036mm/rev feed, and a 3 mm depth of cut, descending in circular interpolation. 

• High-Speed Steel (HHS) cutters are a productive and cost-effective choice. Many large aerospace components, such as landing gear parts, are machined from solid billets of titanium or stainless steel. For these parts, high-performance HSS tools up to 50 mm in diameter are capable of removing large volumes of material. The HSS tools are very effective on low-rpm, high-torque machines for effective roughing and even finishing of titaniums and stainless steel. The ability to use large diameters and widths of cut enables the tools to provide competitive metal removal rates even when run at lower speeds than those achievable with carbide tools. 

An example of an advanced HSS tool is the Jabro JCO710 HSS-Co cutter with 8 percent cobalt content and a hardness of 67 HRC. The tool features polished flutes to reduce friction and edge build-up, and a variable face profile geometry to cut light and reduce the risk of chatter that causes unacceptable surface roughness values. We have seen these cutters provide more than 800 minutes of tool life when applied at a manufacturer producing large titanium parts. 

The goals of aerospace parts production are top quality, reliable consistency and productivity. As metal producers develop new alloys to meet increasingly demanding high-performance applications, we are engineering new cutting tools and strategies to enable aerospace manufacturers to overcome the challenges of machining these materials. Please contact me to learn more.

About the Author 

As Seco’s International Aerospace Specialist, Scott is responsible for supporting the company’s aerospace segment customers, which includes optimizing current processes and defining new technologies. In his spare time, he enjoys spending time with his family and working with horses. 

Balance Cutting Tool Life With Productivity

By Chad Miller, Product Manager – Turning and Advanced Materials

When it comes to cutting tools, machinists often find themselves torn between maximizing tool life and increasing productivity. But it really doesn’t have to be a choice. By understanding and following a few basic rules, it’s possible to achieve a balance of long life and high productivity from the same cutting tool.

1.  Tweak speed slowly within the recommended range.

Speed is the single biggest factor affecting cutting tool life – the higher the speed, the lower the tool life.  Unfortunately, machinists looking to increase productivity often make the common mistake of just cranking up the speed. They may even turn on the machine override and start running at speeds that are 20 or 30 percent higher. While this may increase the number of parts produced, it will drastically reduce the tool life. In fact, we’ve seen that increasing the speed by 50 percent can cause tool life to go down by as much as 90 percent. It’s best to stick within the tool’s recommended range for cutting speeds, tweaking speeds carefully along the way to find the best maximum that gives you both longer tool life and a higher level of productivity.

2.  Increase depth of cut to improve cutting tool productivity.

Increasing depth of cut can have an enormous impact on productivity. At the same time, a larger depth of cut does not have much effect on cutting tool life.  When machining a component that will have numerous passes with an insert, increase the depth of cut to reduce the number of passes thereby increasing productivity. 

3.  Increase feed rate to get more productivity out of the same inserts.

When it’s not possible to decrease the number of passes by increasing the depth of cut, the next best thing is to consider increasing the feed rate. When I am with a customer on his or her shop floor, the first thing I look at is how to increase feed rate without sacrificing surface finish. If we can increase the feed rate and still achieve the desired quality and precision, we can reduce processing time.

4.  Take a closer look.

One of the best ways to find the balance between the variables affecting tool life and productivity is to study how the tool is behaving during operation. Let’s say you’re currently getting about 100 parts per edge and you want to increase productivity to 125 per edge. Stop machining and pull the tool after about 80 pieces or 80 percent of the tool life and look at the edge wear under high magnification. This closer inspection will give you a good idea of what is going on with that insert – its condition or changes that have occurred– and allow you to adjust speed, feed or possibly change to a different grade or chip breaker to be able to increase throughput.

5.  Choose the strongest insert geometry.

Using the strongest possible insert geometry for the application gives you an opportunity to increase productivity by pushing feed rate and depth of cut. According to ISO standards, round insert geometries are going to be the strongest, followed by square, C, P and then V, respectively.

Machinists in different industries view the importance of tool life compared with productivity differently. Machinists in the automotive industry, for example, are often more concerned with achieving a higher volume. They need to get a lot of parts through the door very quickly and tend to run the inserts a bit faster in feed and speed. 

On the flip side, machinists in the aerospace industry are generally more concerned with tool life and quality. When machining high precision parts such as jet engine components, they want to make sure the insert completes the part. They absolutely do not want to have to pull the insert out in the middle of machining, so we see them running at more conservative speeds to extend the life of the tool.  

The most important tip of all is to use the insert within the recommended guidelines for speed and feed. Following these recommendations should result in good tool life. To learn more about how to balance tool life with productivity or discuss a specific challenge, please feel free to contact me.

About the Author

Chad manages Seco's turning and advanced materials product lines, including all CBN and PCD products. When he's not helping customers implement advanced metalcutting solutions, you can find him training for and running 5K, 10K and 1/2 marathon races and triathlons.