By Todd Miller, Manager, Rotating Products
Today, cast irons are more advanced than they were 20 years ago. They are lighter, stronger and more affordable. In fact, cast iron can be an excellent alternative to steel as you face constant pressures to cut costs. However, several variables and challenges exist when trying to determine the right tooling for your cast iron machining operations.
First of all, it’s important to understand the different types of cast iron and realize that each one has a different level of strength, cost and machinability. And that for each of these types, there are also several grades with widely different mechanical properties.
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You must also consider the complexities of cast iron metallurgy. The casting process generates microstructures with properties that vary between a part’s surface and its internal body. Cast iron quality also varies from one foundry to the next.
Here are some of the types of modern cast irons from which to choose:
• Grey cast iron, among the most common and least expensive of all the types, contains carbides in the form of lamellar graphite particles, which gives it excellent vibration damping properties and makes it ideal choice for engine components. It also has the highest level of machinability when compared to other types.
• Vermicular cast iron, also known as compacted graphite iron, offers greater strength and lower weight when compared to grey cast iron. Because vermicular cast iron is suitable for components subjected to both mechanical and thermal stress, automotive manufacturers are using it more in the production of cylinder heads and brake parts.
• Silicon alloyed ferritic ductile cast iron is ideal for the production of wheel hubs and axles. Given its high degree of machinability and excellent mechanical properties, the material is becoming increasingly popular within the automotive industry.
• Nodular ductile cast iron, which consists of spheroid nodular graphite particles in ferrite and/or pearlite matrix, possesses high ductility, good fatigue strength, superior wear resistance and a high modulus of elasticity, and hence have been the choice of material for transmission housings and wheel suspension parts within the automotive and heavy equipment industries.
• Austempered ductile iron offers high strength, high fatigue strength, good wear resistance and high values of elongation to fracture, making it a very competitive material in relation to many cast and forged steels. Because of great strength and elastic properties, austempered ductile iron has the lowest level of machinability when compared to the other types of cast iron mentioned here.
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• Have your workpiece properties under the best possible control because variations can negatively impact total productivity, either directly or indirectly. When workpiece properties are unclear, you can look to tooling systems and cutting strategies to make up for any material quality shortfalls. The trick, however, is knowing what tools and strategies are the right fit for your application.
• In terms of turning cast iron, everything depends on your specific application. You must determine the number of operations necessary to accomplish your goals. If your workpiece properties are unknown, you may opt to include an extra finishing cut, which impacts product lead times. However, by applying the right tooling for the conditions and requirements of the component, you can reduce the number of operations.
• When milling cast iron, there’s a lot more complexity involved when compared to turning the material. While the type of insert grade you use is important, it’s even more critical to look at the total cutting solution. You must consider – in addition to insert geometries and grades – cutter body types and the number of cutting edges as related to your component. Furthermore, heat and coolant are not ideal when milling cast iron.
• In terms of selecting the best type of cutter for cast iron milling, there is no real one-size-fits-all answer. But generally speaking, the type of milling cutter that seems to be making a lot of headway these days would be a negative cutter with inserts that have positive rake angles and in a grade that handles both wet and dry conditions.
• While one type of cutter may be able to successfully cut all the different types of cast irons that does not mean it can effectively machine every type of workpiece shape. You must think about the surface you need to cut, and ask yourself: Is it square in form or very long? Are the wall thicknesses thin or thick, weak or stable? And, how secure is workpiece clamping?
• You must consider your machine tool. When machining cast iron materials, there’s a higher dynamic load, so your machine tool must be highly robust as well as provide high power and high stability – all of which puts strain on the machine. However, in these instances, a negative cutter with the positive rake angle can help lower the power requirements of the machine tool and reduce forces on machine spindles as well.
But in the end, with so many variables to consider, if you want to increase the productivity and predictability of your cast iron machining efforts, the best action for you is to work closely with your cutting tool supplier.
About the Author
Todd is the manager of rotating products for NAFTA, responsible for solutions and applications involving face, square shoulder and disc milling. Todd and his team of product experts are dedicated to providing a consistent, high-level of support to Seco customers throughout the United States, Canada and Mexico. In his spare time, Todd likes to bowl and cheer on the University of Michigan football team.