08/20/2021 Stainless steel provides many properties attractive to industries like automotive and aerospace but finds itself in kitchen products and sporting goods as well. The addition of chromium to iron is what creates the corrosion-resistant and strong product with so many applications. It can be non-magnetic, and its varied alloys have high strength and good plasticity. This composition is also why stainless steel is so hard to machine. It becomes harder and generates a lot of heat. This affects tool wear and surface finish. Feed and speed become critical factors in machining stainless steel.
Why Choose Stainless Steel
With so many alloys available it is important to understand why stainless steel is a popular material choice for various industries. The first is corrosion resistance. A prime example of this feature is in surgical instruments and kitchens where components are in constant contact with heat, chemicals, and water. Stainless steel also has an incredible strength-to-weight ratio which means thinner materials still offer stability under stress. Combined with its strength, it is also very flexible allowing welding, forming, cutting, and machining operations to form its final shape. Its aesthetics and ease of maintenance also help to increase its popularity as a metal of choice. These factors combine to make stainless steel a cost-effective alternative with a longer life span compared to other metals like titanium, aluminum, brass, or nickel.
Machining Stainless Steel
Several factors influence the machining process. The increase of hardness with the application of heat is a prime consideration. The choice of tool and the tool path must focus on chip removal and reduction of chatter. Coolants are often needed, and the work holding fixtures are susceptible to failure due to vibration during the rigorous machining process. To produce top-quality machine parts, consideration of these factors is key.
A generous flow of cutting fluid removes heat from the chip, helps in chip breaking, and improves the dimensional stability of the component. The precise aim and type of coolant are important in its effectiveness. The flow of coolant can be applied directly through the cutting tools, called through coolant, providing the best chip evacuation for stainless steel. Consistency of coolant is critical to avoid temperature fluctuations which can lead to thermal cracking and reduced tool life.
Common cutting fluids are a combination of oil and water. Neat cutting oils are undiluted oils and are also an option for cooling stainless steel. If using an emulsion, an eight to ten percent concentration is recommended.
Tooling must be high quality and extremely robust to work with the hardness of stainless steel. Both tooling and the workpiece must be clamped well. Always keep tool overhang to a minimum, especially when drilling or internal turning. To help reduce vibration and provide quality finishes, use sharp tools or inserts with a small nose or corner radius.
Tools should also be designed with a high flute count and/or high helix, especially for high-quality finishes. This design extracts chips efficiently from the cut.
Stainless steel can vary slightly between suppliers and batches. It is always important to note their recommendations. Feed rate of material and speed of cutting balance tool life and surface finish against fastest machining time. A general SFM is recommended between 100-350, with a chip load ranging between .0005” for a 1/8” end mill up to .006” for a 1” end mill. These rates can be tricky to establish for each operation based on a variety of other factors, including the quality of the cutter and the machine it is in. Operators trying to play safe may be damaging to tools by having feed rate or speed too low. Rubbing will occur, wearing down the sharpness of the cutting tool. Managing small cuts at low speeds will also add to the hardening of the workpiece. High-speed machining on newer equipment follows the principle of faster, shorter cuts with shorter tools working closer to the workpiece. This creates an optimum environment for stainless steel. Heat and friction are reduced, and tool life is extended.
A good machinist will utilize charts, simulations, and manufacturer recommendations, but will always use common sense to develop the best feed rate and speeds for each job. This can only be developed over years of experience.
The high strength of stainless steel combined with its high ductility makes chip breaking more difficult. The inherent risk of vibration can be addressed by ensuring tooling and workpieces are stable and well clamped. There are other mechanics to consider to alleviate the problems stainless steel presents.
Certain operations, like threading, can be improved by pre-drilling a hole 0.1mm larger than recommended. This will reduce torque levels encountered during threading which, in turn, increases tool life and performance.
Corners should also be approached differently, with lower speeds on approach and departure, careful not to slow too much, or heat will build causing a poorer corner finish.
The time it takes to understand the principles of why stainless steel is so hard to machine is well worth it due to the vast amount of applications for it. Precision Tool has 30 years of experience providing quality products. Stainless steel custom parts are one of our specialties. Contact us to learn more or request a quote for your next project.
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