The process of cold forming requires moving a metal slug cut from a continuous coil of wire and forming that slug, under applied force, to a desired shape for further machining. Whether this necessitates threading, grinding, cutting, or even polishing is left to the specifics of the customer, what is always necessary is the use of another consumable in the process to transfer the slug through each stage of forming. This is where the ever-vital transfer finger comes into play into the industry.
Most often, transfer fingers are machined or milled from a cold rolled steel bar into a shape to meet the exact specifications of a required part. Due to the nature of this requirement, transfer fingers are consumable for a specific job, and are often required to be customized for each use. While some tool shops may try to save on cost by investing in a low or medium-carbon steel to mill or modify their own transfer fingers, this can result in premature fatigue or fracturing on the work surface of the transfer finger and can lead to even greater problems from this ill-advised attempt to cut cost. Using a low or medium-carbon steel might save on wear to machine shop tooling equipment, and producing consumables inhouse, is a great way to save money, but without further carburization of the finished product this is not a recommended manufacturing practice.
The preferred practice for long lasting transfer fingers is to machine the form from low-carbon cold rolled bars, specifically of a steel with a carbon percentage of between .15% – .20%. Obviously, the use of low carbon steel bars makes for much easier machining, but then necessitates the use of carburizing to ensure the steel is suitable for manufacturing requirements. Carburizing is used in applications where High Surface near hardness is required. Usually, an alloy with a mix of chromium or manganese is used to ensure a medium hardenability near the surfaces of the part. The most common method in modern manufacturing for carburization is to use a carbon rich gas, such as carbon dioxide, to diffuse into the metal bar before being heat treated.
The emphasis on the need for carburization during tooling may be obvious, but to reiterate, it is to reduce the risk of failure in the finished product. Steel endurance usually reaches its limit under two types of force: fatigue or creep. Creep occurs when a material is placed under a sustained load, which can lead to fracture. This is less of a concern during cold forming, due to the most likely result of failure coming from metal fatigue. Fatigue is when a material is strained or fractured from repeated cycles of stress.
When two parts of equal hardness interact, this will lead to wear or tear that can result in failure. And given the number of cycles that a modern cold former can place on tooling, even the small amount of force directly applied to transfer fingers can result in a failure of the tool during a critical moment in the cycle. This is why using low-carbon steel bars to manufacture transfer fingers makes carburization necessary to ensure that cold formed parts, also made from low carbon steel, do not additionally cause premature failure of machine tooling during forming of a part.
Of course, the simple solution could be to use a higher carbon percentage in steel for manufacturing transfer fingers, but this can make the machining process more difficult and more costly. Harder cutting tools are required and higher costs can result from using this type of steel for manufacturing. The simplest solution is to rely on further carburization of a low-carbon steel, which hardens the outer casing of the metal bar, but allows for easy production.
Manufacturing remains a balancing act between cost and reliability, and the tradeoffs always have outsized impact on the lifecycle of a product. Knowing the reasons for why certain materials are chosen during production often means the difference between a quality product or an extremely unsatisfied customer. For further questions about what solutions our team would recommend to meet your needs contact a representative today.