Fasteners play very important roles in making several objects such as screws and bolts. Most fasteners are manufactured by cold forging (or cold forming). The raw material, a wire or a rod, is fed into a cold-forming press, straightened, and separated into required lengths. These “cut off” blanks are transferred through a series of dies where they get their shape step by step. The name we call dies actually include cold heading dies, header punches and KO pins, cutting knife, shear dies, etc.
In this article, we’ll mainly discuss the cut-off knife for screws, its application, and production notes. Proper cut-off of blanks in the heading machine is a critical operation since blank mass equals the mass of the finished part. During fasteners-making production, after the wire has been successfully drawn to size, the wire enters the inside of the cold heading machine. The wire goes through a cut-off die and continues until it reaches a wire stop. Once it reaches the wire stop, a cut-off knife moves across the wire, cutting it off to a specific length. This is also referred to as the cut-off slug.
The application of cutting knife determines that its working area must be made of tungsten carbide, and the non-working area can use steel. At present, the steel materials used in China mainly include 45# and SKD61. The advantage of SKD61 is that it has better toughness, but the price is more expensive than 45#.
What should we pay more attention to during production of cutting knife
Hardness of steel
The hardness of 45# steel is generally about 10HRC, and it can reach 20-30HRC after quenching and tempering. If it needs to reach 40-45HRC, heat treatment is required. However, the high hardness of the steel will make the screw-cutting knife easily crack.
For using copper welding to fix tungsten carbide and steel, 45# steel is recommended, because its welding adhesion will be better than SKD61. If press-fit inlay is adopted, then SKD61 is more suitable. The whole product can be hardened by heat treatment, which can not only harden the position of the screw hole but also increase the toughness of the steel part.
Quality testing is an essential step during the production process. For products requiring high precision such as fasteners, in addition to using conventional micrometers and vernier calipers to measure dimensional data, depth gauges, pneumatic gauges, or projectors are also used on necessary occasions to ensure that the products meet customer requirements.