Anatomy Of Free Machining Steel

In The Cut

There’s a lot of action in the cutting zone. Plastic deformation of the parent material allows a chip to shear in the primary shear zone. Friction created in the three main shear zones contributes to premature tool failure. Lubrication from machinability additives in the steel and external application of cutting fluids help reduce the effects of high temperatures. The intersection of a cutting tool and workpiece material is a violent and very hot place. A cutting tool is designed to attack a material in a way that lifts a chip. The energy that is put into the cutter, feed and speed is output in the form of heat. The more energy that is put into the cut, the more heat is produced. Chips are actually cut from the material slightly ahead of the tool. This area is referred to as the primary shear zone. It's where the pressure and forces are sufficient to create plastic deformation, which allows the material to yield to the cutter. Also in the cut, a secondary shear zone forms along the face of the cutting tool. This is where the chip actually slides up the face of the cutter. The friction in this area of the cut can raise temperatures to 1,2000 C. Finally, there is a third shear zone that is located under and slightly behind the leading edge of the cutter. This zone is formed by springback material at the bottom of the cut that was depressed as the primary shear zone yielded. Because steel is produced as a liquid and because there are numerous elements suspended in that fluid, how they are distributed in the solid material you're cutting affects how smoothly and consistently this chip formation process occurs. Think, for example, if the cutter is merrily traversing a workpiece and hits an inclusion of very hard carbide, the yield of the material will change dramatically. It could possibly break the edge off the tool. For steelmakers, achieving heat-to-heat consistency, which reduces "surprises" in the cut, is the ongoing technological challenge.