Factors Affecting the Cutting Life of Cutting Tools

Material Machinability

• The first characteristic that determines a cutting tool's lifespan is the machinability of what it is cutting. Machinability means the ability of the material to be cut and shaped by a cutting tool. Softer materials, such as copper, bronze, brass or aluminum, are shaped easily. These materials have a lower density and lower melting point than high speed, or hardened, steel. As a result, like the proverbial warm knife through butter, tools that cut softer materials maintain their sharp edge. As the tooled material hardness increases, the cutter requires frequent sharpening. The friction between the tool and the tooled materials acts like sandpaper on the cutting edge and grinds the edge down.

Cutting Tool Composition

• The composition of the cutting tool also affects how long the tool keeps its sharp edge. Because harder materials cost more to purchase, manufactures must balance the tool's cost against its expected lifespan. High speed steel serves as a suitable cutter for soft metals. Hardened steel, stainless steel or high carbon steel (carbide) must be used for harder steel materials. Ceramic tools and titanium cutting heads are designed to mill hardened and carbide steels.

Cutter Speed and Lubrication

• Friction destroys the knife edge on the cutting tool by creating heat that scrapes and boils individual atoms off the surface of the cutter. As the material is worn down, it becomes dull and loses its ability to mill or shape other materials. Cutting tools are typically lubricated with oil or water during the milling process. The lubricants serve to reduce heat and, thereby, increase the lifespan of the tools. The speed at which the cutter turns also affects its lifespan. Some materials cut well at low speeds, while others require high speed milling. The faster the bit turns, the more friction it creates. In order to get maximum usefulness out of the cutting tools, the milling operators should match the speed of the cutter to the optimal cutting specifications.

Rate of Feed

• Milling tools are designed to spin at high speed, removing a small piece of the milled material with each pass or rotation. If the material is fed into the cutter too quickly, the rate at which it dulls increases as the cutter bogs down trying to cut more material on each pass. The slowed cutter creates more friction, thus dulling the cutting edge. Additionally, if material is fed too quickly, the cutter removes more material with each rotation. Imagine the difference between cutting a sliver of wood from the outer edge of a branch with a pocket knife, or attempting to cut off a 1-inch thick board with the same knife. The latter requires more work from the knife blade and the woodsman. When the cutting tool has to complete more work with each revolution, it loses its cutting edge more quickly because it is working harder than if the feed rate were slow and steady.