Waste Streams - Chips

 Chip Morphology Page

Chips are one of the major waste stream components of machining. Chips are formed as unwanted material is cut away from a workpiece. Chips can be continuous or discontinuous. Continuous chips have a uniform amount of shear strain throughout their length. Discontinuous chips exhibit shear banding. That is, although the entire chip may have exhibit shear deformation, intense shearing is concentrated in narrow bands, causing slippage of planes and a discontinuous surface. As the chip curls, it impacts the uncut work. As the chips is being forced into the workpiece, it develops a bending moment which ultimately causes fracture. This fracture is desirable, facilitating small, well broken, chips which are easier to handle and dispose of. Long unbroken "snarl" chips are undesireable. They may damage the machine or workpiece, and can even be dangerous to the machine tool operator. With advances in chip control technology and knowledge, the need for a "man with a hook" who used to stand by the machine to remove snarl chips, has almost been eliminated.

The chip formation process is highly complex and is governed by mechanisms which are not yet fully understood. Material is not really cut away from a workpiece in the way that cloth is cut with scissors or meat is cut with a knife. It is deformed plastically and sheared away from the remaining metal. As metal approaches the rake face of the tool, it passes through a primary deformation zone which can be modeled as a plane. It is in this region that it changes direction, and undergoes adiabatic shearing. That is, although the region is of an extremely elevated temperature, it is transferring little (or no) heat to or from its surroundings while the material deforms in shear parallel to the shear plane angle. Machining parameters which affect chip morphology include the speed, feed, depth of cut, tool nose radius, edge radius, rake angle, tool and work materials, ambient temperature, cutting fluid, generated vibration in the machine tool and others. This makes predicting the chip morphology extremely difficult.

As a waste stream, chips are a prime candidate for recycling. They can often be melted down and reused. They can be a hazardous waste, in special situations. Grinding dust can be inhaled and cause lung disorders. The machining of exotic materials such as depleted uranium in the defense industry can be highly toxic. The volume of the chips will always be higher than the volume of the removed material. This is because the relative density of the chips is less than the workpiece. The chips will not pack as densely as unmachined metal.

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