Machining removes material by forcing a cutting edge through a workpiece so that a layer of material plastically shears away as a chip.

The undeformed chip has thickness $t_1$. As material crosses a narrow shear region ahead of the tool, it undergoes intense plastic deformation and becomes a chip of thickness $t_2$, usually with $t_2>t_1$.

The chip ratio is

$$r=\frac{t_1}{t_2}.$$

A smaller $r$ means greater compression and deformation of the chip.

Tool geometry strongly affects the process. The rake angle controls the orientation of the tool face along which the chip flows. Friction on that face, material strength and temperature all influence the shear deformation and cutting force.

Chips can be continuous, segmented or discontinuous. A ductile material at suitable cutting conditions often forms a continuous chip. Brittle material, low cutting speed or unstable deformation can produce discontinuous chips. A built-up edge can also form when workpiece material adheres temporarily to the cutting edge, changing the effective geometry and degrading surface finish.

Chip formation explains why machining is not simply geometric scraping. Most of the removed material has undergone severe plastic deformation, frictional sliding and heating before leaving the workpiece.