Guillotine (Metal shearing)
A guillotine is a machine used to accurately cut sheet metal. It may be foot-operated (or less commonly hand-operated), or powered. An angled blade is driven down which slices the metal along the length of the cut, shearing it off very cleanly.
Depending on the capacity of the machine, the angle of the blade may be varied along with the clearance between the upper and lower blades. The thicker the material, the greater the angle and clearance given to the blades.
The angle of the blade is referred to as shear, and this provides a slicing rather than a chopping action; by slicing the material a cleaner cut is produced and less energy is used. This is because the blade contacts only a small area at a time rather than the full length of the cut (which is also how scissors work).
Clearance is defined as the separation between the blades, measured at the point where the cutting action takes place, and perpendicular to the direction of blade movement. It affects the finish of the cut (burr) and the machine's power consumption, and is directly related to the material's composition. The cut of a guillotine is in part a fracturing process; the blade partially penetrates the material but the final separation is due to the material fracturing. Harder materials (such as stainless steel) fracture more readily than softer ones (such as brass).
The design of press tools and guillotine blades is an engineering compromise. A sharp edge is required, along with strength and durability, so to achieve these two objectives the blades for metal work tend to be square-edged rather than knife-edged. The difference between the two angles is called the rake.