A blade is the flat part of a tool, weapon, or machine (such as a fan) that normally has a cutting edge and/or pointed end typically made of a flaking stone, such as flint, or metal, most recently steel. A blade is intentionally used to cut, stab, slice, throw, thrust, position and/or place (an example of this is razor wire), shoot (an example of this is the ballistic knife), scrape (an example of this is an ink eraser) or strike an animate or inanimate object.

Physics

The basic idea of a blade is very similar to a sharp point. The shape concentrates all the force onto a very small area, resulting in a high amount of pressure which allows it to penetrate matter.

A serrated blade (a blade which has many small "teeth") takes this further as each individual tooth concentrates the force on a smaller area which helps cut through denser materials. A serrated knife can cut through objects solely with a sliding motion with little pushing force, this is useful for tools which require these attributes such as bread knives.

Some bladed weapons (and tools) have curved blades. A curve can serve two purposes, the first is that it allows for slicing by continuing to "push" on the surface as it is drawn across it. The other effect is to allow the force to be concentrated in an even smaller area.

As a rule the blade must be made of a substance which is harder than (or as hard as) the material it is intended to cut. If isn't the case the blade will either be unable to cut (as it absorbs all the energy as it is damaged) or will wear away very quickly (if it is hard enough to transfer enough of the energy to damage the material). In practical terms the material must also be tough enough to last (e.g. glass is very hard but it shatters easily and thus isn't very effective as a material for a blade).

The problem is further compounded by the fact that heat treatments, which increase hardness for better edge-holding, inevitably reduce the material's toughness. Essentially speaking, a balance must be found between how well the edge must hold, and how well it can last. Methods that can circumvent this somewhat do exist however; for instance differential hardening allows for an edge that can hold well, and a body that can withstand mechanical stress.

Geometry

An ideal blade would come to a perfect edge—not at all rounded—but that says nothing of the angle of that edge. The ideal angle is a function of the material being cut. For example, a tool bit for cutting metal may have nearly a 90° edge; it would probably not even be considered a blade. With very rigid materials such as metal, cutting deep into a piece with a blade would be impossible so deep cutting is done with a saw or grinder which provides kerf through which the cutting device can pass. With less-rigid materials such as a butternut squash, an acute blade prevents the blade from being pinched by the material. When cutting biomaterials such as tomatoes (which tend to have a low elastic modulus but high yield strain), the angle of the blade is less important since the material will bend, but the sharpness of the edge is important because if too much force is required, the material will be squashed rather than cut (or alternatively a serrated blade could be used as mentioned above).