sheet metal punching

Punching is a cutting process in which material is removed from a piece of sheet metal by applying a great enough shearing force. Punching is very similar to blanking except that the removed material, called the slug, is scrap and leaves behind the desired internal feature in the sheet, such as a hole or slot. Punching can be used to produce holes and cutouts of various shapes and sizes. The most common punched holes are simple geometric shapes (circle, square, rectangle, etc.) or combinations thereof. The edges of these punched features will have some burrs from being sheared but are of fairly good quality. Secondary finishing operations are typically performed to attain smoother edges. 

The punching process requires a punch press, sheet metal stock, punch , and die. The sheet metal stock is positioned between the punch and die inside the punch press. The die, located underneath the sheet, has a cutout in the shape of the desired feature. Above the sheet, the press holds the punch, which is a tool in the shape of the desired feature. Punches and dies of standard shapes are typically used, but custom tooling can be made for punching complex shapes. This tooling, whether standard or custom, is usually made from tool steel or carbide. The punch press drives the punch downward at high speed through the sheet and into the die below. There is a small clearance between the edge of the punch and the die, causing the material to quickly bend and fracture. The slug that is punched out of the sheet falls freely through the tapered opening in the die. This process can be performed on a manual punch press, but today computer numerical controlled (CNC) punch presses are most common. A CNC punch press can be hydraulically, pneumatically, or electrically powered and deliver around 600 punches per minute. Also, many CNC punch presses utilize a turret that can hold up to 100 different punches which are rotated into position when needed.

A typical punching operation is one in which a cylindrical punch tool pierces the sheet metal, forming a single hole. However, a variety of operations are possible to form different features. These operations include the following:

Piercing - The typical punching operation, in which a cylindrical punch pierces a hole into the sheet.
Slotting - A punching operation that forms rectangular holes in the sheet. Sometimes described as piercing despite the different shape.
Perforating - Punching a close arrangement of a large number of holes in a single operation.
Notching - Punching the edge of a sheet, forming a notch in the shape of a portion of the punch.
Nibbling - Punching a series of small overlapping slits or holes along a path to cutout a larger contoured shape. This eliminates the need for a custom punch and die but will require secondary operations to improve the accuracy and finish of the feature.
Lancing - Creating a partial cut in the sheet, so that no material is removed. The material is left attached to be bent and form a shape, such as a tab, vent, or louver.
Slitting - Cutting straight lines in the sheet. No scrap material is produced.
Parting - Separating a part from the remaining sheet, by punching away the material between parts.
Cutoff - Separating a part from the remaining sheet, without producing any scrap. The punch will produce a cut line that may be straight, angled, or curved.
Trimming - Punching away excess material from the perimeter of a part, such as trimming the flange from a drawn cup.
Shaving - Shearing away minimal material from the edges of a feature or part, using a small die clearance. Used to improve accuracy or finish. Tolerances of ±0.001 inches are possible.
Dinking - A specialized form of piercing used for punching soft metals. A hollow punch, called a dinking die, with beveled, sharpened edges presses the sheet into a block of wood or soft metal.