Hemming is the folding over of a short flange upon itself to form a smooth, rounded edge and to facilitate the attachment of mating parts.

Bending is a relatively simple forming operation which provides rigidity and shape to sheet metal parts. Similar to bending is flanging. However a flange is significantly smaller in dimension than the rest of the part. The functions of a flange include:

lGiving a more finished appearance

lRigidity

lEdge strengthening

lProviding a fastening or attachment surface

Multiple stamping operations may be performed within a single stamping die, or at a number of die stations within a stamping die set and with a single stroke of the press.

Single station dies can be either compound dies or combinations dies. A compound die performs basic cutting operations such as blanking and hole punching to produce parts. Combination dies combine shaping and forming functions with cutting operation to manufacture parts.

Multiple station dies are arranged so that a series of sequential operations are accomplished with each press stroke. Two die types are used; progressive and transfer. With progressive dies, coil stock is fed into the press. Individual stamping are connected with a carrier strip as they progress through the various die operations and are ultimately separated and then discharged from the press. In transfer die operations individual stock blanks are mechanically moved from die station to die station within a single die set. Large stamping are done with tandem press lines where the stock is moved from press to press in which specific operations, such as drawing or trimming, are performed.

The resistance of the sheet metal stock to the force exerted by the moving dies creates friction. For this reason, lubrication is vital for successful sheet metal forming. A lubrication’s function is to minimize contact between the tooling and the work piece. This results in reduced tonnage requirements, longer tooling life, and improved product quality.

Lubricants range from light mineral oils to high viscosity drawing compounds. They may be oil base, water soluble, or synthetic materials. These lubricants may be applied in a variety of ways, including:

lManually by roller or brush

lDrip

lMachine roller

lSpraying

lFlooding

Die making is as much of an art as a science. When all the dynamics of stamping are taken into account, the resulting part may not meet all expectations. To help fine tune the stamping process and finalize die design, die makers use an analytical tool called Circle Grid Analysis, or CGA. The application of CGA involves the etching of a pattern of small circles on the surface of the blank. This pattern deforms along with the blank as it is formed, providing point- to-point calculations of the deformation that occurred. Analyzing this stamped grid pattern suggests the location and type of rework that must be performed on the dies to produce easily manufactured parts. The CGA process is repeated on the stamping die until an acceptable part is produced.