Sheet Metal Stamping
Stamping presses andstamping dies are tools used to produce high volume sheet metal parts. Thepress provides the force to close the stamping dies where they shape and cutthe sheet metal into finished parts.
Productionstamping is generally performed using sheet metal materials .020” to .080”thick, but the process also can be applied to foils as thin as .001” or toplate stock with thickness' approaching 1.000”.
Formabilityis the primary attribute of sheet metal material. Formability is furtherdefined as the materials ability to be:
Themetallurgical term for these qualities is “ductility”. Ductility is thematerials ability to deform and elongate without fracture. The extent to whicha stamping is subjected to such deformation is directly related to the part’soverall shape and geometry.
Other factors also influence the material’s formability. They include:
• the die design
• the press
• the press speed
• sheet metal feeding mechanisms
• monitoring and control systems
Sheet Metal Stamping Dies
The word“die” is a generic term used to describe the tooling used to produce stampedparts. A die set assembly consisting of a male and female component is theactual tool that produces the shaped stamping. The male and female componentswork in opposition to both form and punch holes in the stock. The upper half ofthe die set, which may be either the male or female, is mounted on the pressram and delivers the stroke action. The lower half is attached to anintermediate bolster plate which in turn is secured to the press bed. Guidepins are used to insure alignment between the upper and lower halves of the dieset.
The mostcommon types of dies perform cutting and forming. Cutting dies are used toshear sheet material into what is called a blank. These blanks are then exposedto blanking dies which cut the entire perimeter of the part, or to forming dieswhere the blank is stamped into a part. Punching is another function of cuttingdies. Punching is the cutting of a slug from the sheet metal stock to produce ahole or slot. Cutting dies are also used to trim excess metal from around aformed part.
Holepunching and other cutting operations require specific and carefully maintainedclearances between the punch (male component) and the die (female component).The setting of the required clearances is determined by both the stockthickness and temper. In general, die clearances increase as the stockthickness increases. The depth of punch penetration into the sheet metal stockwill also increase as softer stock is used.
Formingis a general term used to describe a stamped part whose shape and contour isreproduced directly from the shape and contour of a die set. The main formingoperations accomplished with press mounted dies are:
Drawing,or draw forming, involves forcing a blank deeply into a die cavity and shapingit into the shape and contour of the punch face and sides. Without sufficientformability qualities, drawn blanks are subject to wrinkling, thinning, andfracturing. Draw forming requires an addition to the die set called a“blankholder”. The function of the blankholder, usually a ring through whichthe punch and ram pass, is to control the metal flow as it is forced into thedie cavity. In practice, the blankholder must exert less pressure against theblank than the punch, so metal can flow into the die; yet it must exert enoughpressure to prevent the material from wrinkling.
Bendingis a relatively simple forming operation which provides rigidity and shape tosheet metal parts. Similar to bending is flanging. However a flange issignificantly smaller in dimension than the rest of the part. The functions ofa flange include:
• giving a more finished appearance
• edge strengthening
• providing a fastening or attachment surface
Hemmingis the folding over of a short flange upon itself to form a smooth, roundededge and to facilitate the attachment of mating parts.
Multiplestamping operations may be performed within a single die, or at a number of diestations within a die set and with a single stroke of the press.
Singlestation dies can be either compound dies or combinations dies. A compound dieperforms basic cutting operations such as blanking and hole punching to produceparts. Combination dies combine shaping and forming functions with cuttingoperation to manufacture parts.
Multiplestation dies are arranged so that a series of sequential operations areaccomplished with each press stroke. Two die types are used; progressive andtransfer. With progressive dies, coil stock is fed into the press. Individualstampings are connected with a carrier strip as they progress through thevarious die operations and are ultimately separated and then discharged fromthe press. In transfer die operations individual stock blanks are mechanicallymoved from die station to die station within a single die set. Large stampingsare done with tandem press lines where the stock is moved from press to pressin which specific operations, such as drawing or trimming, are performed.
Theresistance of the sheet metal stock to the forces exerted by the moving diescreates friction. For this reason, lubrication is vital for successful sheetmetal forming. A lubrication's function is to minimize contact between thetooling and the work piece. This results in reduced tonnage requirements,longer tooling life, and improved product quality.
Lubricantsrange from light mineral oils to high viscosity drawing compounds. They may beoil base, water soluble, or synthetic materials. These lubricants may beapplied in a variety of ways, including:
• manually by roller or brush
• machine roller
Die making is as much of anart as a science. When all the dynamics of stamping are taken into account, theresulting part may not meet all expectations. To help fine tune the stampingprocess and finalize die design, die makers use an analytical tool calledCircle Grid Analysis, or CGA. The application of CGA involves the etching of apattern of small circles on the surface of the blank. This pattern deformsalong with the blank as it is formed, providing point-to-point calculations ofthe deformation that occurred. Analyzing this stamped grid pattern suggests thelocation and type of rework that must be performed on the dies to produceeasily manufactured parts. The CGA process is repeated on the die until anacceptable part is produced.
Computersoftware programs are available that can assure that proposed stamping designscan be manufactured with certainty, with the objective of minimizing thelengthy fine-tuning process of die making. Analytical tools such as finiteelement modeling, or FEM, are also used especially for facilitating deep drawprogressive design with minimum trial and error situations.
Most sheet metal stamping uses materials:
a. .020” to .080” thick
b. .050” to .125” thick
c. .001” to 1.000” thick
d. .010” to .060” thick
Ductility refers to a material’s ability to:
a. resist penetration
b. resist corrosion
c. bend over upon itself
d. deform and elongate
A compound die is a die that performs:
a. hemming and flanging
b. blanking and punching
c. bending only
d. hole punching only
The device(s) used to insure die component alignment are called:
a. roller bearings
c. guide pins
d. "v" grooves
The terms drawing, flanging, and hemming are examples of:
A carrier strip is used with:
a. transfer dies
b. progressive dies
c. tandem presses
d. combination dies
The principle function of a lubricant in press stamping is to:
a. cool the dies
b. enable easy stack removal
c. prevent wrinkling
d. minimize tool contact to the work
Circle Grid Analysis, or CGA, is used to:
a. calculate tonnage exerted on the work
b. predict deformation distances
c. finalize die design
d. determine optimum press speed