Blanking and Punching
31
deformation becomes larger. The material is pulled into the clearance area, and the edges of the punched or blanked zone become more and more rounded. In fact, if the clearance is too large, the sheet metal is
bent and subjected to tensile stresses instead of undergoing a shearing deformation.
4.2 BLANKING AND PUNCHING CLEARANCE
Clearance is the space per side between the punch and the die opening Fig.
such that:
2
Because of the amount of clearance between the punch and the die, tool and die producers enjoy some kind of mystique related to their work as being both an art and a science.
Proper clearance between cutting edges enables the fractures to start ideally at the cutting edge of the punch and also at the die. The fractures will proceed toward each other until they meet, and the fractured
portion of the sheared edge then has a clean appearance. For optimum finish of a cut edge, correct clear- ance is necessary and is a function of the kind, thickness, and temper of the material.
The upper corner of the cut edge of the strip stock and the lower corner of the blank acquire a radius where the punch and die edges respectively make contact with the work material. This edge radius is pro-
duced by the plastic deformation taking place and is more pronounced when cutting soft materials.
Excessive clearance will also cause a large radius at these corners, as well as a burr on opposite corners. When clearance is not sufficient, additional layers of the material must be cut before complete sepa-
ration is accomplished. With correct clearance, the angle of the fractures will permit a clean break below the burnish zone because the upper and lower fractures will extend toward one another. Excessive clear-
ance will result in a tapered cut edge, because for any cutting operation, the opposite side of the material that the punch enters after cutting, will be the same size as the die opening.
The width of the burnish zone is an indication of the hardness of the material. Provided that the die clearance and material thickness are constant, the softer the material, the wider will be the burnish zone.
Harder metals required larger clearance and permit less penetration by the punch than ductile materials; dull tools punch and die create the effect of too small a clearance as well as a burr on the die side of the
Workpiece
Fig. 4.4
Punch and
die
clearance.
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