Page 403 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Section 16.2 Shearing 383

TABLE l6.l
General Characteristics of Sheet-metal Forming Processes (in alphabetic order)
Forming process Characteristics
Drawing Shallow or deep parts with relatively simple shapes, high production rates, high tooling and
equipment costs
Explosive Large sheets with relatively simple shapes, low tooling costs but high labor cost, low-quantity
production, long cycle times
Incremental Simple to moderately complex shapes with good surface finish; low production rates, but no
dedicated tooling required; limited materials
Magnetic-pulse Shallow forming, bulging, and embossing operations on relatively low strength sheets, requires
special tooling
Peen Shallow contours on large sheets, flexibility of operation, generally high equipment costs, process also
used for straightening formed parts
Roll Long parts with constant simple or complex cross sections, good surface finish, high production rates,
high tooling costs
Rubber Drawing and embossing of simple or relatively complex shapes, sheet surface protected by rubber
membranes, flexibility of operation, low tooling costs
Spinning Small or large axisymmetric parts; good surface finish; low tooling costs, but labor costs can be high
unless operations are automated
Stamping Includes a wide variety of operations, such as punching, blanking, embossing, bending, Hanging, and
coining; simple or complex shapes formed at high production rates; tooling and equipment costs can
be high, but labor cost is low
Stretch Large parts with shallow contours, low-quantity production, high labor costs, tooling and equipment
costs increase with part size
Superplastic Complex shapes, fine detail and close dimensional tolerances, long forming times (hence production
rates are low), parts not suitable for high-temperature use

cracks eventually meet each other and complete separation occurs. The rough
fracture surfaces are due to the cracks; the smooth and shiny burnished surfaces on
the hole and the slug are from the contact and rubbing of the sheared edge against
the walls of the punch and die, respectively.
The major processing parameters in shearing are
° The shape of the punch and die
° The speed of punching
° Lubrication
° The clearance, c, between the punch and the die.

The clearance is a major factor in determining the shape and the quality of the
sheared edge. As the clearance increases, the zone of deformation (Fig. 16.3a) be-
comes larger and the sheared edge becomes rougher. The sheet tends to be pulled
into the clearance region, and the perimeter or edges of the sheared zone become
rougher. Unless such edges are acceptable as produced, secondary operations may be
required to make them smoother (which will increase the production cost).
Edge quality can be improved with increasing punch speed; speeds may be as
high as 10 to 12 m/s. As shown in Fig. 16.3b, sheared edges can undergo severe cold
working due to the high shear strains involved. Work hardening of the edges then will
reduce the ductility of the edges and thus adversely affect the formability of the sheet
during subsequent operations, such as bending and stretching.
The ratio of the burnished area to the rough areas along the sheared edge
(a) increases with increasing ductility of the sheet metal and (b) decreases with
increasing sheet thickness and clearance. The extent of the deformation zone in