Page 406 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

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Chapter 16 Sheet-Metal Forming Processes and Equipment

Discarded Parting
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Punching Blanking LanLing"*

(H) (D)

FIGURE l6.4 (a) Punching (piercing) and blanking. (b) Examples of various die-cutting
operations on sheet metal. Lancing involves slitting the sheet to form a tab.

well as those described throughout the rest of this chapter, generally are carried out
on computer-numerical-controlled machines with quick-change toolholders. Such
machines are useful, particularly in making prototypes of sheet-metal parts requir-
ing several operations to produce.

Die Cutting. This is a shearing operation that consists of the following basic
processes (Fig. 16.4b):

° Perforating: punching a number of holes in a sheet
° Parting: shearing the sheet into two or more pieces
° Notching: removing pieces (or various shapes) from the edges
° Lancing: leaving a tab without removing any material.

Parts produced by these processes have various uses, particularly in assembly
with other components. Perforated sheet metals with hole diameters ranging from
around 1 mm to 75 mm have uses as filters, as screens, in ventilation, as guards for
machinery, in noise abatement, and in Weight reduction of fabricated parts and
structures. They are punched in crank presses (see Fig. 14.17a) at rates as high as
300,000 holes per minute, using special dies and equipment.

Fine Blanking. Very smooth and square edges can be produced by fine blanking
(Fig. 16.5a). One basic die design is shown in Fig. 16.5b. A V-shaped stinger or
impingement mechanically locks the sheet tightly in place and prevents the type of
distortion of the material shown in Figs. 16.2b and 16.3. The fine-blanking process,
which was developed in the 19605, involves clearances on the order of 1% of the
sheet thickness and that may range from 0.5 to 13 mm in most cases. Dimensional
tolerances are on the order of ;i:0.05 mm and less than :i:0.025 mm in the case of
edge perpendicularity.

Slitting. Shearing operations can be carried out by means of a pair of circular
blades similar to those in a can opener (Fig. 16.6). In slitting, the blades follow either
a straight line, a circular path, or a curved path. A slit edge normally has a burr,
which may be folded over the sheet surface by rolling it (flattening) between two
rolls. If not performed properly, slitting operations can cause various distortions of
the sheared edges.