Page 450 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)
P. 450

0      Chapter 16  Sheet-Metal Forming Processes and Equipment
                                   Stamping and Progressive-die Operations.  In progressive dies (see Section 16.2.3),
                                   the cost of the tooling and the number of stations are determined by the number and
                                   spacing of features on a part. Therefore, it is advantageous to hold the number of fea-
                                   tures to a minimum in order to minimize tooling costs. Closely spaced features may
                                   provide insufficient clearance for punches and may require two punches. Narrow cuts
                                   and protrusions also are problematic for forming with a single punch and die.


                                   Deep Drawing.   After a deep-drawing operation, a cup invariably will spring back
                                   towards its original shape. For this reason, designs that use a vertical wall in a deep-
                                   drawn cup may be difficult to form. Relief angles of at least 3° on each wall are eas-
                                   ier to produce. Cups with sharp internal radii are difficult to produce, and deep cups
                                   often require one or more ironing operations.




                                    l6.l4    Equipment for Sheet-metal Forming

                                   For most general pressworking operations, the basic equipment consists of mechan-
                                   ical, hydraulic, pneumatic, or pneumatic-hydraulic presses with a wide variety of
                                   designs, features, capacities, and computer controls. Typical designs for press frames
                                   are shown in Fig. 16.60 (see also Figs. 14.17 and 16.23f). The proper design, stiff-
                                   ness, and construction of such equipment is essential to the efficient operation of the
                                   system and to achieving a high production rate, good dimensional control, and high
                                   product quality.
                                        The traditional C-frame structure (Fig. 16.60a) has been used widely for ease
                                   of tool and workpiece accessibility, but it is not as stiff as the box-type pillar
                                   (Fig. 16.6Oe) or the double-column frame structure (Fig. 16.60f). Furthermore,
                                   accessibility has become less important due to advances in automation and in the
                                   use of industrial robots and computer controls.
                                        Press selection for sheet-metal forming operations depends on several factors:

                                      I. Type of forming operation, the size and shape of the dies, and the tooling
                                        required.
                                     2. Size and shape of workpieces.
                                     3. Length of stroke of the slide, the number of strokes per minute, the operating
                                        speed, and the shut height (the distance from the top of the bed to the bottom
                                        of the slide with the stroke down).
                                     4. Number of slides. Single-action presses have one reciprocating slide. Double-
                                        action presses have two slides, reciprocating in the same direction. They typi-
                                        cally are used for deep drawing-one slide for the punch and the other for the
                                        blankholder. Triple-action presses have three slides; they are typically used for
                                        reverse redrawing and for other complicated forming operations.
                                     5. Maximum force required (press capacity and tonnage rating).
                                     6. Type of mechanical, hydraulic, and computer controls.
                                      7. Features for changing dies. Because the time required for changing dies in
                                        presses can be significant (as much as a few hours) and thus affect produc-
                                        tivity, rapid die-changing systems have been developed. Following a system
                                        called single-minute exchange of die (SMED), die setups can be changed in
                                        less than 10 minutes by using computer-controlled hydraulic or pneumatic
                                        systems.
                                     8. Safety features.
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