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Magnesium Steels Austenitic stanless Low-carbon,lowalloy,HSLA Titanium Titanium alloys Note :T——thickness of material
5T 0.5T 0.5T 0.7T 2.6T 13T 6T 4T 3T 4T Bend allowance as shown in Figure 4 is the length of the neutral axis in the bend and is used to determine the blank length for a bent part. However, the position of the neutral axis depends on the radius and angle of bend (as described in texts on mechanics of materials).An approximate formula for the bend allowance, Lb is given by
Lb= α(R十kT)
Where Lb——bend allowance, in (mm). α——bend angle, (radians) (deg). T——sheet thickness, in (mm). R——inside radius of bend, in (mm).
k——0.33 when R is less than 2T and 0.50 when JR is more than 2T.
Bend methods arc commonly used in press tool. Metal sheet or strip, supported by-V bock[Figure 6(a)],is forced by a wedge-shaped punch into the block. This method, termed V bending, produces a bend having an included angle which may be acute, obtuse, or 90°.Friction between a spring-loaded knurled pin in the vee die and the part will prevent or reduce side creep of the part during its bending.
Edge bending [Figure 6(b)] is cantilever loading of a beam. The bending punch forces the metal against the supporting die. The bend axis is parallel to the edge of the die. The workpiece is clamped to the die block by a spring-loaded pad before the punch contacts the workpiece to prevent its movement during downward travel of the punch.
Figure 6 Bending methods
Bending Force can be estimated by assuming the process of simple bending of a rectangular beam. The bending force in that case is a function of the strength of the material. The calculation of bending force is as follows:
P=KLST2/W
Where P-bending force, tons (for metric usage, multiply number of tons by 8.896 to obtain kilonewtons).
K——die opening factor: 1.20 for a die opening of 16 times metal thickness,
1.33 for an opening of 8 times metal thickness.
L——length of part, in.
S——ultimate tensile strength, tons per square in. W——width of V or U die, in. T——metal thickness, in.
For U bending (channel bending) pressures will be approximately twice those required for V bending, edge bending requires about one-half those needed for V bending.
Springback in that all materials have a finite modulus of elasticity, plastic deformation is followed, when bending pressure on metal is removed, by some elastic recovery (see Figure 7). In bending, this recovery is called springback. Generally speaking, such a springback varies in sheet from 0.5°to 5°, depending upon finite modulus of elasticity, modes of bending, clearance of die and so on, but phosphor bronze may spring back from 10°to15°.
Figure 7 Springback during bending
Methods of reducing or eliminating springback in bending operations can be made according to the following operations, shown in Figure 8, and parts produced in bending die are also overbent through an angle equal to the springback angle with an undercut or relieved punch.
Figure8 Methods of reducing or eliminating springback
For the applications, there are many types of the presses, such as the single-action straight-slide eccentric mechanical press, punch press, hydro-former press, hydraulic press, press brake, triple-action press, turret press, two-point press, twin-drive press, two point single-action press, watch press, trimming press, closed-type single-action crank press, knuckle-lever press, one-point single-action press, open-back inclinable press, open-side press, four-point press, four-crank press, flywheel-type screw press, friction screw press, straight-side single-action double-crank press, rocker-arm press, screw press and top-drive sheet-metal stamping automatic press and so on.
A double-action press is used for large, or deep drawing operations on sheet metal parts. This type of press has an outer ram ( blank holder ) and a section inner ram ( punch holder ) . During the operating cycle, the blank holder contacts the material first and applies pressure to allow the punch holder to properly draw the part (Figure 9).
Figure9 Typical versatile press
A triple-action press has the same inner and outer ram as the double-action press, but a third ram in the press bed moves up allowing a reverse draw to be made in one press cycle. The triple-action press is not widely used.
A knuckle press is used for coining operation. The design of the drive allows for very high pressures at the bottom of the ram stroke. This type uses a crank, which moves a joint consisting of two levers that oscillate to and from dead center and results in a short, powerful movement of the slide with slow travel near the bottom of the stroke.
A hydraulic press is used basically for forming operations and has a slower operating cycle time than most mechanical presses. The advantages of hydraulic presses are that the working pressure stroke, and speed of the ram are adjustable (Figure 10). Hydraulic presses belong to the force-constrained type of forming machines . Their main use is found in those areas of forming technology where the force along the path of the ram must remain constant or under accurate control. The drive mechanism of