Springback (old)
The stress state is extremely complex in bending. The complete tensile and compressive stress-strain curves of the material are traversed on rthe tensile and compressive sides of the bend, respectively. This means that around the neutral plane the stresses must be elastic. When the forming tool is retracted, the elastic components, of the stress cause springback, and a residual stress pattern. Springback increases both the angle and radius of the bent part. The elastic zone is more extensive for a relatively gentle bend(large Rb/h ratio) and for a material with high ratio of yield strength σ0.2 to elastic modulus E; therefore springback also increases according to the approximate formula
Rb/Rf=1-3(Rb*σ0.2)/(h*E)+4((Rb*σ0.2)/(h*E))^3
where Rb is the radius of the bending die and Rf is the radius obtained after the forming spressure is released.
Springback established a new force equilibrium with a residual stress distribution typified by a compressive stress on the outer and tensile stress on the inner surface
Several techniques are used to combat springback:
1. If springback for a given material is known and if the material is of uniform quality and thickness, compensation for springback is possible for overbending.
2. Alternatively, the elastic zone can be eliminated at the end of the stroke by one of two means. First, the two ends of the sheet may be clamped before the punch bottoms out, so that the end of the stroke involves stretching of the part, causing tensile yielding in the entire sheet thickness. In the second method the punch nose is shaped to indent the sheet, so that plastic compression takes place throughout the thickness
3. If a counterpunch is used with a controlled pressure, compressive stress are maintained in the bend zone during the entire process. Since this also has the effect of imposing a hydrostatic pressure on the bend zone, bending beyond the limits given by
Rb=h(1/2q-1) for q<0.2
Rb=h((1-q)^2)/(2q-q^2) for q>0.2
4. Less ductile materials may have to be bent at some elevated temperature; because the yield strength is lower, springback is also less.
(Schey, J.A., Introduction to Manufacturing Process, 2nd Edition,pg.299,300,301)
Springback (new) [material property]
Every plastic deformation is followed by elastic recovery. As a consequence of this phenomenon, changes occur in the dimensions of the plastic-deformed workpiece upon removing the load. Permanent deformation is expressed as the difference between the plastic and the elastic deformations.
E(t)=E(pl)-E(e)
While a workpiece is loaded, it will have following characteristics as a consequence of plastic deformation
-bend radius
-bend angle
-profile angle
All workpiece materials have a finite modulus of elasticity, so each will undergo a certain elastic recovery
upon unloading. In bending, this recovery is known as springback.
(Sheet metal forming processes and die design Author: Vukota Boljanovic p.64-65)
-new definition is better because it is more simple and understandable.
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Turret Punch (old)
Turret presses are suited to situations in which a sequence of punching, notching, and related cutting operations must be accomplished on sheet-metal-parts. Turret presses have a C-frame. The conventional ram and punch is replaced by a turret containing many punches of different sizes and shapes. The turret works by indexing (rotating) to the position holding the punch to perform the required operation. Beneath the punch turret is a corresponding die turret that positions the die opening for each punch. Between the punch and die is the sheet-metal blank, held by an x-y positioning system that operates by computer numerical control. The blank is moved to the required coordinate position for each cutting operation. (Mikell P. Groover; Fundamentals of Modern Manufacturing Materials, Processes, and Systems 3rd Edition; pg.467)
Turret Punch (new) [manufacturing method]
For producing identical parts, a fixed template is prepared by means of wihch all holes are accurately punched. The illustration shows variety of work that can be done on a machine of this type.
Turret punch presses of the tape-controlled variety are available in capacities of 15 to 150 tons with multi-station turrets.
(21st Centuty Manufacturing: National Initiative For Product Data Exchange: Product Data Exchange Baseline Activity Author: DIANE Publishing Company p.192)
-new definition is better and more understandable
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Die Punch:(old)
The sheet-metal blank is placed in a punching die, where a pattern of holes is punched simultaneously with one storke of the punching pres. The high-tonnage brake pres is often used to provide the power stroke required on the punching die block.
(Ronald A. Walsh, Electromechanical Design HandBook, page 10)
Die Punch(new) [manufacturing method]
As the material strip is run through the die, the upper punches, fastened to the moving ram of the press, remove blanks from it.
Figure 2 shows the die set, and all parts comprising the die assembly are built within it. Die sets are made by several manufacturers and they may be purchased in a great variety of shapes and sizes. Punch shank (A) is clamped in the ram of the press. In operation, the upper part of the die set (B), called the punch holder, moves up and down with the ram. Bushings (C), pressed into the punch holder, slide pn guide posts (D) to maintain precise alignment of cutting members of the die. The die holder E is clamped to the bolster plate of the press by bolts passing through slots (F).
(Die design fundamentals: a step-by-step introduction to the design of stamping dies including material, punches, die sets, stops, strippers, gages, pilots, and presses Author:J. R. Paquin, R. E. Crowley p.2)
-new definition contains more information and is better.
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Stretcher Bending(old)This process for sheet metal forming uses a machine known as a stretcher bender, wherein a sheet metal formed section such as an angle, channel, or Z section is pulled against a radiused die block to accurately form structural frame sections to a specified radius of curvature. The frame structures of rockets and aeorospace vehicles which have a single radius of curvature are formed on the stretcher bending machine. This is the only cost effective method known for producing such sheet metal frame parts quickly and accurately on a relatively simple machine. (Electromechanical Design Handbook, 3rd Edition, Walsh, p.11.12)
Stretch Bending(new):[manufacturing method]
In sheet metal bending operation, compressive stress is developed, which ultimately leads to local buckling or wrinkling. It can be avoided by keeping the sheet metal strip under the tension during the operation. This process of simultaneous streching and bending is called Strech bending.
In strech bending, the complete deformation is carried out in plastic state only. The material is first brought into plastic state by streching. Hence it is called strech forming.
The sheet is held in the jaws of hyraulic cylinder and is streched beyond the elastic limit. Then the sheet is brought into contact with the die.
(Production Technology Author: Narayana p.220)
-new definition is better and more understandable
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Explosive forming 19 mart 18:22
Explosive forming involves the use of an explosive charge to form sheet metal into a die cavity.One method of implementing the process is illustrated in figure 20,45.The workpart is clamped and sealed over the die , and vacuum is created in the cavity beneath.The apparatus is then placed in a large vessel of water.An explosive charge is placed in the water at certain distance above the work.Detonation of the charge results in shock wave whose energy is transmitted by the water to cause rapid forming of the part into the cavity.The size of the explosive charge and the distance at which it is placed above the part are largely a matter of art and experience.Explosive forming is reserved for large parts, typical of the aerospace industry.
(Mikell P.Groover, Fundamentals of Modern Manufacturing , materials,processes, and systems third edition page 473)
Explosive Forming(new): [manufacturing method]
The invention of explosive forming, which shapes metal against a mould or die, is attributed to an engineer from Kent named Walter Claude Johnson. In 1897 he obtained a patent which he described "a method of generating fluid pressure for tube jointing or embossing, stamping of the metal sheets and such like oprations". The piston forces a liquid into, for example, the tubes of a bicycle frame and expands the ends of the tubes into their various joints.
(Fundamentals of Modern Manufacturing: Materials, Processes, and Systems Author:Mikell P. Groover p.475)
(New Scientist 10 July 1986 p.46)
-new definition is more clear and visual and therefore better.
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