01-01-2013, 11:54 AM
SHEET-METAL FORMING PROCESS
SHEET-METAL FORMING.ppt (Size: 1.65 MB / Downloads: 210)
Shearing :
Sheet metal subjected to shear stress developed between a punch and a die is called shearing
Shearing usually starts with formation of cracks on both the top and bottom edges of the work piece.These cracks meet each other and separation occurs
Process parameters :
Shape of the material for the punch and die
Speed of the punching ,lubrication and clearance
Shearing operation :
Several operations based on shearing performed
Punching – sheared slug discarded
Blanking – Slug is the part and the rest is scrap
Other shearing operations:
Die cutting
Fine blanking
Slitting
Steel rules
Nibbling
Scrap in shearing
Tailor welded blanks
Shearing dies :
Clearance : function of type of material,its temper and its thickness and of the size of the blank and its proximity to the edges
Clearance of soft materials are less than harder grades
Punch & die shapes :
Surfaces of punch and die are flat
Punch force builds rapidly and entire thickness is sheared at same time.
Bending is suitable for shearing thick surfaces
Anisotropy :
Anisotropy is caused by thermo-mechanical processing of sheet.
2-types
Crystallographic anisotropy
Mechanical fibering
Spring back :
In Bending ,after plastic deformation there is an elastic recovery this recovery is called spring back.
Spring back can be calculated approximately in terms if radii Ri and Rf
Ri/Rf = 4 ( Ri Y / ET )3 – 3 (Ri Y /ET) + 1
Spring back Increases as (R/T ratio & yield stress of material ) increases as elastic modulus E decreases
Deep drawing :
Punch forces a flat sheet metal into a deep die cavity
Round sheet metal block is placed over a circular die opening and held in a place with blank holder & punch forces down into the die cavity
Deep drawing Practice :
Blank holder pressure – 0.7% -1.0 % of Yield strength + UTS
Clearance usually – 7% -14 % > sheet thickness
Draw beads are used to control flow of blank into die cavity.
Ironing is a process in which the thickness of a drawn cup is made constant by pushing of the cup through ironing rings.
Redrawing – Containers or shells which are too difficult to draw in one operation undergo redrawing