ENGR 241 – Introduction To Manufacturing Chapter 16: Sheet Metal Forming Processes

Shearing Process that involves cutting sheet metal by subjecting it to shear stresses, usually between a punch and a die. Process Variables punch force punch speed lubrication edge condition Clearance Material thickness

Punching Force Pmax = 0.7(UTS)(t)(L) Approximate empirical formula for estimating maximum punch force: Pmax = 0.7(UTS)(t)(L) Pmax = maximum punch force UTS = Ultimate Tensile Strength t = thickness L = total length of sheared edge

Shearing: Additional Concepts Punching (piercing): sheared slug is discarded. Blanking: slug is the part, the rest is discarded. Fine Blanking (less distortion). Burrs (clearance, ductility, dull tool edges).

Shearing Discard Punching Blanking

Shearing Operations Die Cutting Perforating- punching holes. Parting- shearing sheet into two or more pieces. Notching-removing pieces or shapes from edge. Lancing- leaving a tab without removing the material.

Shearing Operations Parting Perforating Slitting Notching Lancing

Shearing Operations Slitting Driven Pull-through powered blades idling blades

Shearing Operations Steel Rules Nibbling thin strip of metal is bent and held on its edge on a flat base. die is presses against the sheet to cut parts (similar to piercing, cookie cutting). Nibbling successive punched holes.

Shearing Operations (Nibbling) successive holes

Shearing Dies Compound Progressive Dies Transfer Dies several operations on same strip (punching). Progressive Dies different operations performed with each stroke. Transfer Dies sheet metal undergoes different operations at different stations.

Bending One of the most common forming operations, used for producing parts and to impart stiffness. After bending, outer fibers of the material are under tension while the inner fibers are in compression.

Bending Process Variables minimum bend radius bending allowance Smallest radius allowed before cracks on outer surface (2T, 3T, etc.) bending allowance Springback: elastic recovery Overbending

Bending Radius The bending radius is measured to the inner surface of the bend and it is normally expressed in terms of material thickness. r t

Bending Radius Theoretically, the strains at the outer and inner fibers are equal in magnitude. where: eo = outer strain ei = inner strain r = bending radius t = thickness

Bending Allowance t L d=0.4t  r where: L = bending allowance r = bending radius t = thickness

Bendability A material may be reduced in thinkness to increase its bendability A material may be heated to increase its bendability Anisotropy is an important factor in bending operations (Fig. 16.17) The larger the minimum bend radius, the more resistance to bending

Bending Operations Press Brake Beading Flanging machine utilizes long dies in a press Beading edge of sheet metal is bent into die cavity Flanging bending of edges

Bending Operations Hemming edge folded over itself Roll Forming bent by series of rolls Tube Bending and Forming pack inside of tube with sand to prevent buckling and folding. flexible mandrels

Other Methods Stretch, draw, compression bending Rubber Forming one die is replaced with a rubber or polyurethane pad. Spinning axisymmetric parts over a mandrel with tools or rollers.

Other Methods Peen Forming Explosive Forming Magnetic-Pulse Forming Superplastic Forming