MT-284 MANUFACTURING PROCESSES INSTRUCTOR: SHAMRAIZ AHMAD MS-Design and Manufacturing Engineering Shamraiz_88@yahoo.com Topic: Sheet Metal Working

This Lecture Introduction to sheet metal working and types Advantages of sheet metal working Sheet metal characteristics Sheet metal work – Shearing (Cutting) Sheet metal work – Bending (Forming) Sheet metal work – Drawing (Forming)

Sheet Metalworking Definition: Cutting and forming operations performed on relatively thin sheets of metal are called sheet metal working. Thickness of sheet metal = 0.4 mm (1/64 in) to 6 mm (1/4 in) Thickness of plate stock > 6 mm Operations usually performed as cold working (at room temperature) Exception to cold working: (warm working) Thick stock, brittle metal and significant deformation

Basic Types of Sheet Metal Processes Cutting (Shearing) Shearing to separate large sheets into small sheets Blanking to cut part perimeters out of sheet metal Punching to make holes in sheet metal Bending (Forming) Straining sheet around a straight axis Drawing (Forming) Forming of sheet into convex or concave shapes

Sheet Metalworking Terminology Punch‑and‑die - tooling to perform sheet metal processes; cutting, bending, and drawing (Male and Female parts ) Stamping press - machine tool that performs most sheet metal operations. Different from forging and extrusion presses Stampings - sheet metal products Stock - The sheet metal is fed into the press in form of long strips and coils, called stock.

Sheet and Plate Metal Products Sheet and plate metal parts for consumer and industrial products such as Automobiles and trucks Airplanes Railway cars and locomotives Farm and construction equipment Small and large appliances Office furniture Computers and office equipment

Advantages of Sheet Metal Parts High strength Good dimensional accuracy Good surface finish Relatively low cost For large quantities, economical mass production operations are available

Sheet Metal Characterstics: Elongation A mechanical property of metal that is the degree to which a material may be bent, stretched, or compressed before it ruptures. and is expressed as a percentage of the original length. A specimen subjected to tension first undergoes uniform elongation up to the UTS after which it begins to neck This elongation is then followed by further non uniform elongation until the specimen fractures Because the sheet is being stretched during forming, high uniform elongation is thus desirable for good formability

Sheet Metal Characteristics Stress-Corrosion Cracking Residual stresses can develop in sheet-metal parts because of the non uniform deformation that the sheet undergoes during forming When disturbed, such as by removing a portion of it the part may distort Furthermore tensile residual stresses on surface can lead to stress-corrosion cracking of the part unless it is properly stress relieved

Sheet Metal Characteristics Other Important Characteristics in Forming Anisotropy: An important factor in forming particularly in deep drawing ( Having different values of properties when measured in different directions) Grain size: Important for surface finish (smaller grain provide better surface finish) Spring Back: It is important consideration in bending about elastics behavior of material Wrinkling: With tensile stresses, some time compressive stresses are developed which result in buckling(wrinkles)

Sheet Metal Cutting (Shearing) Definition: Shearing- mechanical cutting of material without the formation of chips or the use of burning or melting Shearing Process: Fracture and tearing begin at the weakest point and proceed progressively to the next-weakest location Results in a rough and ragged edge Punch and die must have proper alignment and clearance Sheared edges can be produced that require no further finishing Steps: Contact of two shearing edges Deformation Penetration Fracture

Sheet Metal Cutting Figure 20.1 Shearing of sheet metal between two cutting edges: (1) just before the punch contacts work; (2) punch begins to push into work, causing plastic deformation;

Sheet Metal Cutting Figure 20.1 Shearing of sheet metal between two cutting edges: (3) punch compresses and penetrates into work causing a smooth cut surface; (4) fracture is initiated at the opposing cutting edges which separates the sheet.

Shearing, Blanking, and Punching Three principal operations in pressworking that cut sheet metal: Shearing Blanking Punching

Shearing Sheet metal cutting operation along a straight line between two cutting edges Typically used to cut large sheets Figure 20.3 Shearing operation: (a) side view of the shearing operation; (b) front view of power shears equipped with inclined upper cutting blade.

Blanking and Punching Blanking - sheet metal cutting to separate piece (called a blank) from surrounding stock Punching - similar to blanking except cut piece is scrap, called a slug Figure 20.4 (a) Blanking and (b) punching.

Fine Blanking Operations Fine Blanking - the piece being punched out becomes the workpiece and pressure pads are used to smooth edges in shearing Figure 17-3 (Top) Method of obtaining a smooth edge in shearing by using a shaped pressure plate to put the metal into localized compression and a punch and opposing punch descending in unison.

Other Sheet-Metal Cutting Operations Cutoff (one cutting edge) Parting (with 2 cutting edges Slotting (rectangular hole) Perforating (pattern of holes) Notching (cutting a portion on the edge) Slitting , leaving a tab(mark) on the sheet without removing any material Trimming (remove excess metal) Shaving (smooth cutting edges) Fine Blanking (close tolerances, smooth with pressure pad)

Characteristics of sheared edge Rollover region the region where initial plastic deformation occurs Burnish a smooth cutting region below rollover. Fracture zone Rough surface beneath burnish Burr The sharp edge caused by elongation of metal

Clearance in Sheet Metal Cutting Definition: Distance between punch cutting edge and die cutting edge is called clearance. Typical values range between 4% and 8% of stock thickness If too small, fracture lines pass each other, causing double burnishing and larger force If too large, metal is pinched between cutting edges and excessive burr results

Design Clearance

Design Die and Punch Sizes For a round blank of diameter Db is determined as: Blank punch diameter = Db - 2c Blank die diameter = Db For a round hole (piercing) of diameter Dh is determined as: Hole punch diameter = Dh Hole die diameter = Db + 2c

Clearance Calculation The recommended clearance is: C = at Where c – clearance, in (mm); a – allowance; and t = stock thickness, in (mm). Allowance a is determined according to type of metal. From Mikell P. Groover “Fundamentals of Modern Manufacturing”.

Cutting Forces Cutting forces are used to determine size of the press needed. F = StL Where S – shear strength of the sheet metal, lb/in2 (Mpa); t – sheet thickness in. (mm); and L – length of the cut edge, in. (mm). In blanking, punching, slotting, and similar operations, L is the perimeter length of blank or hole being cut. Note: the equation assumes that the entire cut along sheared edge length is made at the same time. In this case, the cutting force is a maximum.

Calculating Clearance and Force Example: Round disk of 3.0” dia. is to be blanked from a half-hard cold-rolled sheet of thickness 1/8” with shear strength = 45,000 lb/in2. Determine (a) punch and die diameters, and (b) blanking force. (a). From table , a = 0.075, so clearance c = 0.075(0.125) = 0.0094”. Die opening diameter = 3.0” Punch diameter = 3 – 2(0.0094) = 2.9812 in (b) Assume the entire perimeter of the part is blanked at one time. L = p Db = 3.14(3) = 9.426” F = 45,000(9.426)(0.125) = 53,021 lb

Examples If the aluminum sheet metal has a tensile strength = 310 MPa ,determine the force of shearing 155089.2 MPa

Shearing Problem A round disk of 150-mm diameter is to be blanked from a strip of 2-mm, half hard cold rolled steel whose shear strength is 310Mpa. Determine Diameters of Punch and Die Blanking Force Ans: c-0.15mm, Punch-149.52, L-471.2mm F-292020 M Pa

Sheet Metal Bending Straining sheetmetal around a straight axis to take a permanent bend Figure 20.11 (a) Bending of sheet metal

Sheet Metal Bending Metal on inside of neutral plane is compressed, while metal on outside of neutral plane is stretched Figure 20.11 (b) both compression and tensile elongation of the metal occur in bending.

Bending Bending is the plastic deformation of metals about a linear axis with little or no change in the thickness The bend takes permanent set upon removal of stresses that caused it Forming- multiple bends are made with a single die Springback is the “unbending” that occurs after a metal has been deformed Figure 17-19 (Top) Nature of a bend in sheet metal showing tension on the outside and compression on the inside. (Bottom) The upper portion of the bend region, viewed from the side, shows how the center portion will thin more than the edges.

Types of Sheet Metal Bending V‑bending - performed with a V‑shaped die Edge bending - performed with a wiping die

V-Bending In V-bending the sheet metal blank is bent between a V-shaped punch and die For low production Performed on a press brake V-dies are simple and inexpensive Figure 20.12 (a) V‑bending;

Edge Bending Edge or wipe bending involves cantilever loading of the material. A pressure pad is used to apply a Force to hold the blank against the die, while the punch forces the work piece to yield and bend over the edge of the die For high production Pressure pad required Dies are more complicated and costly Figure 20.12 (b) edge bending.

Spring back in bending When the bending stress is removed at the end of the deformation process, elastic energy remains in the bent part causing it to partially recover to its original shape. In bending, this elastic recovery is called springback. It increases with decreasing the modulus of elasticity, E, and increasing the yield strength, Y, of a material. Springback is defined as the increase in included angle of the bent part relative to the included angle of the forming tool after the tool is removed. After springback: The bend angle will decrease (the included angle will increase) The bend radius will increase

Springback in bending Following is a schematic illustration of springback in bending: Manufacturing processes by S. Kalpakjian and S. Schmid αi: bend angle before springback αf: bend angle after springback Ri: bend radius before springback Rf: bend radius after springback Note: Ri and Rf are internal radii

Compensation for Springback Many ways can be used to compensate for springback. Two common ways are: Overbending Bottoming (coining) When overbending is used in V-bending (for example), the punch angle and radius are fabricated slightly smaller than the specified angle and raduis of the final part. This way the material can “springback” to the desired value. Bottoming involves squeezing the part at the end of the stroke, thus plastically deforming it in the bend region.

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