1. MT-284 MANUFACTURING PROCESSES
INSTRUCTOR:
SHAMRAIZ AHMAD
MS-Design and Manufacturing Engineering
Topic: Sheet Metal Working

2. 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)

3. 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

4. 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

5. 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.

6. 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

7. 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

8. 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

9. 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

10. 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)

11. 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

12. Sheet Metal Cutting
Figure 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;

13. Sheet Metal Cutting
Figure 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.

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

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

16. 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 (a) Blanking and (b) punching.

17. 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.

18. 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)

19. 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

20. 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

21. Design Clearance

22. 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

23. 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”.

24. 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.

25. 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) = ”.
Die opening diameter = 3.0”
Punch diameter = 3 – 2(0.0094) = 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

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

27. 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 , L-471.2mm F M Pa

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

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

30. 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 (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.

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

32. 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 (a) V‑bending;

33. 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 (b) edge bending.

34. 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

35. 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

36. 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.

37. Thank you