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Sheet Metal Forming Lecture 6 EMU.

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Presentation on theme: "Sheet Metal Forming Lecture 6 EMU."— Presentation transcript:

1 Sheet Metal Forming Lecture 6 EMU

2 SHEET METAL WORKING Cutting Operations Bending Operations Drawing
Other Sheet Metal Forming Operations EMU

3 Sheet Metalworking Defined
Cutting and forming operations performed on relatively thin sheets of metal Thickness of sheet metal = 0.4 mm (1/64in) to 6mm (1/4 in) Thickness of plate stock > 6 mm Operations usually performed as cold working EMU

4 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 EMU

5 Advantages of Sheet Metal Parts
High strength Good dimensional accuracy Good surface finish Relatively low cost Economical mass production for large quantities EMU

6 Sheet Metalworking Terminology
Punch‑and‑die - tooling to perform cutting, bending, and drawing Stamping press - machine tool that performs most sheet metal operations Stampings - sheet metal products made by press machine EMU

7 Basic Types of Sheet Metal Processes
Cutting Shearing to separate large sheets Blanking to cut part perimeters out of sheet metal Punching/ Piercing to make holes in sheet metal Bending Straining sheet around a straight axis Drawing Forming of sheet into convex or concave shapes Blanking and piercing/punching are shearing processes in which a punch and die are used to modify webs. The tooling and processes are the same between the two, only the terminology is different: in blanking the punched out piece is used and called a blank; in piercing the punched out piece is scrap EMU

8 Shearing, Blanking, and Punching
Three principal operations in press working that cut sheet metal: Shearing Blanking Punching Piercing In blanking the punched out piece is used and called a blank; in piercing the punched out piece is scrap. Punching is like when Mike Tyson hits you. Piercing is when he bites you on the ear. A slug (the material punched out) is produced in punching operations but not in piercing work EMU

9 Sheet Metal Cutting - Shearing
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; EMU

10 Sheet Metal Cutting - Shearing
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. EMU

11 Shearing Sheet metal cutting operation along a straight line between two cutting edges Typically used to cut large sheets Shearing operation: (a) side view of the shearing operation; (b) front view of power shears equipped with inclined upper cutting blade. Engagement of entire blade into cutting need higher forces. Therefore, inclined blade is used to reduce force and to improve cut- edge. Engagement of entire blade into cutting need higher forces. Therefore, inclined blade is used to reduce force. EMU

12 Shearing Shearing is a process for cutting sheet metal to size out of a larger stock such as roll stock. Shears are used as the preliminary step in preparing stock for stamping processes, or smaller blanks for CNC presses The shearing process produces a shear edge burr, which can be minimized to less than 10% of the material thickness. The burr is a function of clearance between the punch and the die, and the sharpness of the punch and the die. Burr: Metals particles produced during shearing operation EMU

13 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 (a) Blanking and (b) punching. EMU

14 Slug is cut and bur is minimum
Punching and Piercing …. …….. Piercing tool Punching tool Bur 3. … This is not to say the punched material will not have a burr, but the pierced holes will have a significantly more pronounced bur Punching Piercing Slug is cut and bur is minimum No slug is cut, only bur EMU

15 Punching Punching is a metal fabricating process that removes a scrap slug from the metal workpiece each time a punch enters the punching die. This process leaves a hole in the metal workpiece EMU

16 Punching is an operation of cutting holes into a sheet blank
Punching: operation 1. Punch, made of hardened steel, is forced through a work-piece. 2. The punch cuts the metal and separates it in the form of scrap 3. The hole size depends on the punch size Characteristics: Ability to produce economical holes in both strip and sheet metal during medium or high production processes. The ability to produce holes of varying shapes - quickly Punching is an operation of cutting holes into a sheet blank The illustration that follows shows a few common punch and die configurations and the workpieces that would be formed by this combination. Multiple punches can be used together to produce a complete part with just one stroke of the press EMU

17 TiN coated tool steel punches
Punch Tools TiN coated tool steel punches To reduce punch wear To increase punch life To increase dimensional accuracy of holes EMU

18 Other shearing processes
EMU Close tolerances and low v

19 Punch and Die Sizes Die size determines blank size Db;
Punch size determines hole size Dh.; c = clearance EMU

20 Clearance in Sheet Metal Cutting
Distance between punch cutting edge and die cutting edge Typical values range between 6% and 15% of stock thickness If clearance is too small, fracture lines pass each other, causing double buffing and larger force If too large, metal is pinched and bent between cutting edges and excessive burr results Buffing is the plastic deformation of a surface due to sliding contact with another object. EMU

21 Clearance in Sheet Metal Cutting
Recommended clearance is calculated by: c = at where c = clearance; a = allowance; and t = stock thickness Allowance a is determined according to type of metal Low “c” for soft materials High “c” for hard materials Typical “a” values for metals range from 0.04 to 0.09 EMU

22 Punch and Die Sizes - For a round blank of diameter, Db
Blanking (Blank diameter is controlled) - For a round blank of diameter, Db Diameter of die = blank diameter ( Db) Diameter of punch = Db - 2c where c = clearance Punching (Hole diameter is controlled) - For a round hole of diameter , Dh Diameter of punch = hole diameter (Dh ) Diameter of die = Dh + 2c In blanking, u give the required diameter of the blank to the DIE, while in punching, u give the required diameter of the hole to the PUNCH EMU

23 Angular Clearance Purpose: allows slug or blank to drop through die
Typical values: 0.25 to 1.5 on each side If u give angle to the full depth of the die, the die size will increase whenever u sharp the die for adjusting the angle EMU

24 Cutting Forces Important for determining press size (tonnage)
F = S t L where S = shear strength of the metal; t = stock thickness, L = length of cut edge (contact length) = 2*3.14* R L is basically perimeter of blank or hole being cut The above formula is based on fact that entire punch face is engaged in cutting. If angled punched is used, cutting force will reduce. EMU

25 In blanking. Die size = blank size
Example Problem C= At In blanking. Die size = blank size Punch size = die size - 2C EMU

26 Example F = S t L EMU

27 Sheet Metal Bending Straining sheet metal around a straight axis to take a permanent bend Bending of sheet metal EMU

28 Sheet Metal Bending Metal below the neutral axis is compressed, while metal above the neutral axis is stretched Metal on neutral axis neither stretched nor compressed The material is stressed beyond the yield strength but below the ultimate tensile strength. The surface area of the material does not change much., why?? Bending usually refers to deformation about one axis EMU

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

30 V-Bending For low production Performed on a brake press
V-dies are simple and inexpensive 2. A press brake, also known as a brake press or just brake EMU

31 Edge Bending For high production Pressure pad required
Dies are more complicated and costly If u move the die in (1) towards right what is gonna happen -> Shearing will occur instead of bending EMU

32 Stretching during Bending
If bend radius is small relative to stock thickness, metal tends to stretch during bending Important to estimate amount of stretching, so that final part length can be obtained as specified dimension EMU

33 Bend Allowance Formula
where Ab = bend allowance;  = bend angle; R= bend radius; t = stock thickness; and Kba is factor to estimate stretching If R < 2t, Kba = 0.33 If R  2t, Kba = 0.50 EMU

34 Springback Increase in included angle of bent part relative to included angle of forming tool after tool is removed Reason for spring-back: When bending pressure is removed, elastic energy remains in bent part, causing it to recover partially toward its original shape EMU

35 Spring back (SB) SB= (α’-α’b)/α’b
Springback in bending is seen as a decrease in bend angle and an increase in bend radius: (1) during bending, the work is forced to take radius Rb and included angle b' of the bending tool, (2) after punch is removed, the work springs back to radius R and angle ‘. EMU

36 Bending Force Maximum bending force estimated as follows:
where F = bending force; TS = tensile strength of sheet metal; w = part width in direction of bend axis; and t = stock thickness. For V- bending, Kbf = 1.33; for edge bending, Kbf = 0.33; D is opening width of a V-die or wiping die EMU

37 Die opening dimension D: (a) V‑die, (b) wiping die.

38 Drawing Sheet metal forming to make cup‑shaped, box‑shaped, or other complex‑curved, hollow‑shaped parts Sheet metal blank is positioned over die cavity and then punch pushes metal into opening Products: beverage cans, ammunition shells, automobile body panels Also known as deep drawing (to distinguish it from wire and bar drawing) EMU

39 Drawing Difference between wire drawing & deep drawing?
(a) Drawing of cup‑shaped part: (1) before punch contacts work, (2) near end of stroke; (b) work-part: (1) starting blank, (2) drawn part. EMU

40 Clearance in Drawing where t = stock thickness
Sides of punch and die separated by a clearance c given by: c = 1.1 t where t = stock thickness In other words, clearance is about 10% greater than stock thickness EMU

41 Mechanics of Drawing Fh: Holding force F: Punch force
Straightening the bent sheet, stretching Bending at die and punch radius Fh: Holding force F: Punch force Wall thickness variation: yes Wall thinning maximum at bottom corner of cup (max: 25%) EMU

42 Measures of Drawing Severity
Drawing Ratio: Db/Dp (Depends on Punch & Die corner radii, Friction conditions, Depth of draw and Material properties) Optimal value: 2. Greater the ratio, more severer the operation Reduction: r= (Db-Dp)/Db Optimal value should be less than 2 Thickness/Blank Diameter: (to/Db): It should be greater than 1%. Lower ratios lead to wrinkling of flange. EMU

43 Forces in Sheet Drawing
Drawing force required by punch: Blank holding force: Higher holding forces don’t allow material to be be pulled into the die, causing undue stretching of blank and tearing. Smaller holding forces cause wrinkling of flange EMU

44 Defects in Sheet Drawing
, Due to Small blank holding force Due to Small Punch force Due to high blank holding force Due to high anisotropy of material Due to friction and lack of lubrication at the sheet/punch interface EMU

45 Drawing Without Blank Holder & Ironing
Condition Large to/Db required Ironing EMU

46 Stretch Forming Sheet metal is stretched and simultaneously bent to achieve shape change Stretch forming: (1) start of process; (2) form die is pressed into the work with force Fdie, causing it to be stretched and bent over the form. F = stretching force. EMU

47 Force Required in Stretch Forming
where F = stretching force; L = length of sheet in direction perpendicular to stretching; t = instantaneous stock thickness; and Yf = flow stress of work metal Die force Fdie can be determined by balancing vertical force components EMU

48 1. How initial straight part of sheet is bent?
Roll Bending Large metal sheets and plates are formed into curved sections using rolls Plastic deformation but no significant material flow Give concepts of pre-bending, rolling cone. Plastic deformation but no material flow 1. How initial straight part of sheet is bent? 2. How cone is rolled? EMU

49 Deformation is in local area
Spinning Metal forming process in which an axially symmetric part is gradually shaped over a rotating mandrel using a rounded tool or roller Products: Automobile parts, Utensils, Aerospace parts Flow of material and material thining Deformation is in local area Conventional spinning: (1) setup at start of process; (2) during spinning; and (3) completion of process. EMU

50 Types of Spinning Three types: Conventional spinning Shear spinning
Tube spinning EMU

51 Conventional Spinning
1. Process is completed in several passes 2. Thinning occurs but not to great extent 3. Blank diameter reduces as process proceeds 4. Thin blanks are used Flow of material and material thining Conventional spinning: (1) setup at start of process; (2) during spinning; and (3) completion of process. EMU

52 Shear Spinning Deformation is performed through shearing of blank, parallel to rotation axis. Final thickness is reduced extensively: Thick blanks are used EMU

53 Tube Spinning Tube spinning is used to produce tubes and cylinders
As the roller moves in axial direction, the thickness reduces and the length increases. The roller touches the parts of the tubular blank, the deformation is local. EMU

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