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1. UNIT-IV SHEET METAL OPERATION Sheet Metal Operation 1. Blanking 2. Piercing 3. Coining 4. Deep Drawing 5. Spinning 6. Stretch Forming 7. Bending 8. Types of presses & Press tools

2. Blanking Blanking is the shearing operation which shears the metal to closed contours. The removed portion which is detached from the metal strip is called as blank and is used for further operations. The remaining metal goes to the scrap. The basic tool required for blanking along with metal working press are punch and die. The blanking operation is performed at one stroke of the press which makes the punch to penetrate into the sheet metal and produces the required blank.

3. Piercing Piercing sometimes also called as punching, is used for making holes in a sheet. Piercing is similar to blanking, except that the punched out portion coming out of the die is discharged as scrap in piercing. In piercing, a pointed punch is forced through the sheet metal to obtain a hole.

4. Piercing or Punching : Clearance Clearance C is the distance between the punch and die. The correct clearance depends on sheet metal type and thickness (t) and temper of the work piece material. Harder material requires larger clearance than soft materials. C = A t, where A is clearance allowance and t is sheet thickness. The clearance allowance A is taken to be 0.075 for most of the steels and 0.045 for Aluminium alloys. The clearance may also determined with the help of the following equation : C = 0.0032 t mm , where τ = Shear strength in N/mm 2 If the clearance is not set correctly, either an excessive force or an oversized burr can occur: Effect of clearance : clearance too small causes less than optimal fracture and excessive forces clearance too large causes oversized burr  

5. The clearance is applied in the following manner :   i ) When the hole has to be held to size i.e , the hole in the sheet metal is to be accurate ( punching operation), and slug is to be discarded, the punch is made to the size of the hole and the die opening size is obtained by adding clearance to the punch size. ii) In blanking operation, where the slug or blank is the desired part and has to be held to size, the die opening size equals the blank size and the punch size is obtained by subtracting the clearance from the die opening size.

6. Die diameter is enlarged with clearance C in punching. In blanking, the punch diameter is decreased to account for clearance. D is the nominal size of the final product. Die-punch clearance is very critical for blanking and punching, as it governs the kind of finish obtained on the final part. Typical recommended clearances for the operations are given in table below : S. No. Material Radial clearance as percentage of thickness General Good Finish 1 Aluminium 10-12 4-6 2 Low carbon Steel 9-11 5-7 3 Stainless Steel 10-12 4-5 4 Brass 7-10 2-5 Radial clearance for blanking and punching :

7. Punch diameter has to be smaller than the die hole. The clearance between die and punch is based on the type of process. For blanking operation for obtaining a blank of diameter D b the clearance is given on the punch. Diameters of punch and die are given by: Diameter of punch = D b - 2C and die diameter = D b , where c is radial clearance. For punching operation on a sheet with a hole of diameter D h , The die hole diameter = D h +2C The punch diameter = D h An angular clearance must be provided for the die hole to allow parts to drop through it. In general, the clearance can be expressed as: For easy removal of slug during punching or blanking, a small angular clearance of 1 degree to 1.5 degree is provided in the die hole.

8. Cutting forces : For circular blank of diameter D mm and thickness t mm, the cutting force will be given as F = τ x t x L = τ x t x π D where τ is the shear strength of material, L is the length of the cut edge (Perimeter) , t = Sheet thickness For approximate solutions, F Max . = 0.7 x σ t x t x L ; Where σ t is the Ultimate tensile strength For Rectangular blank with length L and width b, it is F Max . = 2 ( L+b ) x τ x t

9. Energy in press work :   Energy in press work or the work done to make a cut is given by E = F Max x Punch Travel = F Max x k x t Where K = Percentage of penetration required to cause rupture. To allow for energy lost in machine friction and in pushing slugs through the die etc , the above equation gets modified as,   E = F Max x k x t x c Where the factor c accounts for the amount of extra energy required. This depends upon the circumstances in each case. For general purpose, a factor of 1.16 is recommended.  

10. BENDING Bending is the metal working process by which a straight length is transformed into a curved length. Bending is a non cutting operation performed on a press. Bending requires a press tool to deform a component permanently to a required shape. Bending is one common sheet metal forming operation used not only to form shapes like channels, tanks, trays and brackets. During bending operation, the outer surface of the material is in tension and the inside surface is in compression.  Somewhere, in the cross section, there is a plane which separates the tension and compression zones. This plane is parallel to the surface around which the sheet is bending, and is called neutral axis. The position of neutral axis depends on the radius and angle of bend. The strain in the bent material increases with decreasing radius of curvature. The stretching of the bend causes the neutral axis of the section to move towards the inner surface. In most of the cases, the distance of the neutral axis from the inside of the bend is 0.3 t to 0.5 t where “t” is the thickness of the part.

11. Bending Methods : V- Bending : In V- Bending, a wedge shaped punch forces the metal sheet or strip into a wedge shaped die cavity. The bend angle may be acute , 90 or obtuse. As the punch descends, the contact forces at the die corner produce a sufficiently large bending moment at the punch corner to cause the necessary deformation. Edge Bending : In edge bending a flat punch forces the stock against the vertical face of the die. The bend axis is parallel to the edge of the die and the stock is subjected to cantilever loading. To prevent the movement of the stock during bending, it is held down by a pressure pad before the punch contacts it.

12. MINIMUM BEND RADIUS           As the ratio of the bend radius to the thickness of sheet (R / t) decreases, the tensile strain on the outer fibers of sheet increases. If ( R / t ) decreases beyond a certain limit, cracks start appearing on the surface of material. This limit is called Minimum Bend Radius for the material. Minimum bend radius is generally expressed in terms of the thickness of material, such as 2t, 3t, 4t, etc.  Bending Force :           There are two general types of bending : V- bending (V- Die) and Edge bending (Wiping Die). V-Die ( V- bending) is used extensively in brake die operations and stamping die operations. The bending force can be estimated from the following simple relation. F = k.Y.L.t 2  / D where F is bending force, Y is the yield stress of the material, L is the bend length ( bend allowance ), t is the sheet thickness, D is the die opening and k is a constant whose value can be taken as 1.3 for a V-Die (V- Bending) and 0.3 for wiping die ( Edge bending) .           Bending force varies as the punch progresses through the bending operation. The force is zero in the beginning. It rises and reaches the maximum value as the punch progresses and reaches the bottom of the stroke.

13. BEND ALLOWANCE           When a sheet is bent, the material near the outside of the bend is under tension, and that near the bend radius is under compression. It is the length of the neutral axis in the bend. This determines the blank length needed for a bent part. It can be approximately estimated from the relation L b  = a ( R + k t ) where, L b  = Bend allowance in mm a = Bend angle in radians; R = Inside radius of the bend in mm t = Thickness of sheet in mm, and k = constant, whose value may be taken as 1/3 when R < 2t, and as 1/2 when R > 2t. Example A 15 mm wide and 3 mm thick steel sheet is required to be bent at 60  at bend radius 10 mm. Determine the bend allowance. Solution. Here, Bend radius R = 10 mm      Sheet thickness t = 3 mm     a = { (π x 60) / 180 } Radians Since R > 2t, k = 0.5 Bend allowance = { (π x 60) / 180 } [ 10 + 0.5 x 3] = 12.04 mm

14. Example:1 A 450 mm long and 3 mm thick piece of carbon steel sheet is required to be bent at 90  using a V – die. Assume the yield stress of the material as 550 MPa and the die opening as 10 times the material thickness. Estimate the force required for the operation. Solution : Here, Y = 550 MPa  L = 450 mm   t = 3 mm  k = 1.3 (for V – die) D = 30 mm Bending force P = k.Y.L.t 2  / D   = 1.3 x 550 x 450 x (3) 2  / 30   = 96.5 KN

15. Coining Coining is a closed die squeezing operation except that the flow of the metal occurs only at the top layers and not the entire volume. In coining die and punch of the desired design on both sides of the final object is engraved. First the blank is placed in the die and then the punch descends on to the blank under high pressure (of the order of 1600 MPa.). Under this high pressure the metal flows into the crevices of the engraved design. Very high pressure is required for coining in order to reproduce very fine details of the design . The difference between coining and embossing is that the same design is created on both sides of the work piece in embossing (one side depressed and the other raised), whereas in coining operation, a different design is created on each side of work piece . Coining is employed for producing coins, medals and for designs on decorative pieces.

16. Spinning Spinning is one of the oldest method of sheet metal forming. Parts that have circular cross sections can be made by spinning from sheet metal. When dies are uneconomical to make for forming operation because of small number of parts to be made, spinning is used. In this method, a punch (form or chuck) is made and then metal is formed over it by the spinning operation. The form of chuck made of wood or metal is mounted on the lathe head stock and the live center of lathe is used to hold the metal blank in place. A smooth , hardened, rotating or stationary tool is held by the operator and is pressed against the blank to progressively bend the work piece to confirm to the chuck or mandrel. The mode of deformation of the metal during spinning is a mixture of bending and stretching, making the process most suitable for shaping of hallow parts from ductile metals and alloys . The thickness of the blank is upto 6 mm for soft non ferrous metals and upto 5mm for low carbon steel. Spinning speeds vary from 1.5 m/s for small parts to 25 m/s for large diameter parts. Spinning has been used to produce parts more than 3.6 m in diameter. Before spinning, a suitable lubricant should be applied to the surface of the metal. Soap, bees wax, linseed oil are commonly used for this purpose. The degree of deformation obtainable by spinning depends upon the shape of the finished part, the material, the lubricant and particularly the skill of the operator.

17. Advantages : The cost of tooling is very less. The form block may be of plaster, wood and metal. Low equipment cost. The equipment consists of a speed lathe to the chuck of which the form block is attached. The blank is held between the form block and the tailstock. 3. It is suitable for manufacturing large parts (up to 3.6 m or more) very economically. Limitations : It is suitable for only axially symmetric parts. It is suitable for small quantity production. The method depends to a large extent on the skill of the operator. Finished parts are not always uniform and close tolerance can’t be obtained . Application : Reflectors, kitchenware, bells, musical instruments, light fixtures, funnels, radar dishes and rocket motor cases.  

18. DRAWING           It is a process of cold forming a flat blank of sheet metal into a hollow vessel without much wrinkling, trimming, or fracturing. The process involves forcing the sheet metal blank into a die cavity with a punch. The punch exerts sufficient force and the metal is drawn over the edge of the die opening and into the die. In forming a cup, however, the metal goes completely into the die . The metal being drawn must possess a combination of ductility and strength so that it does not rupture in the critical area (where the metal blends from the punch face to the vertical portion of the punch). The metal in this area is subjected to stress that occurs when the metal is pulled from the flat blank into the die.

19. OPERATION:   A setup similar to that used for blanking is used for drawing with the difference that the punch and die are given necessary rounding at the corners to permit smooth flow of metal during drawing. The blank of appropriate dimensions is place within the guides on the die plate. The punch descends slowly on the blank and metal is drawn into the die and the blank is formed into the shape of cup as punch reaches the bottom of the die. When the cup reaches the counter – bored portion of the die, the top edge of the cup formed around the punch expands a bit due to the spring back  . On the return stroke of the punch, the cup is stripped off the punch by this counter – bored portion.           The term shallow drawing   is used when the height of cup formed is less than half its diameter. When drawing deeper cup (height greater that ½ diameter) the chances of excessive wrinkle formation at the edges of blank increases. To prevent this, a blank holder is normally provided. As the drawing process proceeds the blank holder stops the blank from increasing in thickness beyond a limit and allows the metal to flow radially. The limiting thickness is controlled by the gap between the die and the blank holder, or by the spring pressure in the case of a spring loaded blank holder.           Some lubricant is generally used over the face of the blank to reduce friction and hence drawing load is decreased.

20. Deep Drawing : Deep drawing of sheet metal is performed with punch and die. The punch is the desired shape of the base of the part, once drawn. The die cavity matches the punch and is little wider to allow for its passage, as well as clearance. This step is similar to sheet metal cutting operations. As in cutting, clearance is the lateral distance between the die edge and the punch edge. The blank is placed over the die opening. A blank holder, that surround the punch, applies pressure to the entire surface of the blank, holding the sheet metal work flat against the die. The punch travels towards the blank. After contacting the work, the punch forces the sheet metal into the die cavity, forming its shape. Equipment for sheet metal deep drawing processes would involve a double action, one for the blank holder and for the punch. Both mechanical and hydraulic presses are used in manufacturing industry. The hydraulic press can control the blank holder and punch actions separately, but the mechanical press is faster. Punch and die materials, for the deep drawing of sheet metal, are usually tool steels and iron. However, the range of materials for punch and die can span from plastics to carbides. Parts are usually drawn at speeds of 4 to 12 inches per second.

21. Redrawing: In deep drawing, the percentage reduction in one draw is defined as % reduction = (D - d)/ D x100 D/d = 1.6 to 2.3 To make tall cups of smaller diameter (Such as cartridge cases and closed end tubes), it is necessary to use successive drawing operations. Reducing a drawn cup to a smaller diameter and increased height is known as Redrawing. Redrawing is extensively used for food containers, fountain-pen caps, oil filter housings and shock absorber pistons etc. Clearance Clearance C is the distance between the punch and die and is about 10% greater than the stock thickness: C = 1.1 x t