Sheet Metalworking

Home , Drawing (manufacturing), Electrohydraulic forming, Explosive forming, Punching

1. Cutting operations 2. Bending operations 3. Drawing 4. Other sheet-metal-forming operations 5. Dies and presses for sheet-metal processes 6. Sheet-metal operations not performed on presses

Cutting and forming operations performed on relatively thin sheets of metal § 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

§ 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

§ High strength § Good dimensional accuracy § Good surface finish § Relatively low cost § Economical mass production for large quantities

§ R2 § Punch-and-die - tooling to perform cutting, bending, and drawing § Stamping press - machine tool that performs most sheet metal operations § Stampings - sheet metal products

1. Cutting § Shearing to separate large sheets § Blanking to cut part perimeters out of sheet metal § Punching to make holes in sheet metal 2. Bending § Straining sheet around a straight axis 3. Drawing § Forming of sheet into convex or concave shapes

§ (1) Just before punch contacts work; (2) punch pushes into work, causing plastic deformation; (3) punch penetrates into work causing a smooth cut surface; and (4) fracture is initiated at opposing cutting edges to separate the sheet

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

§ (a) Side view of the operation; (b) front view of power shears equipped with inclined upper cutting blade

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

Distance between punch cutting edge and die cutting edge § 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

§ 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

Metal group a 1100S and 5052S aluminum alloys, all 0.045 tempers 2024ST and 6061ST aluminum alloys; brass, 0.060 soft cold rolled steel, soft stainless steel

Cold rolled steel, half hard; stainless steel, 0.075 half hard and full hard

§ For a round blank of diameter Db:

§ Blanking punch diameter = Db - 2c

§ Blanking die diameter = Db where c = clearance

§ For a round hole of diameter Dh:

§ Hole punch diameter = Dh

§ Hole die diameter = Dh + 2c where c = clearance

§ Die size determines

blank size Db § Punch size determines hole

size Dh § c = clearance

§ Purpose: allows slug or blank to drop through die § Typical values: 0.25° to 1.5° on each side

§ Important for determining press size (tonnage) F = S t L where S = shear strength of metal; t = stock thickness, and L = length of cut edge

R4. cutoff and parting ? Fig. 14.7 A cutoff operation separates parts from a strip by shearing one edge of each part in sequence. A parting operation cuts a slug between adjacent parts in the strip.

R5. notching and seminotching ? Fig.14.8 A notching operation cuts out a portion of the sheet metal from the side of the sheet or strip, while a seminotching operation removes a portion of the sheet metal from the interior of the sheet or strip.

§ (a) Straining of sheet metal around a straight axis to take a permanent bend § (b) Metal on inside of neutral plane is compressed, while metal on outside of neutral plane is stretched

§ V-bending - performed with a V-shaped die § Edge bending - performed with a wiping die

§ (1) Before bending § (2) After bending § Application notes: § Low production § Performed on a press brake § V-dies are simple and inexpensive

§ (1) Before bending § (2) After bending § Application notes: § High production § Pressure pad required § Dies are more complicated and costly

§ If bend radius is small relative to stock thickness, metal tends to stretch during bending § Important to estimate amount of stretching, so final part length = specified dimension § Problem: to determine the length of neutral axis of the part before bending

α A = 2π (R + K t ) b 360 ba where Ab = bend allowance; a = bend angle; R= bend radius; t = stock thickness; and Kba is factor to estimate stretching

§ If R < 2t, Kba = 0.33

§ If R ³ 2t, Kba = 0.50

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

§ Springback in bending is seen as a decrease in bend angle and an increase in bend radius: (1) during bending, work is

forced to take radius Rt and angle ab' of the bending tool, (2) after punch is removed, work springs back to R and a‘

§ Maximum bending force estimated as follows: K TSwt 2 F = bf D 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

§ Die opening dimension D for (a) V-die, (b) wiping die

Sheet metal forming to make cup-shaped, box-shaped, or other complex-curved, hollow-shaped parts. R9

§ 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)

§ Drawing of cup-shaped part: (1) before punch contacts work, (2) near end of stroke § Starting blank and drawn part shown in lower views

§ 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

§ Drawing ratio § Reduction § Thickness-to-diameter ratio

§ Most easily defined for cylindrical shape (e.g., cup) D DR = b Dp

where Db = blank diameter; and Dp = punch diameter

§ Indicates severity of a given drawing operation § Upper limit: DR £ 2.0

§ Defined for cylindrical shape: D - D r = b p Db § Value of r should be less than 0.50

Thickness of starting blank divided by blank diameter

§ Desirable for t/Db ratio to be greater than 1%

§ As t/Db decreases, tendency for wrinkling increases

(a)Wrinkling in the flange (b)Winkling in the wall (c) Tearing (d)Earing (e)Surface scratches Blank Size Determination

§ For final dimensions of drawn shape to be correct,

starting blank diameter Db must be right

§ Solve for Db by setting starting sheet metal blank volume = final product volume § To facilitate calculation, assume negligible thinning of part wall

Fabrication Results Shapes other than Cylindrical Cups

§ Each of the following shapes presents its own unique technical problems in drawing § Square or rectangular boxes (as in sinks) § Stepped cups § Cones § Cups with spherical rather than flat bases § Irregular curved forms (as in automobile body panels)

§ Other sheet metal forming operations performed on conventional presses can be classified as § Operations performed with metal tooling § Operations performed with flexible rubber tooling

§ Achieves thinning and elongation of wall in a drawn cup: (1) start of process; (2) during process

§ Creates indentations in sheet, such as raised (or indented) lettering or strengthening ribs § (a) Punch and die configuration during pressing; (b) finished part with embossed ribs

§ (1) before and (2) after

§ Low tooling cost § Form block can be made of wood, plastic, or other materials that are easy to shape § The same rubber pad can be used with different form blocks § Process attractive in small quantity production

(a) 지속적인 확장/압축 제품 (b) 부분적인 확장/압축 제품 (c) 조정만을 적용하는 제품 Dies for Sheet Metal Processes

Most pressworking operations are performed with conventional punch-and-die tooling § Custom-designed for the particular part § The term stamping die is sometimes used for high production dies

§ Components of a punch and die for a blanking operation

§ Components of a typical mechanical drive stamping press

What are the relative advantages and disadvantages of mechanical presses versus hydraulic presses in sheet metalworking? R16

§ Gap frame § Configuration of the letter C and often referred to as a C-frame

§ Straight-sided frame § Box-like construction for higher tonnage

§ Gap frame press for sheet metalworking (photo courtesy of E. W. Bliss Co.) § Capacity = 1350 kN (150 tons)

Characterized by wide bed (photo courtesy Strippit Inc.)

§ Computer numerical control turret press (photo courtesy of Strippet, Inc.)

§ Sheet metal parts produced on a turret press, showing variety of hole shapes possible (photo courtesy of Strippet Inc.)

§ Straight-sided frame press (photo courtesy of Greenerd Press & Machine Company, Inc.)

§ Stretch forming § Roll bending and forming § Spinning § High-energy-rate forming processes

§ Hydraulic presses - use a large piston and cylinder to drive the ram § Longer ram stroke than mechanical types § Suited to deep drawing § Slower than mechanical drives

§ Mechanical presses – convert rotation of motor to linear motion of ram § High forces at bottom of stroke § Suited to blanking and punching Stretch Forming R14

§ Sheet metal is stretched and simultaneously bent to achieve shape change

§ Large metal sheets and plates are formed into curved sections using rolls

§ Continuous bending process in which opposing rolls produce long sections of formed shapes from coil or strip stock

§ Metal forming process in which an axially symmetric part is gradually shaped over a rotating mandrel using a rounded tool or roller § (1) Setup at start of process; (2) during spinning; and (3) completion of process

§ Processes to form metals using large amounts of energy over a very short time § HERF processes include: § Explosive forming § Electrohydraulic forming § Electromagnetic forming

Use of explosive charge to form sheet (or plate) metal into a die cavity § Explosive charge causes a shock wave whose energy is transmitted to force part into cavity § Applications: large parts, typical of aerospace industry

§ (1) Setup, (2) explosive is detonated, and (3) shock wave forms part and plume escapes water surface

Sheet metal is deformed by mechanical force of an electromagnetic field induced in the workpart by an energized coil § Presently the most widely used HERF process § Applications: tubular parts

(1) Setup in which coil is inserted into tubular workpart surrounded by die; (2) formed part

©2012 John Wiley & Sons, Inc. M P Groover, Introduction to Manufacturing Processes 14.6 Determine the minimum tonnage press to perform the blanking and punching operation in Problem 14.3. The aluminum sheet metal has a tensile strength = 310 MPa, a strength coefficient of 350 MPa, and a strain-hardening exponent of 0.12. (a) Assume that blanking and punching occur simultaneously. (b) Assume the punches are staggered so that punching occurs first, then blanking

Solution:

(a) F = 0.7(TS)tL t =3.5 mm from Problem 14.3. L = 50p + 15p = 65p = 204.2 mm F = 0.7(310)(3.5)(204.2) = 155,100 N = 155,100/ 9.81 = 15. 8 tons 110% 능력을 갖는 프레스 17.4 tons => 18 ton press

(b) Longest length will determine the minimum tonnage press required. Punching length of cut, L= 15π, blanking length of cut, L=50π = 157.1 mm (use blanking) F = 0.7(310)(3.5)(157.1) = 119,300 N = 119,3000/ 9.81 = 12.161 tons 110% 능력을 갖는 프레스 13.4 tons => 14 ton press 14.8 A bending operation is to be performed on the part shown in Figure 14.13 except that the dimensions are changed as follows: stock thickness = 5.0 mm instead of 3.2 mm, inside bend radius = 8.0 mm instead of 4.75 mm, and included angle = 65° instead of 120°. Other dimensions are the same. Determine the blank size required.

Solution: a = 180 - a’ = 180 – 65 = 115°.

Ab = 2p(a/360)(R + Kbat) R/t = (8.0)/(5.00) = 1.6, which is less than 2.0; therefore, Kba = 0.333 Ab = 2p(115/360)(8.0 + 0.333 x 5.0) = 19.40 mm Dimensions of starting blank: w = 35 mm, L = 38 + 19.4 + 25 = 82.4 mm 14.13 A sheet-metal part 3.0 mm thick and 20.0 mm long is bent to an included angle = 60° and a bend radius = 7.5 mm in a V-die. The metal has a yield strength = 220 MPa and a tensile strength = 340 MPa. Compute the required force to bend the part, given that the die opening dimension = 15 mm.

Solution:

For V-bending, Kbf = 1.33.

2 2 F = Kbf(TS)wt /D = 1.33(340)(20)(3) /15 = 5426 N 14.19 A cup-drawing operation is performed in which the inside diameter = 80 mm and the height = 50 mm. Stock thickness = 3.0 mm, and starting blank diameter = 150 mm. Punch and die radii = 4 mm. Tensile strength = 400 MPa and yield strength = 180 MPa for this sheet metal. Determine (a) drawing ratio, (b) reduction, (c) drawing force, and (d) blankholder force.

Solution:

(a) DR = 150/80 = 1.875

(b) r = (Db - Dp)/Db = (150 – 80)/150 = 70/150 = 0.46

2 2 (d) Fh = 0.015Yp(Db - (Dp + 2.2t + 2Rd) )

2 2 2 2 Fh = 0.015(180)p(150 - (80 + 2.2 x 3 + 2 x 4) ) = 0.015(180)p(150 - 94.6 )

Fh = 114,942 N 14.22 A drawing operation is performed on 3.0 mm stock. The part is a cylindrical cup with height = 50 mm and inside diameter = 70 mm. Assume the corner radius on the punch is zero. (a)

Find the required starting blank size Db. (b) Is the drawing operation feasible?

Solution:

Use surface area computation, assuming thickness t remains constant. 2 2 2 Cup area = wall area + base area = pDph + pDp /4 = p(70)(50) + 0.25p(70) = 14,846 mm . 2 2 Blank area = pDb /4 = 0.7855Db 2 Setting blank area = cup area: 0.7855Db = 14,846 2 Db = 14,846/0.7855 = 18,900 Db = 137.48 mm

Test for feasibility: DR = Db/Dp = 137.48/70 = 1.964; t/Db = 3/137.48 = 0.0218 = 2.18%. These criteria values indicate that the operation is feasible; however, with a punch radius Rp = 0, this shape would be difficult to draw because the drawing punch would act on the metal like a blanking punch.