Sheet Metal Design

SHEET METAL DESIGN

Sheet metal • Sheet metal is metal formed by an industrial process into thin, flat pieces. • Sheet metal is one of the fundamental forms used in metalworking and it can be cut and bent into a variety of shapes. Countless everyday objects are fabricated from sheet metal •

Thicknesses can vary significantly; extremely thin sheets are considered foil or leaf and pieces thicker than 6 mm (0.25 in) are considered Plate

• Sheet metal is available in flat pieces or coiled strips. The coils are formed by running a continuous sheet of metal through a roll slitter.

• In most of the world, sheet metal thickness is consistently specified in millimeters. In some standards, the thickness of sheet metal is commonly specified by a traditional, non-linear measure known as its gauge the larger the gauge number, the thinner the metal. Commonly used steel sheet metal ranges from 30 gauge to about 7 gauge

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Gauge differs between ferrous (iron based) metals and nonferrous metals such as aluminum or copper; copper thickness, for example is measured in ounces, which represents the weight of copper contained in an area of one square foot. Parts manufactured from sheet metal must maintain a uniform thickness for ideal results

Hot rolling and cold rolling What is hot rolling and cold rolling? • Hot rolling is a mill process which involves rolling the steel at a high temperature (typically at a temperature over 1700° F), which is above the steel's recrystallization temperature. ... Hot rolled steel is used in situations where precise shapes and tolerances are not required • Cold Rolled Steel A rolling process at temperatures that are close to normal room temperature are used to create cold rolled steel. This increases the strength of the finished product through the use of strain hardening by as much as 20 percent (Cold rolling is a process by which the sheet metal or strip stock is introduced between rollers and then compressed and squeezed. The amount of strain introduced determines the hardness and other material properties of the finished product)

• As one of the most widely used steel metals, carbon steel is highly malleable and comes in a range of carbon content levels. ... Cold rolled steel is more brittle than hot rolled and also causes more concern with forming operations. Hot rolled is used for forming steel and is less expensive than cold rolled finishes • The difference between CRS and 304 stainless steel is that CRS is a process and 304 is an alloy. Steel, hot or cold rolled, will rust and corrode. It is used in applications where that is not a consideration. Stainless steel is an alloy with chromium and nickel that prevents rust and corrosion

Example

Sheet metal forming process

Shearing

Shearing

Shearing

Shearing Operations

Shearing Operations

Operations Cutting Shearing by two sharp cutting edges. Plastic deformation penetration fracture

Shearing, Blanking and Punching Shearing - Cutting on sheet-metal with straight edges. Blanking - Cutting sheet-metal with a closed contour. Punching - Making holes on sheet-metal.

Sheet-metal Cutting c = at c = clearance a = allowance t = thickness Blanking: Blanking punch dia. = Db- 2c Blanking die dia. = Db

0.25 to 1.5

Punch Hole punch dia. = Db Hole die dia. = Db+ 2c

Sheet-metal Cutting

Small clearance Cutting force = StL S = Shear strength of sheet-metal t = Sheet-metal thickness L = Length of cutting edge or, Cutting force = 0.7TtL T = Ultimate tensile strength of sheet-metal

Large clearance

Sheet-metal Cutting Operations Cut-off Parting

Slotting

Perforating

Notching / Seminotching

Sheet-metal Cutting Operations Fine blanking close tolerances and smooth edges in one step.

Trimming - Cutting operation to remove excess metal Shaving - Shearing with very small clearance to obtain accurate dimensions., secondary or finishing operation.

Punches and Dies

Punches and Dies

Sheet Metal Characteristics and Formability

Sheet Metal Characteristics and Formability

Bending

V- bending

Edge - bending

Bending Analysis Spring back

Die-opening dimension

Bending Operations

Straight flanging

Hemming

Stretch flanging

Seaming

Shrink flanging

Curling

Bending Operations

Channel bending

Offset-bending

U-bending

Corrugating

air-bending

Tube forming

Bending Sheets, Plates, Tubes

SPRING BACK-

Miscellaneous Bending and Related Operations

Bending Sheets, Plates, Tubes

Bending Sheets, Plates, Tubes

Bend allowance

To know the BLANK length, BEND ALLOWANCE or BEND DEDUCTION should be calculated. BA/BD requires the location of neutral line, which is defined by the K factor! K- factor is a ratio that represents the location of the neutral sheet with Respect to the sheet thickness. K= t/T BA =  (R + KT) A/180

K-Factor depends mainly on : 1.material property 2.radius of the bend 3.Ambient temperature 4.Direction of the material grain 5.Method of bending. Reverse engineering is the most generic way to find out the KFactor Bend Deduction = Length X + Length Y – Total Flat Length Outside Setback = (Tan(Bend Angle/2)) * (thickness + Bend Radius) Bend Allowance = (2 * Outside Setback) – BendDeduction K-Factor = (-Bend Radius + (Bend Allowance /  * BendAngle/180)))/thickness

BA and BD

Bend allowance

Consider a sheet with a 20 mm thickness and a length of 300 mm as shown in Figure 1. We are going to review three bending scenarios with three different bending angles; 60, 90 and 120, and we will calculate K-Factor, Bend Allowance and Bend Deduction for them. The bending tool has a radius of 30 mm, which means that our Inside Bend Radius (R) is 30 mm.

90 Degrees Bend Angle

At the neutral axis

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we can calculate the Bend Allowance as follows:

We know that BA is the length of the arc on the neutral axis. The length of the arc for this scenario can be calculated as

Where R’ is the radius of the arc on the neutral axis. By inserting the Bend Allowance value in the above equation

Now if we subtract R from R’ we can find the distance of the neutral axis (t) from the inner face

From the K-Factor equation

Bending Angles Less Than 90 degrees

Then we have to calculate Leg Length 1 and Leg Length 2

Where R is the Inside bend radius which is equal to 30 mm in this example. We can calculate Leg Length 1 through a few simple equations

Now let’s calculate Leg Length 2

Now that we have both Leg Length 1 and 2 we can use the following equation again to calculate the Bend Allowance

To calculate R’ which is the radius of the arc on the neutral axis we can use the following equation

A is the bending angle in the above equation so

To calculate the neutral axis distance from the inner face (t) we can subtract inside bend radius from R’

And by having t and the sheet thickness (T) we can calculate the KFactor as follow

Bending Angles Greater Than 90 degrees

Next we calculate Leg Length 2

Now we can calculate the Bending Allowance:

By having BA we can now calculate K-Factor

Bend Deduction Calculation Bend Deduction can be calculated using the following equation:

Where OSSB is the outside setback. OSSB is defined as illustrated in figure 5 for different bending angles and can be calculated using the equation below:

Where A is the bending angle. T is the sheet thickness and R is the bending radius

Bend relief • What is bend relief in sheet metal? • When sheet metal makes a transition from a bend to a flat surface, or to another bend, it tends to rip and tear. To eliminate this, a bend relief is added so the edge of the sheet metal is perpendicular to the bend. In general, a minimum bend relief is equal to the material thickness plus the inside bend radius

Corner relief

Embossing What is embossing in sheet metal? Embossing is a metal forming process for producing raised or sunken designs or relief in sheet material by means of matched male and female roller dies, theoretically with no change in metal thickness, or by passing sheet or a strip of metal between rolls of the desired pattern. Method of creating raised logos or characters on paper without ink. In this process, two metal dies are used; one with raised logo or characters and another with matching but recessed logo or characters. When a sheet of paper is pressed between these dies, blind embossing occurs. See also heat embossing.

• TO INCREASE THE STRUCTURAL RIGIDITY OF THE PANEL AND STIFFNESS

coining What is coining metal? Coining is a form of precision stamping in which a work piece is subjected to a sufficiently high stress to induce plastic flow on the surface of the material. ...Coining is used to manufacture parts for all industries and is commonly used when high relief or very fine features are required.

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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.

lancing What is lancing in a sheet metal? Lancing is a piercing operation in which the work piece is sheared and bent with one strike of the die. A key part of this process is that there is not reduction of material, only a modification in its geometry. This operation is used to make tabs, vents, and louvers. Sheet metal is simply metal formed into thin and flat pieces. It is one of the fundamental forms used in metalworking, and can be cut and bent into a variety of different shapes. ... Shearing processes -- processes which apply shearing forces to cut, fracture, or separate the material.

louvering

Lancing and louvering

Deep Drawing

Design Tips •The Bend radius should, if possible, be kept the same for all Radii in the part to minimize set up changes.

•Ideal minimum inner radius should be at least 1 material thickness. •As a general rule, bending perpendicular to the rolling direction is easier than bending parallel to the rolling direction. Ex: Cold rolled steel sheets. •Minimum flange width = 4 * Thickness + BR. •Slots and holes distances from the bend min D = 3* thickness+BR •Better to use oblong holes for mounting and take care of dimensional stack up. •Minimum Bend relief : SHEET THICKNESS + INNER BR.

Formability Tests for Sheet Metals

Formability Tests for Sheet Metals

Spinning

Forming

Explosive Forming

Magnetic Pulse Forming

Other Forming Processes

Manufacturing of Metal Honeycomb Structures

What is bend allowance in sheet metal? The amount of spring back is dependent on the material, and the type of forming. When sheet metal is bent, it stretches in length. The bend deduction is the amount the sheet metal will stretch when bent as measured from the outside edges of the bend. The bend radius refers to the inside radius

K- factor : What is meant by K factor in sheet metal? The K-Factor in sheet metal working is the ratio of the neutral axis to the material thickness. ... The K-Factor is used to calculate flat patterns because it is directly related to how much material is stretched during the bend. It's used to determine Bend Allowances and Bend Deductions ahead of the first piece.

Drawing c = Clearance Db = blank diameter Dp = Punch diameter Rd = die corner radius Rp = Punch corner radius F = drawing force Fh = holding force

Deep Drawing

Initial step

bending of edge

Thinning and drawing

straightening of side wall

final cup shape

Drawing Analysis Db DR   2.0 Dp

Drawing ratio, where Db = blank diameter Dp = punch diameter t = thickness of the starting blank Db  D p Reduction, r   0.5 Dp

Thickness to diameter ratio, t / Db 1% D Drawing force, F  D p tT ( b  0.7, )max. at 1/3 stroke. Dp Fh  0.015Y Db2  ( Dp  2.2t  2 Rd ) 2

  Holding force, Holding pressure may be set at 0.015 of the yield strength T = Tensile strength, Y = Yield strength, Rd = die corner radius

Drawing Analysis • Blank diameter can be calculated from the conservation of volume based on the final volume of the part. • If the limits on the drawing ratio, reduction and thickness-to-diameter ratio are exceeded, the blank must be drawn in steps or having annealing between the steps. • Process optimization: – – – –

Punch and die corner radii friction depth of draw (per step) material characteristics

Other Drawing Operations Redrawing Reverse drawing – r, 40%-45% first draw – r, 30% second draw – r, 16% third draw

Drawing without blank holder, Db  D p  5t

Drawing Defects

a) Wrinkling in flange - small holding force b) Wrinkling in the wall - insufficient holding force, wrinkling initially occurring on the flange. c) Tearing - high stress, sharp die radius d) Earing - anisotropy of the material e) Surface scratches - Die or punch not having a smooth surface, insufficient lubrication

Other Sheet-metal Operations Ironing - squeezing and drawing in conjunction Corning & Embossing - to produce surface details Lancing - combination of cutting and forming

Other Sheet-metal Operations Rubber forming processes: Guerin process - low cost, small volume, pressure up 10 MPa. Hydroforming - Higher pressure, up to 100 MPa.

Dies

Components of a punch and die for blanking operation

Dies Progressive dies and final part

Press crankshaft

Knuckle joint

eccentric

Press are driven mechanically or hydraulically.

Stretch Forming

Stretching and forming with a die at the same time.

Roll Forming Similar to the the rolling process but working on sheet-metal or tubes.

Spinning Conventional spinning

Shear spinning - thinning and bending occur at the same time.

Spinning

Tube spinning - similar to shear spinning but working on a tube.

High Energy Forming

Explosive forming Electrohydraulic forming Electromagnetic forming

Tube Bending

Stretch bending Drawing bending Compression bending

R/D  3.0 with mandrel R/D  1.5 without mandrel

Bending Analysis

BA = 2  A( R + Kbat ) / 360 where BA = bending allowance, A = bend angle, R = bend radius, t = stock thickness, Kba = stretch factor. For R< 2t, Kba= 0.33 ; for R 2t, Kba= 0.5. Spring back,

A  Ab SB  Ab of sheet-metal part where A’ = include angle

A’b = include angle of bending tool w = width of sheet-metal Bending force D = die opening dimension 2 K bf Twt Kbf = 1.33 V-bending F D Kbf = 0.33 edge bending

Welding of sheets