sheet metal design

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Sheet Metal Design

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Pro e sheet metal design guide of sheet metal

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Page 1: Sheet Metal Design

Sheet Metal Design

Page 2: Sheet Metal Design

Press WorkingAlso called as Chipless Manufacturing and Cold Stamping. Machine Used – PressPress Comprises of :•Frame supporting Ram & Bed•Mechanism to operate ram•Ram with Punch•Die Block

Operations:1.Cutting Operations(work piece stressed beyond ultimate strength)

- Blanking, Punching, Notching, Perforating, Trimming, Shaving, Slitting & Lancing

2.Forming Operations (work piece stressed below ultimate strength)- Bending, Drawing, redrawing & Squeezing

Page 3: Sheet Metal Design

Press Working

CNC turret type Punching Machine

Hydraulic Shearing Machine

Page 4: Sheet Metal Design

Types of PressesClassified on following:

• Source of Power:

Mechanical - The energy of flywheel is utilized which is transmitted to the work-piece by gears, cranks, eccentrics or levers. The flywheel rotates freely on the crank shaft and is driven from an electric motor through gears or V belts.

Hydraulic - In hydraulic press, the ram is actuated by oil pressure on apiston in a cylinder.

Page 5: Sheet Metal Design

Types of Presses

Mechanical presses have following advantages over the hydraulic presses -(1) run faster (2) lower maintenance cost (3) lower capital cost

Advantages of hydraulic presses are: (1) More versatile and easier to operate (2) Tonnage adjustable from zero to maximum (3) Constant pressure can be maintained throughout the stroke. (4)Force and speed can be adjusted throughout the stroke (5) More powerful than mechanical presses(6) Safe as it will stop at a pressure setting

The main disadvantage of hydraulic press is that it is slower than a mechanical press.

A press is rated in tonnes of force, it is able to apply without undue strain. To keep the deflections small, it is a usual practice to choose a press rated 50 to 100% higher than the force required for an operation..

Page 6: Sheet Metal Design

Types of PressesClassified on following:

• Actuation Of Slide:

Rack & pinion

Cam

Eccentric

Crank

Screw

Knuckle

Toggle

Hydraulic

Page 7: Sheet Metal Design

Types of Presses

Page 8: Sheet Metal Design

Types of PressesClassified on following:

• Number of Slides:

Single Action

Double Action

Triple Action

Page 9: Sheet Metal Design

Types of Presses

Double Action Press Triple Action Press

Page 10: Sheet Metal Design

Types of PressesClassified on following:

• Frame Type:

Open

Closed

• Type of Work:

Punching

Blanking

Drawing

Etc.

Page 11: Sheet Metal Design

Types of Presses

Open FrameClosed Frame

Page 12: Sheet Metal Design

Press SelectionFactors to be considered while selecting a pressOverall work size, the stock thickness and material, kind of operation to be performed, power required and speed of operation.

For punching, blanking and trimming operations usually the crank or eccentric type mechanical press is used. This is due to their small working strokes and high production rates. In these operations, there is sudden release of load at the end of the cutting stroke. This sudden release of load is not advisable in hydraulicpresses. So, hydraulic presses are not preferred for these operations.If however these are inevitable, then some damping devices are incorporatedin the press design.

For coining and other squeezing operations, which require very large forces, knuckle joint mechanical press is ideally suited. Hydraulic presses, which are slower and more powerful, can also be used for these operations. Hydraulic presses are also better adapted to pressing, forming and operations, which are slower processes.

Page 13: Sheet Metal Design

Press WorkingDefinitions of the main components of the die and press :

Bed - The bed is the lower part of a press frame, that serves as a table to which a bolster plate is mounted.

Bolster plate - This is a thick plate secured to the press bed, which is used for locating and supporting the die assembly. It is usually 5 to 12.5 cm thick.

Die set - It is unit assembly which incorporates a lower and upper shoe, two or more guideposts and guidepost bushings.

Die - The die may be defined as the female part of a complete tool for producing work in a press. It is also referred to a complete tool consisting of a pair of mating members for producing work in a press. Die block, is a block or a plate which contains a die cavity.

Lower Shoe - The lower shoe of a die set is generally mounted on the bolster plate of a press. The die block is mounted on the lower shoe. Also, the guide posts are mounted in it.

Page 14: Sheet Metal Design

Press Working

Page 15: Sheet Metal Design

Press WorkingPunch - This is the male component of the die assembly, which is directly or indirectly moved by and fastened to the press ram or slide.

Upper Shoe - This is the upper part of the die set which contains guidepost bushings.

Back up plate - Back up plate or pressure plate is placed so that the intensity of pressure does not become excessive on punch hold or the plate distributes the pressure over a wide area and the intensity of pressure on the punch holder is reduced to avoid crushing.

Stripper - It is a plate which is used to strip the metal strip from a cutting or non-cutting punch or die. It may also guide the sheet.

Knockout - It is a mechanism, usually connected to and operated by the press ram, for freeing a work-piece from a die.Pitman - It is a connecting rod which is used to transmit motion from the main drive shaft to the press slide.

Shut Height - It is the distance from top of the bed to the bottom of the slide, with its stroke down and adjustment up.

Page 16: Sheet Metal Design

Cutting OperationsNotching: This is cutting operation by which metal pieces are cut from the edge of a sheet, strip or blank.Perforating: This is a process by which multiple holes which are very small and close together are cut in a flat work material.Trimming: This operation consists of cutting unwanted excess material from the periphery of a previously formed component.Shaving: Edges of a blanked part are generally rough, uneven and unsquare. Accurate dimensions of the part are obtained by removing a thin strip of metal along the edges. This operation is termed as Shaving.Slitting: It refers to the operation of making incomplete holes in a workpiece.Lancing: This is a cutting operation in which a hole is partially cut and then one side is bent down to form a sort of tab or louver. Since no metal is actually removed, there will be no scrap.Nibbling: The nibbling operation which is used for only small quantities of components, is designed for cutting out flat parts from sheet metal. The flat parts range from simple to complex contours. This operation is generally substituted for blanking. The part is usually moved and guided by hand as the continuously operating punch cuts away at the edge of the desired contour.

Page 17: Sheet Metal Design

Forming OperationsBending - In this operation, the material in the form of flat sheet or strip, is uniformly strained around a linear axis which lies in the neutral plane and perpendicular to the lengthwise direction of the sheet or metal.

Drawing - This is a process of forming a flat workpiece in to a hollow shape by means of a punch which causes the blank to flow into a die cavity.

Squeezing - Under this operation, the metal is caused to flow to all portions of a die cavity under the action of compressive forces.

Page 18: Sheet Metal Design

Press OperationsShearing - Shearing is a sheet metal cutting operation along a straight line between two cutting edges by means of a power shear.

Metal sheet is held on top of hardened die, shearing blade cuts downward, usually driven by hydraulic or electric force.

Punching & Blanking - Blanking and punching are similar sheet metal cutting operations that involve cutting the sheet metal along a closed outline. If the part that is cut out is the desired product, the operation is called blanking and the product is called blank.

If the remaining stock is the desired part, the operation is called punching.

Both operations are illustrated on the example of producing a washer:

Page 19: Sheet Metal Design

Blanking & Punching

Cutting of sheet metal is accomplished by a shearing action between two sharp edges. The shearing action is illustrated in the figure:

Page 20: Sheet Metal Design

Punching & Blanking

Page 21: Sheet Metal Design

Blanking & PunchingClearance:Clearance c is the distance between the punch and die. The correct clearance depends on sheet-metal type and thickness t:

c = at (Typically 2 to 10% of thickness)where a is the allowance (a = 0.075 for steels and 0.060 for aluminum alloys)If the clearance is not set correctly, either an excessive force or an oversized burr can occur

Page 22: Sheet Metal Design

Blanking & Punching

Page 23: Sheet Metal Design

Blanking & PunchingThe clearance is a function of the kind, thickness and temper of the work material, harder materials requiring larger clearance than soft materials, the exception being Aluminium.

The usual clearances per side of the die, for various metals, are givenbelow in terms of the stock thickness, t :

For brass and soft steel, c = 5% of tFor medium steel, c = 6% of tFor hard steel, c = 7% of tFor aluminium, c = 10% of t

The total clearance between punch and die size will be twice these figures. These clearances are for blanking and piercing operations.

Page 24: Sheet Metal Design

Blanking & PunchingThe reason behind the application of clearance

The diameter of the blank or punched hole is determined by the burnished area. On the blank, the burnished area is produced by the walls of the die. Therefore, the blank size will be equal to the size of die opening (neglecting a slight expansion of the blank duo to elastic recovery after the cutting operation is completed).Similarly in punching operation, the burnished area in the hole is produced by the punch, therefore, the size of the hole will be the same as the punch. Therefore, the application of clearance on punch or die will depend on whether the punched hole or the cut blank is the desired product. Hence, in punching operation (where hole in the strip is the desired product), the punch is made to the correct hole size and the die opening is made oversize an amount equal to clearance. Similarly, if the blank is the desired product, the die opening size is made to the correct blank size and the punch is made smaller an amount equal to die clearance. In other words, punch controls the hole size and die opening controls the blank size.

Page 25: Sheet Metal Design

Blanking & PunchingLand - It is the flat (usually horizontal) surface continuous to the cutting of a die which is ground and reground to keep the cutting edges of the punch sharp.Straight - It is the surface of a cutting die between its cutting edge and the beginning of the angular clearance. This straight portion gives strength to the cutting surface of the die and alsoprovides for sharpening of the die. This straight portion is usually kept at about 3 mm for all materials less than 2 mm thick. For thicker materials it is taken to be equal to the metal thickness.

Angular clearance - Angular clearance or relief is provided to enable the slug to clear the die. It is provided below the straight portion of the die surface. It is usually ¼ ° to 1 ½ ° per side but occasionally as high as 2°. , depending mainly on stock thickness andfrequency of sharpening.

Page 26: Sheet Metal Design

Blanking & PunchingPunch and Die Clearance considering the elastic recovery of the material:After cutting operation has been completed, elastic recovery of the strip material takes place. In blanking operation, after the release of blanking pressure, the blank expands slightly. The blanked part is thus actually larger than the die opening that has produced it. In punching operation, after the strip is stripped off the punch, the material recovers and the hole contracts. Thus, the hole is actually smaller than the size of the punch which produced it. Difference in size due to elastic recovery will depend upon: blank size, stock thickness and stock material.If the stock is upto 0.25mm, this difference may be taken as zero. For stock thickness between 0.25 mm and 0.75 mm, it may be taken as equal to 0.025 mmFor stock thickness more than 0.75 mm, it may be taken as 0.05mm.Thus to produce correct hole and blank sizes, the punch size should be increased and the die opening size should be decreased.

Page 27: Sheet Metal Design

Blanking & PunchingCutting forces -

Cutting force in all shearing operations is determined byF=StL

where S is the shear strength of material, for approximate solutions,S=0.7UTS

T is thickness of sheet L is the length of the cut edge

Page 28: Sheet Metal Design

Blanking & PunchingTools and dies for cutting operations

Simple diesWhen the die is designed to perform a single operation (for example, cutting, blanking, or punching) with each stroke of the press, it is referred to as a simple die

Page 29: Sheet Metal Design

Blanking & PunchingMulti-operational dies:More complicated press working dies include:

compound die to perform two or more operations at a single position of the metal stripprogressive die to perform two or more operations at two or more positions of the metal strip

Page 30: Sheet Metal Design

Blanking & Punching

Page 31: Sheet Metal Design

Strip LayoutPreparation of Blanking Layout

It is a layout, of position of the work-pieces in the strip and their orientation with respect to one another.

Major considerations are• Economy of material• Direction of material grain • Scrap twisting & wedging

The strip layout with maximum material saving may not be the best strip layout, as the die construction may become more complex which would offsetthe savings due to material economy unless a large number of parts are to be produced.

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Strip Layout

Page 33: Sheet Metal Design

Strip Layout

a = t + 0.015 h

b = approx. 1 to 1.5t

Page 34: Sheet Metal Design

Forming OperationsSome Design Tips:• Dimension the part in a single direction wherever possible

Sequential nature of the forming process and introduction of dimensional variation at each bend

It is in line with the process and helps to control tolerance accumulation

• Allow a more generous bend tolerance (+/- .007”) as tighter tolerances, while achievable, will result in higher costs

• Use consistent bend radius for all bends per part ; it minimize setup changes

• Dimensioning should be done from a feature to an edge. Avoid feature-to-feature

dimensions over two or more planes. Feature-to-bend dimensions may require special fixtures or gauging.

• This also means that tolerances in the title block of a drawing may be unnecessarily restrictive for certain dimensions and angles, while very appropriate for others.

Page 35: Sheet Metal Design

Forming OperationsCritical Dimensions in Sheet Metal Forming –

• Outside dimension should be used unless the inside dimension is critical.

• Emboss and offset dimensions should be to the same side of the material unless the overall height is critical.

Only the truly critical dimensions should be highlighted as such. Placing excessively high tolerances and redundant critical dimensions can dramatically increase the costof the part.

Page 36: Sheet Metal Design

Forming Operations• As a rule, inside bend radius should be equal to material thickness. When theradius is less than recommended, this can cause material flow problems in soft material and fracturing in hard material.

• When a bend is made close to an edge the material may tear unless bend relief is given. Figure "A" shows a torn part. Figure "B" shows a part with the edge a sufficient distance from the form. This distance should never be less than the radius of the bend. Figure "C" shows a bend relief cut into the part, again the depth of the relief should be greater than the radius of the bend. The width of the relief should be a material thickness or greater, preferably a material thickness + 1/64".

Page 37: Sheet Metal Design

Forming OperationsForming Near Holes – When a bend is made too close to a hole, the hole may become deformed. Figure "A" shows a hole that has become teardrop shaped because of this problem. To save the cost of punching or drilling in a secondary operation the following formulas can be used to calculate the minimum distance required:

For a hole < 1" in diameter the minimum distance "D" = 2T + R (see fig. "B")For a slot or hole > 1" diameter then the minimum distance "D" = 2.5T + R (see fig. "C“)

Page 38: Sheet Metal Design

Forming OperationsForm height to thickness ratio –

To determine the minimum form height for sheet metal use the following formula:

D = 2.5T + R (see below) The height can be less but it required secondaryoperations and is far more costly.

Page 39: Sheet Metal Design

Forming OperationsEdge Distortion –

An exaggerated example of edge deformation is pictured in figure "A" below.The overhang caused by this distortion can be as large as ½ the material thickness. As material thickness increases and bend radius decreases the overhang becomes more severe. In situations where an overhang is unacceptable the part can be relieved as in figure "B".

Page 40: Sheet Metal Design

Forming OperationsHole to edge clearance –

A good rule of thumb for hole placement is to keep the hole at least one material thickness away from any edge. If the hole gets too close to an edge a bulge canform as shown below. Also note, if the hole is used for fastening two pieces together, extra web should be used to account for the added stress.

Page 41: Sheet Metal Design

Forming OperationsHole Taper –

When a hole is punched, it does not have a constant radius through the entirethickness of the part. The cross section of a punched hole is shown below. The taper in the bottom side of the hole is relative to the die clearance. Die clearance is the difference between the punch diameter and die diameter. It is usually about 10% of the material thickness. To get a constant diameter through the entire material thickness the part must be drilled, a far more costly operation.For most materials hole diameter should not be less than material thickness. As tensile strength increases the punch diameter must also increase.

Page 42: Sheet Metal Design

Forming OperationsFeature placement restrictions – When placing formed features next to one another, care should be taken to allow clearance between features. If the station does not clear a form alreadyplaced in the part, the form could be flattened out. An example of good vs. bad placement is illustrated below. Relieving the stripper can overcome this problem in some cases.

Page 43: Sheet Metal Design

Forming OperationsCounter sinks – A counter sink can be put in sheet metal by both machining and/or punching.Each of these methods give the finished part different characteristics. The cross sections of the features are shown below, they are listed from least to most expensive (tooling cost not included).• Formed – Low Cost – Used for thin gages, 18 gage and lighter.• Punched – Low Cost – Most common, used for anything heavier than 18 gage.• Punched and Machined – Medium Cost – Used for harder materials that cannot be formed with a punch, e.g. heavy gage stainless.• Machined Complete – High Cost – Rarely used, only for high tolerance applications or materials too thick to be punched.

Page 44: Sheet Metal Design

Forming OperationsCorners- May be sharp, however to reduce tooling costs, specify radii of ½ material thickness or a minimum of .015”.

Notches and Tabs- Should not be narrower than 1.5X the material thickness. Length of notches can be up to 5X length of material thickness.

Page 45: Sheet Metal Design

Forming OperationsCutoffs:There are three kinds of cutoffs in blanking: straight/square, half round or partial radius and full radius. The square cutoff is the most economical. The full radius is not recommended as it leaves an unavoidable “feather edge” burr along the outside material edge.

Piercing:Holes-Minimum diameter of holes should be equal or greater than 1.2 X material thickness, and 2X material thickness for stainless steel or high tensile materials.

Edge-to-hole- Allow 2x material thickness (“web”) to prevent bulging of material

Page 46: Sheet Metal Design

Bending OperationsBending is defined as the straining of the sheet metal around a straight edge.

Bending induces plastic deformation in the material, so material retains its shape after releasing the force.

Page 47: Sheet Metal Design

Bending OperationsBending operations involve the processes of V-bending and edge bending:

• V-bending—sheet metal is bent along a straight line between a V-shape punch and die.• Edge bending—bending of the cantilever part of the sheet around the die edge.

Page 48: Sheet Metal Design

Bending OperationsBend Allowance:

This is the stretching length that occurs during bending. It must be accounted to determine the length of the blank,

Lb =Σ L + Σ BA

where Lb is the length of the blank, L are the lengths of the straight parts of the blank, BA is the bend allowance,

where A is the bend angle; t is the sheet thickness;R is the bend radius; Kba is a factor to estimate stretching, defined as follows:for R < 2t , Kba = 0.33for R ≥ 2t, Kba = 0.50

Page 49: Sheet Metal Design

Bending OperationsSpringback:

Springback is the elastic recovery leading to the increase of the included angle when the bending pressure is removed.

To compensate for springback two methods are commonly used:

• Overbending—the punch angle and radius are smaller than the final ones.• Bottoming—squeezing the part at the end of the stroke.

Page 50: Sheet Metal Design

Bending Operations

Page 51: Sheet Metal Design

Bending OperationsBending forces

The maximum bending force is estimated asF = K bf UTS wt2/Dwhere Kbf is the constant that depends on the process, Kbf = 1.33 for V-bending and Kbf = 0.33 for edge bending; w is the width of bending; D is the die opening dimension as shown in the figure:

Page 52: Sheet Metal Design

Bending Operations

Page 53: Sheet Metal Design

Bending Operations

Page 54: Sheet Metal Design

Bending Operations

Page 55: Sheet Metal Design

Deep Drawing Operations

Page 56: Sheet Metal Design

Deep Drawing Operations

Page 57: Sheet Metal Design

Deep Drawing OperationsDeep drawing is a sheet-metal operation to make hollow-shaped parts from a sheet blank.

Page 58: Sheet Metal Design

Deep Drawing OperationsClearanceClearance “c“ is the distance between the punch and die and is about 10% greater than the stock thickness:

c = 1.1t

Holding forceThe improper application of the holding force can cause severe defects in the drawn parts such as (a) flange wrinkling or (b) wall wrinkling, if the holding force is too small, and (c) tearing if the folding force is overestimated.

Page 59: Sheet Metal Design

Deep Drawing OperationsMeasures of drawing

Two measures of the severity of a deep drawing operation are used,

1.Drawing ratio DR defined asDR = Db/Dp

Here Db is the blank diameter and Dp is the punch diameter

DR must be less than 2.0 for a feasible operation. If it is more than 2.0, the progressive deep drawing is applied .

2.Thickness-to-diameter ratio t/ Db, It is desirable to be greater than 1% to avoid wrinkling.Blanked and drawn parts showing progression of drawing

Page 60: Sheet Metal Design

Deep Drawing Operations

Blanked and drawn parts showing progression of drawing operation

Page 61: Sheet Metal Design

Deep Drawing OperationsDrawing forces

The drawing force F required to perform a deep drawing operation is estimated roughly by the formula,

F = πt DpUTS (DR - 0.7)

The holding force Fh is defined as,Fh = 0.015 Y π[Db

2 - (Dp + 2.2t + 2Rd)2]where Y is the yield strength of the material

Blank size determination

The blank diameter can be calculated by setting the initial blank volume equal to the final volume of the part and solving for diameter Db.

Page 62: Sheet Metal Design

Guerin ProcessThe Guerin process involves the use of a thick rubber pad to form sheet metal over a positive form block:

Page 63: Sheet Metal Design

Guerin Process

Advantages: small cost of tooling

Limitations: for relatively shallow shapes

Area of application: small-quantity production

Page 64: Sheet Metal Design

Hydroforming ProcessIt is similar to Guerin process but instead of rubber pad a rubber diaphragm filled with fluid is used in this process.

(1) start-up, no fluid in the cavity; (2) press closed, cavity pressurized with hydraulic fluid;(3) punch pressed into work to form part. Symbols: v - velocity, F – appliedforce, and p - hydraulic pressure

Advantages: small cost of tooling

Limitations: simple shapes

Area of application: small-quantity production

Page 65: Sheet Metal Design

Stretch Forming ProcessIn stretch forming process, the sheet metal is stretched and bent to achieve the desired shape.

(1) start of the process(2) form die is pressed intothe work causing it to stretched

and bent over the form. Symbols: v - velocityFdie - applied force

Advantages: small cost of tooling, large parts

Limitations: simple shapes

Area of application: small-quantity production

Page 66: Sheet Metal Design

Spinning ProcessSpinning is a metal forming process in which an axially symmetric part is gradually shaped over a mandrel by means of a rounded tool or roller.

Flat circular blanks are often formed into hollow shapes such as photographic reflectors.In a lathe, tool is forced against a rotating disk, gradually forcing the metal over the chuck to conform to its shape.Chucks and follow blocks areusually made of wood for this process

Advantages: small cost of tooling, large parts (up to 5 m or more)

Limitations: only axially symmetric parts

Area of application: small-quantity production

Page 67: Sheet Metal Design

High Energy Rate FormingThese are metal forming processes in which large amount of energy is applied in a very short time. Some of the most important HREF operations include:

Explosive formingIt involves the use of an explosive charge placed in water to form sheet into the die cavity.

Page 68: Sheet Metal Design

High Energy Rate FormingAdvantages: small cost of tooling, large parts

Limitations: skilled and experienced labor

Area of application: large parts typical of the aerospace industry

Page 69: Sheet Metal Design

Electrohydraulic FormingThis is a HREF process in which a shock wave to deform the work into a die cavity is generated by the discharge of electrical energy between two electrodes submerged in water. Similar to explosive forming, but applied only to small part sizes.

Page 70: Sheet Metal Design

Electromagnetic FormingThe sheet metal is deformed by the mechanical force of an electromagnetic field induced in the work-piece by a coil.

Advantages: can produce shapes, which cannot be produced easily by the other processes

Limitations: suitable for magnetic materials

Area of application: most widely used HERF process to form tubular parts

Page 71: Sheet Metal Design

Other Design PointsTo provide strength – Ribbing, Notching

To eliminate Fragility – Embossing

Care to be taken for 90 degree bend ribbing

For small quantity – laser cutting and manual bending

Tooling considerations

Modifications/design revisions – Tooling consideration (material addition or removal in the tool)