sheet metal processes

Sheet Metal Processes

Prof. K.N.WakchaureDept. of Mechanical Engineering

SRES COLLEGE OF ENGINEERING, KOPARGAON

Sheet Metal Basics

• Sheet metal thickness:-0.15 to 6mm• Metal Plates:- >6mm• Foils: <0.15mm

• Sheet and plate have different applications. Plate is generally used for larger structural parts like boilers, turbines, bridges, and ships.

• Sheet metal is used in the manufacture of cars, trains, aircraft, farm equipment, office equipment, furniture, house appliances, computers, machine components, and beverage cans to name a few.

• Sheet metal products typically have high strength, good surface, and accurate tolerances.

Sheet Metal Classification• Cutting: Cutting is the use of shearing forces to remove material from

a work piece. Technically not a metal forming process, but of extreme industrial importance.

• Bending: Bending is the forming of a sheet metal work about an axis.

• Deep Drawing: Deep drawing is the forming of a cup or box with a flat base and straight walls, from a sheet metal blank.

• Other Processes: Other sheet metal working processes such as ironing, spinning, rubber forming, and high energy rate forming are also discussed in latter sections.

Sheet Metal Cutting

• Sheet metal cutting is a major classification of many kinds of press working operations.

• Press working, a term referencing sheet metal operations in general, involves the working of a sheet between two die.

• Cutting operations involve the separation of material of the sheet in certain areas.

• This separation is caused by shearing forces acting on the metal through the edges of the punch and die.

• Industrial sheet metal working operations such as this will usually incline the punch so as to reduce the maximum force needed by distributing the required force over the cutting stroke.

• The angle the punch may be inclined varies from 4 to 15 degrees, however the higher the angle, the greater the magnitude of the horizontal force component acting to displace the work.

• For that reason angles are usually under 9 degrees.

Cutoff And Parting Of Sheet

Blanking• Blanking is the cutting of a sheet metal part along a closed

contour in one step. The piece cut out is called a blank, and may be further processed. Many blanks are often continuously cut out of a sheet or strip.

• Blanking will waste a certain amount of material. When designing a sheet metal blanking process, the geometry of the blanks should be nestled as efficiently as possible to minimize material waste.

• A distinction should be made between blanking and punching, since essentially they are the same process. In punching the piece cut out is waste.

• In blanking, the piece cut out is the work and is kept.

Nibbling• Punches for the cutting of holes and profiles may come in

many different shapes and sizes. • A machine called a nibbler uses a small straight punch to

create the same geometric removal of sheet metal as that of a larger more complicated punch.

• It does this by rapidly punching many overlapping holes to make the larger cut profile. Nibbling is useful in producing intricate cuts with simple machinery.

• Nibbling may take longer than a punch specifically designed for a certain cut, however it may be an efficient alternative for small production runs.

Perforating• Sometimes it is desirable to punch many holes in a piece of sheet

metal, often in a certain pattern. • These holes may be round or some other shape. Perforating sheet

metal will allow for the passage of light or fluid material through the material.

• It can often serve in ventilation and filtration of fluid substances. Perforated sheet is also used in structure and machinery construction to reduce weight and for cosmetic appearance.

• Special equipment is employed that punches many holes at once, and at a high rate.

• In industrial practice the size of these holes is usually from (1-75mm). • The upper rate at which some special perforating machines can punch

holes is 100,000 to 300,000 per minute

Shaving

• Edge surfaces of cut sheet metal are not typically smooth and straight. Even in properly cut metal the fracture zone will be angled and rough.

• In sheet metal cutting, the lower the clearance value the higher the edge's quality.

• Shaving is a secondary process that can be used to improve edges of cuts that have already been made.

• Shaving uses very little clearance to perform a straight, smooth, accurate cut to only the end of the edge.

Mechanics Of Cutting Sheet Metal

• Mechanics of cutting sheet metal should be understood when designing a sheet metal manufacturing process. The work piece in such an operation is secured to the lower die, while the motion of the upper die, (called a punch), enacts the cutting.

Clearance

• In manufacturing industry there is a range of optimum values for specific processes, clearances may span from 1% to 30% of sheet thickness.

• Generally, typical values span from 3% to 8% of sheet thickness.

• A large factor is the type of work material and its temper. Aluminum alloy sheet might have an optimum clearance value of 4%, a brass sheet 6%, and a hard steel sheet 7.5%.

• If the hole size is small relative to total sheet thickness, extra clearance could be needed.

Sheet Metal Bending

Mechanics Of Sheet Metal Bending

Methods Of Eliminating Springback

Stretch Forming• Sheet metal is clamped at edges and stretched

over a die or form block• Dies made of zinc alloys, steels, plastics, wood• Little or no lubrication• Low-volume, versatile, and economic production

Products: aircraft wing skin panels, automobile door panels, window frames

Deep Drawing

• Deep drawing is a process that is used extensively in the forming of sheet metal into cup or box like structures.

• Pots and pans for cooking, containers, sinks, automobile parts, such as panels and gas tanks, are among a few of the items manufactured by deep drawing.

• Clearance in deep drawing manufacture is greater than sheet thickness, usually clearance values are 107% to 115% of sheet thickness.

Drawing Ratio• Measurement of the amount of drawing performed on a blank can

be quantified. This can be done with the drawing ratio. • The higher the drawing ratio, the more extreme the amount of

drawing. • Due to the geometry, forces, metal flow, and material properties of

the work, there is a limit to the amount of drawing that can be performed on a blank in a single operation.

• Drawing ratio is roughly calculated asDR = Db/Dp.

• Db is the diameter of the blank and Dp is the diameter of the punch. • The limit to the drawing ration for an operation is usually 2 or under.

Reduction• Another way to express drawing ratio is the reduction (r). • Reduction is measured using the same variables as

drawing ratio.• Reduction can be calculated by r = (Db - Dp)/(Db).• Db and Dp being blank and punch diameters respectively.• Reduction ratio should be .5 or under. Often expressed as

the percent reduction r = (Db - Dp)/(Db) X 100%. • In this case the reduction should be 50% or under.

• If required percent reduction is over 50%, the part must be formed in multiple operations. Redrawing is the subsequent deep drawing of a work that has already undergone a deep drawing process.

Ironing • Ironing can be incorporated into a deep drawing process or can be

performed separately. • A punch and die pushes the part through a clearance that will act

to reduce the entire wall thickness to a certain value. • While reducing the entire wall thickness, ironing will cause the part

to lengthen. • The percentage reduction in thickness for an ironing operation is

usually 40% to 60%. • Percent reduction can be measured

(ti - tf)/ti X 100%.

• With ti being initial thickness, and tf being final thickness. Many products undergo two or more ironing operations. Beverage cans are a common product of ironing operations.

Spinning

Hydroforming

• Hydroforming is an effective sheet metal forming process. • Hydroforming can typically obtain deeper draws than

conventional deep drawing operations. Hydroforming uses a rigid punch to push a sheet metal work piece into a rubber membrane.

• Behind the rubber membrane is a chamber of pressurized fluid. When the work is pressed into the chamber the rubber membrane surrounds it completely and the pressure of the fluid forces the sheet metal to form on the punch.

• Fluid pressure can be controlled during the operation, and can be as high as 15,000 lbs/in2, (100MPa).

Explosive Forming

Design Consideration

Numerical

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