SHEET METAL

DESIGN

Contents

1 Introduction2 Metals used in Sheet Metal Working3 Gauges4 Sheet Metal Forming Processes4.1 Piercing4.2 Blanking4.3 Fine Blanking4.4 Punching4.5 Trimming4.6 Nibbling4.7 Notching4.8 Drawing4.9 Spinning4.10 Bending4.11 Embossing4.12 Coining5 Comparison to other forming processes6 Die Manufacturing7 Progressive die with scrap strip and stamping8 Die operations and types9 Design calculations10 For Blanking and Piercing11 Draw Die Designing12 Bending Design13 Punching Design14 Die Construction15 Workshop Practice16 Safety Guide for Sheet Metal Workers

Introduction

1. 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.

2. Thicknesses can vary significantly, although extremely thin thicknesses

are considered foil or leaf, and pieces thicker than 6 mm (0.25 in) are

considered plate.

3. Sheet metal is available as flat pieces or as a coiled strip. The coils are

formed by running a continuous sheet of metal through a roll slitter.

4. The thickness of the sheet metal is called its gauge. The gauge of sheet

metal ranges from 30 gauge to about 8 gauge. The higher the gauge, the

thinner the metal is.

5. There are many different metals that can be made into sheet metal, such

as: Aluminium, brass, copper, steel, tin, nickel and titanium. Sheet metal has

applications in car bodies, airplane wings, medical tables, roofs for building and

many other things.

6. Sheet metal working involves manufacturing articles from sheet metal or

thin sheets, which may be of black iron, galvanized iron, copper or

stainless steel.

7. The articles made of sheet metals are lighter in weight, and are less

expensive. With properly designed shapes and structures, sheet metal

articles are replacing castings and forgings in several engineering

applications.

8. Besides the articles such as funnels, hoppers, cans, pipes, elbows and

boxes, sheet metal products are used for the purpose of covering machines

and other structures in the form of safety guards or facade of attractive

shapes.

9. Since sheet metal working involves forming shapes from flat metal sheets,

the ‘development and drawing of shape of the article in actual size’ on the

sheet metal is the most important and prime operation of the work. The

knowledge of geometry, mensuration and properties of metals is therefore

most essential. Nearly all patterns of articles come from the development

of the surfaces of few geometrical models like cylinder, prism, cone and

pyramid. A properly drawn pattern on the sheet metal saves time and

money because if the pattern (or development of surface of the article) is

wrong, then the blank cut from the sheet would just result into wastage of

material, time and labour besides delay in production.

10. The sheet metal working also involves knowledge of various operations of

joining metals like mechanical jointing or soldering and brazing etc.

Metals used in Sheet Metal Working

1. A large variety of metals in the form of sheets and plates used in sheet

metal working include black iron, galvanized iron, copper, brass, tin,

aluminum, lead and zinc.

2. The metal sheets are designated either in terms of gauge numbers (such

as Imperial or Legal Standard Wire gauge) or thickness in millimetres

(along with width and length) given in standard metal reference tables.

3. The thickness of sheets vary inversely as their gauge number, higher the

gauge number, smaller the thickness and vice-versa. For example, for

gauge no. 20, thickness is 0.914 mm, for gauge number 10, the equivalent

sheet thickness is 3.251 mm, for gauge no. 3, it is 6.401 mm, etc.

4. Cold rolled sheets (annealed) are usually available in thickness 0.8 mm to

3.25 mm, width 1000 mm and length 2000 mm.

5. Black Iron Sheet or uncoated sheet is the cheapest material used for sheet

metal work. Since these sheets carry no protection coatings on their

surfaces, these are likely to corrode quickly. These are, therefore, used for

marking those articles, which are later enameled or painted before use.

Articles like pans, tanks, cabinet works, almirahs and furniture’s are

commonly made from black iron sheets.

6. Galvanized Iron Sheets or G.I. sheets are soft iron sheets carrying zinc

coating on their surface to resist corrosion and to add to the aesthetic of

the sheet. Zinc coating in varying thickness is given according to the

severity of corrosive atmosphere to which the sheet metal product is likely

to be subjected. Special care is, however, required in welding or brazing

these sheets to avoid excessive damaging of the coating. G.I. sheets are

used for making articles such as trunks, storage tanks for food grains,

buckets and other containers for water storage, pans, roofing sheets etc.

7. Copper Sheets find application in making radiators of automobile engines,

heating appliances, equipment for chemical plants etc. These sheets are

costlier than aforesaid black iron sheets and G.I. sheets but these have

better resistance to corrosion. These can be easily worked upon, being

highly ductile and soft. Brass Sheets are used for making variety of articles

through cold working processes like pressing, drawing and spinning. These

are often used for making kitchenware’s and utensils.

8. Tin Sheets are the tin coated iron sheets and hence have silvery

appearance. These offer good resistance against rusting and atmospheric

corrosion. Articles made from tin sheets find application in food industry for

making containers for edible oils and ghee, cans, and dairy equipment etc.

Gauge

The sheet metal gauge (sometimes spelled "gage") indicates the standard

thickness of sheet metal for a specific material. As the gauge number

increases, the material thickness decreases.

Sheet metal thickness gauges for steel are based on a weight of 41.82 pounds

per square foot per inch of thickness. This is known as the Manufacturers'

Standard Gage for Sheet Steel. For other materials, such as aluminium and

brass, the thicknesses will be different. Thus, a 10 gauge steel sheet which has

a thickness of 0.1345 inches will weigh 41.82*0.1345 = 5.625 pounds per

square foot.

Examples: 16 ga CRS is 2.5 pounds per square foot. For 18 ga CRS the weight

is 2.0 pounds per square foot and for 20 ga CRS the weight is 1.5 pounds per

square foot.

SHEET

METAL

FORMING

PROCESSES

PIERCING

Standard sheet metal gaugesGaug

e

Steel Galvanized

steel

Stainless

steel

Aluminum Zinc

3 0.2391

(6.0731)

- - - 0.006

4 0.2242

(5.6947)

- - - 0.008

5 0.2092

(5.3137)

- - - 0.010

6 0.1943

(4.9352)

- - - 0.012

7 0.1793

(4.5542)

- 0.1875 0.1443 0.014

8 0.1644

(4.1758)

0.1681 0.1719 0.1285 0.016

9 0.1495

(3.7973)

0.1532 0.1563 0.1144 0.018

10 0.1345

(3.4163)

0.1382 0.1406 0.1019 0.020

11 0.1196

(3.0378)

0.1233 0.1250 0.0907 0.024

12 0.1046

(2.6568)

0.1084 0.1094 0.0808 0.028

13 0.0897

(2.2784)

0.0934 0.094 0.072 0.032

14 0.0747

(1.8974)

0.0785 0.0781 0.0641 0.036

15 0.0673

(1.7094)

0.0710 0.07 0.057 0.040

16 0.0598

(1.5189)

0.0635 0.0625 0.0508 0.045

17 0.0538

(1.3665)

0.0575 0.056 0.045 0.050

18 0.0478

(1.2141)

0.0516 0.0500 0.0403 0.055

19 0.0418

(1.0617)

0.0456 0.044 0.036 0.060

20 0.0359

(0.9119)

0.0396 0.0375 0.0320 0.070

21 0.0329

(0.8357)

0.0366 0.034 0.028 0.080

22 0.0299

(0.7595)

0.0336 0.031 0.025 0.090

23 0.0269

(0.6833)

0.0306 0.028 0.023 0.100

24 0.0239

(0.6071)

0.0276 0.025 0.02 0.125

25 0.0209

(0.5309)

0.0247 0.022 0.018 -

26 0.0179

(0.4547)

0.0217 0.019 0.017 -

27 0.0164

(0.4166)

0.0202 0.017 0.014 -

28 0.0149

(0.3785)

0.0187 0.016 0.0126 -

29 0.0135

(0.3429)

0.0172 0.014 0.0113 -

30 0.0120

(0.3048)

0.0157 0.013 0.0100 -

31 0.0105

(0.3200)

0.0142 0.011 0.0089 -

32 0.0097

(0.2464)

- - - -

33 0.0090

(0.2286)

- - - -

34 0.0082

(0.2083)

- - - -

35 0.0075

(0.1905)

- - - -

36 0.0067

(0.1702)

- - - -

37 0.0064

(0.1626)

- - - -

38 0.0060

(0.1524)

- - - -

1. Piercing is a shearing process where a punch and die are used to create

a hole in sheet metal or a plate.

2. The process and machinery are usually the same as that used in

blanking, except that the piece being punched out is scrap in the piercing

process.

3. There are many specialized types of piercing: lancing, perforating,

notching, nibbling, shaving, cutoff, and dinking.

4. The amount of clearance between a punch and die for piercing is

governed by the thickness and strength of the work-piece material being

pierced.

5. The punch-die clearance determines the load or pressure experienced at

the cutting edge of the tool, commonly known as point pressure.

6. Excessive point pressure can lead to accelerated wear and ultimately

failure.

7. Burr height is typically used as an index to measure tool wear, because it

is easy to measure during production

BLANKING

1. A blanking die produces a flat piece of material by cutting the desired

shape in one operation.

2. The finish part is referred to as a blank.

3. Generally a blanking die may only cut the outside contour of a part, often

used for parts with no internal features.

Three benefits to die blanking are:

1. Accuracy: A properly sharpened die, with the correct amount of

clearance between the punch and die, will produce a part that holds close

dimensional tolerances in relationship to the parts edges.

2. Appearance: Since the part is blanked in one operation, the finish edges

of the part produces a uniform appearance as opposed to varying degrees of

burnishing from multiple die cutting operations.

3. Flatness: Due to the even compression of the blanking process, the end

result is a flat part that may retain a specific level of flatness for additional

manufacturing operations.

FINE BLANKING

1. Fine blanking is a fully automated precision metalworking process.

2. It is a form of precision metal stamping in which extremely tight

tolerances can be held, and usually additional machining steps can be avoided

post-production.

3. Materials that can be fine blanked include carbon steels, alloy and

stainless steels, as well as soft non ferrous alloys like aluminum, brass or

copper.

Typical fine blanking press cross section

Fine blanking presses are similar to other metal stamping presses, but they

have a few critical additional parts. A typical compound fine blanking press

includes a hardened die punch (male), the hardened blanking die (female), and

a guide plate of similar shape/size to the blanking die. The guide plate is the

first applied to the material, impinging the material with a sharp protrusion or

stinger around the perimeter of the die opening. Next a counter pressure is

applied opposite the punch, and finally the die punch forces the material

through the die opening. Since the guide plate holds the material so tightly,

and since the counter pressure is applied, the material is cut in a manner more

like extrusion than typical punching. Mechanical properties of the cut benefit

similarly with a hardened layer at the cut edge from the cold working of the

part. Because the material is so tightly held and controlled in this setup, part

flatness remains very true, distortion is nearly eliminated, and edge burr is

minimal. Clearances between the die and punch are generally around 1% of

the cut material thickness, which typically varies between .5-13mm. Currently

parts as thick as 19mm can be cut using fine blanking. Tolerances between

±.0003"-.002" are possible based on material thickness & tensile strength, and

part layout.

With standard compound fine blanking processes, multiple parts can often be

completed in a single operation. Parts can be pierced, partially pierced, offset

(up to 75•), embossed, or coined, often in a single operation. Some

combinations may require progressive fine blanking operations, in which

multiple operations are performed at the same pressing station however.

Advantages

• excellent dimensional control, accuracy, and repeatability through a

production run.

• excellent part flatness is retained.

• straight, superior finished edges to other metal stamping processes.

• smaller holes possible relative to thickness of material[6].

• little need to machine details.

• multiple features can be added simultaneously in 1 operation[7].

• more economical for large production runs than traditional operations

when additional machining cost and time are factored in (between 1000-20000

parts minimum, depending on secondary machining operations).

Disadvantages

• slightly higher tooling cost when compared to traditional punching

operations.

• slightly slower than traditional punching operations.

Broaching

Broaching is the process of removing material through the use of multiple

cutting teeth, with each tooth cutting behind the other. A broaching die is often

used to remove material from parts that are too thick for shaving.

Bulging

A bulging die expands the closed end of tube through the use of two types of

bulging dies. Similar to the way a chefs hat bulges out at the top from the

cylindrical band around the chefs head.

Bulging fluid dies: Uses water or oil as a vehicle to expand the part.

Bulging rubber dies: Uses a rubber pad or block under pressure to move the

wall of a workpiece.

PUNCHING

1. Punching in metal fabrication is the process of using a machine to press a

shape through a sheet of metal and into a die to create the desired shape in

the metal.

2. This is most commonly done by use of a turret, a computer numerical

controlled machine that houses tools and their corresponding dies in a

revolving indexed turret. These machines use hydraulic, pneumatic, or

electrical power to press the shape with enough force to shear the metal.

3. The shape is formed by pressing the material against a die with a huge

force. The shear forces generated between the material and die separate the

material into the desired shape.

4. The desired shape is not obtained, however, as burred edges and rough

surfaces are formed. These edges and surfaces must be further processed until

the desired shape is achieved.

5. The punch force required to punch a piece of sheet metal can be

estimated from the following equation:

F = 0.7tL(UTS)[citation needed]

Where t is the sheet metal thickness, L is the total length sheared

(perimeter of shape), and UTS is the ultimate tensile strength of the

material.

6. Die and punch shapes affect the punching process. The punch force

increases during the process as the entire thickness of the material is sheared

at once.

7. A beveled punch helps in the shearing of thicker materials. Beveling

reduces the force at the beginning of the stroke. However, beveling a punch

will disort the punched shape because of lateral forces that develop.

8. Compound dies allow multiple shaping to occur. Using compound dies

will generally slow down the process and are typically more expensive than

other dies.

9. Progressive dies may be used in high production operations. Different

punching operations and dies are used at different stages of the operation on

the same machine.

10. Other processes such as stamping, blanking, perforating, parting,

drawing, notching, lancing and bending operations are all related to punching.

11. A punch press is a type of machine press used for forming and cutting

material.

12. The punch press can be small and manually operated and hold one

simple Die set, or be very large, CNC operated, and hold a much larger and

complex die set.

13. A Die set consists of a set of (male) punches and (female) dies which,

when pressed together, may form a hole in a workpiece or may deform the

workpiece in some desired manner.

14. The punches and dies are removable with the punch being temporarily

attached to the end of a ram during the punching process. The ram moves up

and down in a vertically linear motion.

15. Commonly machines are large metal framed equipment having two types

of machine frames. A ‘C’ type frame or a 'portal' type frame.

16. The ‘C’ type commonly has the hydraulic ram at the top foremost part to

enable the punching process to be carried out, whereas the portal frame is

much akin to a complete circle with the ram being centred within the frame to

stop frame deflection or distortion.

17. All punch press machines have a table or bed with brushes or rollers

mounted in the tables to allow the sheet metal workpiece to traverse with low

friction. Brushes are commonly used in production environments where

minimal scratching to the workpiece is required, such as brushed aluminium or

high polished materials.

18. The main bed of most machines is called the 'Y' Axis with the 'X' Axis

being at right angles to that and allowed to traverse under CNC control.

Dependent on the size of the machine, the beds and the sheet metal workpiece

weight, then the motors required to move these axis tables can vary in size

and power. Older styles of machines used DC motors to move, however with

advances in technology, today's machine mostly use AC brush less motors for

drives.

19. The process of operation begins with the CNC controller commanding the

drives to move a particular axis to a desired position.

20. Once in position, the control initiates the punching sequence and pushes

the ram to Bottom Dead Centre and returns it to Top Dead Centre. The Origins

of BDC and TDC go back to older machines where this was a pitman type press

with a Pneumatic or Hydraulic operated clutch system.

21. On today's machines BDC/TDC does not actually exist but is commonly

used as a term to derive the top and bottom of a stroke of the ram. The Punch

enters the Sheet metal, and pushes it through the die, obtaining the required

shape of the punch and die set. This will form a slug of metal that is collected

underneath the die and ejected to a scrap container. The whole punching

process on modern machines is extremely fast compared to older pitman style

machines and thus gives rise to increased production volumes.

22. The sequence takes approximately 0.5 milli seconds to complete

( variant from machine to machine and manufacturer)and signals to the control

the next movement command allowed after the ram has reached the top of its

stroke.

23. As a metal forming process, the punch press is used for the highest

volume production. Cycle times are often measured in sheet yield as a

percentage of waste to parts required ratios per sheet processed.

24. As most programming is done by skilled CAD/CAM operators parts within

the sheet workpiece are commonly nested. Machine setters are mostly used to

set up tooling and programming but thereafter once the machine is running an

operator of low skill can oversee its continued operation. Often one operator

will monitor several punch presses simultaneously making this one of the

lowest cost metal manufacturing processes.

25. Punch presses are usually referred to by their tonnage. In a production

environment a 20 ton press is mostly the prevalent machine used today. The

tonnage needed to cut and form the material is well known so sizing tooling for

a specific job is a fairly straightforward task.

26. Most punch presses today are hydraulically powered, however there

remains a legacy of older machines which are mechanically driven rams,

meaning the power to the ram is provided by a heavy, constantly-rotating

flywheel. The flywheel drives the ram using a Pitman arm. In the 19th century,

the flywheels were powered by leather drive belts attached to line shafting,

which in turn ran to a steam plant. In the modern workplace, the flywheel is

powered by a large electric motor.

27. Mechanical punch presses fall into two distinct types, depending on the

type of clutch or braking system with which they are equipped. Generally older

presses are "full revolution" presses that require a full revolution of the

flywheel for them to come to a stop. This is because the braking mechanism

depends on a set of raised keys or "dogs" to fall into matching slots to stop the

flywheel. A full revolution clutch can only bring the flywheel to a stop at the

same location- top dead center. Newer presses are often "part revolution"

presses equipped with braking systems identical to the brakes on commercial

trucks. When air is applied, a band-type brake expands and allows the flywheel

to revolve. When the stopping mechanism is applied the air is bled, causing the

clutch or braking system to close, stopping the flywheel in any part of its

rotation.

28. Hydraulic punch presses, which power the ram with a hydraulic cylinder

rather than a flywheel, and are either valve controlled or valve and feedback

controlled. Valve controlled machines usually allow a one stroke operation

allowing the ram to stroke up and down when commanded.

29. Controlled feedback systems allow the ram to be proportionally

controlled to within fixed points as commanded. This allows greater control

over the stroke of the ram, and increases punching rates as the ram no longer

has to complete the traditional full stroke up and down but can operate within

a very short window of stroke.

Trimming & Shaving:

1. Trimming dies cut away excess or unwanted irregular features from a

part, they are usually the last Shaving:

2. The shaving operation removes a small amount of material from the

edges of the part to improve the edges finish or part accuracy operation

performed.

3. The shaving process is a finish operation where a small amount of metal

is sheared away from an already blanked part. Its main purpose is to obtain

better dimensional accuracy, but secondary purposes include squaring the

edge and smoothing the edge.

4. Blanked parts can be shaved to an accuracy of up to 0.025 mm (0.001 in)

Nibbling :

The nibbling process cuts a contour by producing a series of

overlapping slits or notches.

This allows for complex shapes to be formed in sheet metal up to 6

mm (0.25 in) thick using simple tools.

The process is often used on parts that do not have quantities that

can justify a dedicated blanking die.

The edge smoothness is determined by the shape of the cutting die

and the amount the cuts overlap; naturally the more the cuts overlap

the cleaner the edge.

Notching

The notching process removes material from the edge of the workpiece. A

notching machine is shown in the below fig.

The machine shown above will create 90 degree notches in sheet metal. This

makes it possible to create profiles that can then be bent into three dimensional

shapes (like boxes). Lay the work on the table. The guides can be used to help

orient the part as desired. Pulling on the actuating lever will cut a notch in the

work.

Drawing:

1. The drawing operation is very similar to the forming operation except

that the drawing operation undergoes severe plastic deformation and the

material of the part extends around the sides.

2. A metal cup with a detailed feature at the bottom is an example of the

difference between formed and drawn. The bottom of the cup was formed

while the sides were drawn.

Spinning

1. Spinning is used to make axis-symmetric parts by applying a work piece

to a rotating mandrel with the help of rollers or rigid tools.

2. Spinning is used to make rocket motor casings, missile nose cones, and

satellite dishes, for example.

3. Metal spinning, or spin forming, is a metal working process by which a

disc or tube of metal is rotated at high speed and formed into an axially

symmetric part using tools.

4. Metal spinning is often performed by hand to produce decorative items,

or using machine tools, such as CNC lathe, when tight tolerances are required.

Metal may be formed into a die to shape the outside diameter or onto a

mandrel to size the inner diameter.

5. Metal spinning ranges from an artisan's specialty to the most

advantageous way to form round metal parts for commercial applications.

Artisans use the process to produce architectural detail, specialty lighting,

decorative household goods and urns. Commercial applications include rocket

nose cones, cookware, gas cylinders, brass instrument bells, and public waste

receptacles.

6. Virtually any ductile metal may be formed, from aluminum or stainless

steel, to high-strength, high-temperature alloys. The diameter and depth of

formed parts are limited only by the size of the equipment available.

Metal spinning by hand

1. Metal Spinning is a process by which circles of metal are shaped over

mandrels (also called forms) while mounted on a spinning lathe by the

application of levered force with various tools.

2. It is performed rotating at high speeds on a manual spinning lathe.

3. The flat metal disc is spun against the mandrel and a series of sweeping

motions then evenly transforms the disc around the mandrel into the desired

shape. It takes a very skilled workman to correctly shape and finish a hand

spun piece.

Safety considerations

When spinning metal by hand, care must be taken to not touch the spinning

metal with one's hands until the metal edge has been "turned over" (rolled to a

rounded edge so that the bare edge of the metal stock is protected). This is

mentioned specifically because wood turners are accustomed to touching the

spinning wood in the lathes (once it reaches relative smoothness) to monitor

their progress. This practice is very dangerous in metal spinning. Lexan/Clear

plastic lathe shields and guards are recommended.

Metal spinning tools

The basic hand metal spinning tool is called a Spoon , though many other tools

(be they commercially produced, ad hoc, or improvised) can be used to effect

varied results. Spinning tools can be made of hardened steel for using with

aluminium or solid brass for spinning stainless steel/mild steel.

Mandrels

The mandrel/chuck can be made from wood, steel alloys, or synthetic

materials. The choice of material is dictated by the hardness of the material to

be spun and by how many times the tool is expected to be used.

Cut-off tools

Cutting of the metal is done by hand held cutters, often foot long hollow bars

with tool steel shaped/sharpened files attached. This is dangerous and should

only be done by skilled tradesmen.

In CNC applications, traditional carbide or tool steel cut-off tools are used.[1]

Rotating tools

Some metal spinning tools are allowed to spin on bearings during the forming

process. This reduces friction and heating of the tool, extending tool life and

improving surface finish. Rotating tools may also be coated with thin film of

ceramic to prolong tool life. Rotating tools are commonly used during CNC

metal spinning operations.

Commercially, rollers mounted on the end of levers are generally used to form

the material down to the mandrel in both hand spinning and CNC metal

spinning. Rollers vary in diameter and thickness depending the intended use.

The wider the roller the smoother the surface of the spinning; the thinner

rollers can be used to form smaller radii.[1]

Lathes

Woodworking lathes are often used, although a wilson lathe is the most

common manual spinning lathe for spinning metal by hand. The mandrel

having been formed from wood on the lathe or steel chuck machined on a CNC

lathe previous to mounting on the metal stock. All stock sizing is done prior to

the spinning.

BENDING

Bending is a common technique to process sheet metal. It is usually done by

hand on a box and pan brake, or industrially on a brake press or machine

brake. Typical products that are made like this are boxes such as electrical

enclosures, rectangular ductwork, and some firearm parts such as the receiver

of the AKM AK-47 variant.

Press Brake

Usually Bending has to overcome both tensile stresses as well as compressive

stresses. When Bending is done, the residual stresses make it re bend or spring

back towards its original position, so we have to overbend the sheet metal

keeping in mind the residual stresses.

The bending operation is the act of

bending blanks at a predetermined angle. An example would be an "L" bracket

which is a straight piece of metal bent at a 90° angle. The main difference

between a forming operation and a bending operation is the bending operation

creates a straight line bend (such as a corner in a box) as where a form

operation may create a curved bend (such as the bottom of a drinks can).

EMBOSSING

1. Embossing is the process of creating a three-dimensional image or

design in paper and other ductile materials.

2. It is typically accomplished with a combination of heat and pressure on

the paper.

3. This is achieved by using a metal die (female) usually made of brass and

a counter die (male) that fit together and actually squeeze the fibers of the

substrate.

4. This pressure and a combination of heat actually "irons" while raising the

level of the image higher than the substrate to make it smooth. In printing this

is accomplished on a letterpress. The most common machines are the Kluge

Letterpress and the Heidelberg Letterpress.

5. The term "impressing" enables one to distinguish an image lowered into

the surface of a material, in distinction to an image raised out of the surface of

a material.

6. The embossing process can be applied to textiles as non-wovens to get

better finished products as sanitary napkins, diapers, tissue paper and others.

7. In printing it is used as an accent process and can be used in conjunction

with ink called colour register embossing or with no ink called blind embossing.

8. It also can be used with foil stamping which when embossed with foil is

known as combination stamping or combo stamping.

9. All of these processes use a die and counter die. Most types of paper and

boards can be embossed and there are no restrictions on size.

10. Embossing involves a separate stage in the production process, after any

varnishing and laminating. This process costs as much as printing.

Notary use: A notary public frequently uses embossing to mark legal papers,

either in the form of an adhesive seal, or using a clamp-like embossing device

used to certify (a signature on a document, contract, etc.) or cause to become

certified through a notary public or bill.

Postage stamps: Embossing has been used regularly on postage stamps.

Notable early examples include some of the earliest stamps of Italy, Natal,

and Switzerland, as well as the early high values of Britain. Modern stamps

still sometimes use embossing as a design element.

Rubber stamp embossing / Heat embossing: Rubber stamp embossing is

another form of embossing popular in scrap booking and card making. A

rubber stamp is used to apply adhesive (often a slow-drying, sticky ink called

pigment ink) to paper in a desired pattern. Embossing powder is dusted onto

the paper and then blown away, so that it adheres only to the stamped

surface. The powder is then subjected to heat, which causes it to melt and

cover the stamped area. When the heat is removed, the liquified powder fuses

into a palpable smooth raised surface in the shape of the stamped pattern.

Embossing powders are available in transparent, translucent, opaque, metallic,

and glitter colors for a variety of artistic effects.

11. A variation on heat embossing stamped images is triple embossing. An

area of paper is covered with pigment ink and embossing powder sprinkled all

over it and heated until molten. This is repeated so that there are a minimum

of 3 layers of heated powder. While this triple layer of powder is still hot, a

rubber stamp can be pressed into it to leave an indented design.

Embossing also refers to an image processing technique which the color at a

given location of the filtered image corresponds to rate of color change at that

location in the original image. Applying an embossing filter to an image often

results in an image resembling a paper or metal embossing of the original

image, hence the name.

COINING

1. Coining is similar to forming with the main difference being that a coining

die may form completely different features on either face of the blank, these

features being transferred from the face of the punch or die respectively.

2. The coining die and punch flow the metal by squeezing the blank within a

confined area, instead of bending the blank. For example: an Olympic medal

that was formed from a coining die may have a flat surface on the back and a

raised feature on the front. If the medal was formed (or embossed), the surface

on the back would be the reverse image of the front.

Compound operations

1. Compound dies perform multiple operations on the part. The compound

operation is the act of implementing more than one operation during the press

cycle.

2. Compound die: A type of die that has the die block (matrix) mounted on

a punch plate with perforators in the upper die with the inner punch mounted

in the lower

3. set. An inverted type of blanking die that punches upwards, leaving the

part sitting on the lower punch (after being shed from the upper matrix on the

press return stroke) instead of blanking the part through.

4. A compound die allows the cutting of internal and external part features

on a single press stroke.

Comparison to other forming techniques

1. Other methods of forming round metal parts include hydroforming,

stamping and forging or casting.

2. Hydro-forming and stamping generally have a higher fixed cost, but a

lower variable cost than metal spinning.

3. Forging or casting have a comparable fixed cost, but generally a higher

variable cost.

4. As machinery for commercial applications has improved, parts are being

spun with thicker materials in excess of 1" thick steel.

5. Conventional spinning also wastes a considerably smaller amount of

material than other methods.

Advantages

These are several benefits of spinning and shear forming. Several operations

can be performed in one set-up.

• Work pieces may have re-entrant profiles and the profile in relation to

the center line virtually unrestricted.

• Forming parameters and part geometry can be altered quickly, at less

cost than traditional metal forming techniques.

• Tooling and production costs are also comparatively low.

• Spin forming is easily automated and an effective production method for

prototypes as well as high production runs.

Die (manufacturing)

A die is a specialized tool used in manufacturing industries to cut, shape

and form a wide variety of products and components. Like molds and

templates, dies are generally customized and uniquely matched to the

product they are used to create. Products made with dies range from simple

paper clips to complex pieces used in advanced technology.

Die forming

Progressive die with scrap strip and stampings

1. Forming dies are typically made by tool and die makers and put into

production after mounting into a press.

2. The die is a metal block that is used for forming materials like sheet

metal and plastic. For the vacuum forming of plastic sheet only a single

form is used, typically to form transparent plastic containers (called blister

packs) for merchandise.

3. Vacuum forming is considered a simple molding thermoforming

process but uses the same principles as die forming.

4. For the forming of sheet metal, such as automobile body parts, two

parts may be used, one, called the punch, performs the stretching, bending,

and/or blanking operation, while another part, called the die block, securely

clamps the workpiece and provides similar, stretching, bending, and/or

blanking operation.

5. The workpiece may pass through several stages using different tools or

operations to obtain the final form.

6. In the case of an automotive component there will usually be a shearing

operation after the main forming is done and then additional crimping or rolling

operations to ensure that all sharp edges are hidden and to add rigidity to the

panel.

Die components

• Die block

• Punch plate

• Blank punch

• Pierce punch

• Stripper plate

• Pilot

• Dowel Pin

• Back gage

• Finger stop

Die operations and types

Die operations are often named after the specific type of die that performs the

operation. For example a bending operation is performed by a bending die.

Operations are not limited to one specific die as some dies may incorporate

multiple operation types.

DESIGN CALCULATIONS

For Blanking and Piercing

Clearances :

Clearances are one of the main factors controlling a shearing process. The

clearance per side is given by

C=0.0032. t . τ

Where

t= thickness and

τ= material shear stress, MPa

Clearance as percentage of stock thickness

Material Round Other contours

Soft aluminum<1 mm 2 3

Soft aluminum>1mm 3 5

Hard aluminum 4 to 6 5 to 8

Soft copper alloys 2 3

Hard copper alloys 4 5 to 6

Low carbon steel 2 3

Hard steel 3 5

Silicon steel 3 4 to 5

Stainless steel 4 to 6 5 to 8

Angular Clearance

Angular clearance or draft in a shearing operation depends on the material,

thickness and shape of the stock used.

Its value ranges from 0.25 to 2 deg per side.

Stripper Force:

As the punching is completed the stock tends to grip the punch as the punch

moves upward which makes the use of a stripper necessary to separate the

punch from the job.

The force required for the same is called stripper force

F(s) =K. L. t

Where

F(s) = stripping force, kN

L = perimeter of cut, mm

t = material thickness

K =stripping constant

=.0103 for low carbon steels with t<1.5mm with cut at the edge.

=.0145 for same material but for other cuts

=.0207 for low carbon steels, t>1.5mm

=.0241 for harder materials

Punching Force:

As the name indicates it is the force of the punch needed to cut the blank or

pierce a sheet

F(p) = L . t . τ

F(p)= punching force, N

τ =Shear strength MPa

For holes with diameter d<t

F(p)=D.t. s/3√D/t

Where, D= diameter of punch,mm

s = tensile strength in Mpa.

Shear Force on the punch:

Sometimes a component is required to be sheared on a smaller capacity

punching then a shear is ground or cut on the face of the die or that of punch

to distribute the cutting action over a period of time . This is done to relieve the

shear of the punch or the die face so that it contacts the stock for some time

period rather than instantaneously.

The relation used for calculating maximum shear stress is

F(sh)=L.t. τ (p/t1)

Where,

p= penetration of punch in fractions

t1=shear on punch or die, mm

Draw Die Design

Corner Radius on Punch:

Customery taken as 4t to 10 t and ideally taken as equal to punch radius.

Draw Radius:

Larger radius causes the metal to be released early by the blank holder and

thus lead to edge wrinkling. Too small a radius causes the thinning and tearing

of the side walls of the cups, generally,

Draw radius=4.t normal

=6 to 8 t when the blank holder is used.

Clearences:

An allowance in the range of 7 to 20% of the blank thickness is provided,

depending on the cup material and cup dimensions.

Clearences in drawing in terms of blank thickness

Blank thickness

(mm)

First draw Second draw Sizing draw

Up to 0.40 1.07 to 1.09 1.08 to 1.10 1.04 to 1.05

0.41 to 1.25 1.08 to 1.10 1.09 to 1.12 1.05 to 1.06

1.30 to 3.0 1.10 to 1.12 1.12 to 1.14 1.07 to 1.09

Above 3.01 1.12 to 1.14 1.15 to 1.20 1.08 to 1.11

Blank Size:

The calculation could be based on volume, surface area or by layout. Some

useful relations in calculating the blank diameter for cylindrical shell for

relatively thin materials are given by:

D=√(d2+4dh) when d ≥20r

D=√(d2+4dh-0.5r) when 15r ≤d≤20r

D=√(d2+4dh-r) when 10r≤d≤15r

D=√((d-2r)2+4d(h-r)+2πr(d-0.7r)) when d<10r

Where

r = corner radius of the punch, mm

h = height of the shell, mm

d = outer diameter of the shell, mm

D = blank diameter, mm

An additional trim allowance could be provided of 3 mm per 25mm of cup

diameters.

Drawing Force:

Drawing force for cylindrical shapes can be given by the below empirical

equation

P= πdts[(D/d)-C]

Where

P= drawing force, N

t= thickness of the blank material, mm

s= yield strength of the metal, MPa

C= constant to cover friction and bending, its value is 0.6 to 0.7.

For other shapes the above formula gives an approximation which can be used

as a guide.

Blank Holding Force:

This force required depends on the wrinkling tendency of the cup which is very

difficult to determine and hence it is obtained more by trial and error. The

maximum limit is generally one-third of the drawing force.

Ironing force,

In ironing the objective is to reduce the wall thickness of the cup, and hence no

blank is required because the punch is fitted closely inside the cup. Neglecting

the friction and shape of the die, the ironing force can be estimated by the

following equation.

F= πd1t1savloge(t0/t1)

Where

F = ironing force, N

d1 = mean diameter of the shell after ironing

t1 = thickness of shell after ironing

t0= thickness of the shell before ironing

sav = average of tensile strength before and after ironing.

Percent Reduction:

There is a limit upto which a material can be strained. The amount of straining

or drawability is represented by the percentage reduction which is expressed in

terms of the diameter of the blank and the shell.

It is convenient to use outer diameter as the cup is normally specified by outer

diameter. The percentage reduction P is given by

P=100[1-(d/D)]

However, practically it is limited upto 40.

Height to

dia ratio

No. of

draws

Percent reduction

First draw Second

draw

Third

draw

Fourth

draw

Up to 0.75 1 40

0.75 to 1.50 2 40 25

1.50 to 3.00 3 40 25 15

3.00 to 4.50 4 40 25 15 10

Maximum reductions possible in single draw

Materials Percent reduction

Almunium alloys 45

Copper 45

Brass, Bronze 50

Low carbon steel 45

Stainless steel 50

Zinc 40

Air Vent:

An air vent is normally provided on the punch to reduce the possibility of

formation of vaccum in the cups when it is stripped from the punch. The size of

the air vent depends on the punch diameter

Punch diameter (mm) Air vent diameter (mm)

Up to 50 4.5

50 to 100 6.0

100 to 200 7.5

Over 200 10.0

Drawing Speed :

The speed with which the punch moves through the blank during drawing is

termed as the drawing speed. This is very important parameter in drawing

because the higher speeds are sometimes detrimental. Particularly harder and

less ductile materials are likely to be excessively thinned out due to excessive

drawing speeds.

Material Drawing speed (m/s)

Aluminium 0.90

Brass 1.0

Copper 0.75

Steel 0.28

Zinc 0.75

Bending:

Bending refers to the operation of deforming a flat sheet around a straight axis

where the neutral plane lies. The nomenclature normally used in bending is

shown in fig. In a bent specimen, since neutral axis remains constant, it is the

required length. Beyond the bend lines, the material is not affected. Hence to

calculate the length required, it is necessary to find out the bend allowance

which is the arc length of the neutral axis between the bend lines.

Bend allowance, B=α(R+Kt)

Where

α = bend angle, radians

R = inside radius of the bend, mm

K = location of the neutral axis from bottom surface

= 0.33 when R< 2 t

= 0.50 when R>2 t

t = sheet thickness, mm

Bend Allowance Overview

Bend allowance is a term which describes how much material is needed between

two panels to accommodate a given bend. Bend allowance, while being

oftentimes tricky to determine for all cases, is fairly easy to predict and calculate

for many standardcircumstances.

Determining bend allowance is commonly referred to as “Bend Development” or

simply “Development”.

Area under compression

Area under tension

Bend allowance

Often bend allowances are calculated for a sheet metal part and used to make

costly tooling or production parts that require a lot of labor to produce. A scrap

tool or production run can be very costly, much more so that a test piece.

So if you are ever not sure of your developed flat length, make a test piece

(laser, turret or sheared piece) to confirm your development.

One of the easiest ways to make a test piece is to shear a piece to an exact

length, and then form it using the exact process that will be used to create the

part. After the part is formed, the part is measured and compared to the

expected lengths and the bend allowance is adjusted as needed. Often times,

when hard tools are produced, laser cutblanks are used to validate the forming

tools and part development before the cutting tools are completed.

No rule will apply to every case. While most bend developments can be predicted

with ease and will develop correctly, there is no perfectly scientific method for

predicting bend allowance due to the many factors like tooling conditions, actual

vs. planned thickness, forming method and the given part tolerance. Many

companies will develop their bend allowances based on standard formulas,

standard forming practices and historical trial and error.

General Principles

The Neutral Axis does not change.

When developing a flat blank length, there is a length of the part that does not

change.

This length is called the neutral axis. Material on the inside of the neutral axis will

compress, while material on the outside will stretch. Based on the material

thickness,

form radius and forming methods, the ratio of compression to tension in the part

will

change.

A part that is bent over a very sharp radius, when compared to the thickness, will

stretch more on the outside, which means that the neutral axis will lie closer to

the

inside of the bend. A part that is gradually bent will have less outside stretch,

which

means that the neutral axis will lie closer to the center of the part.

Compression/Tension Ratio Depends Mostly On Geometry.

K-factor – Effectively 50%T Max / .25%T Min

Where the neutral axis is situated in a bend is commonly called the “K-Factor” as

it is

signified as “K” in the development formulas. Since the inside compression can

not

exceed the outside tension, the k-factor can never exceed .50 in practical use.

This

means that the neutral axis cannot migrate past the midpoint of the material (i.e.

towards the outside). A reasonable assumption is that the k-factor cannot be less

than.25.

The neutral axis migrates based on the compression to tension relationship of

the given

bend.

3. Different Bend Types & K-Factors

Wrapped Hem (.29 k factor)

Machine Bend with Set (.33 k factor)

Machine Bend With No Set (.38 k-factor)

Bend Allowance Overview

V-Bend Or Brake Tool (.42 k-factor)

Rotary Benders (.43 k-factor)

Gradual Bends / Large Radii (.50 k-factor)

Bend Allowance Overview

4. Related Formulas

Radian Formula

When a developed length is calculated in radians, the equation is extremely

simplified

because the radian is the actual arc length, so no additional “translation” into

angles is

needed as in the “standard” formula below. In fact, the “standard” formula is the

radian

formula plus a “built in” angle conversion from radian measure to (base 360)

degrees,

shown in the “Common Formula”.

Common Formula

Since is more common to develop a part based on degrees instead of radians,

the bend

allowance formula commonly incorporates the degrees to radians conversion.

Recalling that 360 Degrees = 2ðRadians, then 1 Degree = 2ðRadians / 360

To convert the radian formula to work with degrees, we make the substitution

2ð/360

5. Special Cases

Single Hit Z-Bend

When a z-bend is hit in one hit, the middle panel will stretch more than expected.

This

is because the middle panel is trapped between two v-forms. A typical example

might

be on a .312 deep zee bend with .060 material, which might elongate .010”.

Wrapped hems

Wrapped hem developments should be treated with caution. While they will

generally

develop with a .29 k-factor, they are at minimum made with a two hit process

and

subject to a bit more variation. If the part has a reasonable tolerance, then the

risk is

minimized. Often times, a wrapped hem is used as a safety edge or a cosmetic

feature.

Additionally, a wrapped hem will see significant “backside” thinning which

usually

influences the leg length. Most parts are designed to a nominal outside thickness

and

not this backside “thinned out” thickness, so this must be accounted for in the

developed length.

Shallow z-bends

When a shallow bend is used, the neutral axis of one bend blends into another

one and

does not completely stay within the form arc. This often means that the

developed

length is only slightly longer than the flat length.

Gusseted Bends

When a part has a gusset in a formed flange, the gusseted area will generally

form

“high” as the gusset drives material beyond the expected development.

Punch Design:

The choice of the type of the punch and its design depends on the shape and

size of the pierced or blanked contour and the work material. Large cutting

perimeters require punches which are inherently rigid and can be mounted

directly. Smalller size holes require punches which may have to be supported

during the operation, and therefore need to have other mechanisms to join th

punch holder. The punches can be broadly classified as: plain punches, pedestal

punches, and punches mounted on punch plate.

Plain Punches:

These are made of solid tool steel block and are directly mounted to the punch

holder. These punches are joined to the holder by means of dowels and screws.

These must be large enough to provide the necessary space for dowels and

screws as well as the necessary strength to withstand the punching force. The

length and width of these punches should be greater than the height of the

punch. Sometime it may become necessary to have a height of the punch

greater than either length or width. This is when the punch is excessivly long and

the work performed is heavy or unbalanced.

Pedestal punches:

The proportions of the pedestal punches are also similar to that of plain punches.

The length and width of the base should be larger than or equal to the height of

the punch. The flange thickness and the fillet radius are to be liberally provided

to withstand the larger forces coming on the punch.

The fillet radius provided in a pedestal punch reduces the stress concentration,

but to strengthen the joining of the punch to the base.

Punches Mounted in Punch Plates:

A simple method of asembling a plain punch in punch plate is that the punch

with the uniiform cross-section is fastened to the punch holder by means of the

screws through the punch itself. The punch plates has the necessary holes for

locating the punch properly. Clearence holes are provided in the punch plate for

the positioning of the squared or sharp cornered punches.

Perforator type Punches:

Punches whose cutting face diameter is less than 25mm are termed as

perforators. The punches need not be round, but the inscribed circle of the punch

should have a diameter less than 25 mm. As a rule, all the perforators are

mounted in a punch plate.

If the cutting face is round, then assemblig the punch in any orientation is

possible. But for the punches with other contours some means of preventing the

rotation of the punch is necessary.

Some popular methods of preventing the punch rotation are by means of the

spring loaded ball, and the use of dowel slot of the size of 3 mm in the head of

the punch.

Quill punches:

For piercing very small holes less than 6 mm, it is desirable to provide extra

support to the punch shank by means of a closely fitting quill. Quilled punches

are more expensive if made individually because of the close fitting required

between the quill and the punch sizes. Therefore they are mass produces in

various standard sizes.

Back-up plate

Hardened back-up plates are normally required to be kept between smaller

perforator punches and the punch holder. Because of the smaller area of the

perforators, they have the tendency to dig into the softer punch holder in the

absence of a back-up plate. The back-up plate is about 6 mm thick.

Slug Ejection:

Sometimes, the slug that is punched, clinges to the punch face and comes along

with it during the return stroke. Normally, the die is supposed to restrain the slug

from moving along with the punch, because of the spring back. But in slugs of

very small sizes such as those from thin sheets, the spring back being too small,

the slug is likely to be drawn along by the punch. Hence, means are to be

provided for the ejection of the slug, particularly in small perforator punches.

Die Construction

Screws and Dowels:

Dowels are used for alignment of dies while as the screws keep the intact.the

head of the cap screw should be kept in arecessed hole to eliminate any

projection. The recess should be deeper than the head depth by about 3 mm to

account for the resharpening of the die.

For proper alignment of components only two dowels are required. If the

component is too small, the dowels would only weaken it and the construction

would not offer much resistance to any lateral deflection which may be caused

by the unbalanced side thrust. When the height of the component is deonted as

H is greater than 4 times the dowel diameter,d, it is good practice to relieve the

dowel holes. The relieveing would help in the finishing of the dowel hole

Die Block:

The die block size essentially depends on the workpiece size and stock thickness

and sometimes on blank contour.

Die block thickness for mild steel workpiece

Stock thickness (mm) Die block thickness (mm)

Up to 1.5 20 to 25

1.5 to 3.0 25 to 30

3.0 to 4.5 30 to 35

4.5 to 6.0 35 to 40

Over 6.0 40 to 50

Die blank thickness based on blanking perimeter

Blanking perimeter (mm) Die block thickness (mm)

Up to 75 20

76 to 250 25

Over 250 30

For softer materials such as non-ferrous alloys, die blocks can be made thinner

and similarily for higher shear strength material they can be made thicker.

A more scientific method is based on the experimental results obtained from a

series of die die tests for breakage under impact loading. The results of which

are given in the below table. First the die thickness is to be selected from this

table which is based on stock thickness and its shear strength.

Die block sizes:

Workpiece thickness (mm) Die thickness for 1Mpa of shear strength

(mm)

0.25 0.05

0.50 0.10

0.75 0.14

1.00 0.18

1.25 0.21

1.50 0.24

1.75 0.27

2.00 0.29

2.25 0.31

2.50 0.32

The values gvien in above table are for smaller blanks with cutting perimeters of

less than 50 mm. for those with larger perimeters, a correction factor given in

below table is to be applied to the thickness obtained from above table

Expansion factors for die thickness

Cutting perimeter, (mm) Expansion factor

Up to 50 1.00

51 to 75 1.25

76 to 150 1.50

151 to 300 1.75

301 to 500 2.00

If the die is properly supported in a die shoe, the thickness can be reduced to a

proportion of as much as 50%. A grinding allowance of 3 to 5 mm is to be added

to the die thickness to account for the necessary die sharpenings during the life

of the die.

WORKSHOP PRACTICE

Aluminium Sheets have now very extensive use in a variety of industries, from

building industry to dairy and milk product industry. Aluminium sheets are used

for roofing, doors andwindow frames, kitchen cabinets, kitchenwares, containers,

milk and dairy industry, electrical appliances and aeroplane bodies. Stainless

Steel Sheets provide very high resistance to corrosion. These now have

superseded brass and aluminium for kitchenwares, dairy equipment, food

processing plants, and chemical plants. Lead Sheets are highly malleable, soft

and have good resistance to acidic corrosion. These are weak in strength and

hence are mostly used as liners for containers of acid tanks and other structures.

Zinc Sheets are used for roofing purpose e.g. CGI sheets or corrugated

galvanized iron sheets. Zinc is also used in making die cast products besides

being mostly used for coating purposes. Now we have a clear idea of the types of

metals used. But this is not sufficient, we need to have the knowledge of the

various kinds of tools for converting these materials into useful products. Here

we can correlate lot many examples discussed earlier.

Hand Tools

Common tools used for sheet metal working include:

• ·Standard Scale

• Trammels

• Circumference Rule

• Prick Punch

• Straight Edge

• Centre Punch

• Steels Square

• Hand Snip or Shear

• Scriber

• Stakes

• Semi-circular Protractor

• Hammers

• Dividers

• Hollowing Block

Some of these tools are shown below:

Standard Scale made of

steel is used for

measuring dimensions and

for laying out profiles or

development of

patterns on the sheet metal. Circumference Rule provides direct linear

measurements of the circumference for the given characters. Straight Edge is a

flat graduated steel bar with one longitudinal edge bevelled. It is available in

different lengths, upto one metre or more and is used for scribing long straight

lines Flat Steel Square, is a L-shapped piece of hardened steel flat with

graduation on both the arms which “are at right angle to each other. It is used

for marking lines in perpendicular direction to any base line.Scriber a long wire or

rod of steel with its one end pointed sharp and hardened for scratching lines

during the process of laying out profiles on the metal sheets.Semi-circular

Protractor resembles with a semi-circular protractor used commonly in

geometrical drawings. It is made of steel mid is used for marking acid measuring

angles. Dividers are used for marking circles and arcs.Trammels are used for

marking large circles or arcs, which are beyond the scope of dividers. Prick punch

is used for indentation marks. The included angle of punch is 300. Centre punch

is used for marking centres for holes to be drilled.The included angle of a centre

porch is kept 900 Hand Snips or Shears are used for cutting sheet metal to the

required shape and size. When size is large, a snip is called Shear’. The snips

may be Straight Snips, Bent Snips and Slitting shears. The straight snips have

straight jaws and are used for cutting along a straight line. The bent snips have

curved blades and are used for cutting along curved lines. The slitting shears

have their lower jaws off-set to allow the metal to pass. The cutoff strip of sheet

creates no problem or abstraction to hand. The shears are used for cutting long

and continuous cuts. Stakes are the gadgets used for bending, seaming or

forming operations carried out on sheet metal. Some commonly used stakes by a

tinsmith are shown m Fig. 8 1(b).

The stakes work as both the supporting tool & the forming tool. Bick iron or beak

hoin slake ts a general purpose tool for straight bending, corner seam closing,

rivetting, etc, 0easing stake with horn has a tapering square horn with grooved

slots at one end mid round horn at the other end. The slots are used for wiring

and heading operations of a horizontal sharp edge and is used for making

straight, shape bends, Square stake ire available in different shapes. A hollow

mandril stake is used for rivetting, seaming and forming operations. Round hard

stake is used for forming or shaping curved surfaces A double seaming stake

with four heads comprises a horizontal bar with a hole at one end in which any of

the four heads may be fitted for carrying out operations of seaming, and

rivetting. Hammers used in tin smithy work are of various types in terms of

shape, size and weight. They have round or square heads to suit forming of

round or square shapes Soft faced hammers are used to avoid damage to sheet

metal. A wooden mallet is an soft hammer. A bumping hammer used for raising

curved shapes and a bossing mallet are two typical hammers used in tin smithy

work.

Different types of stakes used for bending sheet metals Hollowing blocks (or

Swages) are made of hard wood and are made to have curved forms or surfaces

on its top face.It is conveniently used for any ‘hollowing’ process in which sheet

metal is stretched particularly over a large part of the middle area of the blank.

The purpose of rising is to shrink or reduce the circumference of the blank, in

such a way that the metal is worked up or raised to required form.

Soldering Irons or Soldering Coppers consist of a forged tapered piece of copper

bit joined to air iron rod and a wooden handle mid are available in various shapes

and sizes The soldering iron is heated electrically or by some other external

means such as gas stove, smith furnace or even oxy-acetylene flame.

Sheet Metal Joints

Common types of joints used in sheet metalwork

are shown below

(a) Lap Joint (d) Rivetted Joint

(b) Flush Joint (e) Grooved Joint

(c) Edge Over Joint (f) Double grooved Joint

Some of the common types of Sheet Metal Joints

(a) Lap joint, the most common type of general use

(b) Flush joint used where one face of the article is required to be level or flush.

(c) Edge over joint used for readily fixing bottoms

(d) Rivetted joint where in overlap is kept about six to eight times of the

diameter of rivet

(e) Grooved joint wherein one edge of the sheet is folded down and the other up.

(f) Double grooved joint (inside type) is used to avoid projections on the outer

surface of the job. It holds together firmly the edges of round or straight-sided

jobs.

Laying out some typical forms

Development or Pattern layout involves laying the full size pattern of the product

to be made on the metal sheet which, when cut according to the laid pattern and

folded suitably, gives the required product. In general, laying out involves

making of square, rectangular, cylindrical and radial shapes (or the combinations

of these) for making articles of various configurations. Margins or Allowances (in

the form of additional size) in the dimensions of the pattern are kept for taking

care of lapping, folding, making edges or seams. For example, wire edges need

extra metal of 2.5 to 3 times the thickness of sheet. The three common methods

followed for laying out patterns are menstruated below :

a) Parallel line method involves development of surfaces or layouts using parallel

lines particularly in ewe of cylindrical land prismatic pares.

b) Radial line method of development of surfaces is used for components that

have slanting surfaces like that of cone sand pyramids.

c) Triangulation method is used for developing surfaces of the transition or

intermediate

pieces often used for

joining two different

pipes to the bottom end

of a chute or hopper

with square top edge.

SAFETY GUIDE FOR SHEET METAL WORKERS

There are certain things in your job that can lead to fatigue, discomfort, or pain

when you do them repeatedly or without breaks. These include:

• Exerting force to perform a task or to use a tool.

• Working in positions such as bending, kneeling, stooping, twisting, and

overhead reaching.

• Using awkward hand, wrist, elbow, or shoulder postures.

• Remaining in the same position for a long time with little or no movement.

• Continuous pressure from a hard surface or edge on any part of the body.

• Working in very hot or very cold temperatures produced by climate,

equipment, or machines.

• Sitting on, standing on, or holding equipment or tools that vibrate.

In addition, stressful work situations can increase muscle tension and reduce

awareness of proper work technique.

Most common injuries:

Back, Wrists and Hands, Knees, Neck and Shoulders

Prepare Yourself for WorkJust as a runner prepares for a race event by warming up, prepare for work by

warming up and stretching. Warm up by walking, marching in place, or moving

your arms in circles. Once your muscles are warm:

• Stretch S-L-O-W-L-Y and hold each stretch 3-5 seconds.

• Stretch a few minutes before and during your workday.

Caution: Check with your doctor before exercising. If you feel discomfort while

exercising, stop immediately!

While you are off work, keep yourself physically ready for returning to work, whether it’s the next day or later.

Be AwareIf you experience symptoms, you must change the way you work or the tools you

use. If

you don’t change, your symptoms may get worse and may keep you from

working at all.

You may have a problem if you have any of these symptoms:

• Constant fatigue

• Cold hands

• Swelling

• Numbness

• Tingling

Where?

• Back

• Hands

If you develop any symptoms:

• Talk with your supervisor about your symptoms right away.

• Work with your supervisor to identify the cause of the problem.

• Follow your company’s ergonomics program and its Injury and Illness

Prevention Program.

• Always look for better ways to do your job.