UNIT 4 SHEET METAL WORK

Structure

4.1 Introduction

4.2 Sheet Metal Materials

4.3 Sheet Metal Tools

4.4 HEMS

4.5 SEAMS

4.6 Sheet Metal Operations

4.7 Development Procedure

4.8 Experiment No. 1

4.9 Experiment No. 2

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4.1 INTRODUCTION

Sheet metal work is generally regarded as the working of metal from 16 gauges down to 30 gauges, with hand tools and simple machines into various forms by cutting, forming into shape and joining. - -

4.2 SHEET METAL MATERIALS .-

Black Iron

It is also known as uncoated sheet since it carries no artificial coating on its surfaces. However, it is probably the cheapest of all types of sheets used in sheet metal work. Being uncoated, it is prone to corrosion. Consequently, its use is confined mostly to the manufacture of such items which are to be painted before shipment, e.g. block iron is used in tanks, pans, trunks, stove pipes, etc.

Galvanized Iron

Zinc coated iron is called 'Galvanized iron'. This soft steel sheet is popularly known as GI sheet. The zinc coating resist rust, improves the appearance of the metal and permits it to be soldered with greater ease; but welding is not so easy as zinc gives toxic fumes and residues. Because of zinc it can with stand contact with water and exposure to weather, e.g. articles like cabinets, trunks, buckers, pans, etc. are made of galvanized iron sheets

Copper Sheets

These sheets are relatively costlier but having specific advantages in being good corrosion resistant and good in appearance. They are reddish in colour and their cold rolled variety, which is vastly used in sheet metal work, is highly ductile and malleable and therefore can be easily worked. Some representative examples of automobiles, various applications in chemical plants, domestic heating appliance, etc.

.4luminum Sheets

On account of it's inherit weakness it is not used in its pure form. The useful variety of aluminium alloy. Which is rolled into sheet form, carries additions of suitable amount of silicon, manganese copper and iron. It is whitish in colour and light in weight. It offers very high resistance to corrosion and abrasion. Its coinmon applications are aeroplane bodies, kitchen ware, etc.

Workshop Technology Tin Plates Laboratory

The nomenclature tin plates are used for those iron sheets which are coated with pure tin. As a result, these sheets which are coated with pure tin. As a result, these sheets provide a bright silvery appearance. They offer good resistance to corrosion and rusting and are mainly used for making packed food containers, cans, etc.

Stainless Steel

It is highly corrosion resistant alloy steel, which exhibits a bright surface without any coating. Though it is a little tougher than galvanized iron, in sheet form it can be shaped and span. It is widely used for making kitchen ware, food handling equipment, etc.

Brass

It is basically an alloy of copper and zinc and is available in many shades and colours. In sheet and strip forms it is used in many cold working processes, such as deep drawing, pressing, stamping, spinning, etc. Earlier it used to be the principal metal for making kitchen ware and utensils. But, now it is largely replaced by stainless steel and aluminum.

I

I Lead I

It is a very soft, weak, low melting point, malleable and heavy metal and possesses high resistance to acid corrosion. Due to low mechanical strength it is normally used in foil form to provide lining for containers and other articles made from some stronger material. It finds its application in sheet form in radiation shielding and an inner lining for acid tanks.

Zinc

It is bluish white metal and is quite ductile. In sheet form it is widely used for roofing work. But, in many other forms, it has its applications in coating, die casting, etc.

SHEET METAL TOOLS

Measuring Tools

Steel Rule

It is useful in measuring and laying out small work. It can be measure with accuracy of 0.5 mm.

Folding Rule

This is very useful in measuring and laying out larger work, the accuracy being 0.5 mm.

Steel Circumference Rule

This is used to find out directly the circumference of a cylinder.

Swing Blade Protractor

This is used for marking and measuring angles.

Venzier Calliper

This is used for measuring dimensions up to 0.02 mm.

Micrometer Caliper

This is used to measure the thickness of metal sheets accurately up to 0.01 mm.

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Thickness Gauge Sheet Metal Work

This is also called slip gauge and is used to measure the clearance between the parts during assembly.

Sheet Metal Gauge

This is used to measure the thickness of sheets.

Figure 4.1 : Different Parts of External Micrometer

Straight Edge and Steel Square

I Straight Edge i

This is a flat graduated bar of steel with one longitudinal edge beveled. This bar comes in variety of lengths ranging from 1 to 3 meters. It is useful for scribing long straight lines.

Steel Square

It is a T-shaped piece of hardened steel with marks graduated on the edges for measuring. The narrow arm of the square is called tongue and the wider part is known as the body. It is used for marking in the perpendicular direction to any base line.

Figure 4.2 : Vernier Caliper

Scriber, Divider and Trammel Points

Scriber

This is sometimes called the metal workers pencil. It is a long wire of steel I with its one end sharply pointed and hardened to scratch lines on sheet metal

I in laying out patterns.

i Dividers

Dividers are used for drawing circles or arcs on sheet metal. They are also used to mark a desired distance between points and to divide lines into equal

Workshop Technology Trammel Points Laboratory

The trammel points consist pf a bar with two movable heads. It is used to draw large circles or arcs that are beyond the limits of the divider.

Punches

A Punch is used in sheet metal work for marking out work, locating centers, etc. in a more permanent manner. Two types of punches are generally used :

Prink Punch

It is used to make small marks on layout lines in order to mark the prick punch marks longer.

Center Punch

It is used on1 y to make the prick punch marks larger at the centers of holes that are to be drilled. Solid and hollow punches are very similar the other two puncher the inner and outer faces of the punch meeting at an angle of 400.These are used for making small holes from 2.5 mm to 10 mm.

A hand level punch is sometimes used for making holes with a punch and die incorporated in the tool when a large number of holes are to be punched.

Chisel and Hammers

Chisels

They are generally used in sheet metal work for cutting sheets, rivets, bolts and chipping operations. A good number of cold chisels are used. The flat chisel and round nose chisel are most widely used in sheet metal work.

Hammers

They are used for forming shapes by hallo\;ling, raising, stretching or throwing off. These are many types of hammers but the most commonly used hammers are :

Riveting Hammer

Uscd for riveting.

Setting Hammer

Useful for setting down the edge, when making a double seam.

Raising Hammer

Used for forming of a flat sheet of metal into a curved or hallow shape such as a square, bowl, tray, etc.

' Mallet

These are soft hammers and made of saw hide, hard rubber, copper brass, lead or mostly of wood, used to strike a soft and light blow on the metal.

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Rivet~ng Hammers

Figure 4.4 : Hammers for Sheet Metal Work

Figure 4.5 : Mallet

Sheet Metal Work

Snips of Shears

A snip, also called a hand shear is used like a pair of scissors to cut thin, soft metal. It should be used only to cut 20 gauge or thinner metal. There are several types of snips available for making straight or circular cuts, the most common being straight snip have straight blade for straight line cutting while curved or bent snips have curved blades for making circular cuts. Both these snips are very light and can be easily handled by one hand. These are also double cutting shear, squaring shear, ring shear and circular shear used for particular requirements as the name indicates. The heavier classes are known as bench shears and block shears where one handle may be held in vice bench plate while the other handle is moved up and down to do the cutting.

Cut Only 20 Gauge of Thinner Metal

A Hand Shears w

Bent Shears

Straight Shears

Figure 4.6 : Snips and Shears I

Stakes

Stakes are the sheet metal workers evils used for bending, seaming or forming, using a hammer or mallet. They actually work as supporting tools as well as forming tools. They also help in bending operation. They are made in different shape and sizes to suit the requirements of the work.

1 Double Seaming Stake

i Use to make double seam I

i Beak-hum Stake

Used for riveting, fonning around and square surfaces, bending straight edges and making corners.

Bevel Edge Square Stake I Used to form comer and edges. 1 Hatchet Stake

Used to make straight, sharp bends and for folding and bending edges.

Needle Case Stake

I Small tubes and pipes are performed on it.

Workshop Technology Elow Horn Stake Laboratorv

Cone shaped articles are formed on it.

Hallow mandrel Stake

Used for riveting, seaming and forming.

Needle case stake

Blowhorn stake

Hatchet stake

Wlow mandrel stake

Figure 4.7 : Common Forms of Stakes

Groovlng with a hand groover

A Rivet set

Pliers

Figure 4.8

Common forms of soldering iron

Pliers are used for holding, cutting and bending work

Flat Nose Pliers

IJsed for forming and holding work.

Round Nose Pliers

Used for holding and forming various shape and patterns.

Groover's and Rivet Sets

Hand Groover

This is used for groove and flattern a seam. It is available in many shapes.

Rivet Set

This is hardened steel tool with hollow in one end. It is used to shape the end of a rivet into round, smooth head.

Soldering Iron Sheet Metal Work

They are used for soldering work and consis1 of a forged piece of copper joined to an iron with a wooden handle. These are also called soldering coppers. They are made in various shapes and sizes.

Half moan Blck lron Funnel stake Convex

Horse head

Figure 4.9 : Soldering Iron

Other General Tools Brick Iron

Used in forming long tapered cylindrical items.

Funnel Stake

Used for conical works.

Side Stake

Similar to brick iron but smaller in size.

HalfMoon Stake

For working the edges on discs.

Pipe Stake

Used for forming tubes.

Creasing Stake

Used for forming beads.

Convex Stake

For spherical work.

Horse Bead

Used for bending and general work for supporting and holding other stakes.

Bumping Hammer

Curved shapers are raised in sheet metal with the help of this tool. - - -

4.4 HEMS

A Hem is an edge or border made by folding. It stiffens the sheet of metal and does away with the sharp edge. Three common types of hems are :

Single Hem

It is made by folding the edges of the sheet metal over the make it smooth and stiff.

Double Hem

It is mad- by folding the edges over twice to make it stiff and smooth.

Wired Edge

Workshop Technology 4.5 SEAMS Laboratory

A Seam is a joint made by fastening two edges together. Most common types of seams are :

Lap Seam

It is the simplest type of seam and can be prepared as lap joint by means of soldering.

Grooved Seam

It is made by hooking two single hems together and then locking them by a groover.

Single Seam

It is used to join a bottom to vertical bodies of various shapes.

Double Seam

It is similar to single seam with the difference that its formed edge is bent upward against the body.

Dovetail Seam

It is similar to dovetail joint in carpentry and is used to join flat plate to a cylindrical piece.

Burred Bottom Seam

It is also called Flanged Seam. It is used to join the bottom of a container to its body. The flange on cylindrical jobs is often referred to as a burr and the process of making a narrow edge is known as burring.

Step 1 Step 2 Step 3 Open Fold Making Wired Edge

a Wired Edge

Step 1

Step 1 Step 2 Step 3 Setting Making Double Down a Double Seam Seam

A o r n e r Fold

Step 1 Step 2

Figure 4.10 : Pattern Making a Corner Fold

4.6 SHEET METAL OPERATIONS

4.6.1 Measuring and Marking

The standard market sizes of metal sheets are quite large. But the required sheet size for making component may be much smaller. So, a standard size sheet may have to he cut into several smaller pieces, each piece being sufficient for making one such component. These smaller sizes are first decided and then these sizes are marketed on the larger sheet to cut the latter into small pieces along the masked lines. A little allowance for cutting is always added to the required overall sizes so that the cut pieces are not undersize. Overall dimensions, length and breadth of the required smaller pieces are marked on the large sheet with the help of marking tools, including a steel rule, a straight edge, a steel square etc and a scriber. The sheet surface may have to be coated with a colouring medium, such as cellulose lacquer, so that the scribed lines are clearly visible. If circular pieces are needed a divider or trammel may have to be used to mark the circles. Their sharp points (tips of legs) work as scriber. Similarly, in mass production of identical small items the blank can be marked using a template and a scriber.

4.6.2 Cleaning

Many a times the blank surfaces need proper cleaning before being processed. This requirement is more prominent in case of non-ferrous metal sheets, like those of copper, brass and silver. For cleaning the surfaces of these blanks pickling process is used. It involves immersing the blank in a pickle both, consisting of one part of dil. &So4 and twenty parts water. This bath is heated and the blanks immersed in the hot bath. After allowing sufficient time for pickling the blanks are thoroughly washed in a stream of water and then allowed to dry. Cold pickling is also sometimes used in such cases where cleaning operation is not required very frequently, but in such cases either the pickling personal is to be increased or a smaller acid to water ratio of the pickle has to be used so as to ensure an effective cleaning of the surface.

4.6.3 Laying Out

It means the operation of scribing the development of the surface of the component on the sheetlsheet blank, together with the added allowances for overlapping, bending, hammering, etc. which when cut out of the blank and folded and joined will give the required component. Such a layout when made on the sheet is called a pattern and the process as pattern layout. If, however, the job to be made are small and in large number it is better to use a template for repetitive marking of the development and then cutting it along the marked contours. The template size will, obviously, include the required allowances.

Shearing Down Cutting Off Parting

Blanking Punching or Piercing

Lancing Nibbing

/ / , / / I f / / / / / / Llne of Cut

Notching Slitting

Drawn Pan

Figure 4.11

Sheet Metal Work

Workshoe Technology 4.6.4 Cutting and Shearing Laboratory

The word cutting is used when the sheet metals is cut by means of a chisel a hammer manual. The term shearing stands for cutting of sheet metal by two parallel cutting edges moving in opposite directions. This can be done either manually by using hand shears or snips or by means of machines called shears. The operation is then known as machine shearing or mechanical shearing. The selection of a particular method and means of cutting will depend on several factors, like thickness of sheet metal, sizes of blanks to be cut, amount of cutting required to be alone, number of blanks to be cut; type of production-jobbing, lot (batch) or mass production, available means of cutting etc.

4.6.5 Hand Forming

The term metal forming stands for shaping andor bending of sheet metal in three dimensions in order to give it the desired shape and size of the final product. For this, metal is either required to be stretched or shrunk in all directions or may need a combination of both. Best example of this operation can be seen if one observes the traditional metal workers manufacturing cooking utensils of brass out of blanks by hand hammering and shaping them into different shapes and sizes with out any joint. For producing hallow shapes through hand forming the centre of the metal has to be thinned or the edges have to be thickened. The former will involve sinking and hallowing work while latter will need raising of edges. Both hammers and mallets of different shapes are used in this operation according to whether the metal has to be stretched needing a solid blow, formed (needing an elastic blow by a mallet or a soft hammer) or sunk (requiring a floating blow).

4.6.6 Machine Shearing *

It is done by means of shearing machines. These machines can be hand operated (bench type) or Guillotine shears. The former is used for smaller and thinner sheets while the latter for larger and thicker ones. Other types of shearing machines are the rotary shears for continuous cutting along a straight line.

4.6.7 Nibbling

It is a process of continuous cutting along a contour, which may be a straight line or an irregular profile. The machines used in this operation are known as Nibblers, which are portable type shearing machines. These machines can be either electrically operated or pneumatically operated.

4.6.8 Circle Cutting

It is the operation of cutting circular blanks or curved contours with the help of a circle cutting machine. It is also a continuous cutting operation.

4.6.9 Piercing and Blanking

Piercing is basically's hole punching operation while blanking is an operation of cutting out a blank. In both the cases blanks will be produced, but in the fornler case obtaining a blank is not the objective. It is the hole of the desired size which is the objective and the blank produced may or may not be used further. In the latter operation the production of a blank of the desired size is the main objective, which is a useful part for further processing.

4.6.10 Edge Forming and Wiring

The edges of sheet metal products are formed (or folded) to ensure safety of hands, while handling these products, and to provide stiffness to the products in order that L!ey will retain their shapes during handling, i.e. will not get buckled or compressed by simple hand or fingure pressure during handling. When still stronger edges are needed they are reinforced by inserting a metal wire or rod and then forming the edge by curling the edge of the sheet metal around it. This not only increases stiffness of the joint but also improves its appearance.

4.6.1 1 Joint Making

Several means are used for joining sheet metal parts together or securing them to other metallic or non-metallic bodies. In case of very large size parts, specially when they are to be fastened to other metallic or large non-metallic bodies, screwed fastening can be used. Sheet metal parts can also be joined by riveting, welding, brazing and adhesives, self tapping screws are more popular screwed fastners in sheet metal work. However, still the most commonly used methods are joining the sheet metal parts by means of folded joints or self securing joints, followed by soldering and adhesive joining.

Original Stock

4' Thickness /F

2'. / \

I' Compression . side 1

Angle of bend

Figure 4.12 : Bending in Sheet Metal

Single Bend Double Bend %rai@ fib Edge Hem

- Embossing Enlarged Curling or M e & n o r

Section Beading wring Lockseam

Figure 4.13 : Common Kinds of Sheet Metal Bend

4.6.12 Bending

Sheet metal is required to be bent at several angles and curvatures to shape it into the required form. The bending operation involves stretching of metal on the outer surface and compressing it on the inner surface along a neutral line, which remains unchanged in length. Sheet metal can be bent by hammering over a base by hand or by means of . bending or rolling machines.

4.6.13 Drawing

Thin walled hallow shapes are produced in sheet metal through the drawing operation. The operation is carried out with the help of a die and a punch on a suitable press. If the drawn length of the component is less than its width or diameter it is called box drawing or shallow drawing. When the drawn length is more than the width the operation is known as deep drawing.

4.6.14 Soldering

It is a very common method of joining sheet metal parts. It involves spreading of a low melting point alloy, i.e. solder in molten condition between the surfaces or edges to be joined and allowing it to solidify these detailed study of soldering process and also of welding and brozing processes is important in sheet metal work.

Sheet Metal Work

Workshop 4.7 DEVELOPMENT PROCEDURE Laboratory

In sheet metal work the usual requirement is to lay the full size pattern on metal sheet that when the latter is cut according to the laid pattern and then folded or bent it will give the required object. This laying out of complete surface on the metal sheet is known as the development of the surface of the object or pattern layout for doing this job successfully the shape of the object should be carefully examined and then it should be divided into simple geometrical solids or portions of these solids. Then only it would be easy to develop the whole surface of the object for this, the surfaces of the simple solids, obtained as a result of the above division, can be developed separately and sheet cut accordingly. All these pieces when joined together will give the required object three common methods of laying out a pattern are :

lines

ParaHel Line Development

Figure 4.14 : (a) Procedure, and (b) Stretchout

4.7.1 Parallel Line Method It is that method of development of surfaces in which parallel lines are used in preparing the layouts. This method is generally applicable for pattern layouts of cylindrical and prismatic components

4.7.2 Radial Line Method This method of development of surfaces is used for such components which carry radial edges or slanting surfaces, like those of pyramids and cones.

4.7.3 Triangulation Method This method is used for developing the surfaces of transition pieces, i.e. for such components in which transition pieces are used to join different cross-sections, e.g. a circular pipe to a chute which has rectangular cross-section or a circular pipe to an air duct of square cross-section.

Application of Sheet Metal Work in Electronics Industry

(a) For fabricating wires to carry electricity.

(b) For manufacturing body parts of various electronic devices likes refrigerator, washing machines, etc.

(c) Used in communication industry

Source : B. S. Raghuvanshi and Hajra Choudhury (Workshop Technology). -- 4.8 EXPERIMENE NO. 1

Aim

To make a cubical box of given dimensions from given metal sheet using various sheet metal operations.

Apparatus

Metal sheet, mallet, shear, scribber, flattner, development on sheet.

Theory

The material of the sheet used in making box is glavanised iron (GI). The detailed theory is given in pages preceding this job.

Procedure

(a) Make, i.e. draw the development of the given job on the sheet with all allowances marked on it.

(b) After the approval of development, trace it on metal sheet using scribber with allowances.

(c) Cut the sheet using shear.

(d) Mark the six notches as shown in development attached on adjacent page using shear.

(e) Bent the allowances at 90" on proper side using stake and mallet.

(f) Bent the sheet at marked places to get four sides of the box.

(g) prepare the seam to lock the sides

(h) Prepare single hem on top sides.

(i) Bent the bottom sides at 90' outwards.

(j) Cut out metal sheet for base of the box using development, shear and scribber.

(k) Bent the outer allowances at 90" and then further by 60".

(1) Place the four sided hollow box on the base and prepare double seam using the allowances provided for it.

(m) Check for any sharp edges and dents.

(n) We get the required box.

Figure 4.16 : General Appearance of Bnx

Sheet Metal Work

Workshop Technology Laboratory

Result

(a) The box of sheet metal has been prepared having given dimensions.

(b) Precautions and Sources of Errors.

(c) Take the proper allowances, low value of allowances may lead to poor locks of metal while high value of allowances leads to metal wastage.

(d) Any bleeding caused by cut from metal should be immediately treated.

(e) Precaution should be taken while hammering to avoid dent formation on the metal surface.

4.9 EXPERIMENT NO. 2

Aim

To make a cylindrical beaker of given dimensions from given metal sheet using various sheet metal operations.

Apparatus

Metal sheet, Mallet, shear, scriber, flattner, development on sheet, stakes, rivets, punch, iron hammer, scale, etc.

Theory

The material of the sheet used in making box is galvanized iron. The detailed theory is given in pages preceding first job.

Procedure

(a) Cut the metal sheet according to development of the cylindrical portion with appropriate allowances.

(b) Mark the lines (required ones) .on sheet using scriber.

(c) Bent the 5 mm and 10 mm allowance on same sides. Bent of 5 mm of 10 mm one on reverse and then roll the sheet.

(d) Lock the seam tightly.

(e) Bent the 5 mm allowance provided on top and bottom of cylinder outwards using iron bars.

(f) Cut the base of cylinder from sheet as per its development.

(g) Make the seam (double) using this base, one outward bend of cylinder, iron bar.

(h) Completely bend the other outward allowance.

(i) Now cut at metal strip of dimension 110 x 25 rnrn which includes 5mm allowance on both sides along 25 mm breadth.

Cj) Bend the allowances inward using stakes, mallet, iron bar.

(k) Punch a hole in cylinder and strip using nut, punch and iron hammer.

(1) River the two using rivets and riveting set which has a hole for adjusting a ' flat for pressing.

(m) Roll the handle to required position and rivet the other end of strip.

(n) This gives the complete beaker, harnmerlfile down any pointed metal sites present on tha oeaker.

Sheet Metal Work

Figure 4.n : General Appearance of Beaker

The beaker of given dimensions has been prepared.

Precautions and Source of Errors

(a) Take the proper dlowances low value of allowances may lead to poor locks of metal while high value of allowances leads to metal wastage.

(b) Any bleeding caused by cut from metal should be immediately treated.

(c) Precaution should be taken while hammering to avoid dent formation on the metal surface.

(d) Care should be taken while preparing first seam to make the cylindrical surface as it can open up in latter stages.

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