Sheet metal drawing operations

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Drawing operations are classified according to the

shape of the part drawn, as follows: cupping, box

drawing, panel drawing shallow and panel drawing

deep.

Cupping

Cupping (Figure 5.24) is the drawing of parts having

a circular or cylindrical shape. The cup may be

perfectly cylindrical, or be composed of several

cylinders of different diameters, or have tapered

walls with spherical bottoms or flat bottoms.

Changing the flat blank to the cup is called the

drawing operation. Reducing the cup to a smaller

diameter with greater height is called redrawing. If

the cup is turned inside out during reduction, the

operation is called reverse redrawing.

Box drawing

The drawing of square or rectangular shapes is

called box drawing. Greater blank holder force is

required in box drawing than in cupping to prevent

wrinkling at the corners of the box. Box drawing

Figure 5.22Metal movement during forming

Metal forming processes and machines 181

Figure 5.23Metal flow during drawing

Figure 5.24Drawing operations

182Repair of Vehicle Bodies

consists basically of forming at the straight sections

and drawing at the corners. Some flow of

metal does, however, occur at the straight sections.

Panel drawing

Drawing of irregular-shaped parts is called panel

drawing. The contour of the parts changes irregularly,

causing a wide variation of stresses; for

example, wrinkling or excess of metal may occur

in one section while the metal tends to cause tears

in other sections. It is very difficult to predict these

stresses, as wrinkling is not limited to the flange

area as in cupping, and tearing or failure is not limited

to the bottom of the panel as in cupping.

Hence panel drawing dies must go through a tryout

period before production is possible.

Adjustment in blank holding pressure, surface contour

and pressure, draw radii and blank size are

made during the try-out period to correct for variations

in stresses from the predicted values.

If the panel height is great the operation is called

deep drawing; low panel heights indicate shallow

drawing (Figure 5.25). Shallow panels often incorporate

a great deal of stretch forming of metal.

Deep drawn panels are used for automobile front

wings, rear wings, rear quarter panels and instrument

panels. Shallow drawn panels are used for

automobile roofs, doors, bonnets and boot lids.

5.14 Sheet metal cutting for press work

There are four types of sheet metal cutting for

press work: shearing, cut-off, parting and blanking.

In shearing the cutting action must be along a

straight line. Shearing does not use a press or die

and is therefore not a stamping operation. It is used

for the following purposes:

1 To cut strip or coiled stock into blanks

2 To cut strip or coiled stock into smaller strips to

feed into a blanking or cut-off die

3 To trim large sheets, thereby squaring the edges

of the sheets.

A cut-off is made by one or more single-line cuts.

The line of cutting may be straight, angular, curved

or of a broken design. It is important to note that

the blanks must interlock perfectly before a cut-off

operation is carried out so that no scrap is produced

during cutting. The operation is performed

on a press by a die and therefore may be classified

as a stamping operation.

Parting is used when the blanks will not interlock

perfectly. Some scrap is inevitable since the

blanks do not fit together, and so parting is not as

efficient as shearing or cut-off.

In blanking the cutting action cuts out the whole

shape of the part in one operation. Excessive scrap

is formed during blanking and therefore it is a very

inefficient operation. The scrap is in the form of a

ribbon commonly called the ‘skeleton’. Blanking is

performed in a press by a die and is a stamping

operation.

Questions

1 Describe the forming and fabricating properties of

the steels used in vehicle body manufacture.

2 With the aid of sketches, describe how a lowcarbon

steel can be shaped into four different

forms to provide strength in body construction.

3 Which properties of a low-carbon steel allow it to

absorb energy resulting from impact through

Figure 5.25Drawn parts (Rover Group Ltd ) collision?

Metal forming processes and machines 183

4 Explain the following treatments that are

associated with aluminium alloys: (a) solution

treatment (b) age hardening (c) precipitation.

5 Explain the following heat treatments to carbon

steel: (a) normalizing (b) hardening (c) tempering

(d) case hardening.

6 What is meant by the term ‘work hardened

material’?

7 Explain the process of annealing.

8 State the important conditions to be observed

when using a wheeling machine.

9 List three safety measures to be observed when

using a treadle operated guillotine.

10 What is meant by the principle of shearing metals?

11 Describe the differences between a bending

machine and a rolling machine.

12 Explain the operation of the swaging machine.

13 Explain the three methods by which heat can be

transferred from one body to another.

14 Define the importance of the property ‘ductility’

in a material.

15 Explain the changes which affect metal when

heated and then cooled.

16 Describe the condition of a metal panel which

has been excessively wheeled.

17 After sheet metal has been work hardened,

describe which process will allow further working

of the sheet metal.

18 What is the minimum carbon content required

for steel which will be hardened by heat

treatment?

19 Describe a simple workshop method which can

be used to estimate the temperature when

tempering carbon steels.

20 When working with carbon steel, what is the

difference between normalizing it and annealing it?

21 List the properties contained in commercially

pure aluminium.

22 What practical factors limit the width of sheet

metal which may be formed on a wheeling

machine?

23 By means of a sketch, draw one type of swage

which could be produced on a flat piece of sheet

metal.

24 What is meant by the upper critical point of

steel?

25 Explain the blanking operation.

Measuring and

marking-out

instruments

6.1 Marking out

A body repair worker must have a good general

knowledge of engineering drawing and surface

developments in order to mark out replacement

parts on sheet metal, as this requires accurate

calculation of sizes and angles. While the majority

of body workers who work in the repair side of

the industry do not have to perform a lot of difficult

marking out from elaborate drawings on to

sheet metals for the making of the articles, those

employed in the manufacturing side of the industry

(e.g. coachworks) have to mark out all units and

assemblies from drawings, whether for a one-off

item or for ‘mass’ production. Consequently a thorough

knowledge is essential of measurement and

marking tools, and also of how to check these tools

so that a standard of accuracy can be achieved and

maintained throughout.

Blueprints, as working or production drawings

are generally termed, are photographic reproductions

of an original drawing. They take their name

from the colour of the finished print when the

drawing is reproduced on ferroprussiate paper.

This paper produces copies of the drawing in white

lines on a blue background. Several other types

of coloured prints are also produced by industrial

contact photo equipment, but in spite of their colour

these are still generally known as blueprints. The

practical body worker is rarely called upon to produce

a finished drawing, but he will find it necessary

in many cases to understand and work from

both general and detailed drawings as set out by a

skilled draughtsman. The main object of reading a

drawing is to obtain a clear mental picture of what

another person has represented on paper by means

of a conventional arrangement of lines and symbols.

To this end the value of hand sketching cannot

be too fully emphasized, as well as the making of

simple models, for a much clearer impression of

form can be obtained when movement and touch

are combined with sight. The skilled craftsman,

particularly in the small workshop or garage, is

generally responsible for seeing his work through

all stages of manufacture, commencing with

careful study and accurate interpretation of the

drawing. Before actual manufacture can begin, it is

necessary to set out the desired shape by marking

the outline on the surface of the sheet of material.

Automobile work demands a thorough knowledge

of the methods of sheet metal work, and

although the machine has displaced the man for

certain operations there is an increasing demand

for craftsmen who are capable of developing the

large variety of work met in the industry. Much of

the work done deals with double-curvature shapes

which are in some cases fabricated by hand in

sections. This type of work needs templates made

to the exact curvature required so that the work

can be checked at each stage of its progression.

Also, very accurate alignment jigs have to be manufactured

to hold these parts, after they are formed

to the template size and shapes, so that they can

be welded together into the finished component.

Marked-out templates, which are usually in thin

metal sheet, cut and then filed to a very accurate

size, are used when one is working to rolling radii

so that the curve can be checked as it is rolled.

Angle templates are useful when bending in the

bending machine to check the angle of bends so

that they are constant.

Manufacture of templates

These can be made first of a paper or hard card

known as template paper, then using materials such

as thin-gauge aluminium, tin plate, and in some

cases thin-gauge mild steel. These metals are used

for their ease of cutting. The next stage in the

manufacture of the template, if it is to be marked

straight on to metal, is to take the sizes from the

appropriate drawings and, using such instruments

as scribes, dividers, straight edges, trammels and

chalk lines where long sizes are involved, accurately

to mark them directly on to the metal. When

the template is fully marked out the profile is

centre punched very carefully on the scribed lines

at close intervals, and then cut, either by machine

or by hand using tin snips, to the centre-punched

marks. The metal is next filed very carefully to

obtain the final finish and checked from the

drawing for its dimensional accuracy. The template

is now ready to be used either for checking or

for marking out predetermined shapes directly on

to sheets. Metal templates have the advantage that

they can be kept and used again if necessary.

6.2 Basic marking-out and measuring

instruments

Rules

A length may be expressed as the distance between

two lines (line measurement) or as the distance

between two faces (end measurement). The most

common example of line measurement is the

rule. Checking by way of comparing with a rule

is called measuring. Measuring rules can be

adjustable or non-adjustable. The adjustable type is

usually of the folding variety, but there are also

thin spring steel rules which are referred to as steel

tapes. Rules are made of hardened and tempered

high-grade steel and are usually graduated in the

English and/or metric systems of measurement.

Rules should not be misused as feeler gauges,

scrapers, screwdrivers or levers; the end of the

rule particularly should be preserved from wear

because it usually forms the basis of one end of the

dimension. In use, a rule should be held so that the

graduation lines are as close as possible, preferably

touching, the faces being measured. The eye which

is observing the reading should be vertically above

the mark being read; this avoids what is known as

‘parallax’, which results in an inaccurate reading

due to the graduation lines being viewed from a

very oblique angle. Rusting of the rule should be

avoided by oiling or an occasional rub with metal

polish. Chalk is good for cleaning the rule.

Scribers

A scriber is used when marking out work, and will

leave a fine line on the metal surface (Figure 6.1).

This will more easily be seen if the surface of the

metal is first prepared by lightly rubbing it with

white chalk or a special colouring agent. Scribers

are made of hardened and tempered high-grade

steel, with a knurled body which facilitates handling.

They are ground at one or both ends to a fine

point which should be kept sharp and protected

when not in use (see Figure 6.2). Some of the

double-ended scribers have points which can be

Measuring and marking-out instruments 185

Figure 6.1Marking out with a scriber and rule (Neill

Tools Ltd (Eclipse))

186Repair of Vehicle Bodies

replaced if damaged or worn. A handy pocket

scriber is also available. When using the scriber,

it is inadvisable to mark any lines other than

cutting lines on coated metals, thus avoiding the

destruction of the protective coating, as this may

result in corroding of the parent metal. It is also

inadvisable when working on soft metals, such as

zinc, aluminium and copper, to scribe along the

full length of fold or bend lines, because this

could result in cracking during the bending of the

metal. Short marks at each end are sufficient for

bend lines.

Straight edges

These are used to test that surfaces are level or that

parts are in line one with another, and are ideal

for lining up and jig work. There are two types

of straight edge. One (Figure 6.3), made from goodquality

hardened and tempered steel about 3.175 mm

thick with one bevelled face, is made in lengths up

to 2 m. The other type, made from cast iron, is

ribbed along its length, and has a flat face. This type

can be more accurate than the steel straight edge

because its weight and robust construction reduce

the possibility of flexing. It is usually made in

lengths of straight edge up to 2.5 m. To test the surface

contour of a metal panel or sheet, the straight

edge is held against the panel and then moved over

the surface, and any distortion or unevenness is

shown as gaps between the edge of the straight edge

and the edge of the article. Straight edges can also

be used in sets of three for sighting parts into line.

A straight edge should never be used for any purpose

other than that for which it is intended.

Figure 6.2Engineer’s scriber (double-ended)

(Neill Tools Ltd (Eclipse))

Figure 6.3Straight edges (with square or bevelled

edges) (Neill Tools Ltd (Eclipse))

Measuring and marking-out instruments 187

Punches

Centre punch

This cast steel tool (Figure 6.4a) is driven into the

metal with a hammer blow. For marking out work,

locating centres for curves and defining profiles,

the centre punch is struck only lightly. Sometimes

lighter pattern punches, called prick or dot punches,

are used for marking out. A centre ‘pop’ should be

made as deeply as possible where a hole is to be

drilled. It is as well to mark hole positions lightly

at first, check them for accuracy, and then increase

their depth by heavier hammer blows, holding

the work on a firm foundation such as an anvil.

Punches should be ground so that the grinding

scratches lead to the point. The tapering point of

the marking punch is usually ground to an angle of

40°, whereas the centre or pop punch has a more

obtuse angle of 60° which facilitates the application

of a drill.

Automatic centre punch

This is hand operated, and a hammer should not be

used. It delivers a punch when hand pressure is

applied to the knurled head. The depth of impression

can be adjusted by turning the knurled head.

Nail punch

This is designed to drive nail heads below the

surface of wood (Figure 6.4b). The square head

prevents rolling and provides a larger face for

striking with the hammer, while the cupped and

bevelled point limits skidding off the nail head.

Parallel pin punch

This is used to punch out pins and dowels on

mechanical assemblies (Figure 6.4c).

6.2.5 Try square (engineer’s square)

This is the most common tool for testing squareness,

and is used for internal and external testing to

check whether work is square (Figure 6.5). It can

also be used for setting out lines at right angles to

an edge or surface. A try square consists of a stock

and a blade, and these may be made separately and

joined together, or the whole square may be formed

from a single piece of metal. Squares are of cast

steel, hardened and tempered and ground to great

accuracy. The size of the square is the inside length

of the blade. Engineers’ squares are made with a

groove cut in the stock where the blade enters the

stock; this allows for any burr on the edge of the

metal being tested.

Before a square can be used, one surface of

the workpiece must be made level and true. This

surface is termed the ‘master’ or ‘face’ side. To use

a square to scribe a line at right angles to the face

side, the blade of the square is laid flat on the workpiece

with the inside of the stock pressed firmly

against this face side; the line can then be scribed

along the outer edge of the blade at the position

required. To check that an adjacent surface is at right

angles to this face side, the square is held upright

on the workpiece on the corner and the inside edge

of the stock is pressed firmly against the face side;

any discrepancy in the angle will be indicated by a

gap or gaps between the inside edge of the blade

and the surface being tested. Engineer’s squares

are available in three grades of accuracy: B (workshop),

A (inspection) and AA (reference).

Figure 6.4(a) Centre punches (b) nail punches

Figure 6.5Try square (Neill Tools Ltd (Moore and

Wright))

188Repair of Vehicle Bodies

Bevel gauge

When two surfaces are at any angle other than 90°,

the angle between them can be tested with an instrument

called a bevel. Bevels consist of a stock with

one or two blades which are adjustable and can be

locked at any angle by means of a thumbscrew.

There are three main types. First is the plain

engineers’ bevel, which has an offset slot for testing

angles which are not possible with a straight slot.

Second is the universal bevel, which has offset

blades and can be set at any angle. The thumbscrew

on the bevel is recessed into the stock, allowing it, if

necessary, to lie flat on the work. Third is the combination

bevel, which consists of stock, split blade

and auxiliary blade. The split blade swings to any

angle and, with the auxiliary blade attached, can be

transferred from one surface to another, work can be

laid out or any desired angle checked and measured.

Bevel protractors

The alignment of planes or surfaces designed to

form a definite angle may be checked by the

universal bevel protractor. This is a graduated disc

with a fixed adjustable blade and a stock with

a straight-edge extension. The disc with the blade

is graduated from 0° to 90° each way and rotates,

together with its blade, through an entire circle in

relation to the stock. For very accurate testing a

vernier bevelled protractor must be used.

Adjustable protractors

When the edge of the workpiece forms an angle

which deviates from the usual angles of 90° and

45°, this angle may be determined with the aid of

an adjustable protractor. The protractor consists of a

semicircular segment covering 180°. The movable

blade which indicates the angle reading is secured

by a locknut which can be set and tightened at any

angle, thus allowing angles to be read directly from

the workpiece. Such a protractor may also be used

for marking out when the marking lines do not

form one of the usual angles.

Combination square

This is used to test that work is true, to measure

and check angles, and to locate the centres of round

bar or centre lines of tubes. It consists of a blade

(Figure 6.6) which is usually marked in English

and/or metric scales, and which may be used in

conjunction with any one of three heads. The various

heads enable the tool to be used as a square,

a protractor or a gauge. A central groove along

the length of the blade enables each of the heads

to slide along this blade and to be locked in any

position on the blade. The three heads used on this

square are:

1 The square head, which has two surfaces, one

at 90° and the other at 45° to the base, so that it

can be used as a normal or a metre square, and

also as a depth or height gauge. This head is

usually fitted with a spirit level and a scriber,

which is contained in the head.

2 The protractor head, which is used to measure

or check any angle up to 360° by simply reading-

off the scale.

3 The centre head, which is V-shaped with two

internal surfaces. These form an angle of 90°, and

very quickly find the centres of round objects.

The complete set forms a very useful workshop

accessory and fulfils many needs, but when a high

degree of accuracy is required a normal solid try

square should be used instead of the square head of

the combination set.

Centre square

This is a flat square made of hardened and tempered

tool steel. It has two blades of different thicknesses

which are designed for the accurate location of the

centres of faces of round bar or discs. The centre

Figure 6.6Combination square (Neill Tools Ltd

(Moore and Wright))

Measuring and marking-out instruments 189

square tool is used in conjunction with a scriber.

It is held against the circumference of the round

bar so that one blade touches the bar on its edge

and the other blade lies across the surface, giving a

diameter line which is then scribed. When two or

more of these lines have been scribed the point of

intersection is the centre of the bar.

Dividers

These are metal workers’ compasses, made of steel

with hardened points. They are used for scribing

circles and arcs, and for marking off lengths. The

most common type are spring dividers (Figure 6.7).

These comprise a circular spring bow which joins

the legs at the top and acts as a spring and pivot.

The distance between the points is set by means of

a fine adjustment screw mounted across the legs. In

use, the centre point of the circle to be scribed is

lightly marked and is used for the static position of

the pivot leg, while the other leg is rotated to scribe

the circle. The points of the divider must be kept

finely ground and without making one leg shorter

than the other, and the points must be protected

against damage when not in use.

Trammels (beam compasses)

Trammels (Figure 6.8) are used to describe arcs

and circles of large radii and mark off lengths

which are beyond the range of dividers. They comprise

two heads fitted with scribers which slide

along a bar. The bar can be any length. To use a

trammel, one leg is placed at the centre point of the

required arc or circle. The other leg is then set and

locked at the correct distance by the aid of a locking

screw fixed in the head. Light pressure is kept

on the pivot leg with one hand while the other hand

describes the arc with the other leg. Additional legs

can be supplied with curved tips and can replace

the scriber legs for use as calipers.

Figure 6.7Spring dividers (Neill Tools Ltd (Moore

and Wright))

Figure 6.8Trammel heads (Neill Tools Ltd (Eclipse))

Calipers

These are used to measure or compare distances

or size (Figure 6.9). There are two designs: inside

calipers, which have straight legs than turn outwards

towards the points and are used for measuring

bores and internal spaces; and outside calipers,

which have curved legs that turn inwards towards

190Repair of Vehicle Bodies

the points, and which are used for external diameters

and other outside measurements. In their simplest

form calipers are usually termed ‘firm joint’

calipers and consist of two legs riveted together at

the top. Also available are the ‘spring joint’ type

where the legs are joined at the top by a circular

spring band and a fine adjustment screw is fitted

across the legs. In use, the legs of the calipers are

first opened out to the approximate size and held

square with the object being measured. The points

are next screwed towards each other so that they

just touch both sides of the part to be measured.

The calipers are then carefully withdrawn from the

workpiece and the distance between the two points

is measured.

Oddleg calipers

This measuring instrument is half caliper and half

divider (Figure 6.10). It consists of two legs riveted

together at the top, one of which is ground to a

point while the other is turned inwards or outwards

at its tip. In use, the curved tip is held against the

edge of the workpiece and the line is scribed with

the point of the other leg. These instruments are

used for marking-out operations such as scribing

parallel lines, following perimeters, and drawing

intersecting arcs on cylindrical objects.

Gauges

Whereas measuring is the establishing of the

actual size of a component, gauging is the positive

comparison of the size of a component with a

standard. Gauges are normally made from a hard

wear-resisting metal, and find extensive use in the

mass production of car bodies where many

components are made to standard sizes which

require frequent checking to ascertain their accuracy.

Handy ‘limit gauges’ are available for the

routine testing of parts specified to be within certain

limits, and come in various sizes and shapes

such as plugs, rings, slotted plates and tapered

plugs.

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