sheet metal joints

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Chapter 4:

Sheet Metal Forming

Shafizan Bt. ShariffuddinSchool of Manufacturing Engineering

UniMAP

4.4 Folding Edges andMaking Seams

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Introduction

• Folding sheet metal to form edges and seams

of various kinds is one of the most important

operations in sheet metal work.

• The edges and seams have several purposes:- – to improve the appearane of finished produts

 – to strengthen the work piee

 – to fasten piees of metal together 

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Folding Edges

• There are two t!pes of mahine ommonl!

used in "ending or folding metal to form

edges or loks for seams :-

 – #ar folders

 – Folding "rakes

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Bar folder 

• This mahine is adapted for "ending edges of

$$ gage %&.'(4 mm) metal or lighter. There are

si* important steps that must "e remem"ered

when using the "ar folder.+. ,llowane for the thikness of the metal

$. Sharpeners of the folded edges

. idth of the lok edge

4. ,d/ustment for the thikness of metal

0. ,ngle of fold

1. T!pes of metal

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Figure 1 #ar Folder 

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• Figure $ and show some operation that an "e

performed on "ar folder

Figure ! Cross-setion view showing "ar folder ad/usted for

sharp fold and for rounded fold.

Figure ": Cross-setion view of "ar folder showing how to form

hemmed edge

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Folding Brakes

• There are man! t!pes of "ending "rakes

used "! sheet metal workers. The most

widel! used mahine is the ornie "rake.

• The average ornie "rake has a apait!

of +1 gage %+.022 mm) and an also "end

the lightest sheet metal.

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Edges

• henever a sheet metal o"/et is made3

some t!pe of edges must also "e formed.

n addition to providing a finish3 an edge

eliminates the raw edge of the metal that

is likel! to ut someone and provides

additional strength for the edge.

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Figure 4 Some of the ommonl! used sheet metal edges.

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Single #em

• The single hem is a folded edge on the metalmade in order to inrease its strength and tomake a smooth finished edge.

• t is one of the most ommon of all edges sineit is the simplest to form.

• The hem is folded over in the "rake andsmashed flat.

• The allowane is generall! 1 mm. 5owever3 onmetal heavier than $$ gage3 it is ommonpratie to inrease the hem to 2 mm to +&mm.

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$ou%le hem

• The dou"le is a simple single hem done

twie. t provides muh greater strength

than the single hem.

• The allowane for a dou"le hem is twie

the hem si6e less +.0 mm3 so that the

outside allowane is short and does not

over up the seond "end line. 7eferFigure 0.

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Figure & nside and outside allowanes for dou"le hem.

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'ired Edge

• For a greater amount of strength than that provided "!the dou"le hem3 the wired edge is used.

• This is done "! wrapping the sheet metal around a pieeof wire.

• The allowane added to the pattern depends upon thediameter of the wire.

• For $1 gage and lighter3 $8 times the diameter of thewire is added to the pattern.

• For $4 gage metal and heavier3 allowanes must "emade for the thikness of the metal in addition to thediameter of the wire.

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Seaming

• n sheet metal onstrution3 there are a

variet! of methods for /oining the edges

of sheet metal. The hoie of /oint3 or

seam3 is determined primaril! "! :-

a) thikness of the metal

") the t!pes of metal

) the ost of fa"riation

d) the e9uipment

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()*es of seams

• n planning the fa"riation of sheet metal

artiles3 the worker should "e a"le to

visuali6e the t!pe of seam that is "est

fitted for the speifi /o".

• arious t!pes of seams are

diagrammatiall! shown in Figure 1.

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Figure + Common seams used in sheet metal work.

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,roo-ed seam

• t is one of the ommon t!pe of seam used in

light or medium gage sheet metal. Figure '

shows how a grooved seam is formed.

Figure ;roove seam formed "! two <lok= shown at ,3 hooked together at #3

and loked together as in C.

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• hen making a grooved seam3 it is

neessar! to make allowane for the

amount of material that is to "e added for

the lok. The amount depends upon thewidth of the lok and the thikness of the

metal.

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• The formula for finding the allowane is afollows :- – $4 gage or lighter > * width of lok

 – $0 gage or heavier > * width of lok plus 0 *thikness of metal

• 5alf of the a"ove allowanes are to "e

added to eah side of the pattern. ;rooveseams are rarel! used in metal heavierthan $& gage.

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Pitts%urgh seam

• Sometimes alled a hammer lok or ho"o lok.

t is used as a longitudinal orner seam for

variousl! shaped pipes as the dut. The seam

onsists of two parts as shown in Figure 2.

Figure / ?itts"urgh seam

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• The ?itts"urgh seam is the most ommonl! usedof an! seam in the sheet metal shop. t an "eformed on roll-forming mahines or on the "rake.@ne of the advantage of this seam is that the

single lok an "e turned on a urve and thepoket lok an "e formed on a flat sheet andthen rolled to fit the urve.

• The allowane needed for the ?itts"urgh seamwhen formed in the "rake is $ mm.

• Figure ( and +& show how ?itts"urgh seams aremade.

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Figure 0. ?itts"urgh seams an "e turned on urves.

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Figure 1. ?itts"urgh seams ma! "e formed on the "rake "! the steps shown.

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$ri-e 2 cli* seam

• This seam is generall! used in onnetion

with S lips for onneting ross seams on

dut. The edges var! with /o" onditions3

however3 the ommon width is + mm.The atual drive lip is formed as shown in

Figure ++.

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Figure 11. Arive-lip seams are made "! %,) turning edges3 %#) formingdrive and %C) attahing.

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S 2 cli* seam

• The S – lip is an S – shaped piee of

metal that forms two poket loks for the

 /oining metal to slip in. @ften in overing a

wall with sheet metal3 the S – lip is "enton the edge of the sheet and is used as

shown in Figure +$ and +.

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Figure 1! S-lips are used to /oin duts or to sheet metal piees

overing a wall.

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Figure 1" S and drive lips used to /oin dut setions.

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Sli* 2 3oint seam

• This seam is used for a longitudinal orner

seam3 as shown in Figure +4. t onsists of

a single lok and dou"le lok. The single

lok is slipped into the dou"le lok3ompleting the assem"l! of the seam.

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Figure 14 Slip /oint seams.

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Figure 1& ?roper and improper /oining of slip /oint seams in pipe onstrution.

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$ou%le seam

• There are two t!pes of dou"le seams. @ne

t!pe is used for making irregular fittings

suh as s9uare el"ows3 offsets3 "o*es3

et. The other t!pe is used to fasten"ottoms to !lindriall! shaped artiles

suh as pails3 tanks3 et.

• #oth t!pes of seams are shown in thefollowing Figure +1 and +'.

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Figure 1+ Making a dou"le seam.

Figure 1 Making a "ottom dou"le seam to fasten !lindriall! o"/ets.

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$o-etail seam

• This seam is an eas! and onvenient

method of /oining ollars to flanges. There

are three t!pes of dovetails :-

 – ?lain dovetails

 – #eaded dovetails

 – Flange dovetails

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Figure 1/ 5ow to form a dovetail seam.