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Chapter 4:
Sheet Metal Forming
4.4 Folding Edges and
Making Seams
Shafizan Bt. Shariffuddin
School of Manufacturing Engineering
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 appearance of finished products
to strengthen the work piece
to fasten pieces of metal together
Folding Edges
• There are two types of machine commonly
used in bending or folding metal to form
edges or locks for seams :– Bar folders
– Folding brakes
Bar folder
This machine is adapted for bending edges of
22 gage (0.794 mm) metal or lighter. There are
six important steps that must be remembered
when using the bar folder.
Allowance for the thickness of the metal
Sharpeners of the folded edges
Width of the lock edge
Adjustment for the thickness of metal
Angle of fold
Types of metal
Figure 1: Bar Folder
• Figure 2 and 3 show some operation that can be
performed on bar folder
Figure 2: Cross-section view showing bar folder adjusted for
sharp fold and for rounded fold.
Figure 3: Cross-section view of bar folder showing how to form
hemmed edge
Folding Brakes
• There are many types of bending brakes
used by sheet metal workers. The most
widely used machine is the cornice brake.
• The average cornice brake has a capacity
of 16 gage (1.588 mm) and can also bend
the lightest sheet metal.
• Whenever a sheet metal object is made,
some type of edges must also be formed.
In addition to providing a finish, an edge
eliminates the raw edge of the metal that
is likely to cut someone and provides
additional strength for the edge.
Figure 4: Some of the commonly used sheet metal edges.
Single Hem
• The single hem is a folded edge on the metal
made in order to increase its strength and to
make a smooth finished edge.
• It is one of the most common of all edges since it
is the simplest to form.
• The hem is folded over in the brake and
smashed flat.
• The allowance is generally 6 mm. However, on
metal heavier than 22 gage, it is common
practice to increase the hem to 8 mm to 10mm.
Double hem
• The double is a simple single hem done
twice. It provides much greater strength
than the single hem.
• The allowance for a double hem is twice
the hem size less 1.5 mm, so that the
outside allowance is short and does not
cover up the second bend line. Refer
Figure 5.
Figure 5: Inside and outside allowances for double hem.
Wired Edge
• For a greater amount of strength than that provided by
the double hem, the wired edge is used.
• This is done by wrapping the sheet metal around a piece
of wire.
• The allowance added to the pattern depends upon the
diameter of the wire.
• For 26 gage and lighter, 2½ times the diameter of the
wire is added to the pattern.
• For 24 gage metal and heavier, allowances must be
made for the thickness of the metal in addition to the
diameter of the wire.
In sheet metal construction, there are a
variety of methods for joining the edges
of sheet metal. The choice of joint, or
seam, is determined primarily by :a)
thickness of the metal
the types of metal
the cost of fabrication
the equipment
Types of seams
• In planning the fabrication of sheet metal
articles, the worker should be able to
visualize the type of seam that is best
fitted for the specific job.
• Various types of seams are
diagrammatically shown in Figure 6.
Figure 6: Common seams used in sheet metal work.
Grooved seam
• It is one of the common type of seam used in
light or medium gage sheet metal. Figure 7
shows how a grooved seam is formed.
Figure 7: Groove seam formed by two “lock” shown at A, hooked together at B,
and locked together as in C.
• When making a grooved seam, it is
necessary to make allowance for the
amount of material that is to be added for
the lock. The amount depends upon the
width of the lock and the thickness of the
• The formula for finding the allowance is a
follows :– 24 gage or lighter = 3 x width of lock
– 25 gage or heavier = 3 x width of lock plus 5 x
thickness of metal
• Half of the above allowances are to be
added to each side of the pattern. Groove
seams are rarely used in metal heavier
than 20 gage.
Pittsburgh seam
• Sometimes called a hammer lock or hobo lock.
It is used as a longitudinal corner seam for
variously shaped pipes as the duct. The seam
consists of two parts as shown in Figure 8.
Figure 8: Pittsburgh seam
• The Pittsburgh seam is the most commonly used
of any seam in the sheet metal shop. It can be
formed on roll-forming machines or on the
brake. One of the advantage of this seam is that
the single lock can be turned on a curve and the
pocket lock can be formed on a flat sheet and
then rolled to fit the curve.
• The allowance needed for the Pittsburgh seam
when formed in the brake is 32 mm.
• Figure 9 and 10 show how Pittsburgh seams are
Figure 9. Pittsburgh seams can be turned on curves.
Figure 10. Pittsburgh seams may be formed on the brake by the steps shown.
Drive – clip seam
• This seam is generally used in connection
with S clips for connecting cross seams on
duct. The edges vary with job conditions,
however, the common width is 13 mm.
The actual drive clip is formed as shown in
Figure 11.
Figure 11. Drive-clip seams are made by (A) turning edges, (B) forming
drive and (C) attaching.
S – clip seam
• The S – clip is an S – shaped piece of
metal that forms two pocket locks for the
joining metal to slip in. Often in covering a
wall with sheet metal, the S – clip is bent
on the edge of the sheet and is used as
shown in Figure 12 and 13.
Figure 12: S-clips are used to join ducts or to sheet metal pieces
covering a wall.
Figure 13: S and drive clips used to join duct sections.
Slip – joint seam
• This seam is used for a longitudinal corner
seam, as shown in Figure 14. It consists of
a single lock and double lock. The single
lock is slipped into the double lock,
completing the assembly of the seam.
Figure 14: Slip joint seams.
Figure 15: Proper and improper joining of slip joint seams in pipe construction.
Double seam
• There are two types of double seams. One
type is used for making irregular fittings
such as square elbows, offsets, boxes,
etc. The other type is used to fasten
bottoms to cylindrically shaped articles
such as pails, tanks, etc.
• Both types of seams are shown in the
following Figure 16 and 17.
Figure 16: Making a double seam.
Figure 17: Making a bottom double seam to fasten cylindrically objects.
Dovetail seam
• This seam is an easy and convenient
method of joining collars to flanges. There
are three types of dovetails :– Plain dovetails
– Beaded dovetails
– Flange dovetails
Figure 18: How to form a dovetail seam.
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