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116- Trade Skills
Hems, Edges, and Seams
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• Sheet metal pieces, such as ductwork, down spouts, and
washing machine housings, begin with a flat pieces of
metal.
• Patterns of the stretched out pieces are transferred or
made directly on to the flat metal.
• These patterns include allowances, or additional
material, for hems, seams, and edges.
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• The amount of allowance is determined by the
thickness of the metal, the radius of the bend, and the
type of bend being made.
• Once outlined on the metal piece, the pattern is cut out.
• Hems, seams, and edges are important in the fabrication
of the metal pieces.
– They join opposite ends of a pattern to make a complete
piece.
– They can also join completely separate pieces.
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Hems
• A hem is a folded over edge on a
piece of metal.
• It is used to finish off rough, sharp
ends of metal sheet and to
strengthen and improve the
appearance of a project.
• Allow about ¼” for the hem.
– When using metal heavier than 22
gage, increase the hem to 5/16” or
3/8”
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• A double hem is a single
hem, repeated.
• A double hem provides
much greater strength
than single hems.
• Double hems are
generally 5/16” or 3/8.
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• Hems can be made using a bar fold or a brake.
– A bar fold can make a hem in one motion.
– A brake requires two motions.
• One to bend the metal
• One to press the metal flat.
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Seams
• Seams are used to fasten sheet metal sections together
using locks, rivets, screws, brazing, or soldering.
• They can be made by hand, on a brake, or on a bar fold.
• The type of seam made and the fastening method
depends on the type of metal, its thickness, and its
fabricating machinery available to the worker.
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• A wide variety of seam types have been designed for
use on sheet metal.
• About 20 types of seams are used in ductwork
construction and for other purposes.
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Variety of seams
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Pittsburgh Seam
• The Pittsburgh is commonly used in sheet metal shops.
• The seams has two parts:
– A single, 90 degree lock and a pocket lock.
• The single lock is inserted into the pocket lock, after
which the edge of the pocket lock is folded down to
seal the seam.
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• Pittsburgh seams are often used in HVAC ductwork.
• They can be formed on all gage metals, from 16 to 40
gage.
• This seam is easy to form on the hand brake or a
Pittsburgh machine.
• A distinct advantage to the Pittsburgh seam is that it can
be formed on curves.
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Steps to make a Pittsburgh seam
Step 1
Step 2
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Dovetail Seams
• The dovetail seam is used to primarily on round or
elliptical pipes. It is seldom used on rectangular.
• There are three types of dovetail seams.
– Plain
– Beaded
– Flanged
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• The plain dovetail seam is used to join a collar and
separate flange.
– This seam is made without rivets, screws, solder.
• Instead, tabs are cut at the end of the collar and every
other tab is then bent down.
• The separate flange is then added, with the bent tabs
holding it in place.
• The remaining tabs are then bent over the flange to
secure it.
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Slip Joint Seam
• A slip joint seam consists
of the edges to be joined
and a drive clip that joins
them.
• The edges are turned to
form pockets.
– The edge on the drive clip,
made from a separate piece
of metal, are also turned to
make pockets.