1. 116- Trade Skills
Hems, Edges, and Seams

2. 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.

3. Once outlined on the metal piece, the pattern is cut out.
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.

4. 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”

5. 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.

6. 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.

7. 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.

8. 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.

9. Variety of seams

10. 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.

11. 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.

12. Steps to make a Pittsburgh seam

17. 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

18. 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.

20. 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.