Introduction to Basic Manufacturing Processes and ... - always yours

Sheet Metal Work 357
18.4 FOLDING TERMINOLOGY OF SHEET METAL JOINT
18.4.1 Edge
The edges on part need to be folded to increase the strength and to eliminate the sharp
edges of sheet metal component.
1. Types of folded edges. The common types of folded edges are: (i) single hem, (ii)
double hem and (iii) wired edge
2. Single hem. It is made by folding the edge over. To layout such a hem, a line is
drawn at a distance equal to the desired hem width.
3. Double hem. It is a single hem with its end bent under. To layout such a hem,
draw two parallel lines each equal to the width of the hem.
4. Wired edge. It consists of an edge, which has been wrapped around a piece of wire.
This edge is used where more strength is needed. To layout wired edge the diameter
of wire is to be determined. The steel metal will be needed to roll around the wire.
18.4.2 Seam
A seam is the section where pieces of sheet metal are joined together. Most common
types of seams are:
(a) Single seam, (b) Double seam, (c) Grooved seam, (d) Lap seam, (e) Dovetail seam, and
(f) Burred bottom seam
1. Single seam
It is used to join a bottom to vertical bodies of various shapes. To layout such a seam, draw
a line parallel to one edge of the sheet metal body stretch out at a distance equal to the width
of the seam. Now draw two lines parallel to the edges of the bottom stretch out. The first line
should be drawn at the distance from the edge of sheet metal equal to the width of the seam
minus 1 mm. approx. Second line should be drawn at a distance from the first equal to the
width to the seam on sheet metal plus 1 mm approx. The plus and minus dimensions of 1 mm
is used to prevent the folded bottom edge of sheet metal from interfering with the body’s folded
bottom edge. If the bottom is round, then mark the lines on sheet metal part.
2. Double seam
The layout process for this seam on sheet metal part is similar to that used for a single
seam on sheet metal part. It differs from single seam in a manner that its formed edge is
bent upward against the body.
3. Grooved seam
It is made by booking two folded edges of sheet metal part together and then off-setting
the seam as shown in Fig. 18.19. On one piece draw one line equal to half the width of the
seam from outer edge. Then draw second line at a distance equal to the width of the seam
from the first line. Same way draw two lines on the other piece of sheet metal part.
4. Lap seam
It is the simplest type of seam made on sheet metal part because one edge laps over
another and is soldered or riveted. To layout lap seam on sheet metal part, draw line on the
edge of piece at a distance equal to the width of the required seam.