Page 52 - Module DMV20173

3.0 SHEET METAL PROCESSES

5. To reduce this possibility, sheared or punched edges may be shaved off

with a sharp tool to improve the surface finish of the edge.

3.3.5 HEMMING AND SEAMING

1. In the hemming process (also called flattening), the edge of the sheet is
folded over itself.

2. Hemming increases the stiffness of the part, improves its appearance,
and eliminates sharp edges.

3.3.6 BULGING

1. This process involves placing a tubular, conical, or curvilinear part into a

split-female die and then expanding it, usually with a polyurethane plug as
shown in Figure 3.24.
2. The punch then is retracted, the plug returns to its original shape (by total
elastic recovery), and the formed part is removed by opening the split

dies.
3. The major advantages of using polyurethane plugs is that they are very
resistant to abrasion, wear, and lubricants; furthermore, they do no
damage the surface finish of the part being formed.

3.3.7 SEGMENTED DIES

1. These dies consist of individual segments that are placed inside the part
to be formed and expanded mechanically in a generally radial direction.
2. They then are retracted to remove the formed part.
3. Segmented dies are relatively inexpensive, and they can be used for large

production runs.

Example 3.3 Manufacturing of bellow
Explain the process of manufacturing of bellow.

Solution
Bellows are manufactured by a bulging process, as shown in Fig. 4.24. After the tube is
bulged at several equidistant locations, it is compressed axially to uniformly collapse the
bulged regions, thus forming bellows. The tube material must be able to undergo the large

strains involved during the collapsing process without developing cracks.

BPLK 84 DMV 20173