Hemming and seaming are two similar metalworking processes, where a sheet of metal edge is rolled over and over onto itself until it achieves a tight fit. In hemming, the edge is rolled to itself, while in seaming the edges of two materials are joined. Hems are typically used to reinforce edges, improve the appearance, connect parts together, and to hide burrs and rough edges. Seams are mainly used in the food industry for canned goods, on amusement park rides, and in the automobile industry.
Hemming and seaming involve the same processes; only the tonnage requirement is greater in case of seaming.
The process begins by bending the edge to an acute angle and then using a flattening die to flatten the hem.
Accuracy is a very important factor in hemming operation as it affects the appearance of the surface as well as surface quality. Material deformations can lead to defects in parts.
The two hemming processes are:
Conventional die hemming
Conventional die hemming is the one suitable for mass production. In die hemming, the flange is folded over the entire length with a hemming tool. Usually, the actual hemming happens as a result of a forming operation where the flange is formed with a hemming tool, after completing the drawing and trimming operations. This formed flange is then hemmed through several process steps, including the pre-hemming and final hemming stages for instance. This depends on the respective opening angle of the flange. Production plants for conventional die hemming are normally very expensive, but the cycle times are very low.
Roll hemming is done incrementally using a hemming roller. Industrial robots are used by Diamond Brite Metals in the process to guide the roller and form the flange. Roll hemming operations can be further divided into several pre-hemming and final hemming process steps. Roll hemming is very flexible for usage. Plus, the tool costs are also much lower than those of conventional die hemming. However, the cycle times are much higher here, since the hemming is carried out using a roller that follows a definite path.
Types of hemmed edges
There are two types of hemmed edges:
· closed hems, and
· open hems
Closed hems are completely flush, as compared to open hems, which have an air pocket in the bend. Also, closed hem required greater tonnage than open hem.
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