After a cup is formed in the deepdrawer, it is time to close the cup.
This is done by either hemming or seaming. Hemming is done on the top side of a cup. It is less complicated to do but the main reason it is hemmed is because the flange is used to prevent the cup from dropping down the ring on pendulum lamps like the Tralwer lamp.

First a thread is pressed in the lid, because it is easier to do this when it's not joined together with the cup. Then cup is flanged. Now a lid is placed over the flange and with a press and the use of a special die and punch they are joined together. Another term for hemming is single flanged .

However, when the “lid” is on the bottom of the cup, like on our Galley lamp, a leak proof joining technique called the cup joint is used. This is the same joint that is used on canned food. It is done on the same machine used for flanging. But after the flanging is done, the edge is folded another time to create a seam. The Cup joint is also called a duct seam, double seam or double lock. 

On this picture you can see a cut out of a cupjoint.