Sheet Metal Hems
Hemming is the bending or rolling process in which a bend achieves 180° or more to create a new edge profile.
Hems are commonly used to re-enforce, hide imperfections and provide a generally safer edge to handle. When a design calls for a safe, even edge the added cost of material and processing of a hem is often preferable to other edge treating processes.
The process of producing a flange on a press brake begins with an initial bend between 30° and 45°. A tighter initial bend will produce a more accurate hem. The open flange is then placed under a flattening bar and closed. The tonnage required to fully compress a hem is typically 2.5 times greater than that of an open hem. There are a number of combination type tooling to create standard sized hems with a single setup. The flange length of a hem is limited by the minimum flange length of the first bend (consult your bend force chart) though a typical rule of thumb is to try for 10 times material thickness. When flattening or finishing a hem it is good practice to load it into the brake flange up, this can help prevent marking (the animation shows the correct method).
Designers should look beyond a single small flat hem to treat edges. Doubling a hem can create an edge perfectly safe to be handled without regard for the initial quality. Adding a hem in the ‘middle’ of a bend profile can open the doors to a variety of profiles not possible without fasteners or welding methods. Even without sophisticated seaming machines a combination of two hems can create strong, tight joints with little or minimal fastening. Hems can even be used to strategically double the thickness of metal in areas of a part which may require extra support. Hems used in thefood service industry should almost always be closed for sanitary purposes (very difficult to clean inside the opening).
Double Hem Edge - Hem And Double Metal Thickness Bend For Support - Using A Hem To Create Advanced Profiles
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