What is sheet metal bending, and when should you use it?

Sheet metal bending makes a part by cutting a flat shape from metal sheet, then folding it along straight lines on a press brake. Use it for thin-walled metal parts such as brackets, panels, trays and enclosures, and choose machining or metal 3D printing for solid or complex shapes.

Sheet metal bending at a glance: what it makes and what it suits

The accuracy figures come from the SendCutSend bending guidelines and the Xometry sheet metal guide. Mark the dimensions that must be exact and we confirm what we can hold.

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How sheet metal bending works: cut flat, then fold

A sheet metal part starts flat. The outline, holes and slots are cut from a sheet, usually by a laser built for metal or a CNC punch. Every hole goes in while the sheet is flat, because it is far easier to cut there than on a finished box.

The flat piece then goes into a press brake. This is a long press with a blade-like punch on top and a V-shaped groove (the die) below. The punch pushes the sheet into the groove and the sheet folds along a straight line. Each bend is one press stroke, so a tray with four sides takes four.

Two things make bending a skill rather than a guess:

• Springback. Metal springs back a little once the punch lifts, so each bend is pushed slightly past the angle you want.
• Stretch. The outside of a bend stretches and the inside squashes, so the flat piece must be cut to a length that allows for every bend. See the K-factor rule below.

See what we make in sheet metal

Stainless steel, mild steel or aluminium: which metal to bend

Most sheet metal parts are made from one of three metals. For anything else, such as brass, copper or galvanised steel, ask us.

• Stainless steel resists rust and cleans easily, so it suits food, medical and outdoor use. It takes more force to bend than mild steel.
• Mild steel is strong and the lowest-cost choice, but it rusts. It should always be primed and painted.
• Aluminium is much lighter than steel and does not rust. Some harder grades crack on a tight bend and need a larger radius, so tell us the grade if your drawing names one.

Sheet can be rolled into curves as well as folded. For a custom stainless steel housing we cut the sheet with its window cutouts, rolled it into nested cylinders, then fitted welded hinges and toggle latches.

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Design rules for bent parts: five that avoid a remake

These are general rules from published design guides. The exact figures depend on the metal, the thickness and the tools, so we check every drawing before it is cut.

• Bend radius: at least the sheet’s thickness. The inside curve of a bend should be no tighter than the sheet is thick, or the outside can crack, says the Xometry sheet metal guide. Thicker or harder metal needs more.
• Flange length: long enough to grip. A flange is the flat side that a bend makes. It must be long enough for the press brake to hold. The Protolabs sheet metal toolkit sets the minimum at 2.5 to 4 times the thickness, plus the bend radius, depending on the metal and thickness. On 2 mm sheet with a 2 mm radius, that is about 7 mm or more.
• Holes: keep them clear of the bend. A hole too close to a bend stretches out of round. Protolabs uses the same distance as its minimum flange length, measured from the bend.
• Same radius on every bend. One radius means one tool for the whole part. Protolabs’ design tips suggest keeping bend radii the same where you can.
• K-factor: the number behind the flat length. Inside a bend, one layer neither stretches nor squashes. The K-factor says where that layer sits, as a fraction of the thickness, as the SOLIDWORKS help explains. Software uses it to size the flat piece. Send a 3D model and the flat piece is worked out from it; if you draw the flat pattern yourself, tell us the K-factor you used.

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When to use something else: machining or printing

Sheet metal is the right answer when a part is mostly thin walls of even thickness. It is the wrong one when the part is not:

A thin bracket bends from a strip of sheet, where machining the same shape would cut most of a block away. A part whose walls change thickness, or curve in two directions at once, cannot be folded at all. That is where machining or printing takes over. See metal 3D printing compared with CNC machining, and what CNC machining is.

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Ordering a bent part: what to send us

Send a 3D model as STEP, or a flat drawing as DXF with the bend lines marked. Tell us:

• the metal and its thickness;
• the quantity;
• the finish: bare, brushed, or primed and painted;
• which dimensions must be exact, and what the part fits against.

A person reads every drawing and replies with a price and anything that should change before cutting. If no drawing exists yet, our design team can draw the part from a sketch or a sample. Collect the finished parts from our Bendemeer studio in Singapore, or have them couriered islandwide.

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