From concept to product
From prototyping to production, our production sites can handle all your manufacturing projects.
Home / All / Sheet Metal Fabrication / What are the most common metal bending techniques used in custom sheet metal fabrication?
What are the most common metal bending techniques used in custom sheet metal fabrication?
The short answer
Air bending is the default and covers most custom sheet metal work, holding roughly 0.5 to 1.0 degrees with one tool set. Bottoming and coining tighten that to about 0.1 to 0.5 degrees on angle-specific tooling, roll bending forms large curves, and hemming folds an edge for safety and stiffness.
Five techniques cover almost all press-brake work
Bending looks like one operation and is really a family of them, selected by how the material is loaded and how far it is allowed to deform. Air bending pushes the sheet into a V-die without bottoming out, so the final angle is set by how far the punch descends and one tool set can produce many angles. Bottoming presses the sheet until it touches the die faces, forcing the material to take the die angle. Coining goes further and presses with enough force to deform the material through its thickness, which removes most springback. Roll bending forms a large radius by passing the sheet between rollers rather than pressing it over a die. Hemming folds an edge back on itself to produce a smooth or stiffened flange. Rotary bending and wipe bending sit alongside these and are used where the sheet is pre-painted or the flange is short.
Air bending: the default, and what it costs you
Air bending is the most common press-brake operation for three reasons. It needs the least force of the three V-die techniques, because the material never touches the bottom of the die. One punch and one die can produce a wide range of angles by changing the punch depth, which removes tooling changeover from the schedule. And it works across a wide range of thicknesses with the same equipment. The cost is springback: because the material is deformed less completely, it relaxes after the punch withdraws, so a nominal 90 degree bend may settle anywhere inside about half a degree to a degree of target unless the machine compensates.
Two setup figures matter more than any other when air bending is specified. The die opening should be sized to the thickness, and the minimum flange is governed by the die opening rather than by a fixed multiple of thickness, because the flange has to be long enough to sit across the die shoulders. A practical working figure is a minimum flange around 0.7 times the die opening. A flange shorter than that will not form squarely, and a corner that is not square is a weld that will not close.
Bottoming and coining: when the angle has to repeat
Bottoming presses the sheet until it contacts the faces of the die, so the bend angle is set by the tool rather than by the punch depth. That makes the angle repeatable across a run and reduces springback, at the cost of higher tonnage and a tool set dedicated to that angle. Coining goes further: enough force is applied to plastically deform the material through the thickness at the bend line, which produces almost no springback, a highly stable angle and a slightly thinner section at the bend. Published practice puts coining at roughly five to ten times the force of air bending, which is why it is reserved for parts where the angle tolerance genuinely matters and why tool wear is a real maintenance item.
Roll bending, rotary bending and hemming
Roll bending uses three rollers, usually in a pyramid arrangement, to curve a sheet progressively rather than press it over a die. It is the right process for cylinders, cones and large-radius panels, and it is the only one of the group that can produce radii far too large for a press brake. It cannot produce a sharp bend, and the achievable radius depends on the roller diameter and the roll position, so the sheet gauge and the target radius have to be checked together before quoting.
Rotary bending wipes the material around a cylindrical die rather than forcing a punch into it, which leaves fewer marks on the surface and makes it a good fit for pre-painted or polished sheet and for angles beyond 90 degrees. Hemming folds the edge back on itself, either flat for a fully closed edge, open for about 135 degrees, or as a tear-drop where clearance is needed. Hems exist for two reasons: a folded edge has no burr and cannot cut, and a folded edge is much stiffer than the same flange left flat. On high-volume parts hemming is done with a dedicated tool; on low volume it is a two-step operation on a press brake.
Where a technique becomes the wrong choice
Each of these carries a failure mode that is worth knowing before the drawing is released. Air bending cannot hold a tight angle callout without compensation or a different technique, so specifying plus or minus 0.25 degrees across a long air-bent run is asking for something the process does not promise. Coining thins the material at the bend line and wears tooling quickly, which makes it a poor choice on a short-run part or on a material whose finish must not be disturbed. Roll bending cannot produce a sharp corner and struggles with short radii on thick sheet. Hemming needs more than one tool change, so it is uneconomic on a handful of parts unless the edge requirement is a safety requirement. And any bend formed from a flange that is shorter than the die opening will be inconsistent no matter which technique is used, because the material never fully engages the tool.
There is one more boundary that belongs with the process rather than the drawing. Springback is not a defect; it is a property of the material, and it grows with yield strength. Harder materials, thicker sheet and larger bend radii all increase it, so the same nominal angle can require different compensation on two parts that look identical on paper. That is why bend-to-bend consistency on a multi-bend part should be specified as a tolerance on the finished angle and checked after forming, not inferred from the machine setting.
How to specify a bend on a drawing
Four items remove most of the ambiguity. State the inside bend radius, not just the angle, because the tooling that produces the radius also determines the flat pattern. State the bend angle and its tolerance, and check that the tolerance is achievable by the technique you are prepared to pay for. Give the flange lengths, and confirm each one clears the die opening rule at the specified thickness. Say whether the bend line must run across the grain or whether orientation is free, because bending parallel to the grain on a hard material is the commonest cause of cracking on the outside of the radius.
See sheet metal fabrication for how bending sits between cutting and welding, metal bending for the forming processes in detail, and custom sheet metal parts for the wider part design rules that bending depends on.
Scope and sources. The definitions of air bending, bottoming and coining, the relative force requirement and the thinning effect of coining come from a metal bending techniques guide (air bending leaving the sheet floating between punch and die with lower force but higher springback, bottoming pinching the sheet to the die angle with better accuracy and less springback, coining using five to ten times the air bending force to deform the material through its thickness with practically zero springback, roll bending forming cylinders and cones from three rolls, and rotary draw and wipe bending behaviour). Tooling selection, application fit and the observation that each angle may need its own tooling for bottoming come from a press brake applications guide and from a bending methods review. The minimum flange figure of about 0.7 times the die opening, the constraint that a short flange does not form square and leaves a variable joint gap, and the note that a bend angle tolerance of about one degree is common industry practice come from a sheet metal joining and forming guide. Springback as a function of yield strength, the overshoot-and-relax method of compensation, cracking from a radius that is too sharp for the material, and bending parallel to the grain as the weakest orientation come from a sheet metal bending methods guide (air bending needing less force with one tool set for many angles and moderate precision, bottoming more accurate and repeatable with angle-specific tooling, coining very high precision at high tooling cost, roll bending for tanks, cones and large curved panels, and rotary bending leaving no marks and suiting bends beyond 90 degrees). The angle tolerance bands are published practical ranges for press-brake work; confirm them against the tooling, the material and the machine on your own part.
Are you looking for a reliable manufacturer of down home textile products?
We can quickly provide customers with market analysis, technical support and customized services.From concept to product
From prototyping to production, our production sites can handle all your manufacturing projects.
subscription