How To Curve Sheet Metal
Content Menu
● Material Behavior When Curved
● Roll Bending – The Workhorse Method
● Press Brake Techniques for Curved Parts
● Tooling and Equipment Selection
● Practical Step-by-Step Example – Rolling a Cylindrical Section
● Q&A
Introduction
Curving sheet metal is one of those operations that separates hobby work from real production capability. A flat blank can become a fuselage skin, a roof panel, a custom fuel tank, or an architectural feature, but only if the person doing the work understands how the material actually behaves under load. The difference between a clean curve and a pile of cracked scrap often comes down to radius selection, tooling choice, lubrication, and knowing exactly how far the alloy can be pushed before it fails.
In daily shop practice, most curved parts are produced by one of four basic routes: roll bending, stretch forming, press-brake work with special tooling, or incremental forming. Each route has its place, and the best fabricators know when to switch from one to another. The goal here is to walk through all of them in enough detail that an engineer or lead fabricator can pick the right process the first time and get repeatable results without wasting material.
Material Behavior When Curved
Sheet metal does not bend the way a wire does. In a curve, the outside surface stretches and the inside surface compresses. The neutral axis — the line that experiences neither stretch nor compression — sits somewhere inside the thickness, usually around 0.33 to 0.5 times the thickness depending on the alloy and temperature.
Aluminum 6061-T6, for example, has roughly 10–12 % elongation before necking in the as-received condition. Push the outer fiber strain above that limit and cracks appear immediately. High-strength steels can tolerate less tensile strain but accept more compression before buckling. Knowing the forming limit curve for the specific grade and temper is essential before laying out any curved part.
Springback is the other constant companion. After the load is removed, elastic recovery opens the radius slightly. Cold-rolled stainless can spring back 8–15°, while annealed copper might only move 1–2°. Compensation is built into tooling or done by deliberate over-forming.
Roll Bending – The Workhorse Method
Three-roll and four-roll machines remain the most common way to produce cylindrical and conical sections. The sheet is passed between an adjustable top roll and two fixed lower rolls. Each pass reduces the radius a little more until the target is reached.
A typical setup for a 1.5 mm mild steel panel 1200 mm wide might start with the top roll 6 mm lower than the bottom rolls. After the first pass, the operator checks the radius with a template, lowers the top roll another 2 mm, and runs the sheet again. Three to five passes are normal. Lubrication with light oil keeps the surface clean and prevents galling on stainless.
Coning — when one end tightens more than the other — happens if the rolls are not parallel. Modern CNC roll benders eliminate most of that by driving each lower roll independently.
Stretch Forming
Stretch forming grips the sheet at both ends, pulls it into tension past yield, then wraps it around a male die. Because the entire sheet is in tension, compressive buckling cannot occur, so very tight radii are possible without wrinkling.
Aerospace shops use this process for leading edges and wing-to-body fairings in aluminum and titanium. Typical pre-stretch is 1–3 % permanent set, followed by wrapping to the final contour. The resulting part has almost no springback and excellent surface finish straight from the machine.
Press Brake Techniques for Curved Parts
Standard press brakes make straight bends, but several tooling tricks produce true curves. Rotary bending dies, adjustable V-dies with radius inserts, and bump-forming all work.
Bump forming is the simplest: the operator programs dozens of small hits along the bend line, each one a few millimeters apart and slightly deeper than the last. A 1000 mm long curve with a 500 mm radius might need 80–120 individual hits. Modern CNC brakes do this automatically and can follow a CAD-defined radius profile.
Incremental Sheet Forming
Single-point and two-point incremental forming use a round-nosed tool on a CNC mill or robot to push the sheet down in small steps. The tool path follows a spiral or zigzag pattern, gradually building the shape. No dedicated die is required, which makes it perfect for prototypes and low-volume parts.
Magnesium and high-strength aluminum alloys show 30–50 % higher formability in incremental forming than in conventional stamping because the deformation is highly localized.
Tooling and Equipment Selection
Hand tools still matter. A good English wheel with a range of upper wheels (flat to 150 mm radius) and matching lower anvils can produce compound curves that machines struggle with. Shot bags, sand-filled leather bags, and hardwood forms are used for shrinking and stretching by hand.
For production, a 4-roll machine with hydraulic preload gives the best control over edge wave and barreling. Stretch-forming machines need accurate gripper jaws and a polished die surface; any scratch transfers to the part.
Practical Step-by-Step Example – Rolling a Cylindrical Section
Start with a 1.2 mm 5052-H32 sheet, 1000 mm × 2000 mm, target radius 600 mm.
- Calculate developed length: circumference segment = π × (600 + 0.4 × 1.2) ≈ 1887 mm. Add 50 mm grip length each side.
- Shear and deburr.
- Set bottom rolls 1200 mm apart, top roll parallel and 8 mm lower.
- Feed the sheet leading edge first, run full length.
- Lower top roll 2 mm, reverse direction, run again.
- Repeat until template matches 600 mm radius (usually 4–6 passes).
- Check for coning; correct by adjusting one lower roll slightly.
- Trim ends square.
Total time on a manual machine: about 15 minutes once the operator is experienced.
Common Problems and Fixes
Orange peel on aluminum → anneal to O-temper first. Inside wrinkles on thick material → add a urethane pad or switch to stretch forming. Cracks on the extrados → radius too tight for the alloy; increase minimum 3× thickness. Excessive springback → over-form by measured amount or bottom the bend.
Real Production Examples
An automotive supplier produces aluminum hood outers on a double-stretch former: 2.0 mm 6111-T4, 450 mm minimum radius, 800 parts per shift, less than 0.5 % scrap.
A shipyard rolls 6 mm AH36 steel plates into 3 m diameter pipe sections on a four-roll machine with pre-bending dies; weld prep edges stay straight within 1 mm.
A small custom shop makes motorcycle tanks from 1.2 mm 5052 using only an English wheel and a plywood buck — four days per tank but perfect fit and finish.
Conclusion
Curving sheet metal successfully is about choosing the process that matches the part geometry, material, volume, and budget. Roll bending handles most cylindrical work quickly. Stretch forming delivers the tightest radii and best surface on long aerospace sections. Press brakes with bump or rotary tooling cover medium-volume curved flanges. Incremental forming and hand wheeling fill the gaps for prototypes and artistic pieces.
The key is never to fight the material. Respect the forming limits, compensate for springback, keep surfaces lubricated and dies polished, and the curve will come out clean the first time. Once the basics are second nature, the real fun begins — pushing alloys and processes to make shapes no one thought possible.
Q&A
Q1: Can I roll stainless steel on the same machine I use for aluminum?
A: Yes, but increase lubrication and expect more passes because of higher strength.
Q2: How tight can I stretch-form 2024-T3 aluminum cladding?
A: Usually down to 2–3 times thickness if pre-stretched correctly and the die is polished.
Q3: My rolled cylinder has flat spots in the middle. What happened?
A: Top roll pressure too low or not enough passes; increase pressure gradually.
Q4: Is heat needed when rolling 2 mm titanium?
A: Warm forming at 200–300 °C reduces springback and roll load dramatically.
Q5: Which process gives the best surface finish for visible architectural panels?
A: Stretch forming over a polished die or careful English wheel work.