How To Fold Sheet Metal
Content Menu
● Sheet Metal Fundamentals That Affect Folding
● Tooling Options in Everyday Use
● Primary Folding Methods Explained
● Material Preparation Before the First Bend
● Design Rules That Prevent Problems
● Bend Allowance and Deduction Calculations
● Step-by-Step Folding Procedure (Example: Simple Tray)
● Safety Practices That Actually Work
● Advanced and Emerging Methods
● Real Production Case Studies
● Q&A
Introduction
Sheet metal folding sits at the core of almost every fabrication shop. A flat blank goes in, a finished part with flanges, ribs, hems, or full enclosures comes out. The process looks simple until the first batch comes back with cracked corners, wrong angles, or parts that refuse to fit together. Then it becomes clear that successful folding depends on material behavior, tooling choices, bend sequencing, and a handful of calculations that cannot be ignored.
The goal here is straightforward: give manufacturing engineers the practical knowledge needed to take a design from CAD to a correctly folded part on the first try, whether the run is five pieces or five thousand. The information comes from daily shop experience combined with findings from peer-reviewed work on forming limits, springback prediction, and modern folding equipment.
Every section includes examples that have actually happened on the floor — duct corners that leaked until the radius was changed, battery trays that cracked until the grain direction was corrected, brackets that were 2 mm short because someone used the wrong K-factor. These are the lessons that journals confirm and shops learn the hard way.
Sheet Metal Fundamentals That Affect Folding
Start with the raw material. Thickness is specified in gauge or millimeters, but uniformity matters more than the nominal value. A coil advertised as 1.5 mm can vary ±0.08 mm across its width; that variation shows up as different springback from one edge to the other.
Common alloys have predictable folding characteristics:
- Cold-rolled steel (DC01) accepts tight radii down to 0.5 × t when annealed.
- 5052-H32 aluminum routinely folds to 1 × t without cracking.
- 304 stainless in 2B finish needs at least 2 × t or it orange-peels on the outside radius.
- 6061-T6 in thick gauges (>3 mm) often requires hot forming or a 3–4 × t radius.
Grain direction cannot be overlooked. Bending parallel to the rolling direction gives the smallest allowable radius and the least springback. Bending across the grain increases the minimum radius by roughly 50 % and raises springback 2–4 degrees in steel.
Surface condition also plays a role. Mill scale on hot-rolled steel acts like sandpaper and marks polished tooling. Pickled-and-oiled or cold-rolled stock folds cleaner. Pre-painted or brushed stainless demands protective film or urethane die pads to avoid scratches that become visible after forming.
Tooling Options in Everyday Use
Most shops rely on three main machines for folding.
Manual box-and-pan brakes remain common for prototypes and short runs. A 48-inch, 16-gauge brake can form small electrical boxes all day. The operator controls angle by eye and feel, so consistency depends on skill.
Hydraulic press brakes with CNC backgauges dominate medium to high volume work. A 110-ton, 10-foot machine with segmented tooling can switch from a 0.8 mm bracket to a 6 mm frame in minutes. Air bending is the default because one set of tools covers a range of angles and materials.
Fully automatic panel benders (Salvagnini, RAS, Trumpf TruBend Center) handle enclosures and panels up to 3 mm thick at rates of several bends per minute. They excel when the part has many short flanges and the run justifies programming time.
Simple hand tools still have their place: a good set of hand seamers and a 24-inch folding bar can close hems on pre-cut duct sections faster than setting up a press brake.
Primary Folding Methods Explained
Air Bending
The punch pushes the sheet into the die but never touches the bottom. The angle is controlled by penetration depth. Springback is always present, typically 2–6 degrees depending on material and thickness-to-radius ratio.
A real production example: 1.2 mm galvanized steel electrical cabinets. Die opening 8 × t, punch radius 1 mm. The CNC overbends to 86° so the part relaxes to 90°. Changing to a different coil lot with slightly higher yield strength shifted the final angle to 92°. The operator adjusted penetration by 0.15 mm and regained tolerance.
Bottom Bending
The punch forces the sheet completely into the die, eliminating most springback. Tonnage requirement roughly triples compared to air bending. Surface marking is heavier unless polyurethane film or hardened tooling is used.
Used extensively for automotive visible parts (door hems, hood edges) where angle tolerance must stay within ±0.3°.
Coining
Extreme pressure actually thins the material in the bend zone. Springback becomes almost zero, but tooling life drops and tonnage skyrockets. Reserved for thick high-strength steels or when radius < 0.5 × t is mandatory.
Folding with Wiping Dies (Panel Bender Style)
The sheet is clamped and a rotating upper beam wipes the flange up or down. Inside radius is fixed by the blade edge, usually 0.5–1 mm. Excellent for thin material and return flanges because there is no sliding contact on the visible surface.
Material Preparation Before the First Bend
Remove burrs. A 0.1 mm burr on the tension side becomes the starting point for a crack.
Clean the surface. Oil or drawing compound left on the sheet causes slippage and inconsistent angles.
Check flatness. A sheet stored vertically for months can have edge wave that shows up as flange twist after folding.
Cut blanks slightly oversize in the bend zones. Laser and turret punch operators routinely add 0.5 × bend allowance to each side so the trim operation after folding removes any migration.
Design Rules That Prevent Problems
Minimum inside radius guidelines (common shop values):
- Mild steel ≤ 1.5 mm thick → 1 × t
- Aluminum 5052/6061 → 1–1.5 × t
- Stainless 304/316 → 2 × t
- High-strength steel (>600 MPa) → 3–4 × t
Flange length should be at least 4 × t plus the radius to avoid distortion.
Notches or reliefs are required when two bends meet at a corner. A rule of thumb: relief width = 1 × t + inside radius.
Keep bends away from material edges by at least the inside radius plus thickness.
Bend Allowance and Deduction Calculations
The neutral axis shifts inward during bending. Location is defined by the K-factor:
K = distance from inside face to neutral axis ÷ thickness Typical values:
- Mild steel air bend → 0.38–0.42
- Aluminum → 0.40–0.44
- Stainless → 0.45–0.50
Bend allowance = π/180 × bend angle × (radius + K × t)
Many shops maintain spreadsheets with measured allowances for each material-thickness-radius combination because textbook values are only starting points.
Step-by-Step Folding Procedure (Example: Simple Tray)
Blank size: 400 × 300 mm, material 1.5 mm CRS, four 25 mm flanges.
- Cut blank 403.2 × 303.2 mm (adds two bend allowances).
- Deburr all edges.
- Load into press brake, backgauge set to 26.6 mm (flange + allowance).
- Fold first two opposite sides to 90° (actually program 87° for springback).
- Rotate part 90°, fold remaining sides. Outer flanges are now trapped; sequence matters.
- Check diagonal dimensions; they must be equal within 0.5 mm.
- Close hems if required using a flattening die.
Safety Practices That Actually Work
Two-hand controls or light curtains on every press brake. Daily check of hydraulic pressure and ram parallelism. No folding above rated tonnage — a 110-ton brake folding 6 mm steel across 500 mm is already near limit. Mandatory cut-resistant gloves when handling sharp blanks. Training records kept current; new operators start on manual brakes.
Advanced and Emerging Methods
Continuous roller folding (Elsayed and Basily, 2004) feeds strip through progressive roller sets to produce corrugated or 3D patterns without stopping.
Robotic dieless folding uses a stylus or roller on a six-axis arm. Programming is done by teaching the path on a sample sheet rather than building dedicated tooling. Common now for architectural panels and one-off prototypes.
Incremental sheet forming with local heating allows titanium and Inconel to be folded at radii previously impossible in cold condition.
Real Production Case Studies
HVAC contractor switched from manual breaks to a panel bender for rectangular duct sections. Cycle time dropped from 8 minutes to 45 seconds per piece, leakage rate fell below 2 %.
Electric vehicle battery tray made from 2.5 mm 6181 aluminum. Initial prototypes cracked at every corner. Redesign increased radius from 2 mm to 6 mm and added 3 mm × 3 mm relief notches. Yield rose from 30 % to 98 %.
European appliance maker producing 8,000 refrigerator door liners per day. Tight radius coining was replaced by two-stage wiping on a panel bender. Tooling wear disappeared and surface marking on pre-painted steel was eliminated.
Typical Problems and Fixes
Problem: Flange leans after folding Cause: Backgauge not square to die or uneven material thickness Fix: Shim the backgauge fingers or rotate the sheet 90°
Problem: Corner bulge on boxed parts Cause: No relief or insufficient relief Fix: Add 0.5 mm extra clearance at corners
Problem: Cracks on outside radius Cause: Radius too tight for the alloy or bend across grain Fix: Increase radius or re-orient blank layout
Problem: Final part length short Cause: Wrong bend deduction Fix: Measure test coupons from the same coil and update the table
Conclusion
Folding sheet metal remains equal parts science and craft. Material properties, tooling geometry, bend sequence, and accurate allowances determine whether a part fits the first time or ends up in the scrap bin. Modern CNC equipment and simulation software have removed much of the trial-and-error, but the underlying physics has not changed.
The most successful shops treat every new job as a small research project: cut test strips, fold them in the actual machine that will run production, measure the results, and lock those numbers into the program. Do that consistently and folding becomes predictable instead of mysterious.
Master the basics covered here, keep a log of measured allowances for each material you run, and stay current with equipment capabilities. The result is faster quoting, higher first-pass yield, and parts that assemble without forcing.
Q&A
Q: Can I fold pre-painted steel without damaging the paint?
A: Yes. Use urethane lower die film or a panel bender with wiping blades. Radius at least 2 × t.
Q: Why do my 90° bends measure 92° after unloading?
A: Normal springback. Either overbend in the machine or switch to bottom bending for zero springback.
Q: What is the cheapest way to make 50 pieces with many short flanges?
A: Manual box-and-pan brake with a skilled operator is usually faster and cheaper than programming a CNC machine for small runs.
Q: How tight can I fold 3 mm 304 stainless without cracking?
A: Minimum safe inside radius is 6 mm (2 × t). Tighter requires hot forming or V-die bottoming with heavy tonnage.
Q: Is there a quick way to check grain direction on a new coil?
A: Bend a 50 mm wide test strip both ways. The direction that accepts the tighter radius and shows less springback is parallel to grain.