Bend Allowance, K-Factor and Bend Tolerances for Custom Sheet Metal: What to Specify and What to Demand from the Supplier

Who owns the K-factor on your drawing

The K-factor describes where the neutral axis sits while the material stretches through a bend, and it is the number that turns a folded 3D part back into a flat blank¹². One design guide publishes the range as 0–0.5 and pairs it with a chart of values by material and bending method¹².

That span is wide enough that any single number you pick from it is a guess until the supplier's tooling and the actual material lot are fixed.

Treat the K value as the fabricator's input: ask for their K-factor and the flat pattern they generated from it, and approve the flat pattern as a controlled document rather than re-deriving it. The sources in this set give no bend-allowance or bend-deduction formula and no per-material K table.

So the arithmetic behind the flat length is something the supplier must show you, not something you can check against a published standard here.

Setting the inside radius and flange baseline

Put a minimum inside bend radius of one material thickness on the drawing as the design baseline¹². What the shop can actually deliver at that radius depends on the die: in air bending the sheet is pressed into a V-die without bottoming out.

And a wider V-opening produces a larger inside radius⁵. Thicker material needs a wider opening to spread the bending stress and avoid cracking, which pushes the radius up and lengthens the minimum flange the part needs to span the die⁵.

Bending force moves with the alloy too — one supplier puts aluminum at roughly half and stainless at about 1.5 times the force of steel. Which matters when a thick stainless flange sits near the edge of a press brake's capacity⁵.

So the radius callout and the V-die opening are one decision, not two: ask the supplier which die opening they will run and what inside radius and minimum flange that produces on your thickness.

Which bend tolerances survive scrutiny

The only bend figures in this evidence are vendor-tier, and none of them is tied to a part size or thickness. Use them as claims to qualify, not as physics.

• ✓Accept ±1° bend angle as a standard callout — a design guide recommends it and a fabricator states it as capability.
• ✓Treat ±0.254 mm bend-to-edge and ±0.2 mm bend-to-hole as one supplier's offer, and ask for the part size and thickness under which they hold.
• ✓Ask which general-tolerance standard governs the drawing; one fabricator cites ISO 2768-c and -m for general metal tolerances.
• ✓Reject any quote that promises tighter than ±1° without naming the material, thickness and bend length it applies to.

Matching springback control to material and criticality

Springback is elastic recovery after the punch retracts, and it scales with how hard the material is. One supplier ranks it from low in copper and brass, through moderate in mild steel and 5052 aluminum, to higher in 304 stainless and 6061-T6, and high in AHSS and dual-phase steels^11,13.

The same source notes 6061-T6 is less tolerant of severe bending than softer tempers¹³. That ranking, not a degree figure, is what should drive your choice of control method.

Overbending is the default for air bending: the punch goes past the target angle so recovery lands on nominal, and a CNC brake with online angle measurement can correct bend by bend¹¹. Bottoming presses the sheet fully into the die and suits stainless and harder alloys, but needs matched tooling¹¹.

Coining nearly eliminates springback at the highest tonnage and tooling cost, which the same source limits to high-value or safety-critical parts¹¹. For a 304 or 6061-T6 bracket, ask which of these the quote assumes; for a mild-steel cover, overbending is the reasonable default.

Press brake, folding machine or stamping die

At low quantity, laser cutting plus press-brake forming usually wins on flexibility and minimal tooling cost¹⁴. As volume rises, the cost of a dedicated stamping die is spread across more parts.

And at high quantities the economics can shift strongly toward stamping because the press cycle stays fast while tooling cost is distributed¹⁴. Stamping's own weakness is the same tooling: high initial die cost and limits on size and geometry²³.

Folding machines work on the same V-die principle but tend to be used on thinner material¹⁹. The sources give no tooling cost figures and no volume threshold in pieces, so the crossover is yours to calculate from your supplier's die quote against your annual quantity.

Inspection and documentation to write into the RFQ

Ask how the supplier will measure bend angle and flat length, and require first-article approval of the flat pattern before the run. The material condition belongs in that approval, not just in the paperwork: cold rolling raises strength and hardness while reducing ductility compared with hot rolled.

So the temper on the certificate changes how the part forms²⁸. Where the part serves regulated construction, EN 10204 3.1 certificates are as a rule required for stainless steel components, per one GMP-compliance source⁷.

That requirement is sector-specific, so confirm it applies to your end use before you make it a blanket callout.

Bend tolerances versus machining tolerances

Machining repeatability figures get copied onto bend drawings by mistake. Keep them apart: the left column is what a bend can be held to, the right is a machining number that does not transfer.

Before you request a quote

• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Request certificate numbers, issuing bodies, scope and validity for the exact model and destination market.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.
• Ask for relevant export destinations, installed-base references and comparable customer contacts.
• Ask the supplier for model-specific evidence and record the condition that would change your buying decision.