Tôlerie Conception Guide: Bend Radius, K-Factor & Hole Spacing

Points Clés

• Inside bend radius ≥ 1× Épaisseur du matériau is the rule that prevents cracked bends — aluminum 5052 wants 1–1.5×, annealed stainless 1–2×, and harder tempers need more.
• K-factor (typically 0.3–0.5, commonly 0.33–0.44) locates the neutral axis and drives your flat-pattern math: BA = (π × angle/180) × (R + K × t). Get it wrong and every flange lands short or long.
• Keep holes at least 2.5× thickness + bend radius away from bend lines — closer than that, they stretch into ovals. Slots need 4× t + R.
• Flanges must be at least 4× Épaisseur du matériau tall, or the press brake has nothing to grip and the bend needs costly secondary operations.
• Springback is physics, not a defect — every air bend relaxes a few degrees when released; your fabricator over-bends to compensate, and harder materials spring back more.
• One uniform thickness, generous radii, features clear of bends — parts that follow the rules quote faster, cost less, and never come back with “can you move this hole?”

Most Tôle parts that get flagged in DFM review fail on the same four points: a bend radius the material can’t take, holes crowding a bend line, flanges too short to form, and flat patterns that don’t account for how metal actually stretches. None of these are exotic — they’re all covered by rules of thumb every fabricator uses daily.

This Tôle design guide collects those rules in one place — bend radius, K-factor, hole spacing, flange height, and springback — with the numbers, the reasons behind them, and a checklist you can run before any drawing goes out for usinage CNC.

Rayon de Pliage : La Règle qui Empêche la Fissuration

When Tôle bends, the outside surface stretches. Ask it to stretch around too tight a radius and it cracks — visibly on a bad day, as invisible micro-fissures that fail in service on a worse one. The baseline rule: keep the inside bend radius at least equal to the Épaisseur du matériau (1T). A 2 mm sheet gets a 2 mm inside radius, minimum.

Matériau moves the number. Ductile Acier laminé à froid handles 1T comfortably. Aluminium 5052-H32 prefers 1–1.5T — thin gauges tolerate tighter, thick and harder tempers need more — while 6061-T6 is the classic cracking candidate and wants generous radii or a bend across the rolling grain. Annealed stainless 304 runs 1–2T thanks to work-hardening at the bend. Two habits keep you safe: don’t design a smaller radius than the material family allows, and where a bend runs near the sheet’s rolling direction, orient it across the grain when possible. For material-level guidance, pair this article with our Tôle material selection guide.

K-Factor and Bend Allowance: Why Flat Patterns Don’t Add Up

Bend a sheet and the outer surface stretches while the inner surface compresses. Somewhere between them lies the neutral axis — the layer that keeps its original length. The K-factor is simply where that layer sits, as a fraction of thickness measured from the inside: K = 0.44 means 44% of the way through. Typical values run 0.3–0.5, with 0.33 common for tight air bends and 0.44 the default in most CAD systems.

K-factor feeds the bend allowance — the arc length of material consumed by each bend, which determines your flat-pattern size:

Bend Allowance = (π × bend angle / 180) × (R + K × t)
where R = inside bend radius, K = K-factor, t = Épaisseur du matériau. Example: 2 mm steel, 90° bend, R = 3 mm, K = 0.44 → BA = 1.571 × (3 + 0.88) ≈ 6.1 mm per bend.

The practical advice: don’t fight your fabricator’s numbers. K-factor varies with material, tooling, and Cintrage method, and every shop calibrates against its own press brakes. Conception with your CAD default, Envoyer STEP + flat-aware drawings, and let the shop apply its measured bend tables — flanges then land on dimension. What you must not do is hard-code flat patterns from an uncalibrated K and dimension the blank as gospel.

Règles d'espacement et de placement des trous

Cintrage pulls material toward the bend. Any feature inside that pull zone deforms with it, which is why hole placement rules exist:

• Holes: ≥ 2.5T + R from the bend line. Closer holes stretch oval and threads stop gauging. If function forces a closer hole, cut it after Cintrage — and expect the cost line to say so.
• Slots: ≥ 4T + R from the bend. Long openings surrender more edge to the pull and distort sooner than round holes.
• Hole diameter: ≥ 1T for laser-cut features, and ≥ 2T when punching stainless or high-tensile sheet — small punches in hard material break.
• Hole to edge: ≥ 2T of web, or the edge bulges when the hole is cut or the part is formed.
• Bend reliefs at flange corners: at least 1T wide and longer than the bend radius — they stop the tear that otherwise starts where a flange meets the parent face.

These clearances cost nothing at design time and remove the two most common DFM comments we Envoyer on bracket and enclosure drawings: “move hole away from bend” and “add bend relief.”

Retour élastique et hauteur de bride

Springback is the metal’s elastic recovery after the punch lifts: a bend formed to 90° relaxes open by a few degrees, more with harder materials and larger radii. Fabricators compensate by over-Cintrage against measured tables — you don’t need to design for it, but you do need to accept its consequence: bend angles carry tolerance (±1° is a normal expectation in air Cintrage), so avoid dimensioning schemes where three stacked bend angles must all land perfectly for a hole pattern to line up. Dimension critical features from a common face instead — the same logic as our guide to standard vs tight Tolérances.

Flange height has a hard floor: the press brake needs material to grip on both sides of the die. Keep every flange at least 4× Épaisseur du matériau tall (measured to the outside). Shorter flanges either can’t be formed at all or need secondary operations that multiply the bend’s cost.

Liste de vérification avant devis

1. One uniform thickness across the whole part — Tôle is folded, not sculpted.
2. Every inside radius ≥ 1T (1.5T for 5052, up to 2T for stainless), ideally one common radius for the whole part so one tool setup covers every bend.
3. Holes ≥ 2.5T + R and slots ≥ 4T + R from bend lines; holes ≥ 2T from edges.
4. All flanges ≥ 4T tall, bend reliefs ≥ 1T wide at flange corners.
5. Critical dimensions referenced from one face, not chained across multiple bends.
6. Envoyer STEP + PDF drawing with material, thickness, finish, and quantity, per our technical drawing guide — and let the fabricator apply calibrated K-factors to the flat pattern.

Real-world example: an automation client sent a 1.5 mm 5052 sensor housing with six M4 clearance holes sitting 2 mm from a bend line — well inside the 2.5T + R zone. Rather than bounce the design back, our DFM review shifted four holes 3 mm outboard and converted the two that couldn’t move to post-bend drilling. The formed parts gauged round on every hole, and the fix cost one email instead of one scrapped batch.

Concevez-le une fois, pliez-le correctement

Tôle rewards designers who respect five numbers: 1T radius, 2.5T + R hole clearance, 4T flanges, a calibrated K-factor, and a degree or two of springback. Follow them and your parts quote fast and form clean the first time. If you’d rather have a second pair of eyes, Envoyer your drawings: BravoFabs runs DFM review on every Tôle RFQ — Découpe laser, CNC Cintrage, Soudage, and finishing under one roof in Dongguan, China — for industrial clients across Europe and North America, free with every quote.