Strip layout for a stamping die

The surface is developable and the flat pattern is solved in closed form. Cut the part to the drawing and it fits — the whole uncertainty sits in the K-factor, which belongs to the material and the machine, not to the geometry.

Dimensions are entered as: Outside dimensions. The flat pattern itself is always computed on the neutral fibre — at Ø800 in 3 mm sheet the difference between the inside and the outside dimension is 18.8 mm of developed width, and then the seam does not meet.

The words and formulas the trade uses for a flat pattern. The symbols are the same throughout Discebo: α is the bend angle in degrees, R the inside bend radius, t the sheet thickness and K the K-factor — all in millimetres.

Flat pattern (development)

The part's surface laid out flat as one continuous cutting outline — the way the sheet looks BEFORE it is bent or rolled. Only a developable surface can be unfolded exactly; a doubly curved surface cannot be laid flat without stretching the material.

Flat length

The flat length the part must be cut to so the dimensions come out right once it is bent. It is either the sum of the outside dimensions minus the bend deductions, or the sum of the flat legs plus the bend allowances — both routes must give the same number.L = Σ(udvendige mål) − Σ(BD) = Σ(flade ben) + Σ(BA)

Bend allowance

The length of material that sits in the bend itself, measured along the neutral fibre. The bend allowance is always positive and is added to the flat legs.BA = (π/180) · α · (R + K · t)

Bend deduction

The amount subtracted from the sum of the outside dimensions, because the two outside dimensions between them count the corner twice. Here the bend deduction is taken as POSITIVE and subtracted; DIN 6935 writes the same quantity as the compensation value v with the opposite sign.BD = 2 · OSSB − BA

Outside setback

The distance from the theoretical outside corner point in to where the bend starts. Above 90°, tan(α/2) is clamped to at most 1 per DIN 6935, so the number does not explode towards 180°.OSSB = tan(α/2) · (R + t)

Compensation value v to DIN 6935

The standard's own way of writing the same corner loss: the flat length is a + b + v, where v is negative. Our bend deduction is therefore the same quantity with the opposite sign — never flip one without the other.L = a + b + v ⇒ v = −BD

K-factor

Where the neutral fibre sits in the sheet, given as a fraction of the sheet thickness. DIN 6935 writes the correction factor k, which applies to the neutral line in the form (R + t·k/2); the American CAD convention writes (R + K·t), so K = k/2.k = 0,65 + 0,5 · log₁₀(R/t), k ≤ 1,0 ⇒ K = k/2

Neutral fibre

The layer in the sheet that is neither stretched nor compressed when the part is bent. The flat pattern is always computed on the neutral fibre — which is why it matters whether the drawing's dimensions are inside or outside.

Bend angle

The angle the material is turned through, not the angle between the legs. The bend angle is 180° minus the included angle, so a square corner is 90° and a seam fold is 180°.α = 180° − åbningsvinklen

Inside bend radius

The radius on the inside of the bend. It is set by the tooling and the material, not by the drawing — too small a radius cracks the part in the outer fibre.

Sheet thickness

The thickness of the material. It enters every single bend allowance, so sheet delivered 0.2 mm thicker than ordered shifts the flat length at every bend.

Fold line

The line on the flat pattern where the part is to be bent. It is drawn but never cut through — in a laser file it therefore sits on its own layer.

Test bend

A strip of known raw length, bent in the material and the tooling that production will use. Measure the two outside legs afterwards and the bend deduction and K-factor can be worked back. The machine's own measured bend table beats any table.BD_målt = (A + B) − L₀

DIN 6935 covers only cold bending of flat products in structural steel. Stainless, aluminium, copper, brass and titanium zinc are not included — there the formulas are an extrapolation, and a test bend in the actual material settles it.

24 fields. The starting values belong to the figure itself — they live in one place in the code, so the tool, the share link and the smoke test can never disagree about what «default» means.

A preset sets every dimension at once. The name is often the word the part goes by in the workshop — the library's search knows them all.

Read them before the part is cut. They are here because they cannot be calculated away — not as legal text, but as craft.

• This is a LAYOUT, not a tool design. The punch's wear, the stripper's force, the location of the centre of pressure, the station sequence, the pilot holes, the spring force and the tool's guides are NOT covered. The figure states where the pieces lie and what the material costs — not whether the tool will hold up.
• Bridge and edge strip are WORKSHOP PRACTICE (bridge approx. 1.0–1.5 × the material thickness, at least approx. 1.0 mm; edge strip approx. 1.5 × the material thickness). There is no DS/EN standard for these figures, and they depend on the material's toughness, on the punch's wear and on how hard the feed pulls on the skeleton. The tool's own bridge table always takes precedence — the tool does not invent the figure.
• The press force has not been calculated. F ≈ perimeter × material thickness × shear strength, and the shear strength belongs to the material, not the geometry. If the press is to be sized, that belongs under the supplier's data for that specific alloy and that shear speed — and the stripper force comes on top.
• The contour in the pattern and in the DXF is NOMINAL. The shear gap between punch and die (approx. 5–10% of the material thickness per side) has not been deducted, and a punched hole gets the die's dimension while a punched-out piece gets the punch's. Which of the two should have the nominal contour is determined by whether the piece is the slug or the remainder.
• The number of strips and pieces is GUARANTEED ACHIEVABLE, not optimal (the same limitation as the engine's sheet layout). The shadow profile overestimates the piece's extent within each band, so two pieces can never come closer together than the bridge — but an expensive optimiser may find a few more pieces.
• No calculation has been made for flatness, planarity or how the strip behaves during feeding. A thin strip with large punch-outs will buckle, and a skeleton with too-narrow bridges will jam in the tool. Both are determined on the machine, not in a spreadsheet.
• Load-bearing, pressure-bearing and certified pieces are not covered. If the piece is to carry load, the calculation belongs under EN 1993-1-1/-1-8; if it is a load-bearing construction product, also under EN 1090 with CE marking and execution class. Lifting gear is certified equipment to the Machinery Directive 2006/42/EC and must not be manufactured according to this figure.