Chimney flashing, 4-part

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.

21 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.

• Udfoldningen er EKSAKT — alle fire dele er plane flader forbundet af rette bukkelinjer, og en kæde har ingen geometrisk tilnærmelse. Hele usikkerheden ligger i K-faktoren, og den er materialets, ikke geometriens. Lav altid en prøvebuk på det parti der skal bruges.
• Figuren regner på en LODRET skorsten med retvinklet grundplan på ÉN plan tagflade. Skorstene der gennembryder rygningen eller en skotrende, skæve skorstene og skorstene på buede tagflader er ikke dækket.
• Opkanthøjder, tagoverlæg og hjørneoverlæg er PARAMETRE med værkstedsmæssige startværdier. De er ikke normtal i dette værktøj — tagdækningens og skorstenens egne montageanvisninger vinder.
• Skorstenens højde over tagfladen og over rygningen, aftrækkets funktion og kravene til certificerede skorstensrør (EN 1856) ligger UDEN FOR figuren. Det er skorstensfejerens og bygningsreglementets område.
• DIN 6935 dækker KUN konstruktionsstål. For titanzink, kobber, aluminium og rustfrit er K-faktoren ekstrapolation — maskinens egen bukketabel vinder over enhver formel.
• Kontaktkorrosion er ikke regnet igennem for hele afvandingen. Kobber må ALDRIG ligge opstrøms for titanzink, aluminium eller galvaniseret stål (EN 12502-2); den modsatte rækkefølge er harmløs.
• Fastgørelse, vindlast og forankring er IKKE dækket. En inddækning på en fritstående skorsten er en vindudsat bygningsdel og skal dimensioneres efter EN 1991-1-4 og tagdækningens anvisninger.
• Undertag, dampspærre og tætningen mod tagkonstruktionen under blikket er uden for figurens område. Blikket er den yderste, ikke den eneste, tætning.