Rectangular elbow

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.

En retvinklet, firkantet kanalbøjning bygget af to lige rektangulære ben, der mødes i et fælles hjørne. Tværsnittet er konstant rektangulært hele vejen igennem.

Klassisk i firkantede ventilations- og klimakanaler, hvor luften skal dreje 90° langs en væg eller i et teknikrum. Bruges også til afkast- og indblæsningskanaler i blik.

VVS-energi og teknisk isolatør på firkantede klimakanaler; smed og metalsmed på blik-kanaler i teknikrum.

Galvaniseret stålplade 0,7–1,25 mm til klimakanaler; rustfri i køkken og proces. Falses op langs kanterne.

Source: Materialer/tykkelser: src/lib/kasse/materialer.ts (DIN 6935 for stål; massefylder og handelstykkelser; springback). Ventilation: DS 447:2021 + BR18 §§ 420–452 (src/lib/vvs/artikler.ts). Fremstillingsfælder: docs/PLAN-UDFOLDNINGSBIBLIOTEK.md §9.2/§4.

Bredde — Kanalbredde

Kanaltværsnittets bredde i mm (den ene af de to sider i rektanglet).

Højde — Kanalhøjde

Kanaltværsnittets højde i mm (den anden side i rektanglet).

Benlængde — Leg length

Længden i mm af hvert af de to lige ben målt fra hjørnet og ud til enden.

Step by step with compass and square — in cardboard first, in sheet metal after.

1. 01Tegn kanalens rektangulære tværsnit i fuld størrelse: Bredde × Højde — det er det profil de fire sider skal følge.
2. 02Aftegn de to lige ben fra siden som et L: hvert ben har længden Benlængde, og de mødes i et 90°-hjørne.
3. 03Læg gjæringslinjen ind i hjørnet — diagonalen der halverer det rette hjørne (45°) — for de to sider der danner ydersiden og indersiden af bøjningen.
4. 04Rul de fire sider ud som ét sammenhængende bånd: afsæt Bredde, Højde, Bredde, Højde i rækkefølge langs en lige grundlinje for hvert ben.
5. 05Overfør hjørnets gjæringssnit til båndet, så top- og bundflade får deres 45°-snit, mens sideflader følger benenes fulde længde.
6. 06Tilføj false- og samletillæg langs den ene langkant og i enderne, så pladen kan lukkes og kanalerne kan samles.
7. 07Klip mønstret i pap, ombuk langs alle fire kanter til det rektangulære profil, og kontrollér at hjørnegjæringen lukker pænt i 90°.

Placér samlefalsen i et hjørne af rektanglet og ikke midt på en flade — så bliver kanten stiv og samlingen pæn. Buk altid efter en ridset linje, så det firkantede profil holder mål hele vejen.

• ·Snitkurven på et rektangulært tværsnit er en trapez-savtak (lineær langs siderne, konstant på ryg og hals) — IKKE en sinus som på et rundt rør. Bruger man cos(φ) fra det runde gjæringsrør, drejer bøjningen ikke den ønskede vinkel.
• ·Pittsburgh-falsen er asymmetrisk: 25,40 mm på lommesiden, 7,94 mm på hansiden. Lægger man 25,40 mm på begge kanter, bliver kanalen 17,5 mm for stor i omkreds.
• ·Læg samlefalsen i et hjørne af rektanglet, ikke midt på en flade — kanten bliver stiv og samlingen pæn.

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.

13 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.

• A × B are EXTERNAL dimensions. The internal clear dimension is 2 · t smaller on each side, and it is the clear dimension that determines the air volume — not the dimension the duct is ordered by.
• The figure does not size the duct. Which material thickness, which stiffening and which leakage class a given duct side requires is given in DS 447 and EN 1505/EN 1507 — that is an input here.
• Pressure loss, sound attenuation and turning vanes are NOT covered. A mitre bend without turning vanes has a significantly higher resistance coefficient than a radius bend; the ζ-value is looked up in duct data.
• Fire dampers, fire-insulated ducts and smoke ventilation ducts are not covered. They are certified components to EN 15650 and EN 12101 respectively and must not be manufactured from a development pattern.
• Seam and flange allowances have not been added to the blank (see the notes). The seam's material consumption depends on the seaming machine's roll set and is given in the machine's data sheet — the tool does not invent it.
• Springback is not compensated for. The development is calculated on the DESIRED angle; how much must be over-bent depends on material, thickness and V-opening and belongs on the machine.
• Hangers, expansion and support are not covered. A bend in a main duct is a fixed point in the routing and must be hung in accordance with the installation's own instructions.