Tee and lateral

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: Inside 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.

Lige T-stykke: et lige hovedrør med en lodret gren, der står 90° på hovedrøret. Grenens ende er saddelskåret, så den slutter tæt om hovedrørets runde overflade.

Aftrækninger og forgreninger på runde kanaler i ventilation, og på procesrør hvor en sidegren skal tappe af eller føde ind på en gennemgående hovedstreng.

VVS-energi (ventilationstekniker) og teknisk isolatør — aftrækninger og forgreninger på runde kanaler; smed/metalsmed på procesrør.

Galvaniseret spiroplade 0,5–1,5 mm til ventilation; rustfri 1.4301 til proces og fødevarer.

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.

Hoved-Ø — Hoveddiameter

Det gennemgående hovedrørs udvendige diameter i mm — bestemmer saddel-kurvens krumning som grenen skal skæres efter.

Gren-Ø — Grendiameter

Sidegrenens diameter i mm. Grundlinjen i mønstret er grenens omkreds π·d, og kun grenen saddelskæres.

Hovedlængde — Main pipe length

Hovedrørets længde langs aksen i mm — afsætningen af selve hovedcylinderens kappe.

Grenlængde — Grenens længde

Grenrørets længde i mm fra den fri ende ned til hvor den møder hovedrøret (målt langs grenens akse).

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

1. 01Tegn grenrørets tværsnit som en cirkel med grenens diameter, og del omkredsen i 12 lige store dele (klokke-metoden 1–12).
2. 02Tegn ved siden grundlinjen lig grenens omkreds π·d, og del den i de samme 12 dele; rejs en lodret stationslinje op fra hvert delepunkt.
3. 03Tegn hovedrøret i opstalt (en cirkel med hoveddiameteren) der hvor grenen rammer, så du kan aflæse hvor højt hver generatrix støder mod hovedrørets bue.
4. 04For hver af de 12 stationer måles den sande længde fra grenens fri ende ned til hovedrørets overflade, og målet sættes af på den tilsvarende lodrette stationslinje.
5. 05Forbind toppunkterne med en jævn, bølget kurve — det er saddel-snittet hvor grenen møder hovedrøret.
6. 06Tegn grenens fri ende som en ret linje i toppen, læg fals/sammenlægskant til langs den ene side, og klip mønstret ud.
7. 07Rul mønstret til et rør og kontrollér saddel-kurven mod hovedrøret før falsning.

Sømmen kan flyttes rundt om grenen via klokkemålet. Læg den øverst eller på siden (kl. 12 eller kl. 3) frem for nederst (kl. 6), så samlingen ikke ligger i bunden hvor den ellers ville samle og slippe vand igennem.

• ·Hullet i hovedrøret er hverken en cirkel eller en ellipse: X = a·cos φ, Y = R_m·arcsin((a/R_o)·sin φ). Ellipse-tilnærmelsen holder kun for a/R_o < 0,3.
• ·Deler du omkredsen i for få stationer, bliver mønstret for lille: korden er kortere end buen. 12 dele er nok til pap, men til plade skal antallet højt (relativ mangel ≈ π²/6N²).
• ·Grenens søm kan flyttes rundt via klokkemålet — læg den væk fra vandets løb og et sted, der er nemt at svejse/nitte.

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.

• Fire resistance is NOT covered. Fire dampers and fire-classified ducts are certified equipment (EN 15650, tested to EN 1366-1) and must not be manufactured according to this figure.
• A branch weakens the main pipe. The ligament κ is given as a geometric figure, but how small it may become, and when reinforcement is required, is given in EN 13480-3 (metallic piping systems) or EN 13445-3 (pressure vessels) respectively — the figure supplies geometry, not a strength calculation.
• Certified flue and chimney pipes (EN 1856-1 / EN 1856-2) are type-approved equipment and must not be manufactured according to this figure.
• The joint preparation at the lateral's acute crotch is a welding-craft choice, not a geometric one. At θ below approx. 60° it requires a qualified welding procedure in accordance with EN ISO 15609 / EN ISO 15614 — the figure draws the cutting curve, not the joint geometry.
• The leakage class of a ventilation duct (EN 12237 for round ducts, class A–D) depends on the seam's execution, sealant and installation — not on the development. The class must be agreed, not derived.
• Hygienic execution follows EHEDG's principles (set-in, fully welded, dead leg ≤ 2·d_i). They do not replace an approval under Regulation (EC) 1935/2004 or a HACCP assessment.
• The cutting curve is calculated on the branch's NEUTRAL axis against the surface the mounting choice designates. The outer surface's and the neutral axis's cutting curves are not the same; the difference is of the order t/2 and disappears into the fitting joint — but it is there.
• The insulation shell's diameters are measured OVER the insulation. Insulation thickness, vapour barrier and condensation risk are not part of this figure.