Square tube cross

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.

Firkantrør-kryds: et firkantet hovedrør med et rundt grenrør stående lodret oppe på den plane overside. Fordi hovedrøret er fladt, slutter grenen mod en lige flade i stedet for en saddel-kurve.

Runde afgreninger og studse på firkantede kanaler i ventilation, hvor et rundt rør skal tilsluttes en plan kanalside.

VVS-energi og teknisk isolatør — runde studse på firkantede kanaler; smed/metalsmed på firkantprofil-samlinger.

Galvaniseret stålplade 0,7–1,25 mm til firkantkanaler; rustfri i proces.

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-side — Hovedrørets sidemål

Sidelængden i mm på det firkantede hovedrørs tværsnit — afsætningen af kanalens plane sider.

Gren-Ø — Grendiameter

Det runde grenrørs diameter i mm. Grenens omkreds π·d danner grundlinjen i mønstret.

Hovedlængde — Main pipe length

Det firkantede hovedrørs længde i mm langs aksen.

Grenlængde — Grenens længde

Det runde grenrørs længde i mm fra fri ende ned til hvor det møder hovedrørets overside.

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

1. 01Det firkantede hovedrør udfoldes som fire plane sider: afsæt en grundlinje lig 4 gange sidemålet og del den i de fire flader, plus fals langs den ene kant.
2. 02Marker hvor det runde grenrør sidder på oversiden, og tegn et rundt hul med grenens diameter centreret der.
3. 03Tegn grenrørets tværsnit som en cirkel og del omkredsen i 12 dele med klokke-metoden.
4. 04Afsæt grundlinjen lig grenens omkreds π·d, delt i 12, og rejs en lodret stationslinje fra hvert delepunkt.
5. 05Fordi grenen møder en plan flade og står lodret, er enden et lige snit: alle 12 generatricer har samme længde, så toppen bliver en ret linje (ingen saddel-kurve).
6. 06Læg fals til langs den ene side, klip grenmønstret ud, rul til rør og tilpas mod hullet i oversiden før samling.

• ·Fordi hovedrøret er fladt, slutter grenen mod en LIGE flade (ren cirkel), ikke en saddel-kurve — hullet i den plane side er en ren cirkel, mens grenens ende er lige.
• ·Studsen skal have en fligekrave (opbukkede flige), hvis den skal svejse-/nittesamles mod den plane flade.
• ·Læg aldrig sømmen i bunden (kl. 6) — kondens og vand samler sig i falsen. Læg den kl. 12 eller på siden (kl. 3).

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.

20 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.

• The figure covers the square profile — flat sides. Round pipe tees and laterals (saddle cuts) belong to the figure ‘T-piece and lateral’, not here.
• Pressure drop, sound attenuation and air distribution between the main run and the branch are NOT calculated. A branch increases turbulence and pressure drop; the sizing belongs to the project's own duct calculation.
• Stiffening against buckling of large, thin duct sides is not sized. The relationship between side dimension, material thickness and stiffening is given in DS 447 / EN 1505 and is an INPUT — the figure only warns with figures.
• Fire dampers, fire protection and smoke ventilation are not covered. A branch piece in a fire-protected duct system must be executed in accordance with EN 1366 / EN 15650 and the authorities' instructions.
• Set-in mounting (the branch pushed THROUGH the main duct) is not modelled — the hole is calculated as a set-on pass-through. For set-in, the hole must be made 2·t + root gap larger.
• For an oblique cut (θ < 90°), the branch is laid out as a flat blank WITHOUT bend lines; the three bend lines are given as check dimensions and must be scribed on. The seam allowance on the oblique branch must be added according to the machine's roll set.