Segmented pipe 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: 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.

En rund rørbøjning sat sammen af flere lige rør-stykker (segmenter), der hver er skåret skråt i enderne (gjæringssnit) og samlet til en glat bue. Jo flere segmenter, jo blødere følger bøjningen den ønskede radius.

Bruges overalt hvor en rund kanal eller et rør skal skifte retning: ventilations- og udsugningsanlæg, spirorør, procesrørføring og smede-/blikkenslagerarbejde. Segmenterede gjæringsbøjninger laves i hånden når der ikke kan trækkes en færdig bøjning.

VVS-energi (ventilationstekniker), teknisk isolatør, smed og metalsmed — hvor en rund kanal eller et spirorør skal skifte retning uden en færdigtrukket bøjning.

Galvaniseret stålplade (DX51D+Z) 0,5–1,5 mm til ventilation, rustfri 1.4301 til proces og fødevarer, aluminium til lette kanaler. Tyndplade, der samles med fals eller blindnitter.

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.

Rør-Ø — Pipe diameter

Rørets udvendige diameter i mm — bestemmer tværsnittets størrelse og dermed den udrullede grundlinje (omkredsen π·d).

Bøjeradius — Bøjeradius

Afstanden i mm fra bøjningens centrum til rørets centerlinje — styrer hvor blød eller skarp buen er.

Bøjevinkel — Bøjevinkel

Hvor mange grader (°) røret skifter retning, fx 90° for en ret vinkel.

Segmenter — Number of segments

Antal lige rør-stykker (uden enhed) buen deles i — flere segmenter giver en glattere bøjning og flere gjæringssnit at samle.

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

1. 01Tegn rørets tværsnit som en cirkel med diameter lig Rør-Ø, og del omkredsen i 12 lige store dele med passeren (ur-metoden): nummerér delepunkterne 1–12.
2. 02Tegn opstalten af ét segment fra siden: afsæt centerlinjen og marker gjæringssnittets vinkel — for en jævn bøjning er hvert ende-snit Bøjevinkel/(2·Segmenter), og mellem-snit det dobbelte.
3. 03Træk lodrette stationslinjer op fra hvert af de 12 delepunkter til gjæringssnittet, så du kan aflæse hver generatrices sande længde op til snitlinjen.
4. 04Ved siden afsætter du en vandret udrullet grundlinje lig omkredsen (π · Rør-Ø) og deler den i de samme 12 dele med samme nummerering.
5. 05Rejs en lodret stationslinje i hvert delepunkt på grundlinjen, og overfør hver generatrices sande længde fra opstalten ud på den tilsvarende stationslinje.
6. 06Forbind toppunkterne med en jævn kurve gennem en bøjelig lineal — det er mønstret for det halve segment; spejl det om midten for at få det fulde gjæringsstykke.
7. 07Læg saml-tillæg (false/overlæg) til langs den ene generatrix, klip i pap og rul til en cylinder for at kontrollere at gjæringssnittene flugter.
8. 08Gentag for hvert af Segmenter stykker (mellemstykkerne er ens og dobbelt så skrå i begge ender) og saml dem skiftevis til den fulde bue.

Sømmen kan flyttes hele vejen rundt om røret via klokkemålet i værktøjet. Læg sømmen øverst eller på siden (klokken 12 eller 3) — aldrig i bunden klokken 6 — så samlingen vender væk fra vandet og leder fugt udenom i stedet for at suge den ind.

• ·Regn udfoldningen på den NEUTRALE fiber (middeldiameter), ikke ydre eller indre — på Ø800 med t = 3 mm er forskellen 18,8 mm, nok til at sømmen ikke mødes.
• ·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).
• ·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.

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.

• The figure is pure GEOMETRY. It does not size the pipe: wall thickness, pressure, temperature and support have not been calculated, and a pressure-bearing piping system must be calculated in accordance with EN 13480 or PED 2014/68/EU respectively.
• The flow resistance in a mitred bend is greater than in a drawn one, and it increases as the number of segments decreases. The tool calculates NO pressure loss and has no resistance coefficients — use the piping system's own ζ-values or equivalent lengths.
• Certified chimney and flue-gas pipes (EN 1856) and fire-resistant ventilation ducts (EN 1366/EN 15650) are type-approved equipment and must not be manufactured according to this figure.
• The flange ring is a GENERIC plate ring with the user's own dimensions. It is not a standard flange: the bolt pattern, gasket face and pressure class follow neither EN 1092-1 nor EN 12220, and it must not be used as a mating flange to a catalogue flange without the dimensions being checked.
• Edge distance and pitch in the hole pattern follow the repo's rivet rules (edge distance ≥ 2·d) and workshop practice for a minimum pitch of 3·d. It is guidance-only practice from the blind-rivet suppliers' installation instructions — NOT a structural code. Load-bearing and pressure-bearing joints must be sized separately.
• The clearance in the socket-and-spigot fit is an INPUT. Look up the fit's tolerance with the supplier of the pipe the bend is to be joined to; the tool does not invent it.
• Insulation shells in food and cleanroom zones must be cleanable. Riveted overlap and seam joints are crevices; choose a butt-welded seam and switch on ‘Hygienic execution’ if the piece is located in such a zone.
• ‘Bend type A/B/C/D’ is PRACTICE from the trade, passed on by a technical insulator — not a standard, a trade agreement or a supplier table. No written source for the naming has been found, and none is therefore cited. The source should be traced in the AMU booklet for technical insulation or in the insulation trade's teaching materials, before the naming is used as curriculum.
• The bend type changes NOTHING in the cutting pattern. The mitre cuts are sharp edges without a circumferential overlap, so the choice is an assembly instruction in the text — the circumferential overlap at the head or tail must be added by hand by whoever assembles the piece.