Breeches piece

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.

Y-stykke: et hovedrør der deler sig i to ens grene, spredt symmetrisk om aksen. Begge grene har samme diameter som hovedrøret og mødes i en symmetrisk gjæring.

Symmetriske gaffel-forgreninger i ventilation og procesrør, hvor en strøm skal deles ligeligt i to retninger.

VVS-energi og teknisk isolatør — symmetriske gaffel-forgreninger, hvor en strøm deles ligeligt i to; smed/metalsmed på procesrør.

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

Rør-Ø — Pipe diameter

Den fælles diameter i mm for hovedrør og begge grene — alle tre rør er lige store i dette Y.

Spredningsvinkel — Spredningsvinkel

Den samlede vinkel i grader mellem de to grene; den deles ligeligt, så hver gren spredes ±halvdelen ud fra aksen.

Grenlængde — Grenlængde

Længden i mm af hver gren (og hovedrøret) målt langs aksen.

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

1. 01Tegn samlingen i opstalt: hovedrøret op gennem midten og de to grene spredt ±halvdelen af spredningsvinklen, så de mødes i en symmetrisk gjæringslinje.
2. 02Aflæs gjæringslinjen — fordi rørene er lige store, halverer den vinklen mellem hvert rør og bliver en ret linje i opstalten.
3. 03Tegn rørets tværsnit som en cirkel og del omkredsen i 12 dele med klokke-metoden; rejs stationslinjer fra hvert delepunkt op til gjæringslinjen.
4. 04Afsæt grundlinjen lig rørets omkreds π·d ved siden af, delt i 12, og rejs lodrette stationslinjer.
5. 05Overfør hver generatrices sande længde fra tværsnittet op til gjæringslinjen ud på den tilsvarende stationslinje.
6. 06Forbind toppunkterne til en jævn kurve — det er gjæringssnittet for ét grenrør; samme mønster gælder begge symmetriske grene.
7. 07Læg fals til, klip ud, og rul til rør; de to ens grene gjæres mod hinanden om hovedrøret.

• ·De to grene mødes i en symmetrisk gjæring — gjæringsvinklen er halvdelen af grenenes indbyrdes spredning, ikke deres fulde vinkel.
• ·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).

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.

18 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 describes the GEOMETRY. It does not size the installation: air volume, velocity, pressure drop and noise must be calculated according to the installation's own design, and the cross-section division is a check, not a sizing.
• Fire dampers and fire-resistant ducts are certified equipment (EN 15650, EN 1366-1/-2). A branch in a fire-compartment separation must be executed according to the system supplier's approved detail — not according to this figure.
• Certified chimney and flue systems (EN 1856-1/-2) are type-approved as a SYSTEM. A home-made branch piece in such a system voids the approval.
• Pressure-bearing pipes and vessels above PED 2014/68/EU's class limits must be calculated in accordance with EN 13445 or EN 13480 respectively. The figure does not calculate any material thickness from pressure.
• The joint is not stiffened or reinforced. At large diameters, thin sheet or pulsating flow, the step is the piece's weakest point, and reinforcement must be sized separately.
• The straight edge and any flanges, sockets and gaskets at the ends are NOT part of the workpiece. The ends are supplied raw and sharp.
• Hygienic execution follows EHEDG's principles (no crevices, fully welded and ground root). This does not replace an approval under Regulation (EC) 1935/2004 or a HACCP assessment.