Bypass
Pipe jacket pressed flat where there is no room — straight, in a bend, in a T and in a cone.
How true is the flat pattern?
The surface is faceted or triangulated, so the pattern is an approximation. The error is known and printed as a number on the drawing — it is not hidden.
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.
The vocabulary of bending
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.
The dimensions you type
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.
| Dimension in mm | Interval | Standard | Explanation |
|---|---|---|---|
| Where is the bypass?variant | Straight pipe, tapered · Straight pipe, plain · In a bend · In a T-piece · In a cone (reducer) | Straight pipe, tapered | — |
| Diameter (inside dimension of the jacket)D | 40–1.000 mm · step size 1 | 120 mm | Outside diameter of the insulation. In a reducer: the diameter at the end where the minus measure sits. |
| Minus measureP | 1–300 mm · step size 1 | 25 mm | How far the pipe is pressed in: from the circle in to the flat side. Must be less than the radius. |
| Bypass lengthconditionallaengde | 20–2.000 mm · step size 1 | 120 mm | — |
| Flaps at the endsconditionalflapper | on / off | on | Circular segment with radius R that closes the end. With flaps, the end gets an under-bead. |
| Bend angleconditionalboej_vinkel | 15–90 ° · step size 0,5 | 45 ° | — |
| Number of full segmentsconditionalboej_hele | 0–6 stk · step size 1 | 0 stk | In addition to the two half end segments. 0 = a knee in two pieces. |
| Bend radius (to the axis)conditionalboej_R | 0–3.000 mm · step size 1 | 100 mm | A knee has no real radius; the school picks one so there is something to build around. |
| Centre lengthconditionalboej_cl | 20–3.000 mm · step size 1 | 200 mm | From the straight end along the axis to the intersection of the two legs’ axes. |
| Fully flat segments in the middleconditionalboej_flade | 0–6 stk · step size 1 | 0 stk | The bypass sits in the middle of the bend with one tapered segment on each side. |
| Branch centre lengthconditionalt_cl | 30–2.000 mm · step size 1 | 140 mm | From the main pipe’s axis to the free end of the branch. |
| Main pipe bypass lengthconditionalt_laengde | 50–3.000 mm · step size 1 | 250 mm | — |
| Flange collar on the branchconditionalbertel | 0–20 mm · step size 1 | 10 mm | At least 10 mm, laid down 87–88° on the main pipe. |
| Diameter of the other endconditionalk_D2 | 40–1.500 mm · step size 1 | 230 mm | The full end of the reducer — without minus measure. |
| Reducer lengthconditionalk_hoejde | 20–3.000 mm · step size 1 | 300 mm | — |
| Position of the longitudinal seamsoem | 3 o’clock (at the side) · 6 o’clock (opposite the bypass) · 9 o’clock (at the side) | 3 o’clock (at the side) | The middle of the bypass is at 12 o’clock. The seam must not sit in the bypass. |
| Bead at the endsende_sik | Over-bead (3 mm) · Under-bead (5 mm) · No bead | Over-bead (3 mm) | — |
| Goods thicknesst | 0,4–1,5 mm · step size 0,05 | 0,7 mm | Sheet jackets are 0.5–1.0 mm. |
| Materiallegering | Aluminium Al99.5 (1050A) · Aluminium AlMg3 (5754) · Hot-dip galvanised steel DX51D+Z · Stainless 1.4301 / AISI 304 · Stainless 1.4404 / AISI 316L | Aluminium Al99.5 (1050A) | — |
Advanced fields
Hidden by default — they have a sensible default, and most parts do not need them.
| Dimension in mm | Interval | Standard | Explanation |
|---|---|---|---|
| Overlap on the longitudinal seamoverlaeg | 10–50 mm · step size 1 | 20 mm | The school adds 10 mm on both sides of the circumference = 20 mm overlap. |
Ready-made presets
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.
Tapered bypass Ø120, minus 25
Lektion 1 bypas: flat at one end, tapered up at the other; chord 97.5 mm against arc 113.7 mm
Also known as: strålet bypass · stråle bypas · bypass lige rør
Plain bypass Ø120, minus 25, with flaps
Lektion 1 bypas: flat all the way, flaps with radius 60 and under-bead at the ends
Also known as: almindelig bypass · bypass med flapper · flad bypass
Plain bypass Ø100, minus 10
The formula sheet’s example: K 60, V 73.74°, S 64.35, circumference 309.81 mm
Also known as: bypass formel · bypass regneeksempel
Tapered bypass in 45° knee Ø120, minus 25
Lektion 2 bypas bj: two segments with 22.5° mitre, centre length 200, bypass in the back
Also known as: bypass i knæ · bypass i bøjning · bypass knæbøjning
Tapered bypass in 90° bend Ø135, minus 30
Exercise booklet p. 17: R 120, centre length 200, 3 full + 2 half segments
Also known as: bypass i bøjning · stråle bypas i bøjning · bøjning med bypass
Tapered T-piece Ø120 with bypass, minus 30
Exercise booklet p. 18: equal diameter, branch centre length 140, main pipe bypass 250 mm
Also known as: bypass i T-stykke · bypass i tee · stråle tee
Bypass in reducer Ø230 → Ø130, minus 30
Reducer dimensions from the exercise booklet p. 24, minus measure 30 at the small end
Also known as: bypass i kegle · bypass i reduktion · konisk bypass
Reservation
Read them before the part is cut. They are here because they cannot be calculated away — not as legal text, but as craft.
- For safety reasons, a bypass must not be fitted on high-pressure steam (Pladeudfoldning specielle, AMU Syd 2009, p. 9).
- A bypass «cheats» on the insulation thickness, so it can be a good idea to get the customer’s approval (Pladeudfoldning specielle p. 9). A bypass does not comply with DS 452; there is only a tolerance for small fittings (Lektion 1 bypas).
- Oblique bypass on a straight pipe has not been built: it only exists as a scanned drawing without text or dimensions.
- A bypass in a cone follows «the same method» as on a straight pipe; the source does not describe the development itself. Source reliability is medium.
- The T-piece is only built for equal diameter with the same minus measure in branch and main pipe (exercise booklet p. 18).
- The flaps are drawn as separate parts. In the school’s pattern they are in one piece with the jacket and are bent down at the end.