Guides · 14 September 2026 · 7 min

How to develop a lobster back bend for insulation cladding

The segmented bend, worked by parallel lines: where the numbers come from, why the end pieces are half segments, and the two things that go wrong on the bench.

A lobster back is a bend built from straight segments, each cut on a mitre so that the run of them turns the corner. On lagged pipe it is the fitting you make most, because a swept elbow in cladding is rare and a two-piece mitre only suits the small stuff. The development is parallel-line work, and it is worth knowing by hand even if the app does it for you, because the moment you understand it you can look at a printed pattern and know whether to trust it.

What you are working to

Cladding is developed to the outside of the lagging, not the pipe. So the first number is the cladding diameter: pipe outside diameter plus twice the insulation thickness. A 6-inch pipe (168.3 mm) with 25 mm of lagging is cladded at 218.3 mm. Half of that is the radius you carry through everything below.

The second number is the centre radius of the bend, measured to the pipe centre line. The trade works in multiples of the pipe size: 1D, 1.5D, 2D, 3D, 5D. A long-radius elbow on 6-inch is 1.5D, which is 1.5 times the nominal bore. Whatever the drawing says, the centre radius has to be bigger than the cladding radius, or the throat of the bend pinches into itself and no pattern on earth will fit it.

Dividing the bend into segments

A 90-degree bend in four segments is two full segments in the middle and a half segment at each end. The half segments are what let the bend land square on the straight cladding either side: a full segment at the end would leave a mitre face pointing out into space. So a four-segment bend has three mitre joints, and each joint turns the pipe through 90 divided by 3, which is 30 degrees.

Each mitre face sits at half the joint angle to the pipe. For that 30-degree joint the mitre is at 15 degrees. That 15 degrees, half the joint angle, is the number the whole development hangs on. Call it the half-mitre.

The throat and the back

On a full segment the shortest line is at the throat and the longest at the back, and the difference between them is fixed by the half-mitre and the cladding radius. The extra length at the back, over the centre-line length of the segment, is the cladding radius times the tangent of the half-mitre. On our 218.3 mm cladding at a 15-degree half-mitre that is 109.15 times 0.268, which is 29.2 mm. The throat is the same amount shorter. Going round the pipe from throat to back the length follows a cosine curve between those two values, which is why every lobster pattern has that same soft wave along its edge.

The centre-line length of a segment is the arc the pipe centre travels through the joint angle: centre radius times the joint angle in radians. Add your swage allowance at each joint on top of that, because the segment has to reach into its neighbour.

Setting it out by hand

1. 01Draw the half-mitre triangle: a horizontal line the cladding radius long, and the mitre line up from its end at the half-mitre angle. The vertical it makes is the back-to-throat difference.
2. 02Draw a half circle on the cladding diameter and divide it into six equal parts, so twelve stations round the full pipe. Number them 1 to 12 with 1 at the throat and 7 at the back.
3. 03Drop each station up onto the mitre line and across to the development. Station 1 gets the short length, station 7 the long, and the ones between get the cosine values.
4. 04Lay out the girth of the cladding (pi times the diameter) along the base line, mark the twelve stations equally along it, and stand the ordinates up at each. Join the tops with a fair curve.
5. 05Mirror the curve for the other end of the segment, add the lap down one edge and the swage allowance on each mitre, and that is one full segment. The half segments are the same curve with a square cut at the plain end.

Where it goes wrong

• Developing the whole bend as one sheet. A bend with gore joints is not developable in one piece; unrolling across a mitre shears the segments and the girth comes out wrong. Each segment is its own strip.
• Taking the centre radius inside the cladding radius. It happens when a 1D bend on a heavily lagged pipe is called up without anyone checking; the throat crosses itself and the pattern is nonsense. Refuse the job or change the radius.
• Forgetting the lap goes on the seam edge only, and that the rivet holes go on the fold lines, both edges, so the rolled segment comes out at the exact diameter and lands on its neighbour without a fight.

TurboClad does all of this from three numbers: the pipe, the insulation and the bend, with the centre radius picked as a multiple of the bore. It develops each segment on its own and prints the lot at 1:1 with the laps, holes and swage lines drawn. Print it at 100 percent, check the 100 mm bar with a rule, and offer the first segment up to the metal before you cut the rest.