Making and checking branched fittings

Seven fittings have been developed as mean-line patterns, every crotch found and every element checked. What remains is what turns patterns into fittings: allowances for the joints, the order in which the pieces come together, the checks that prove each fitting before it is closed, and the list of what the job consumes. Branched fittings add one thing that plain fittings do not have, the crotch seam, and most of this chapter’s care goes there.

This chapter sets out the allowances, the crotch seam and its riveting order, a check sheet for each fitting, the job’s sheet and mass, and the records kept, and ends with a crotch that was riveted from the peak down.

Allowances

Every allowance here is an illustration, to be replaced by the shop’s own figures for its own seams and sheet. They are added to the mean-line patterns only after those patterns have been checked, and they are offset square to each edge, as Transitional Elbows by the Numbers sets out.

Figure 1Allowances on the FX-4 fittings

Joint	Where	Allowance	Added to
Crotch seam, riveted lap	every breeches and manifold	15 mm	one prong's crotch edge only
Prong seam at the crotch's peak	each prong	15 mm	one edge
Slip joint, spigot end	main and prong ends	30 mm	the end's edge
Hood seams	G1 back and front	15 mm	one edge of each
Pipe tabs	G7 into G1	15 mm, cut into 20 mm tabs	the pipe's cut curve

The crotch seam’s allowance goes on one prong only, so that one prong’s crotch edge laps over the other’s. On a collecting fitting, the lap is arranged so that, seen from inside, each upstream edge lies over the edge downstream of it, as Chapter 1 explained; at the crotch, where the two prongs’ streams meet, the lap faces the main end.

The crotch seam

The crotch seam is the defining joint of a branched fitting. It is curved, it runs from one foot over the peak to the other foot or to a triple point, it is often in an awkward place to reach, and it is where two separately formed prongs must meet exactly. Four rules make it go together well.

1. Cut the crotch edges flat, before forming. The crotch edge on each pattern comes from calculated points and is cut on the bench with the sheet flat. Cutting it after forming, on a curved surface, is how Chapter 4’s scribed prongs went wrong.
2. Mark matching points on both crotch edges. The same division points that defined the crotch, at each element or ruling, are marked on both prongs’ crotch edges. At assembly, each mark on one edge meets its partner on the other, and the seam cannot creep.
3. Rivet from the feet up. The feet are fixed by the main end, which must be round; the peak is free. Riveting from each foot towards the peak lets any small mismatch run out at the peak, where it can be eased, instead of piling up at the main end.
4. Seal before closing. Sealant goes on the lap before each pair of rivets is set, not afterwards, so that it fills the joint rather than sitting on it.

A check sheet for every fitting

Each fitting is checked after forming and trial assembly, before its seams are closed, against numbers calculated in its chapter.

Figure 2FX-4 fitting checks before closing

• Do: G1: back vertical on the wall line; duct Ø250 level at 450 mm; half-height section to its Ø425 templateChapters 2 and 3
• Do: G2: crotch to its ellipse template, peak 350.6 mm; prong ends Ø250, centres 300 apartChapter 4
• Do: G3: crotch peak 370.8 mm; prong ends at 30°, both tilting in the plane y = 0Chapter 5
• Do: G4: feet 29.6 mm towards the small prong; crotch template leaning 3.04°Chapter 6
• Do: G5: feet at θ = ±79.2°; turning prong's end at 45°; crotch within about 1 mm of its plane templateChapter 7
• Do: G6: three feet 120° apart; triple point on the axis at 367.3 mm; prong centres on a 160 mm circleChapter 8
• Do: G7: pipe cut curve on the hood all round, no gap over 1 mm; pipe square to the wall at 200 mmChapter 9

The checks share the logic of every check in this book. Each fitting is checked at its ends, because that is where it meets the rest of the system, and at its crotch, because the crotch is where its two halves meet each other. Templates turn the crotch check into a look rather than a measurement, which is why the job makes them before any fitting is formed.

Templates for branched work

The FX-4 job uses four kinds of template, all cut from hardboard or plywood offcuts, all made from the coordinates.

• End templates: a hole of each end’s size on the mean line plus a small clearance, Ø355, Ø400, Ø300, Ø250, Ø230 and Ø200, with the feet marked round the edge of each main-end template so that the crotch seams can be checked for position as well as the ends for roundness.
• Crotch templates: each crotch’s true shape, cut as a flat profile, with its feet and peak marked. G2’s and G3’s stand on the main’s centre line; G4’s and G5’s lean, and come with a batten and an angle to set them by; G6’s single template serves all three crotches.
• Prong-end gauges: a board with each prong end’s centre marked at its calculated offset and length, so that the formed fitting can be stood on its main end and each prong end checked for position at once.
• Angle gauges: for the tilted prong ends of G3 and G5, a plate cut to the tilt, laid across the prong end’s face.

Templates are labelled with the job, the fitting and the line they represent, mean line or inside, and kept with the workbook.

Marking the prongs

A breeches has pieces that look alike and are not. G4’s two prongs are different sizes and obvious; G2’s are identical, and either can go on either side; G6’s three are identical. But every prong carries crotch marks that must meet its neighbour’s, and every prong has an outer side and an inner side that must face the right way. Each pattern is therefore marked, on the reverse, with the job, the fitting, the prong and its position, and with an arrow showing the direction of flow. The crotch marks are numbered from the foot, so that mark 3 on one prong meets mark 3 on the other.

Testing the joints

Once assembled, each fitting is tested for leaks at its seams before it leaves the shop. The shop’s method, an illustration, is a smoke test: the fitting’s ends are closed with blanking plates, a smoke source is introduced at a slight pressure, and every seam is watched for smoke. The crotch seams are watched most closely, especially at the peaks and G6’s triple point, where three or four layers of sheet and sealant meet. A leak is marked, the fitting opened, the joint cleaned, resealed and re-riveted if needed, and the test repeated.

The test’s pressure, duration and acceptance are matters for the system’s specification, and the shop follows whatever the specification calls for. A fume-extraction system runs below atmospheric pressure, so a leak draws air in rather than letting fume out; but every leak wastes the fan’s effort and reduces the capture at the hoods, and a leaking crotch is a place where fume deposits collect.

Where CAD fits

Sheet metal CAD can model most of these fittings, by lofting each prong and trimming it against its neighbour, and can flatten the result. The coordinate method remains useful in the same three ways it was for the elbows in Transitional Elbows by the Numbers: as a check on the CAD pattern, for fittings the CAD system handles awkwardly, and on the shop floor, where a table of crotch points and element lengths can be checked with a tape. For branched work, the formulas for a crotch’s feet and peak are a fourth use: they let a fitting’s proportions be checked in seconds, before anyone models anything, and they show at once whether a proposed set of prong ends will collide.

The job’s sheet and mass

The mean-line areas of the patterns, before allowances, give the job’s net sheet. At 0.8 mm, galvanised steel sheet weighs about 6.28 kg/m².

Figure 3FX-4 fittings, net sheet area

G1 wall hood0.663 m²

G2 breeches, parallel0.535 m²

G3 breeches, 30°0.733 m²

G4 unequal breeches0.535 m²

G5 wye0.579 m²

G6 three-way manifold0.698 m²

G7 pipe0.097 m²

The seven fittings total about 3.84 m² of net sheet, about 24.1 kg. With a nesting yield of about 65%, an illustration of what the shop achieves on branched work once allowances and offcuts are counted, the job needs 3.84 ÷ 0.65 ÷ 2.88 = 2.05 sheets of 2,400 × 1,200 mm. The shop orders three, because 2.05 is too close to two to rely on, and keeps the offcut.

The prongs’ notched patterns nest well against each other: a crotch notch on one prong takes the outer side of another. Of all the patterns, G1’s halves are the largest, about 669 × 817 mm each, and they go on the sheets first.

Order of work

The fittings are made in the order that lets each be checked against what it joins. G1 comes first, because G7’s pipe joins it and G7’s hole is cut in its patterns. The breeches and manifold follow, each with its prongs cut, formed, matched on their crotch marks, trial-assembled to their templates and riveted from the feet up. G7’s pipe is cut last, after G1 is formed, and offered up to the formed hood before its tabs are turned.

Within each fitting, the order is the same: cut every piece flat, with its crotch edges and allowances; mark the crotch points; form the prongs; clamp the main end round in its template; bring the prongs together on their crotch marks; check against the crotch template and the prong-end gauge; and only then rivet. Nothing is riveted until everything has been checked, because a riveted crotch is slow to undo, and the checks are quick.

Keeping the records

Each fitting’s records are kept together: the coordinate workbook, with the prong ends, the apexes, the crotch points and the pattern coordinates; the check sheet with its readings; and the templates, labelled with the fitting and the job. A breeches developed once can be made again from its workbook in a fraction of the time, and a replacement fitting years later can be checked against the same templates.

The error: a crotch riveted from the peak down

This is an illustration. The first G4 was assembled by a worker who started riveting the crotch seam at its peak, where the two prongs first met cleanly, and worked down towards the feet on each side. Each rivet pulled the prongs a fraction of a millimetre together, and the small differences between the two prongs’ forming accumulated downward. By the feet, the two prongs’ main-end edges were out of line by about 4 mm, and the main end had a step at each foot. It would not enter its Ø355 duct.

The rivets near the feet were drilled out, the main end was clamped round in its template, and the seam was re-riveted from the feet upward, letting the remaining mismatch run out at the peak, where it was eased with a mallet. The shop’s assembly note for every breeches now begins: “Clamp the main end round. Rivet the crotch from the feet up.”

The patterns had been right throughout; the fault was entirely in the order of assembly, which is why it was not caught by any of the pattern checks. The lesson is about where errors are allowed to go. Every assembly has small mismatches. The order of work decides where they end up, and the right order sends them to the place where they do least harm and are easiest to correct.

The method, summarised

Every branched fitting in this book was developed the same way:

1. Write down the main end and every prong end as coordinates, and check that the prong ends do not collide.
2. Decide which prongs are cones (ends parallel to the main) and which are triangulated (ends tilted).
3. Find each crotch: by formula for a pair of cones, by symmetry for equal prongs, ruling by ruling otherwise. Find the feet and the peak first.
4. Develop each prong: cones by radial lines from their apexes, others by triangulation, each to its crotch, with the feet as division points.
5. Check the patterns on the mean line, add allowances square to their edges, and mark matching points on the crotch edges.
6. Assemble with the main end clamped round, rivet the crotch from the feet up, and check each fitting at its ends and its crotch.

Exercise: G7’s pipe

G7’s pipe runs from the reference plane at x = 500 mm to the hood, its lines ranging from 200.0 to 213.0 mm long and averaging 206.5 mm round its girth. Find its net sheet area and mass, and explain why the average line length gives the area.

Worked answer.

• The pipe’s girth is π × 150 = 471.2 mm.
• Its pattern is a stretch-out of that girth with a curved cut end, and its area is the girth times the average length of its lines: 471.2 × 206.5 = 97,300 mm², or 0.097 m².
• At 6.28 kg/m², that is about 0.61 kg.

The average line length gives the area because the pattern is a strip of constant width, the girth, whose height varies along it; its area is the width times the average height, exactly as for any shape between a straight edge and a curve. The tabs’ allowance, added after, increases it by about 471.2 × 15 = 7,070 mm².