Allowances, checks and the material list

Six fittings have been developed as mean-line patterns, each checked line by line against its coordinates. None of them is yet ready to cut. Each needs allowances for its seams and end connections, each must be checked after forming against the numbers that made it, and the job as a whole needs a list of what it consumes. This chapter does those things for DX-3, and in doing so collects the checks that the earlier chapters introduced one at a time.

It sets out the allowances and how to add them to curved edges, gives each fitting’s check sheet, totals the sheet and mass, discusses where CAD fits, and ends with a collar that was measured on the wrong line and then “corrected”.

Allowances

Every allowance in this section is an illustration, to be replaced by the shop’s own figures for its own seams, sheet and machines. The Library’s Sheet Metal Patterns by the Numbers covers seam allowances in general; here the concern is how they are added to the patterns in this book.

Figure 1Allowances on the DX-3 fittings

Joint	Where	Allowance	Added to
Riveted and sealed lap seam	F1, F2, F5 and the round pieces of F3 and F6b	15 mm	one edge of the seam
Lock seam, rectangular	F4 wrapper edges, F6a corner seam	20 mm and 8 mm	wrapper edge and cheek edge
Slip joint, spigot end	round ends that slip into the next duct	30 mm	the end's edge
Bead for a hose clamp	F2's top end	10 mm	the end's edge
Flange to the hood face	F1's rectangular end	25 mm	each rectangle edge, notched at the corners

Three rules apply to adding them.

1. Add allowances after the mean-line pattern is checked. Every check in this book is made on the mean line. A pattern with allowances added must have them subtracted again before any check, and that is where errors creep in.
2. Offset square to the edge. On a curved edge, such as F2’s bottom edge or F3’s ellipse edges, the allowance is an offset curve, each point moved square to the edge by the allowance’s width. A strip of constant width drawn in one direction across the sheet is narrower where the edge slopes. On F2’s bottom edge near the seam, where the edge slopes at about 17° on the sheet, a 30 mm strip added straight down would be only 28.7 mm wide measured square to the edge.
3. Notch where allowances meet. Where a flange or lap meets another at a corner, as F1’s flange does at the rectangle’s corners, the allowances overlap and must be notched so that the formed corner is not three layers thick.

A check sheet for every fitting

Each fitting has a short list of checks, made after forming and before seaming or riveting, against numbers calculated in its chapter. They are grouped here so that the inspection of the whole job can be planned.

Figure 2DX-3 fitting checks before seaming

• Do: F1: rectangle 400 × 200 and square; round end level, Ø250; throat seam 235.8 mm; back line 548.3 mmChapters 2 and 5
• Do: F2: ends at 30°; top end's high and low points 340 and 240 mm above the bottom; seam 243.6 mmChapters 3 and 4
• Do: F3: collars' mitre lengths 251.6 and 238.2 at the heel; seams in line; outlet square to inletChapter 6
• Do: F4: outlet square to inlet; 400 and 300 mm wide; 200 mm deep throughout; cheeks symmetricChapter 7
• Do: F5: oval end in its template; straights straight; round end 250 out and 300 upChapter 8
• Do: F6a: corner lines 410.7 mm; folds on the short diagonals; ends rotated 30°Chapter 9
• Do: F6b: each joint closes all round; outlet 400 mm from the column axis, 300 mm up, sloping 25.5°Chapter 9

The checks share a pattern. Each fitting is checked by its ends, their size, shape, plane and position relative to each other, because the ends are where it meets the rest of the system. Its lines, the seam and the centre line, are checked because they are the easiest single measurements that confirm the pattern was laid out correctly. Nothing is checked by eye against a drawing.

Measuring on the right line

Girths are the commonest check on round work, and the commonest source of a false alarm. A tape round the outside of a formed duct measures the outside girth, which is longer than the mean-line girth by π times the sheet’s thickness; a tape round the inside measures the inside girth, shorter by the same amount. For 0.8 mm sheet, the difference is π × 0.8 = 2.5 mm each way.

F3’s inlet collar, Ø315 on the mean line, therefore has three correct girths: 987.1 mm inside, 989.6 mm on the mean line, and 992.1 mm outside. The check sheet states which one is meant. The shop measures outside girths, because a tape goes round the outside easily, and writes the outside value on the check sheet.

Templates for the trial fit

Every chapter’s error was found at a trial fit, and the trial fits depended on templates. The DX-3 job uses four kinds, all cheap to make from offcuts or plywood.

• End templates. A flat plate with a hole of the end’s shape and size, round, rectangular or flat oval, on the mean line plus a small clearance. The formed end must enter it all round without forcing.
• Plane templates. Two plates fixed at the angle between a fitting’s end planes: 30° for F2, 90° for F1, F3, F4 and F5. The formed fitting is offered up with one end on each plate.
• Position gauges. A plywood board with the two ends’ centres marked at their calculated offsets, for the fittings where position matters as much as angle, such as F5’s 250 mm out and 300 mm up.
• Radius templates. For F3’s collars and F4’s wrappers, an arc of the mean-line radius on a short chord, checked against the rolled piece.

Templates turn checks that need calculation into checks that need only a look. They are made once from the coordinates and kept with the job’s patterns, so that a replacement fitting years later can be checked the same way.

Marking and identifying the parts

DX-3’s six fittings are fifteen separate cut pieces: one each for F1, F2, F5 and F6a, three for F3, and four each for F4 and F6b. Each is marked on the reverse with the job number, the fitting, the piece and the seam line, and every piece with a mirror-image partner, such as F4’s two cheeks, is marked with its hand. Pieces that look alike but are not, such as F6b’s end pieces and inner pieces, whose twists differ, are marked with their position in the elbow. A piece without a mark is set aside until it is identified, because the cost of fitting the wrong one is a remake.

The order of work

The fittings are made in the order that lets each one be checked against its neighbours. The exact pieces are cut and formed first: F3’s collars, F4’s wrappers and F6b’s straight pieces, because their shapes are fixed and the triangulated pieces are built to meet them. The triangulated pieces follow, each trial-fitted to the exact pieces it joins. Fittings that meet a fixed object, F1 at the hood, F5 at the purlins and F6a at the collector, are trial-fitted on site before seaming if access allows, or against a template taken from site if it does not.

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 per square metre, taking steel at 7,850 kg/m³ and ignoring the coating’s small contribution.

Figure 3DX-3 fittings, net sheet area

F1 hood elbow0.348 m²

F2 connector0.210 m²

F3 tapering elbow0.663 m²

F4 rectangular elbow0.433 m²

F5 flat-oval elbow0.292 m²

F6a twisted transition0.403 m²

F6b helical elbow0.435 m²

The six fittings total about 2.79 m² of net sheet, about 17.5 kg. A 2,400 × 1,200 mm sheet has an area of 2.88 m². The patterns are irregular, and the shop’s experience with fittings of this kind, an illustration, is a nesting yield of about 65% once allowances and offcuts are counted, so the job needs about 2.79 ÷ 0.65 ÷ 2.88 = 1.5 sheets. The shop orders two, and uses the offcut for the next job’s small parts.

The largest single pattern decides the minimum sheet size. F1’s pattern is about 959 mm wide and 548 mm high before allowances, and fits a 1,200 mm sheet width with room for its flange. F3’s middle piece is about 949 mm wide, and F6a’s folded band about 982 mm long. None needs a wider sheet, but F6a’s band, if nested with its long edge along the sheet, leaves an awkward triangular offcut; nesting it across the sheet’s width is better.

Where CAD fits

Most of these fittings can be modelled in sheet metal CAD by lofting between the two end profiles and flattening the result. For a shop with CAD, that is often the fastest route to a cut file. It is not a substitute for knowing what the loft has assumed, and the shop’s rule is that a CAD pattern for a new kind of fitting is checked once against coordinates before it is trusted. The coordinate method remains useful in three ways.

• As a check. A CAD flat pattern and a coordinate pattern of the same fitting should agree within a millimetre or two. Where they do not, one of them has a different idea of the fitting, usually because the CAD loft matched the ends’ points differently, faceted the surface differently, or developed on the inside rather than the mean line.
• For the fittings CAD finds awkward. Twisted faces folded along chosen diagonals, as on F6a, and helical elbows made as twisted mitres, as on F6b, are easier to define and check from coordinates than to persuade a loft feature to produce.
• On the shop floor. A table of line lengths and pattern coordinates can be checked by anyone with a tape, and a job sheet with coordinates says exactly what the fitting is, whatever software made the cut file.

The error: a collar measured on the wrong line

This is an illustration. F3’s inlet collar was rolled and seamed, and an inspector measured its girth with a tape round the outside: 992.1 mm. The check sheet, an early version, gave the mean-line girth of 989.6 mm without saying which line it meant. The inspector concluded that the collar was 2.5 mm oversize and had it reseamed with a slightly larger lap.

The collar had been right. After the correction, its outside girth was 989.6 mm, its mean-line girth 987.1 mm, and its diameter about 0.8 mm under size on the mean line. Offered up to the tapering elbow’s middle piece, which had been made correctly, the collar’s mitred edge fell inside the middle piece’s inlet edge by about 0.4 mm all round, and the joint could not be closed without distorting it. The collar was remade.

The fix to the process was simple. Every girth on the check sheet now states its line: “Ø315 inlet collar: outside girth 992.1 mm (mean line 989.6)”. The lesson is the one that runs through the whole book: a number is only useful with its meaning attached. A true length is a length on the mean line between two stated points; a girth is a girth on a stated line; an allowance is added square to a stated edge. Coordinates make those meanings explicit, which is their real advantage.

Keeping the job’s records

When the fittings are delivered, the job’s records are kept together: the coordinate workbook for each fitting, its check sheet with the readings taken, the templates, and a note of any change made during the job and why. Most of DX-3’s fittings are one-offs, but a dust-extraction system is extended and repaired over its life, and a replacement F1 for a new saw, or a second F2 for another machine drop, is far quicker to make from a workbook than from a measured old fitting. The records also let a later reader see what each number meant, which, as the collar showed, is the part most easily lost.

The method, summarised

Every fitting in this book was developed the same way, and the same steps serve fittings not in it:

1. Fix axes and an origin, and write down each end’s centre, plane, shape and size.
2. Look for the parts that are exact, cylinders, cones and flat faces, and develop those by the simpler methods.
3. Divide the remaining surface’s ends into matching points, calculated from the true shapes, never from a view.
4. Choose the triangles, with short diagonals running consistently, and calculate every true length.
5. Lay the triangles out flat from the seam, with steps along the true curves of the ends.
6. Check the pattern on the mean line, add allowances square to its edges, and check the formed fitting by its ends.

Exercise: the helical elbow’s sheet

F6b is four pieces of Ø200 duct, each 173.2 mm along its axis and cut at both ends. Find the net sheet area and mass of the four pieces at 0.8 mm. Why can each piece’s area be found without knowing its twisted cuts?

Worked answer.

• Each piece’s mean-line girth is π × 200 = 628.3 mm.
• A cylinder cut at both ends by planes through points on its axis has an area equal to its girth times its axis length, because what the cut removes on one side of the axis it adds on the other. Each piece is therefore 628.3 × 173.2 = 108,800 mm², or 0.1088 m².
• The four pieces total 0.435 m², and at 6.28 kg/m², about 2.7 kg.

The twist moves the cut curves round the girth, but it does not change how much sheet lies between them. The same rule gave F3’s collars their areas in Chapter 6.