Architectural Flashing Supply
Start a Quote
Advanced geometric pattern development · Lesson 4 of 4

Advanced geometric pattern development: defects, verification and case study

The errors that ruin a pattern and how each one is caught, the checks to make before cutting, a case study of a sleeve pattern that came up short, and the evidence to hand over for review.

You will be able to: Diagnose common pattern development errors, verify a pattern with arithmetic and construction checks before it is cut, and document the evidence for review.

10 min · 11 slides · 7 questions

All lessons in this module
Slide 1 of 11

Four errors that ruin a pattern

In the drawing, right against wrong; the errors:

  • Vertical heights (10.0 and 4.0 in) as radii, not slant heights (11.18 and 4.472 in), give a 180.0 deg sector, not 161.0 deg: a different cone.
  • Elbow seams on one side waste metal; alternate 180 deg.
  • A pattern cut on the developed line has no metal for its seams or edges.
  • A 6.325 in plan length where the true length is 8.718 in cuts the piece short.
Four pattern errors, right and wrongFour rows, correct on the left with a green tick, wrong on the right with a red cross. 1: frustum sectors to scale, swung with the slant height, 161.0 degrees, against the vertical height, 180.0 degrees, a different cone. 2: two elbow patterns nested on a strip with seams 180 degrees apart sharing one cut, against both seams on one side leaving a band of waste. 3: a pattern with a dashed allowance band outside it against one cut on the line. 4: a line cut at its true length, 8.718 inches, against its plan length, 6.325 inches, with a dotted line under it showing the 8.718 inches needed.1234slant: 161.0°vertical: 180.0°seams 180° apartsame side: wasteallowance addedcut on the line8.718 indotted line: needed6.325 intrue lengthplan length

Four pairs, correct on the left with a tick and wrong on the right with a cross: a frustum sector swung with the slant heights from the apex, 11.18 and 4.472 inches (161.0 degrees), against one swung with the vertical heights, 10.0 and 4.0 inches (180.0 degrees); two elbow patterns nested with seams 180 degrees apart against both seams on one side; a pattern with an allowance band outside the developed line against one cut on the line; a line cut at its true length, 8.718 inches, against its plan length, 6.325 inches, with a dotted line under it showing the 8.718 inches needed.

1
Radius: slant height, 161.0 deg sector, not vertical height, 180.0 deg
2
Elbow seams: alternate 180 deg, not all on one side
3
Allowance: added outside the developed line, not cut on the line
4
Length: the true length, 8.718 in, not the plan length, 6.325 in
Row 1: NAVEDTRA 14040 and 14251; row 2: NAVEDTRA 14040; row 3: NAVEDTRA 14040 and 14251; row 4: the square-to-round of worked example W3. Sectors and lengths drawn to scale; the rest not to scale.

Sources

Glossary

Chord
A straight line between two points on an arc; it is always shorter than the arc, so stepping a circle off in chords comes up short of the circumference.
Turn angle
How far the metal turns from straight at a bend, as AFS's flashing calculator states it: 180 deg minus the included angle between the legs.
Signed interior angle
The angle between the two legs at a bend as FlashDraft states it: 180 deg is straight, 90 deg is a right-angle corner, and the sign gives the direction of the fold.
Symmetry check
A check on a triangulated square-to-round: perpendicular bisectors of the square's sides meet at a centre, and arcs from it must pass through the matching points.