Rectangle to round

Concentric rectangle to round transitions on duct and breeching. True lengths are worked out, and every pattern point is given as a tape measurement.

Inputs

Rectangle side A-AActual size including insulation. This side is the middle of the pattern.

Rectangle side A-BActual size including insulation

Round line size

Round insulation

Round ODFrom the Knauf chart, or type your own

Height, weld to weld

0Step-off (1/12 round circ.)

0True length C-1 (seam)

True lengths

Line	True length 1/16"	Decimal

A-1 = B-1, A-2 = B-2 and so on, because the round is centred.

Pattern points, measured

Point	Along A-A from left A	Up from A-A

Same points as swinging arcs, as tape measurements: along line A-A from the left corner, then square up. Negative means past the corner or below the line.

Pattern, to scale

Draw line A-A the length of side A-A, , and square a centreline up from its middle.
From each A swing the A-1 true length to the centreline. That is point 1.
Step (1/12 of the round) each side of 1, and cross it with the A-2 true length from A. Carry on to 3 and 4.
From each A swing side A-B, , and cross it with the A-4 true length from 4. That is B.
From each 4, step off again and cross with the B-3, B-2 and B-1 true lengths from B.
From each B swing half of side B-B, , and cross it with the C-1 true length from 1. That is C, the seam.
Join the round points with a smooth curve. Add a 2" lap parallel to C-1 on one side only.

Method: Advanced Metal Jacketing (IIIATF), Task D.5, Rectangle to Round. 12 spaces on the round, seam at the middle of side B-B. Check the first piece on the job before cutting a batch.