Elbow butterfly
One-piece throat for a jacketed 90° elbow, usually short radius where the throat is too tight for gores. The side, heel and side are done with the gore layout; the butterfly covers the throat half.
Inputs
From the Knauf chart, or type your own
Field measurement: the throat measured on the insulation
0Step-off (arc 1 to 2), 8 times
0Layout length (½ circumference)
0Wing radius (C to 1)
Transfer points
| Line | Above and below 1/16" | Decimal |
|---|
Butterfly pattern, to scale
- Draw a centreline and step off eight times. Square a line through each mark, above and below.
- Number the lines 1 2 3 4 5 4 3 2 1. That's half the insulation circumference.
- Mark each distance from the table above and below the centreline on its line. Join the marks.
- Set dividers from C (the centre, on line 5) to the top of either line 1: . From C, swing that arc through both ends of line 1 on each side. These are the wings.
- Add laps and notches as needed. If you add laps for notches, add the extra length and width to the stretch-out.
Reference drawing
Quarter circle at ½ the insulation OD, divided into 4, with 1 at the bottom and 5 on the horizontal. Line A is ½ the butterfly throat, measured on the insulation; the 45° line runs from A to B. Each point goes straight up to the 45° line.
Method: IIIATF Metal Layouts, Elbow Butterfly, and Advanced Metal Jacketing Op. Unit B.5.3 (one-piece butterfly throat). For 90° elbows. Check the first piece on the fitting before cutting a batch.