Sheet metal layout, by hand
Every fitting you lay out by hand uses one of three methods: radial line for cones, parallel line for anything cut from pipe, and triangulation for everything else. Each guide below works one fitting start to finish with real numbers, and ends in a free calculator for your own sizes.
The three methods
Radial line
Use it for: Anything that comes to a point on its center: a cone, a concentric reducer, a round hopper.
Swing every edge from one center, the cone’s apex. Two radii and an arc angle are the whole pattern.
Parallel line
Use it for: Anything cut from a straight pipe or duct: saddles, miters, lobster-back (gored) elbows.
Unroll the pipe into a strip as long as its circumference, divide it into lines, and measure the cut up each line.
Triangulation
Use it for: Everything else that changes shape or runs off center: eccentric reducers, square to rounds, hoppers, offset transitions.
Split the surface into thin triangles, find each side’s true length, and build the pattern one triangle at a time from the seam.
Drawn by Stretchout: a cone, a pipe saddle template and an eccentric cone.
The guides
- How to make a pipe saddle templateTo make a pipe saddle template, cut a paper strip as long as the branch pipe’s outside circumference, divide it into equal lines, and mark each line’s cutback: how far back from the long point the cut sits. Join the marks with a smooth curve, cut on it, wrap the template on the branch and scribe the cut.
- How to lay out a cone flat patternTo lay out a cone flat pattern, find the slant height from the height and the difference in the end radii. From one center, swing the outside radius (large radius × slant ÷ radius difference) and the inside radius (small radius × slant ÷ radius difference), step off the arc angle between two lines from the center, and cut.
- How to lay out an eccentric reducerTo lay out an eccentric reducer, divide both ends into the same number of points and find the true length of every line and diagonal between them from the height and the plan distance. Starting with the flat side as the seam, swing each new point from the last two, one triangle at a time, then join the points.
- How to lay out a square to roundTo lay out a square to round, divide each quarter of the round into equal spaces and find the true length from each corner to every point: the square root of height squared plus plan distance squared. From the seam, swing each point from the last two, one triangle at a time, and join the round with a curve.
- How to lay out a lobster back bendTo lay out a lobster back bend, find the cut angle: the bend angle divided by twice the number of joints. A full gore measures twice the bend radius times the tangent of that angle along its center, longer at the heel and shorter at the throat. Lay out one gore by parallel line, then cut every gore from it.
- How to lay out a miter cut on pipeTo lay out a miter cut on pipe, wrap a strip around the pipe to get its outside circumference, divide it into equal lines, and on each line mark the cutback from the long point: the pipe radius times the tangent of the cut angle times one minus the cosine of the line’s angle. Join the marks, wrap and scribe.