Pattern Development
Parallel line, radial line, and triangulation methods - stretchout calculations, true length lines, and miter lines.
- Develop patterns using the parallel line method for rectangular duct
- Apply radial line development for cones and tapered fittings
- Use triangulation to develop patterns for offset transitions
- Calculate bend allowance and stretchout for sheet metal bending
Lesson 1
Parallel Line Development
What Is Parallel Line Development
Parallel line development is the most common pattern layout method in sheet metal work. It is used for objects that have parallel elements - meaning all the fold lines or surface lines run in the same direction. Rectangular duct, square-to-round transitions with square sides, and cylindrical shapes all use parallel line development.
Stretchout Calculation
The stretchout is the total width of the flat pattern before bending. For rectangular duct, the stretchout equals the perimeter of the duct cross-section plus allowances for seams. For a duct measuring W x H:
- Stretchout = 2W + 2H + seam allowances
For a round pipe, the stretchout equals the circumference:
- Stretchout = pi x diameter (3.14159 x D)
Developing a Rectangular Elbow Pattern
To develop a pattern for a rectangular elbow (miter elbow), divide the elbow into sections using miter lines. Each section is a straight piece with angled cuts at each end. The miter line is drawn at the bisecting angle of the turn.
Seam Allowance
Always add seam allowances to the stretchout. A Pittsburgh seam requires approximately 1 inch on one edge and 3/8 inch on the mating edge. Forgetting seam allowances is one of the most common layout errors.
Parallel line development is used for shapes with parallel fold lines. The stretchout is the total unfolded perimeter plus seam allowances. For rectangular duct: stretchout = 2W + 2H + seam. For round pipe: stretchout = pi x D.