Reducer Tool
Use the Reducer Tool to draw the flat pattern of a round reducer (a cone frustum), concentric or eccentric (flat on one side, so the top or bottom of the duct or pipe stays level), to cut from sheet and roll. It is a tool of the pipework and HVAC trades.
Usage
- Choose the Reducer: Concentric or Eccentric.
- Fill in the Large Diameter, the Small Diameter and the Height (between the two ends, along the axis). The pattern is developed on the diameters as typed: for a thick sheet type the mean diameters (inside plus the thickness).
- Fill in the Seam Allowance: a lap added beyond the closing edge, folded on its line (0 for none, for a butt weld).
- Set the number of Steps round each circle (72 to start with; made a multiple of 12).
- Choose the layer to cut on.
- Click* where the seam's large end goes, then a point along the seam towards its small end. The seam's line is shown as you move; the pattern is laid out to its right.
How it is developed
Both circles are divided into the same number of steps; a line joins each point on the large circle to the point at the same angle on the small one, and a diagonal splits each strip between two lines into two triangles. Each line's true length is worked out from its length in plan and the height (√(plan² + height²)), and the triangles are laid down flat one after another, each from its three true sides, sharing a side with the last. So the same method serves a concentric reducer (whose pattern is part of a ring) and an eccentric one. An eccentric reducer's seam is on its flat side, as long as the height.
The lines every 30° round are marked on the Dimension layer (if the drawing has one), to help the rolling; the seam allowance's fold goes on the Crease layer.
It follows the drawing
The diameters, height and seam allowance are kept in new labelled variables (RDL1, RDS1, RDH1, RDA1), and the pattern is drawn by a block on the two points you marked: change a variable, or move a point, and it is developed again.
*Depending on the chosen snap mode, several clicks may be required to define the point parametrically. Read more about Snap Modes
See also: Mitre Segment, Rectangle to Round, HVAC, Pipework.