FabLayout

Round to oval ventilation neck calculator

For a neck from a round base to an elliptical mouth, level or tilted up to 45°: enter the base diameter, both mouth axes, the height and the tilt. One-piece flat pattern.

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Your dimensions are measured to the

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1:1 PDF and DXF files of this flat pattern are made in Trueflat, ready for print, plasma, laser or waterjet. It works offline on site.

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700.4 mm1,119.2 mm
Flat pattern: cut on the bold outline, bend on the dashed blue lines, the thin lines are marking lines

Seam

801.6mm

Approximate, within ±9.0 mm

Cut length
In the app
Top edge (chords)
In the app
Sheet size
In the app

Every result is in Trueflat

The free web version shows the main result and the drawing. Trueflat gives you every mark and measurement, and exports a cut sheet. It is free to download.

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How to lay it out

  1. Divide the round base

    Divide the base circle into four times the chosen number of sections, starting at the point on the B axis.

  2. Find the mouth points

    For each base point, find the mouth point whose tangent is parallel to the base tangent there; on a tilted mouth, the point where a plane through the base tangent just touches the ellipse.

  3. Take the true lengths

    Measure each joining line and one diagonal per four-sided piece from its plan distance and its height difference.

  4. Lay out from the seam

    Start with the seam on the B axis and add the pieces in turn, each split along its outward diagonal.

  5. Close

    Finish on the second seam line, curve both edges through their points and add your allowances.

Good to know

  • This pattern is approximate; the result card shows how close it is.
  • Source: No published source checked for this shape. Method: outer envelope of the base circle and the (tilted) elliptical mouth, points joined at parallel tangents, triangulated; checked against a separate reference implementation in Trueflat.

Method and formula

A ventilation neck rises from a round base of diameter D to an elliptical mouth with full axes A and B at height H. Base and mouth points are joined where their tangents are parallel, so the surface unrolls without stretching. It is the oval to round construction with the ends swapped, plus an optional tilt.

  • Tilt α, 0° or 1° to 45°, turns the mouth about its A axis so that one B side rises. H is the height of the mouth centre.
  • Clearance: H − (B/2) sin α ≥ max(D, A, B)/10, so the low edge of the mouth keeps a usable slope.
  • The mouth may be at most 4:1.
  • Accuracy: both edges are drawn as chords. With a level mouth each edge is short by at most π³R ÷ (48 n²), with R = (larger semi-axis)² ÷ smaller semi-axis.

With sheet thickness and inside or outside dimensions, each opening is moved by one amount to the middle of the sheet. This is approximate: the calculator shows how far the opening can be off, and asks for middle-of-sheet dimensions if that is more than 1 mm.

Worked example

D = 300, A = 500, B = 250, H = 400 mm, level mouth, six sections per quarter. Both seam edges total 801.56 mm. The base is 300π = 942.48 mm round and the mouth 1211.06 mm; the chord bound is 8.97 mm. Tilted to 30°, the low point of the mouth is 337.5 mm high and 108.25 mm from the centre in plan.

Questions

How is this different from oval to round?

Here the round end is the base and the ellipse is the mouth on top, and the mouth can be tilted. Oval to round has the ellipse at the bottom and both ends level.

When do I tilt the mouth?

When the mouth has to face at an angle instead of straight up. Tilt it up to 45°; beyond that the opening faces sideways and is a branch job.

Is the pattern exact?

The surface unrolls without stretching; the only error is drawing both curved edges as chords. On a level mouth each edge is short by at most π³R ÷ (48 n²). Inside or outside dimensions with thickness are converted approximately and the deviation is shown.

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