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Measuring Arched Openings for 2 Types Stretch-Formed Metal

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Stretch forming is a profile bending process in which s metal extrusion is kept under constant tension, while stretched and bent simultaneously over a die to create contoured parts.

Stretch forming capabilities typically include portions of circles including half-circles and eyebrows, ellipses, and arched shapes. These shapes can be formed with straight leg sections at one or both ends of the curve.

Close and consistent tolerances, along with lack of surface marring, distortion, or ripples, and surface misalignment of complex section profiles, are the benefits inherent in stretch forming(section bending). The process yields a smooth and even surface.

Read More: The best guide to the Stretch Forming Process in 2022

Type 1#: Half-circle or “Eyebrow” Openings

Determine at what points the curve begins and ends. These points are called tangent points (or straight-line points). Measure the distance between these points ideally with a laser measurer. This distance is called the chord. If the opening is a complete half-circle (180°), this distance will also be the diameter and half of this is your radius.

From the exact center of the chord, measure straight up to the top of the arc. Do NOT include any straight legs. This distance is called the rise (or height). If the rise is one-half of the chord, then you have a complete half circle or 180°. See “Notes” below for a comment on inside vs. outside measurements.

If straight legs are required, measure from the tangent point (where the curve ends) down to the lowest point needed.

stretch forming machine
stretch forming machine

Type 2#: Gothic Openings

Measuring Arched Openings for Stretch-Formed Metal
  1. Hang a plumb bob from the point of the Gothic opening.
  2. Determine at what point along the side the curve begins. This tangent point acts as the top of an arc that begins at the top of the pointed opening and extends through this tangent point and continues as an imaginary line until it intersects with the plumb line coming straight down from the upper point. This distance on the plumb line between these points is the chord.
  3. The distance from the center of the chord (on the plumb line) measured horizontally to the beginning of the curve (or tangent point) is the rise.
  4. If there are straight legs, measure from the tangent point (where the curve ends) straight down to the bottom of the piece.


The rise, the chord, and straight legs (if needed) are the only measurements the stretch-forming shop needs to determine the arc length and provide you with a quote. Stretch-Forming Shop can also work with the chord and radius if your drawing offers this information.
The above procedure gives Stretch-Forming Shop the outside dimensions so Stretch-Forming Shop will deduct the width of the material you are using to determine the inside radius. If using an inside dimension, use inside dimensions throughout, i.e. be consistent and advise which you are using.

Stretch-Forming Shop will assume standard cut-length material (24’) unless you tell us otherwise.

5 Tips for Successfully Stretch Forming Aluminum

Here are five tips that will help relieve your tension when stretch forming and finishing your architectural aluminum

Tips 1#: Stretch Form Before Finishing

Nearly all manufacturers’ and applicators’ warranties require that the aluminum extrusions are finished after curving. Finishing before bending often results in marring or damage to the finished surface due to the parts being stretched across the hard form during the curving process.

Tips 2#: Clear anodize

Clear anodize remains the most popular finish to emphasize aluminum’s inherent appeal. Standard colors of anodizing also include champagne, bronze tones, and black.

Read More: Painted or anodized finishes

Tips 3#: Extended Learning Terminology

Decipher the terminology of stretch forming and finishing aluminum. As examples:
• The chord is the distance, or width, between the points where the curve begins and ends.
• The rise is the distance, or height, from the exact center of the chord to the top of the arc.

The chord, rise, and straight legs (if needed) are the only measurements needed to determine the arc length.

Read the above measuring guidelines to assist you.

Tips 4#: Be Mindful Of The Max

Any stretch forming shop has the maximum arc length that can be stretched form(Example: Linetec is 260 inches.). Larger lengths are reviewed on an individual basis. Curved shapes can be up to 10 inches in width.
A template is needed for compound curves –a single extrusion that has more than one radius – and will likely require a custom die. Also, please keep in mind that the standard cut length of 24 feet is assumed unless you tell us otherwise.

Tips 5#: Add Value Where It Matters

After your aluminum material is stretch formed, not only does the stretch forming shop enhance its appearance and performance with the finishing, but it also can thermally improve your fenestration framing. You can choose from either a thermal pour-and-de bridge or thermal strut barriers. Thermal strut provides the added benefit of dual finishing, where the interior and exterior framing surfaces can be specified with different colors or finish types.

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