STOR STOR CONET STOS TINK TAPB RTY MLoft Segmental Bend Software (SEGB)- Lobster Back Patterns first created 27/12/05 - last modified 02/04/12 Page Author: Ty Harness

Current release version 1.4 available from ShareIT.

From V1.032 exports the full segmental bend assembled into a DXF file.

All upgrades and beta versions free to full version users. Latest info.: SEGBbeta15

[Back] [Min. Specification] [Features] [Additional material] [FAQ's] [Downloads] [Home Page]

Using SEGB

SEGB is not just an alternative method to the manual drawing of patterns but an engineering tool to help design the transformations. You can control key geometrical parameters and perform advanced calculations like finding the internal volume. Once you have designed your segmental bend then the pattern has already been developed out for you.

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Figure 1 - SEGB Version 1.4 Windows 7 - Software
to help design both oblique and right segmental bends.

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Figure 2 - Oblique segmental bend or frequently referred to as
lobster back bends because of the shape of the pattern.

Figure 2 shows that you can stagger your joints to minimize the amount of material waste and cutting time with the 'Gilbow' shears. Note the distortion of the sheet from the shears which means you need to 'break-in' the segments by passing them through the rollers. Once rolled they can be tacked up and finally welded.

The key design properties associated with a segmental bend are:
  • In/Out Diameter - Both right and oblique bends have circular inputs/outputs.
  • Divisions - the greater the number of divisions the less truncation error and greater pattern curve accuracy.
  • Bend Radius - not an easy site measurement to take. Use a folding rule or two flats to find the centre of the radius.
  • Bend Angle - even if the bend angle is not a right angle you can still use a right segmental bend. You can use a protractor against your folding rule.
  • N segments - A right bend always has two half segment in and out which is automatically calculated by the software.
  • Area In/Out - useful for engineers who need to consider the fluid inside.
  • Minor Diameter - This parameter is useful when designing oblique segmental bends where the cross section of the tube becomes elliptical and the In/Out Diameter is the major axis. The perimeter and area of the ellipse can be calculated in the advanced properties section.

    The segmental bend is often used for HVAC duct work. Frequently, you will be able to buy off the shelf bends made up for all the common diameters and various angles like 22.5,45,60 and 90 deg but it's important to be able to design and make a bend of unusual dimensions and to understand the difference between a right cylinder cut obliquely and an oblique cylinder. Ultimately we would all like smooth elbow bends but they can only be made with limited sizes, and the next best thing is the segmental bend. The greater the number of segments the closer the approximation to a smooth elbow. 6 Segments per 90 degrees is usually acceptable for sheet metal work. Any more than 6 and you've got a lot of welding.

    text here 3D Segmental Bend
    Figure 3 - Right Segmental Bend (3 Segments) - note the start and end segments are half the full segment.

    Right segmental bends have a circular cross section which allows you to use stock Circular Hollow Section (CHS). If you have a large enough band saw the right segmental bend is just a mitring exercise but for large diameter CHS then wrapping the paper template around the pipe allows you to centre dot the cut line for cutting by (say) flame. The oblique cut produces an ellipse but because you use a half segment on either end then the input and output areas are circular.

    Oblique segmental bends have an elliptical cross section but maintain a circular input and output. You can buy elliptical tube stock but it is not readily available like CHS. If the material thickness is under 16 SWG then it's easy to form the elliptical cross section and in some cases the oblique angle of the first segment is more aesthetically pleasing to the design like in the case of a breeches piece. Figure 4 shows an oblique bend welded to a breeches piece and in this case a right bend would be ill suited.

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    Figure 4 - A 3 segment oblique bend welded to a breeches piece

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    Figure 5a , 5b- A 3 segment oblique bend - note that the input/output area is circular.

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    Figure 5c - D&D Engineering Ltd. - 3mm thick stainless back from the laser cutters.

    The greater the number of segments the less elliptical the cross section becomes, and again this is another reason why 6 segments per 90 degrees is so often recommended. One of the worst cases which can arise is a 1 segment circle to circle connection - you would normally mitre the in and out pipes in effect producing a right bend to ease the problem. The 1 segment example does, at least, emphasize the elliptical cross section shown in Figure 6.

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    Figure 6a,6 b - Rotating the oblique segment to show the elliptical cross section

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    Figure 7 - Maybe something a bit more ornamental like a 25 segment toroid - You would have to be bonkers!

    Limited Version Features
  • Draw a oblique or rectangular segmental bend
  • Stand Alone Windows application. CAD software not required.
  • New snap measuring feature in the user interface.
  • Print a simple specification sheet for job cards or quotations
  • Print a full size or scaled template for fabrication purposes.
  • Additional geometric properties calculated like internal bend volume

    Full Version Features
  • 2D DXF Export of front elevations and pattern - useful for CAD operators
  • 2D DXF pattern only and 1 continuous closed pline designed for CADCAM applications
  • 3D DXF Export using 3DPOLYFACE objects for immediate rendering.
  • 3D VRML Export for web browsers and polygon renderers.
  • Export TYFACE format which can incorporate into any 3rd Party software development.
  • Export JPG and BMP graphics
  • HTML export to enable you to communicate your design quickly over the web.
  • Free version for educational establishments and students home PC.
  • Commercial users can install the software on as many PC's at their work premises with only 1 licence.

    Example Output from SEGB
    Print Simplex Method
    Print Complex Method
    Example HTML output that includes TYSEGB,DXF,VRML,TYFACE href's

    Minimum Specification
    Version 1.4 and above was designed for XP,VISTA and W7. Earlier versions are available for Windows 98SE, Windows Millennium, Windows NT4 SP6a,Windows 2000 Professional, and Windows XP Home or Pro. SP2
    No special graphics card required - 800x600x256 min.

    A printer must be installed (local, network, virtual or generic print to file).

    Additional Material
    Segmental bend FAQs
    Segmental bend mathematics

    [Using the Interface]
    [sheet metal homepage]
    [Ty Harness HomePage]


    DXF file format is a registered Trademark of Autodesk, Inc.

    Windows is a registered trademark of Microsoft Corporation in the United States and other countries.

    If I've omitted or incorrectly used any trademark please contact ty<-at> and I'll correct any errors or omissions, Ty Harness

    Errata SEGB V1.02
    Apologies - SEGB V1.02 does not produce a true oblique segmental bend. A true oblique segmental bend should have circular input and outputs thus causing the cross section of the pipe to become elliptical.

    Version 1.03 has addressed the above error and also calculates the perimeter, area and volume of the elliptical cross section for the oblique mode. Please uninstall version 1.02 and the latest version V1.03 can be obtained from the downloads link above.

    The term rectangular was used on the interface radio buttons where I meant to use the term Right which has now been rectified for Version V1.032