Lobsterback Duct Bends Sheetmetal Method
This is the method I use to construct lobster back bends with sheet metal tools that I can produce flat patterns from, These flat patterns can then be used to produce files for CNC profile cutting machines. I have come across other ways of doing this using surfaces, thickened and turned into sheet metal parts. The software I am using is Alibre Design. As the version I have is low on surfacing tools I have found this a good way to model lobster back bends, which I have been involved with the design and manufacture for many years.
This is the completed 7 segment bend. I started detailing these types of
bends before CAD, then with the first 2D CAD software and now with 3D modeling software. The method I use is made up from my knowledge of producing developed patterns using these three approach. As the 3D model is parametric, once you have a basic model you can easily generate required sizes by editing the parameters. The Alibre Design package comes with a feature where you attach a parameter file to a design file and driving a template with this file can make short work of generating new sizes.
I have not used this method with other software such as Solid Works, Solid Edge or Inventor but for anyone familiar with these packages, should find it not too difficult to adapt this aproach.
As with all standard lobster back bends, the start and finish segments are basically a normal segment cut in half. The location of the segment seam, see image of finished bend, assists the fabricator in lining up the parts to assemble the bend.
We start off by modeling sheet metal cylinder, diameter same as required duct diameter. This needs to have a height bigger than required for segment.
Next we flatten this cylinder, and a sketch is places onto the plane of the flattened cylinder. This sketch is the image above. This is a generated sine curve. The method used to generate this curve. In amongst my collection of vintage engineering text books, some from the Victorian and Edwardian era, there are many examples. I was first taught this basic method of developing lobster back patterns when at school in the 1970's. Combining this old manual pattern deveploment method with a modern 3D CAD package makes producing a useful, flatenable part simple. I have not gone into detail laying out this curve as there are many books on sheetmetal patern developements. When I was a student we were taught sheetmetal patern development.
And now we have completed flat pattern. We can then roll back into modeled sheet metal part with the flat pattern for generating CNC files to cut on profile cutting machines.
The full pattern segments are produced the same way. The sketch used to cut the sine curve can be mirrored to produce both sides. Though I have not tried it, the required sine curve could be generated in a spreadsheet, then used to drive the cut sketch.