SEGB FAQs first created 03/12/08 - last modified 25/01/09 Page Author: Ty Harness
Work in progress.............................................
I am struggling with the difference between the oblique and right options in the seqb software. The oblique option just seems to change it so the first and last segments are the same as the rest of them rather than half of the other segments.

A right segmental bend has a true circular cross section. i.e. you could use tube stock or cirular hollow section (CHS) and mitre the segments in a bandsaw if possible. An oblique segmental bend has circular entry and exit but the cross section is elliptical.

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Figure 1 - 2 right 4 segment bends. Left: Total bend angle of 90 deg. Right: Bend angle of 60 degrees


If you cut a circular cross section tube at an angle other than square to the central axis then the resulting end is an ellipse. This is the confusing part because the cut is often termed an oblique cut which is wrongly confused with an oblique segmental bend. Figure 2 shows a circular tube cut obliquely at 45 degrees. The only other cuts that would allow you to connect another tube is one that's identical. You can connect an identical cut in the two configurations where the middle figure is termed a mitre joint and the left hand end is a scarf joint.

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Figure 2 - Mitring a right cylinder at 45 degrees.


You can see from figure 1 it's the mitre joint we utilise in the segmental bend.

Is it correct a right bend can only be used for 90 degree elbows?

No, Figure 1 shows 2 right segmental bends: one has an inclusive angle of 90 degrees and the other 60 degrees. It's a common misconcpetion is that a right segmental bend must always be a 90 degree bend. The term right actually refers to the tube's cross section being at right angles to the central axis. In order to keep the input and output circular they must be 90 degrees to the central axis and by clever subdivion of the bend angle you can make the 2 end segments half the main(or full) segments. Figure 1 has 4 segments but we shall divide the bend angle of 90 degrees by 3 = 30 degrees. A half segemnt would then be 15 degrees and thus now adding up the 4 segments with half segments at the end 15+30+30+15 = 90 degree. Again 60/3 = 20 and the sequence becomes 10 + 20 + 20 + 10 = 60 degrees. By making the end segemnts half the main segments you use the same pattern and just flip it over to mark out the material.



Note the length of the pattern is 2 x pi x r which is circumference of a circle.

The oblique segmental bend end of segments are circular so you can join a square cut circular tube to any of the segments. Unfortunatly the cross section of the segments are elliptical and you can not buy elliptical tube and it's impossible to roll an ellipse with thick plate. Sheet metal workers (thin sheet variety) roll it and squash it a bit.

The more segments the less elliptical the cross section becomes with the worst case being a 1 peice circle to circle connection. This may or may not cause problems with some applications but the oblique bend sometimes suits the application as in the case of a connection to a breeches piece, as shown in figure xx.

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Figure XX - Sample picture description here


Note the length of the pattern is not 2pi r it's the perimeter of an ellipse and that's a very difficult calculation. Only around the curve is the length 2PI R
Can the segBend program produce a nested set of segments that can be laser cut with small tags holding them together so they can be all rolled together? Also can you move the join to other side of the bend ?

The SEGBv1.032 and below software can't produce a pattern with all the segments nested togethor. Where the joints are staggered each side.

Full version users can download SEGB beta version 1.5 where you can export nested patterns: SEGBbeta15 information.

You can move the joint to the opposite side by changing the bend radius to negative bend radius. i.e. 400 becomes -400.

You could then export both patterns to DXF and then nest in a CAD app.

Or you could provide both patterns and a sketch to the laser cutters.

Like you say, tell them to leave the tags in for you so you can roll them all at once.