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Straight Length For Bent Bar


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We had a custom part needed to be fabricated, guy bent bar stock to desired shape, gave to me. Now I am trying to reverse engineer to get manufacturing drawings and try to figure out the best way to get a straight length. I created a 3d sketch and sweep on the line for the shape. I can measure the straight lengths and arc lengths, but is there a better way to calculate this? I tried doing as sheet metal so I could create a flat pattern, but with round bar it doesn't seem logical, also tried making square and then fillet the edges but that didn't work well either. There are bends at different angles and planes so I am thinking about this for future use of wiring. For example, if I run wires through points with a 3d sketch how will I easily calculate how much wire I used? Saw lots of old posts but no solutions, thought this would be a good one to discuss further.


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The bending allowance for formed components depends on quite a few things.


The main things to consider are-

  • The material being used. (Tensile Strength, ductility, etc.)
  • The material profile. (Sheet, round bar, flat bar, square bar, hexagon bar, I beams, channels, etc.)
  • The radius of the bend.
  • The angle of the bend.

You can find formulae and/or tables on line that will cover all of these variables for different angles of bend, most will be for sheet stock but you can find them for bars and beam profiles as well.


In practice for bars and sections, I have always found that if you calculate (or measure in cad) the length of the neutral axis after bending you will be within 1% (exceptionally up to 3%) of the straight length before bending.


Again you can find formulae and tables for finding the neutral axis of any particular profile online.


You won't get it spot on by this method, because the material will stretch and compress on either side of the bend.

(But the neutral axis is called that for a reason, its the thoeretical axis that (theoretically) doesn't get stretched or compressed during bending).


If it's for a one off component then calculate the length of the neutral axis, add a bit extra, and trim it after bending.


If it's for regular production then calculate the length of the neutral axis, cut a bar to that length, bend it up, measure the finished component, and adjust the cut length as necessary for the next one.

(This is a big part of what prototyping is about).


You mention calculating wiring lengths.

A wire is typically extremely thin in relation to its overall length and so bending it does not produce any noticable stretching.

Just calculate the length of the bend at the centreline of the wire and add it to the length of the straights.


And just for info here is an article on bend allowance and deduction for sheet metal in Inventor:


(There may be one there for bars as well but I didn't do a search).

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