Specifying Multi-Radius Bending: CAD to the Rescue

When considering how information is communicated between different firms/companies, CAD or Computer Aided Design is probably one of the biggest milestones in the history of Industry. In the bending/rolling industry, the most important dimensions to convey of any part or structural member are the part’s radius and arc length. These are two specifications that are easily communicated through any type of print or drawing.  In the rare case that drawings are not spec’d with pertinent radial and arc length information, the detailer usually provides a group of related dimensions that when used in combination with one another are able to provide the specified radius and arc length that the member is to be formed to…like a chord and a mid-ordinate dimension, for example. This is usually due to the fact that the curved and straight members are detailed alike and are usually contained within the same print. It is typically the case that the detailer who detailed the job detailed all parts curved and straight, the same. So even though radial and arc dimensions are not provided, the detailer does include enough information from which to derive those essential dimensions from.

On other occasions we see prints lacking information essential to the roller bending process due to the software being used in the detailing/modeling of the project. CAD software such as Tekla & Rhino take a 3D model of the project as a whole, which was created by a draftsman, and breaks out each part/member of the model as an individual detailed drawing. In the past this type of software wasn’t able to detail curved parts/steel sections, but as more and more architects found out about the possibilities of curved steel, the software has been forced to keep up with the many advances made in the industry. So then why is it that the bender/roller industry still sees prints lacking these easily spec’d details and dimensions? And why is CAD considered an essential tool to so many of the specialty bender/roller fabricators?

Prints that are provided without the necessary bending dimensions are usually due to the fact that the part or member being formed is out of the ordinary. Mulit-radial or elliptical parts require complex bending, which requires a higher skill level and a greater knowledge about the nature of curved steel shapes and the requirements needed when detailing these complex curved parts.  Since software like Tekla & Rhino, that are used in the detailing of each member does not know how to accurately convey the dimensions of a multi-radial or elliptical bend, which is technically an ever changing radius, it is then up to the detailer to correctly convey the drawing’s dimensions in a way that makes sense for roller/bender to fabricate from. To do this correctly it takes a certain level of awareness and familiarity with curved steel shapes, but this is not usually the case with detailers working for a structural steel fabricator. So we as bender/rollers use CAD internally to bring the prints of these complex bends back into usable form.

CMRP engineers use CAD to redraw these complex parts in such a way so that they are able to be detailed accurately and formed correctly in the shop. When considering elliptical or multi-radial bends, a single radius could be best fit or “cheated” to fit a smaller segment of an ellipse, which contains a multitude of continually changing radii. This would be done time after time along the multi-radial bend making sure that each best fit radial arc is tangent to the next until the entire elliptical bend is best fit with a manageable amount of individual radii. This ultimately simplifies the bend allowing it to be accurately formed, also giving the part the illusion that it is formed to an ever changing elliptical radius.

Another instance where CAD helps in the roller/bender industry is when a multi-bent part with tangents is called out to be formed to a specific radius. In most cases the detail on these kinds of bends call for the standard 2D or 3D bend (2x or 3x diameter), but in some occurrences the print calls for a much different, more specific radius that would require a set of precise dies in order to be able to form the parts accurately. Unless a set of dies have previously been purchased or are to be purchased, which could run in the $10’s of thousands of dollars range, then the radius of the part will most likely have to conform to what die sets or tooling is available. Ultimately CMRP uses CAD in order to refit these specific radius bends with a different radius that dies are available for. But if this is done without taking into consideration that changing the radius is going to change the overall dimensions of the part/member then when the part is formed it will be formed incorrectly. In order to change the radius of a bend and still keep the overall dimensions of a multi-bent part with tangents, you must adjust the lengths of the tangents themselves. With the use of CAD, adjusting these and other parameters are as easy as a few clicks of a button.


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