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The Need for Precision Bent Tubing A fair amount of industrial equipment consumes a significant amount of metal tubing. Some of this tubing gets bent into specific shapes as often described by a 2D drawing. A 2D drawing contains the necessary information used to program a CNC tube bending machine. The means in which this information contained on the drawing is programmed into a CNC bender is changing. Manufacturers are now starting to export CAD generated solid models of bent tubes direct from a CAD system right into a CNC tube bending machine.
A Historical View - What Feeds a CNC Tube Bending Machine Most modern tube bending machines are programmed using LRA (a.k.a. YBC) data. The L being the distance between bends, the R being the plane rotation between bends, and the A being the bend angle. Add to this the center line radius (CLR) and you now have enough information to describe a bent profile to most CNC benders.
Most often the LRA data is extracted from a 2D drawing that is sent to the shop floor. In this case the machine programmer would enter the LRA data into the controller via a keypad and display. Some manufacturers help streamline the data entry process by allowing a programmer to enter the data via a Windows environment.
Sometimes LRA bender data is not supplied on the drawing. Another common format sent to the shop floor on a drawing is XYZ data points. The XYZ data points are based on the cartesian coordinate system. It is these data points that describe the various points along the tube’s profile. However, most benders don’t run directly from XYZ data points.
The Conventional Way of Converting XYZ to Bender Data A system for converting from XYZ to LRA bender data is often found within most modern CNC controllers. Free XYZ to LRA data converters can also be found on the web. The process often requires the programmer to enter the XYZ data points into a Windows environment via a human interface.
Once the XYZ data points are entered including the CLR(s), the bender’s software can then convert the XYZ data to LRA bender data. The actual conversion process itself requires a little vector math. However this all takes place in the background and within a millisecond or two, the LRA bender data table is updated with the converted data.
Another way to get LRA data into a bender is thru a CMM machine. This approach requires a tube be measured after it has been formed. The corrected bend data is then sent back to the bender for further processing. Nevertheless, somewhere prior to measuring the tube on the CMM, the original programmed information had to of been entered into the benders controller via some sort of human interface.
The Military and LRA Bender Data Not all tube drawings make use of either LRA or XYZ data to describe a tube. The military makes use of tube bend data that describes how to make a tube on a manual type tube bending machine. To drive a CNC tube bender with the Boeing data, a programmer can enter the Boeing supplied data into a Windows interface see below.
This interface, found on some high end CNC controllers, is used to generate the LRA data that drives a CNC tube bending machine.
However, as with all the previous data entry methods described above, a programmer must enter the Boeing data into the bender’s controller by hand. This takes time and is open for human error.
Cutting Edge Programming for the 21st Century It is clear from above that at the very least the established process of getting bender data into a bender’s controller the first time involves a programmer typing in a series of values. This is no longer necessary.
The average CAD system today (Pro/E, Solid Works, Solid Edge, Inventor, etc.) can output an industry standard STEP file. A STEP file, with respect to a bent tube, is a file that describes numerous properties that a tube bending machine’s controller can utilize. For example, some of the properties that can be extracted from a STEP file are:
• Type of material
• Temper of material
• Weld seam information
• Tube diameter and wall thickness
• Centerline radius
• Geometric data
• Part number
By far the biggest advantage of importing a STEP file into a CNC tube bender is zero data entry errors. There are no errors because the imported STEP file contains the necessary information about the tube to be bent.
From the STEP file, a CNC bender’s controller can extract the type of material, wall thickness, outside diameter, centerline radius, and LRA bender data. With this information, springback information can be determined before a part is even bent. This process of reading in a STEP file can save time not only in a prototype environment but also in a short run production shop.
Sometimes getting a machine programmed for the first time can take much longer then say to run 20 parts. It’s a matter of how complicated the part is to program. A CNC controller that can read a CAD generated STEP file doesn’t discriminate when converting a 2 bend part from a 20 bend part; it takes just a few milliseconds at the most.
To read in a STEP file, the file format is often filename.stp. From the Windows File pull down menu, select the STEP file format, select a STEP file name, and that’s it.
Disadvantages The single biggest disadvantage of using a STEP file converter is the cost associated with tracking the revision of a STEP file being sent to the CNC bender. This process requires additional database management efforts between design engineering and manufacturing engineering.
Conclusion No mater how you slice it, a tube is going to be bent. The process of using a STEP file converter to eliminate data entry errors can go a long way to reducing the cycle time of getting a bent tube off the machine. For a 2 bend part, a STEP file conversion just might be overkill. However, as the number of bends increases, so to does the time it takes to enter the necessary data from a 2D drawing.
Another way of looking at it is how many time have you stood there trying to figure out the correct plane rotation between bends? That’s the power of reading straight from the CAD system and directly into the bender. It comes down to where you want to spend you time; typing in the LRA data our running production.
George Winton, P.E. designs and builds CNC tube bending equipment for Winton Machine in Suwanee, GA. He can be reached at firstname.lastname@example.org or 888.321.1499.
About the Machines We Build All tube bending machines are designed and tested in house. We use standard purchased components to build every tube bending machine. We at Winton are proud of the quality of every tube bender sold. Please, call us today and ask how the quality of the tube bending machines we build will translate into quality bent tubing on your shop floor.