July 15, 2008
Conventional tube bending data, regardless of format, is entered manually and therefore susceptible to errors. A modern approach involves using a CAD system to generate a STEP file, which the CAD program exports directly to the bending machine. This method is fast and eliminates errors. The drawback is that such a system requires additional database management efforts.
A 2-D drawing, which is the conventional method for describing components such as bent tubes, contains all the necessary information for programming a CNC tube bending machine. However, conventions don't hold forever, and modern technological developments are changing the way this information is programmed into a CNC bender. The modern method is to export a solid model of a bent tube from a CAD system directly to a CNC tube bending machine.
Most modern tube bending machines use LRA data, also known as YBC data. In the LRA format, L is the distance between bends, R is the plane rotation between bends, and A is the bend angle. (In the YBC data scheme, the Y, B, and C stand for the same attributes, respectively.) Four characteristics—length, rotation, angle, and centerline radius (CLR)—are enough information for most CNC benders to create a bent profile.
Usually a 2-D drawing, which contains LRA data, is sent to the shop floor and the machine programmer uses a keypad and display to enter the data into the controller (see Figure 1). Some manufacturers help streamline the data entry process by allowing the programmer to enter it in Windows®.
In some cases, the drawing doesn't have LRA data. A common alternative is XYZ data, which is based on the Cartesian coordinate system. The XYZ data set describes the points along a tube's profile (see Figure 2). The drawback is that most benders can't run directly from XYZ information.
Another format is used by the military for manual tube bending machines. The programmer enters the data into a Windows interface and a TM converter generates the LRA data that drives the tube bender (see Figure 3).
Regardless of format, most machine programmers enter the data by hand. This is time-consuming and open to errors.
Another data format, STEP, is more comprehensive than LRA, YBC, or XYZ. Specified by ISO 10303, STEP (STandard for the Exchange of Product model data) is intended to assist manufacturers in tracking product data throughout the life cycle of a product. A STEP file contains many other characteristics that a modern tube bending machine's controller uses, such as:
Most CAD systems today, such as Pro/ENGINEER®, SolidWorks®, Solid Edge®, and Inventor™, can export a STEP file. A Windows environment simplifies file transfer (Click here to view Figure 4).
The biggest advantage in importing a STEP file is that it eliminates data entry errors. A second advantage is that the software can use the material's characteristics to predict the tube's springback behavior before the machine bends the part. While this can save time for any production environment, the savings is especially helpful in prototyping and low-volume shops.
Another advantage is programming time. In some cases, manually programming the machine takes longer than producing the parts, especially if it is a short run of a complex part. When using a CNC that can read a CAD-generated STEP file, the programming time doesn't vary much; whether converting the data for a two-bend part or a 20-bend part, it usually takes just a few milliseconds.
The biggest disadvantage in using a STEP file converter is the cost associated in tracking the revisions of the files sent to the bender. This requires additional database management efforts between the design engineering department and the manufacturing engineering department.
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