The Limitations of Qualifying Tube Shapes using Bender Data

This page explains the major limitations of using bender data for qualifying tube shapes. Back to VTube-LASER

What is Bender Data?

Bender data is the data used to setup tube bending machines. Usually, bender data has at three major columns of data - the LENGTH between bends, ROTATION planes between bends, and BEND ANGLE columns. These columns can be used to define the shape of a tube and setup a tube bender. The three columns are referred to in many different ways by different bender manufacturers and software developers. "LRA" or "LENGTH, ROTATION, ANGLE" = VTube and Supravision "YBC" = Eaton Leonard Standard Axes, BendPro Controls "PRB" = MiiC "FRB" or FEED, ROTATE, BEND = CNC Bender ProControl for SMT "XYZ" = Pedrazzoli, BLM "FPB" = Chiyoda, KEINS, and COMCO

The Limitations of Bender Data for Qualifying Tube Shapes

The tube fabrication industry rarely uses bender data to qualify part shapes because doing that requires the use of angles in the qualification. There is the major problem with using angles for qualification of tube shapes: Properly qualifying tube shapes with the angules in the bend data is a process that is always dependent on the magnitude of the lengths between bends.

Illustration of the Limitation Look at the bender data on the right. The two sets are not the same because I've made the MEASURED rotations to be exactly one degree off of the nominal or MASTER data. Try to answer this question: Given a tolerance envelope of 0.1", does this part qualify or not?

Visually Demonstrate the Problem of Qualifying with Angles

Unless you can perform 3D trigonometry mentally on-the-fly, the answer to the question above isn't obvious. Even if we guess the answer, we can not accurately guess at what tolerance envelope value the part would be considered acceptable. It's easy to visually demonstrate the limitation of using bender data to tell us if a part shape falls within the tolerance envelope. Compare the two similar parts and their alignments. The image on the right is from the alignment of the two tubes from the LRA data shown in the report above. The white tube is the MASTER. The pink tube is the measured. The blue transparent cylinders that surround the tube are the TOLERANCE ENVELOPES. The tube shape must be within each straight's tolerance envelope to be considered a good shape - or a shape that qualifies. This first part has 4 inch straights for every straight. (See the LRA data above.) All the centerlines fall within within the tolerance envelope. The "T1 dev" column values are TANGENT 1 DEVIATIONS, and the "T2 dev" column values are TANGENT 2 DEVIATIONS. The tangent points are where the straights meet the bend arcs along the centerline.

This second alignment image shows the part with IDENTICAL ANGLES - but the two middle straights are lengthened to 10 inches between bends. You can see red cells in the Tangent point/Midpoint grid, and yellow tolerance envelopes where the pink is breaking through. The part is no longer within tolerance - even though all the angle deviations are identical between the two parts.

The Best Data for Qualification

The best data for qualification is centerline TANGENT POINT and MIDPOINT data in the Inspection Data menu and in the Reports menu.

Typical Industry Tangent Point Tolerances

In working with thousands of customers over the past few decades, we've seen some trends in accepted envelope deviation tolerances. Here are what we commonly see: Aerospace and Automative Fluid Lines Diameter Range Envelope Tolerance 12.7 mm (0.5 inch) diameter tubes or less +/- 1 mm (0.039 inches) Greater than 12.7 mm (0.5 inch) +/- 2 mm (0.078 inches) Automotive Exhaust Pipes Diameter Range Envelope Tolerance 50 mm to 76 mm From +/- 2 mm to +/- 3 mm 76 mm to 102 mm +/- 3 mm Larger then 102 mm +/- 3 mm or greater Shipbuilding Diameter Range Envelope Tolerance All Diameters +/- 6 mm HVAC Diameter Range Envelope Tolerance All Diameters +/- 2 to +/- 3 mm Structural Tubes (Frames) Diameter Range Envelope Tolerance All Diameters +/- 2 to +/- 3 mm Tighter Tolerances Sometimes customers will required +/-0.75 mm - but this is very rare. We've never seen tube shapes that must be qualified with a deviation tolerance of less than +/- 0.75 mm.

Other Pages

• About VTube-LASER End Point Deviations
• What are Centerline Tangent Points and Why Are They Important in VTube-LASER?
• About VTube Intersection Point Tolerances
• About VTube End Length Offsets
• Back to VTube-LASER