The Limitations of Qualifying Tube Shapes using Bender Data

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Vtube-laser logo 1.96.png This page explains the major limitations of using bender data for qualifying tube shapes.

Vtl screen hd scanner without logo.png


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

VTube-LASER Master LRA Data.png


ENVELOPES Qualifying Tube Shapes Are Much Better Than Bender Data Deviations Qualifying Tubes

The tube fabrication industry is rapidly abandoning the use of bender data deviations to qualify part shapes for a very good reason.

The problem with using bender data deviations for the qualification of a part shape is that it does not help you know if a part is inside or outside of 3D envelopes (or GD&T profiles) for each straight section in a tube shape without additional polar to linear calculations. Spatial envelopes represent the specific path in 3D space where the tube must be inside of in order to not cause a collision or rub itself to failure from vibration.

In VTube-LASER, this allowed space is called the ENVELOPE as defined by the TANGENT points along each straight in the defined tube (see the image on the right). (Tangent points are where the straights meet the bends.)

The ENVELOPE always includes a tolerance of deviation allowed. Think of the deviation as a spherical radius true position from the MASTER centerline.

Exhaust envelope tolerance.png
The above image is an example of an exhaust pipe with a blue band around the straight section. This blue band is the envelope tolerance.

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 away from MASTER data rotations.

Try to answer this question: Given a tangent point envelope tolerance envelope of 0.100", does this part qualify or not?

This is the problem: There is no way to guess if this part is actually inside its allowed envelope in 3D space without performing some moderately complex trig calculations.

VTL compare master and measured LRA.png

Visually Demonstrate the Problem of Qualifying with Angles

So, unless you can perform 3D trigonometry on-the-fly, the answer to the question above isn't obvious. Even if we make a guess, we can't 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 aligned parts on the image. The white tube is the MASTER. The pink tube is the MEASURED ALIGNED. 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 as defined by the Tangent Grid. 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.

VTL image 4 inch straights.png


This second alignment image shows the part with IDENTICAL ANGLES - but the two middle straights are increased in length to 10 inches between bends.

The important point to notice is that both parts have the same diameters, the same rotations, and the same bend angles. However, when increasing the lengths, the parts are in danger of rubbing themselves to failure (or colliding with some part of the application). We only changed the lengths, and the part is now likely to fail if the envelope tolerances represent the maximum allowed space that the tube can use before it collides with something else.

See red cells in the Tangent point/Midpoint grid, and yellow tolerance envelopes where the pink (representing the measured aligned part) is breaking through the allowed tolerance.

The part is no longer within the tolerance allowed in 3D space - even though all the angle deviations are identical between the two parts.

VTL image 4 inch and 10 inch straights.png

Example 2

We show another smaller part with two tolerance envelope setups.

Both parts are the identical shape. Look at the bender data deviations. They appear to be quite large according to the standards for companies that like to qualify with angles. However, one part is acceptable, and the other part is not. Depending on the envelope tolerance in the application, the same part may be acceptable.


The Best Data for Qualification

The illustrations above show that the best data for tube shape qualification inside an envelope is centerline TANGENT POINT data in the Inspection Data menu and in the Reports menu.


For more information, see What are Centerline Tangent Points and Why Are They Important in VTube-LASER?


Typical Industry Tangent Point Envelope Tolerances

In working with thousands of customers over the past few decades, we've seen some trends in accepted envelope deviation tolerances.

GD&T Profile Note: GD&T tube profile tolerances are always DOUBLE the VTube-LASER Envelope tolerances. So, a GD&T profile tolerance of 0.120" is VTube-LASER's 0.060" envelope tolerance. All tolerances that we show below are half the GD&T profile 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

Automotive Fluid Lines

Length Range

Envelope Tolerance

Up to 1000mm long after bending

From +/- 1 mm to +/- 2 mm

Over 1000mm long after bending

+/- 3 mm or greater


Diameter Range

Envelope Tolerance

All Diameters

+/- 6 mm


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 require +/-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.

Aerospace envelope tolerance.png
Exhaust envelope tolerance.png
Shipbuilding envelope tolerance.png

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