The Limitations of Qualifying Tube Shapes using Bender Data
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What is Bender Data?
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Bender data is the data used to setup many 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.
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So tangent points are important because they represent the best set of points along the centerline to qualify the shape of a tube. The general rule is this: If the tangent points are within tolerance, then it follows that the shape of the part - based on the location of the centerline straights - fits well enough inside the tolerance envelope in order to qualify. The tolerance envelope is is referred to as the centerline profile tolerance. COMPARE XYZ Tangent Point Deviations to XYZ Intersection Point DeviationsXYZ intersection points (not the same as XYZ tangent points) are sometimes used for tube shape qualification. However, intersection points are not as good as tangent points for tube-shape qualification because:
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Tangent Point Deviations in the Inspection Data Grid
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The Tangent chart is represented by a grid of straights for each row with tangent points and midpoints for each straight:
Note that the two end points are also included in the tangent charts are reports (T1d in straight 1, and T2d in the last straight). They are an exception to the technical tangent definition given above because there is no bend attached to these points. But these points still have value in determining if the part is the correct shape, so it is convenient to include them in this chart and grid - even though they are not really tangents. Midpoint deviations are always less than the highest corresponding tangent deviation, and higher than the lowest corresponding tangent deviation. They are included in traditional reports so that you can have three separate deviation tolerances in a straight. (T1-MP-T2)
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The Same Data In Reports
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The same tangent data can be shown in the reports like this. |
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How to Understand the Tangent Data
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The image on the right shows the visual representation of the chart and report above. The deviations in the grid match the part in the image. The part is made transparent so that you can see the two centerlines inside the tube. (It's easy to make parts transparent by setting the transparency value about 0.75 inside the Parametric Tube control menu under Models.) In the image below shows how the distance T1d is measured in the second straight:
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How to Understand the End Point Deviations
Automatic Internal Trimming of End Points for Shape
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Even though the end points are not tangents, we can still use them in the chart because they qualify the part the same way that tangent points do. |
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Untrimmed End Points for Lengths
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However, the end length is 90.2mm too long. In this application, the customer bent the part 90mm too long on purpose in order to give the bend arm clamp die enough material on the first straight to grip. Notice that, even though the part is significantly too long, the BEST FIT algorithm didn't use the actual measured end point in the alignment. The alignment was based on the trimmed point on the measured centerline that was nearest the master end point. So, in this case the part shape in space is qualified - but it needs trimming by 90.2mm to also qualify the end length (another critical qualifier). |
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Typical Industry Tangent Point Tolerances
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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
Automotive Exhaust Pipes
Shipbuilding
HVAC
Structural Tubes (Frames)
Tighter TolerancesSometimes 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. |
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