What are Centerline Tangent Points and Why Are They Important in VTube-LASER?

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Vtube-laser logo 1.96.png All About Tangent Points and Why They Are Important

Vtl screen hd scanner without logo.png


Why are Tangent Points Important in Qualifying Tube Shapes?

Centerline tangent points are important points used to qualify tube shapes when they are compared to a master part shapes. This page describes why they are important for qualifying tube shapes, and how to properly read the tangent deviation reports and charts in VTube-LASER.

A tangent point is the point where any straight meets any bend. These are considered the best points for qualifying a tube shape because they are the points that most closely approximate the walls of a tube gauge.


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 Deviations

XYZ 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:

  • Intersection deviations tend to exaggerate the deviations mathematically.

  • The reason why they exaggerate the deviation is because they are not as close to OD wall in the straight as tangent points are.

  • The higher the bend angle at an intersection, the further the intersection points are from the actual part - which means that the deviation exaggeration is greater as the bend angles increase. Tangent points don't have this problem, because they are always closely connected to the straight sections of the tube shape.

See VTube Intersection Point Tolerances for more information about intersection deviations.

Vtube-laser-1.90-centerline tangent points.png

Tangent Point Deviations in the Inspection Data Grid

The Tangent chart is represented by a grid of straights for each row with tangent points and midpoints for each straight:

  • T1 = Tangent 1 Deviation
  • MP = Midpoint Deviation
  • T2 = Tangent 2 Deviation

  • T1t = Tangent 1 Deviation Tolerance
  • MPt = Midpoint Deviation Tolerance
  • T2t = Tangent 2 Deviation Tolerance

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)

Vtube-laser v2.7 tangentpoint deviations.png

The Same Data In Reports

The same tangent data can be shown in the reports like this.

Some customers prefer to modify the report to show only their critical data. For example, they may remove the midpoints or the end angles from the reports(which can be done by changing the report templates).

(For those with active VTube Software Maintenance Plans: We are happy to help you modify the report templates if you request it.)


How to Understand the Tangent Data

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:

In this case, the T1d value is 0.9mm for straight 2.


How to Understand the End Point Deviations

Automatic Internal Trimming of End Points for Shape

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.

A key in understanding the T1d of straight one and the T2d of the last straight is to remember that the deviation is not the same as how long or short the straights are relative to the master tube shape. See the illustration on the right to understand why.

The MASTER to MEASURED end point deviation in the Tangent grid is 1.9mm. The measurement is the distance between the two lines at the corresponding end points - as if the MEASURED WERE TRIMMED.

(The Measured part is the pink part. The Master part is white.)


Untrimmed End Points for Lengths

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).


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

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 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.

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

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