Centerline tangent point deviations are important because they represent the best set of points along the centerline to qualify the shape of a tube.

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.

In tube fabrication, a tangent point is a centerline point where a straight meets a bend. These are considered the best datum points for qualifying a tube shape because they are the reduced set of points that best represent the position of a cylinder in space.

The tangent point deviations are even directly applicable to a GD&T profile tolerance of the tube wall. VTube-LASER tolerance envelopes are spherical radius true positions from the centerline tangent points. GD&T profile tolerances are tolerances for an entire diameter true position (rather than the radius of diameter).

Applying GD&T profile tolerances to VTube-LASER is easy: Always cut the GD&T profile tolerances in half to get the equivalent tolerance in VTube-LASER. For example, a GD&T profile tolerance of 3 mm is identical to a VTube-LASER tolerance envelope of 1.5 mm.

COMPARE XYZ Tangent Point Deviations to XYZ Intersection Point Deviations

Centerline XYZ intersection points (not the same as centerline XYZ tangent points) are sometimes used for tube shape qualification. However, intersection points are not a good choice for tube-shape qualification because:

• Intersection deviations tend to exaggerate the deviations mathematically. The exaggeration grows was the bend angle grows.
  
  
• For example, with larger bend angles, the intersection points can be much further from the tube wall. As they move further away from the tube wall, the intersection point deviation grows. This is the nature of polar geometry. One degree of change at 10 mm is 0.17 mm. One degree of change at 1000 mm is 17.45 mm. 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.

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)

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.)

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.

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.)

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).

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

Please remember that GD&T tube profile tolerances are always double the VTube-LASER envelope tolerances. So, a GD&T profile tolerance of 3 mm is VTube-LASER's 1.5 mm 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

Automotive Exhaust Pipes

Automotive Fluid Lines

Shipbuilding

HVAC

Structural Tubes (Frames)

As mentioned above, GD&T tube profile tolerances are always double the VTube-LASER envelope tolerances. So, a GD&T profile tolerance of 3 mm is VTube-LASER's 1.5 mm envelope tolerance. All tolerances that we show below are half the GD&T profile tolerances.

The call out in the green circle shows a simple way to call out GD&T profile for a tube shape.

This is an actual example from a real print that was shared with us from a Fortune 500 company. This method of specifying tube shape tolerances is becoming the standard in every industry.

Read the call-out like this:

• Each straight is considered a "Datum A" because the call-out says "9 X the OD of 9.53.
  
  
• The circular symbol that looks like a target indicates "true position." True position means that the tolerance is interpreted as a 3D position and not to be locked to an X, Y, or Z axis.
  
  
• The tolerance is 6 mm large than the 9.53 mm OD - because the circle with an M indicates that this is MAX CONDITION. The tube wall cannot exceed the tolerance.