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

Why are Tangent Points Important in Qualifying Tube Shapes?

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.
A Visual Example of the Problem With Using Intersection Points for Qualification See these two images to help understand the problem with using intersection points for qualification. The white tube is the MASTER part. The pink tube is the MEASURED ALIGNED part. The blue envelopes are the tolerance envelopes for this tube qualification. The two tangents show 0.054 and 0.046 inches in deviation. It's easy to see that the pink straights are well inside the blue envelopes. This part qualifies according to the tangent point deviations.
However, the intersection points are separated by 0.332 inches. The intersection deviation is 6 times larger than the profile deviation of the tube. Intersection deviations do not act as a good representative of the actual profile deviation. It is possible to overqualify a part using them. See VTube Intersection Point Tolerances for more information about intersection deviations.

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)

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.

About 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 Envelope Tolerances

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 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 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

GD&T and VTube-LASER Tolerance Envelopes

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.
See this end profile image to visualize why GD&T profiles are double the VTube-LASER tolerance envelope.
This image shows a tube with a 0.060" diameter with a 0.030" envelope in order to visualize the scale of the envelope in VTube.

GD&T Feature Control Frame Examples

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. The call-out in the green circle shows GD&T symbols that indicate a datum and tolerances using a feature control frame (the boxes with information). The GD&T tolerance of 6 mm becomes a VTube-LASER envelope tolerance is 3 mm in space for the center legs of the tube. The GD&T tolerances at the end in the red circle show a profile tolerance of 3 mm and a perpendicularity tolerance of 2 mm for the end component. These are not referring to tube shape - but to the end component's shape and position relative to the tube. See the comments below about the end component.
Read the GD&T feature control frame for the tube shape 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 larger than the 9.53 mm OD - because the circle with an M indicates that this is MAXIMUM MATERIAL CONDITION. This means that the tube wall cannot exceed the tolerance in space. The equivalent VTube-LASER tolerance for this tube in the middle straights is 3 mm - because VTube-LASER deviations are considered spherical radius true positions.
Read the GD&T feature control frame at the end component of the tube is understood like this: The half-circle indicates a profile tolerance. The inside of the end component must conform to the profile of the end of the tube's diameter (9.53) because the datum is "A" - which is the tube diameter. The tolerance for the end profile is 3 mm. This is a maximum material condition - which means that the ID of the end component cannot exceed more than 3 mm outside of the 9.53 diameter. In VTube-LASER terms, the ID of the fitting is constrained to 1.5 mm of tolerance. It would be best to measure the ID with calipers and confirm that it does not exceed an ID of 1.5 mm away from the diameter of the in any direction. The next feature control frame shows an upside-down T. It indicates a perpendicularity tolerance of the face of the end component. This tolerance cannot exceed a total of 2 mm from perpendicularity from datum B. So, the component face may not wobble more than 2 mm from the axis formed between points 1 to 2 - because the circle with an M indicates that this is MAXIMUM MATERIAL CONDITION. This means that the fitting face cannot twist, relative to the tube, more than that tolerance in space. To find the exact equivalent in VTube-LASER, it would be best to use a machined adapter with a 90-degree bend attached to the end component. Then set the tangent tolerance for that adapter's centerline at 1 mm - because VTube-LASER deviations are considered spherical radius true positions.

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