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. |
COMPARE XYZ Tangent Point Deviations to XYZ Intersection Point DeviationsCenterline 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:
|
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:
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. |
How to Understand the Tangent Data
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. |
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. Here are what we commonly see: Aerospace and Automative Fluid Lines
Automotive Exhaust Pipes
Automotive Fluid Lines
Shipbuilding
HVAC
Structural Tubes (Frames)
|
GD&T Profile and VTube-LASER Tolerance Envelopes
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. |
GD&T Feature Control Frame Examples
The call out in the green circle shows a simple way to call out GD&T profile for a tube shape. |
|
Read the GD&T feature control frame like this:
|
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