Difference between revisions of "What are Centerline Tangent Points and Why Are They Important in VTube-LASER?"

All About Tangent Points and Why They Are Important

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Centerline tangent points are important foundational points used to qualify the shape of the tube. Even so, they are not automatically understood. So this page describes why they are so important to measuring, and how to properly read the tangent deviation reports and charts.

Technically a tangent point is the point where any straight meets any bend. These are considered the best points for qualifying a tube shape because the 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 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).