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

Reading the T1d, MP, T2d Chart

• Back to VTube-LASER

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

• T1d = Tangent 1 Deviation
• MPd = Midpoint Deviation
• T2d = Tangent 2 Deviation
  
  

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.

The two end points are an exception to the technical tangent definition given above. There is no bend attached to these points. But these points (T1d in straight 1, and T2d in the last straight) 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.

The general rule is this: If the tangent points are within tolerance, then it follows that the shape of the part - based on the straights - fits well enough inside the tolerance envelope in order to qualify. In GD&T, this is often referred to as the centerline profile.

The image on the right shows the visual representation of the chart above. The deviations in the grid match the part in the image. The part is made transparent so that you can more easily see the two centerlines inside the tube.

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 end is 90.2mm too long (see the length portion of the inspection grid above), but the measured first straight passes within 1.9mm of the master end point. So the T1d value is 1.9mm - not 90.2mm.

In this application, the customer bent the part 90mm too long on purpose in order to give the clamp 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 point along 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).

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

Some customers prefer to remove the midpoints from the reports (which can be done by changing the templates).

(If your VTube Software Maintenance Plan is active - and you want us to help you to modify a report, please let us know and we will help you.)