VTube-LASER Cylinder Fit Tolerances

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Why Do We Need Cylinder Fit Tolerances?

VTube-LASER uses surface points scanned with laser or measured with ball probes to calculate tube centerlines. Regular tube scans assume that the primitive shape to solve for is a cylinder. But we also assume that no cylinder that we measure in the real world is a perfect cylinder. VTube-LASER has to overcome this issue when measuring tube straights. Also, because of this, it's important for you to know the estimated accuracy of the calculation when fitting to the centerline. The estimated accuracy helps you know how confident you can be with the centerline position.

The Cylinder Fit Data Window Reports How Well the Centerline Fits the Surface Data

After calculating a cylinder centerline from surface points, VTube-LASER reports how well the centerline FITS the surface points of the cylinder. These values are shown in the Cylinder Fit Data window. The values reported are... RADIUS: Estimate the precision of half the diameter. X or Y POSITION: Estimate the precision the X or Y positions of the end points of the cylinder. These estimate values are calculated using statistics. The values are standard deviation x 2, which means that it gives a 95% assurance that the these values are the maximum deviation.

Wobble Deviation - The highest of the X or Y values

To simplify visualizing the X and Y Position deviation, we say that the highest of the X and Y values is the "wobble deviation". The wobble deviation in the example on the right is 0.21 inches - because it is the higher of the two values. If the centerline calculation is very good, then wobble deviation decreases. If the calculation or fit is poor, then the wobble deviation is too high. This Cylinder Fit Data window is YELLOW because VTube-LASER found that the X and Y Position tolerances were exceeded in the cylinder centerline fit. In this case, VTube asked the operator to remeasure the straight because the wobble deviation was too high as defined by the tolerances in System Options/Measure 1.

The Default Cylinder Fit Tolerances

To allow VTube-LASER to warn for poor cylinder measurements, it stores tolerances for the allowed deviations during cylinder fit. The default values shown here often the installed default values in the Measure 1 tab of the System Options menu. For large-diameter applications like exhaust pipes, these settings usually work well. If you measure highly oval or deformed parts, it may be necessary to increase especially the X and Y Position tolerances.

Recommended Aerospace-Grade Cylinder Fit Tolerances

Aerospace-grade materials are often very precise compared to other applications. For aerospace applications, we strongly recommend that you use the tighter tolerances shown here. The XY Positions are set to 0.030" for longer straights, and 0.050" for shorter straights. ("Short" is also definable with the variable in the lower right corner. It is set to 2" length by default.)

Why Are There Separate Values for Shorter Straights?

Math engines cannot calculate short straights as accurately as long straights. This has to do with the nature of cylinder math. It's a common issue with all measuring centers. Because of this, we know that VTube needs to be more patient with when fitting a cylinder centerline when the straight length is below a certain value. In the default setup for VTube-LASER, a straight is considered "short" when it is 2 inches (50.8 millimeters) or shorter.

How to Test for Your Ideal Cylinder Fit Tolerances and Set That as Your Default Setup

You can adjust the tolerances in System Options to better fit your application and increase the assurance that your centerline is a good fit to the surface data. Test for ideal cylinder fit tolerance settings by measuring tubes and watching the tolerance values in the Cylinder Fit window. You will learn that your material regularly measures with certain maximum fit deviations. As an example, if you never see deviations over 0.014" (like shown on the right), you could set the tolerance value to 0.030" (which is double the max observed). If a cylinder fit deviation exceeds double the norm, then you can safely assume that there is a problem in the measurement, and you should be warned. When watching for these values during measurements, keep in mind that you need to consider both the longer straight and shorter straight tolerances. Short straights will nearly always need higher tolerances. To change the default values, be sure to CLEAR the VTube-LASER project first, then press the "Set Project Default State to the current state" button. This will use the new default cylinder fit tolerances for all new projects.

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