How Optical Measuring Technique Enables Lateral Probing of Components

Until recently, geometries such as bore holes of injection valves in the automotive industry were hard to measure optically. The lateral probing of components with vertical surfaces was restricted to CT solutions, complex customized solutions, or tactile measuring systems.

This is different with Vertical Focus Probing, an extension of Focus-Variation technology. Based on areal measurements, it becomes possible to optically probe components over the entire surface.

Various optical measuring methods enable components with different flanks to be measured. The range of measurable slopes or flanks slopes has so far covered 0° - 85°, whereas in industrial practice, Focus-Variation has become the most appropriate method for steep flanks.

However, this technology has also hit its limits for components that have flanks steeper than 85°. Nonetheless, for fifteen years, Bruker Alicona has continuously developed Focus-Variation and has supplemented their optical measurement principle with a novel technique, Vertical Focus Probing. It is now possible to optically touch and measure in 3D surfaces that have slopes of more than 90°. 

Different technologies allow the measurement of components with different flanks.

Figure 1. Different technologies allow the measurement of components with different flanks. Image Credit: Bruker Alicona

Here: Surfaces with 0°, 60° and 90°

Vertical Focus Probing is based on utilizing partial light. Meaning that, in addition to coaxial light, light from various directions is employed. Consequently, individual light rays diffusely reflected from vertical surfaces are captured again by the objective, which allows the repeatable and traceable measurement in a high resolution of flanks with more than 90°.

In Vertical Focus Probing, individual reflected light beams are captured again by the objective, making surfaces with more 90° optically measurable.

Figure 2. In Vertical Focus Probing, individual reflected light beams are captured again by the objective, making surfaces with more 90° optically measurable. Image Credit: Bruker Alicona

How great the proportion of reflected light rays is depends on the roughness and geometry of the measured surface as well as on the light source utilized. The objective also plays a part, as depending on its diameter, an objective can also trap reflected light from surfaces that indicate flanks steeper than 90°.

This is where the numerical aperture (AN) comes into the foreground, which is defined by the objective diameter and the working distance. It affects the extent to which the measurable slope of a surface can still go beyond the 90° mark.

AN = n.sin(∝)

(∝) defines the angle between the scanning direction and the maximum light beam that can be captured:

ImageForArticle_19265_15889422937733633.png

The graphic below shows the measurement of a surface with a slope of more than 90°. It illustrates that even with this geometry reflected light is still captured by the objective.

Reflected light can also be detected by the objective when measuring slopes steeper than 90°.

Figure 3. Reflected light can also be detected by the objective when measuring slopes steeper than 90°. Image Credit: Bruker Alicona

Difference Between Vertical Focus Probing and Focus-Variation

Vertical Focus Probing, similar to Focus-Variation, is based on the vertical scan of the surface to be measured. The focus information curve is evaluated for each position. Vertical Focus Probing is different to Focus Variation in that it calculates not only one, but several Z-values for each measuring point (XY). These Z-values represent the vertical surface.

Accuracy, Benefits and Fields of Use

Vertical Focus Probing can be utilized for a broad range of applications in dimensional metrology, all areas of the manufacturing industry and production. Among others, precision manufacturing, the tooling industry, the automotive industry and the aerospace sector all benefit from new measurement possibilities whenever it comes to components with vertical surfaces. Features such as bores, holes, reference surfaces, lengths, contours etc. can therefore be optically measured with high precision, in high resolution and quick measuring times.

PMI verifications (Product and Manufacturing Information) including position and dimensional tolerances (GD&T characteristics) are realized by measuring numerous positions from only one direction of measurement, as it is with tactile systems.

It is not essential to un- or reclamp components to be able to measure parameters such as lateral distances, diameter etc. Because of the area-based measuring principle and the resulting high measurement point density, many measuring points may be used for the evaluation of e.g. form deviations, which allows for the reliable measurement of particularly small geometries.

Usual applications of Vertical Focus Probing include, as the example, the measurement of micro bore holes such as cooling holes or injection nozzles. The diameter to depth ratio of holes varies from 1:3 to 1:10, the measurable diameter is 0.1 mm to 2 mm. Users measure parameters such as inner and outer diameter and opening angle.

Additional examples are listed below.

Hole Measurement, Diameter 0.2 mm

How Optical Measuring Technique Enables Lateral Probing of Components

Image Credit: Bruker Alicona

Table 1. Source: Bruker Alicona

Measurement METAS1
[mm]
Measurement Vertical Focus Probing
[mm]
Deviation
[mm]
0.20065 ± 0.00013 0.200568 0.000082

 

Measurement of a Pin, Diameter 0.6 mm

 

How Optical Measuring Technique Enables Lateral Probing of Components

Image Credit: Bruker Alicona

Table 2. Source: Bruker Alicona

Measurement DAkks
[2mm]
Measurement Vertical Focus Probing
[mm]
Deviation
[mm]
6.00033 ± 0.00050 6.00092 0.00059

 

Measurement of a Calibrated Ball, Diameter 1mm

ImageForArticle_19265_15889417779687827.png

Image Credit: Bruker Alicona

Coordinate measuring machines are verified in accordance with ISO 10360. Part of this process is the measurement of the bidirectional length measuring error of e.g. ball bars. Usually, tactile methods are extremely suitable for this purpose, due to the fact they are able to probe the ball laterally. This was not possible for optical methods until recently. With Vertical Focus Probing, this is shifting: balls can be probed at the equator, which makes determining the difference possible.

References

[1] Swiss Federal Institute of Metrology

[2] German Accreditation Body GmbH

This information has been sourced, reviewed and adapted from materials provided by Bruker Alicona.

For more information on this source, please visit Bruker Alicona.

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