Comparison of the Cylindricity Deviation Using Different Evaluation Methods

Veiw figure View Figure

In the measurement analysis (Figure 1c) d)), the extracted features are used. These features are obtained from the finite points of the measurement of a real workpiece. Minimum number of points is given by the definition of features. ^{[2, 11]}

2.2. Concept of the Cylindricity Tolerance

Cylindricity tolerance belongs to the group of form characteristics. Standard ISO 1101 recognises six different types of form tolerances as shown in the Figure 2. ^[3]

Figure 2. Types of form tolerances

Download as

Veiw figure View Figure

By the definition of cylindricity, the minimum zone requirement defines such arrangement of two coaxial cylinders that the radial distance between them is minimised. Tolerance zone is defined by two coaxial cylinders. In order to inspect the form deviation, the entire surface of the real cylinder must not exceed minimum distance of two coaxial cylinders regardless the dimension of the cylinder Figure 3.

Figure 3. Concentric cylinders (ISO 1101) [4]

Download as

Veiw figure View Figure

2.3. Methods of the Evaluation of Cylindricity Deviation

Design form of the tolerated geometrical features is defined by features of geometrical ideal form of cylinders. For the cylindricity deviations, it is important to note that the default evaluation must be performed by the minimum zone method as defined in the standards (ISO 1101) Figure 4, i.e. MZCY - minimum zone (feature) cylinder (ISO 1101), MICY - maximum inscribed (reference feature) cylinder, MCCY - minimum circumscribed (reference feature) cylinder, LSCY - least-squares (reference feature) cylinder

Figure 4. Methods of cylindricity evaluation

Download as

Veiw figure View Figure

3. Experiment

A 3DCT system as shown in Figure 5 consists of:

-an X-ray source,

-a rotary table,

-an X-ray sensitive detector.

During a full discrete turn of the specimen, penetration im-ages which contain the spatial X-ray attenuation are recorded.

Using these images it ispossible to reconstruct the fully inner and outer structure of the specimen. The reconstruc-tion method used within this work is a Filtered Back-projection.

Figure 5. Principle of cone beam CT system [12]

Download as

Veiw figure View Figure

The measurement has been performed by CMM Metrotom 1500 made by ZEISS Figure 6 with following metrological parameters:

-maximum permissible error for length measurement with Metrotom 1500 is

-cylindrical measuring volume has the size of and flat detector panel is with the resolution of

-power of X-ray tube can be adjusted up to

-performance value depends on the size and the material density of measured workpiece ^[5].

Figure 6. Coordinate measuring machine

Download as

Veiw figure View Figure

The industrial CT scanning machine, such as the Zeiss Metrotom, a tube generates X-rays that penetrate into the workpiece while it rotates on a mat. An X-ray detector on the other side of the workpiece absorbs and interprets the X-rays. The results obtained by the measurement depend on the combination of the size of the workpiece and its material density. ^[6] For the evaluation of cylindricity deviations, we used VG Studio and Calypso software. Initial conditions of the setting and evaluation of the grey value stayed the same. Grey value threshold is a segmentation technique that is commonly used when reconstructing the tomography machine. The threshold is defined as a portion of the maximum grey value from the interval between 0 and 1.^[7] In case of VG Studio software, ISO Surface = 30399 was converted to threshold of 0,464. In case of Calypso software, threshold was 0,464. In order to visualise the scanned object, Calypso software uses parameter of triangles. The number of triangles is usually given by the dataset. ^{[7, 8]} In our experiment, the number of triangles is 2 000 000.

Figure 7 shows the SolidWorks 3D model workpiece in four representations.

Figure 7. 3D CAD model of workpiece

Download as

Veiw figure View Figure

Figure 8 shows the drawing of workpiece with nominal dimensions.

Figure 8. 2D CAD model of workpiece

Download as

Veiw figure View Figure

To evaluate the measured data, we used the LSCY method. Figure 4 The data was processed in VG Studio and Calypso software. The diameter of the workpiece is and the scanning time is.

In VG Studio software, the point cloud parameter is used only for volume visualisation.. The number of points depends on the size of the workpiece.

Protocol Calypso and VG Studio protocol Figure 9 shows the measured cylindricity deviation- resp. and nominal cylindricity tolerance.

Figure 9. Protocol Calypso and VG Studio

Download as

Veiw figure View Figure

In Calypso software, there are two methods for the evaluation of cylindricity deviation, i.e. method of roundness profile and method of generatrix lines Figure 10.

Calypso protocol presents the values of cylindricity deviation Figure 10. The roundness profile method obtains deviation cylindricity of and nominal cylindricity tolerance of . The generatrix lines method obtains the cylindricity deviation of and nominal cylindricity tolerance of.

Figure 10. Calypso graphical protocol of cylindricity deviation

Download as

Veiw figure View Figure