High Precision CT Scanning for Dimensional Metrology

Metrology grade CT scanning is a relatively new technology in the dimensional inspection world. The CT scanners are expensive currently, but over time the prices will continue to drop. AA Jansson offers CT scanning services on a contract basis, as an alternative for customers who have occasional needs for CT scanning, but not enough volume to justify the investment in-house. This service is for applications like flaw detection, dimensional inspection, and reverse engineering.

Think of a CT scanner as a CMM with an X-Ray probe. The machines are built to metrology standards, and the accuracy statements are similar to CMM accuracy statements. The beauty of a CT scanner, is that these systems capture part geometry on both the outer and inner surfaces of the part. This allows features to be measured that are not accessible to touch probes, lasers, or optical systems. The data can then be used for dimensional inspection, or can be exported as an STL file for reverse engineering and visualization. Assemblies can also be inspected to check for flaws like voids, cracks, gaps or surface contact where there should be no contact.

Shown here is a cut-away view of a bottle and cap shown as assembled. The 3D imaging allows the fit to be inspected to make sure that there are no leakage areas.
Transparent materials are not a problem for a CT scanner, since it is X-Ray based. This is very useful for many applications in plastic or glass, as other types of sensors like lasers and white light scanners cannot be used on transparent surfaces.

CT scanners capture large amounts of high quality data. This allows the results of a scan to be used for tasks like “Part to CAD” comparison using a chromatic graphical report, showing the deviation from nominal (CAD) in color. This is more intuitive and much easier to explain to inexperienced staff than rows of numbers.

Flaw detection and analysis checks can be made to quantify flaw sizes and quantities. Shown here are images of reports with both3D and cross-section views of voids in plastic parts.

This is also very useful for checking porosity in metal castings, and to look for large cracks and other flaws in welds and assemblies.

If you ever had to do a layout inspection on a plastic connector, especially if you were required to measure all the little locking elements inside the connector, then you will understand how much more wonderful life can be with a CT scanner. Just stick the part in a piece of foam and let the scanner find all those hundreds of features that used to be very difficult to access.

Because a CT scanner is a non-contact inspection method with very low inertia forces created during the inspection process, the holding fixtures can literally be chunks of foam cut to hold a part in place on the table.
Some parts (like silicone knee implants for example) are very challenging to hold with a fixture, while still allowing access to the features of interest. Being able to simply stage the part in a chunk of foam greatly simplifies the inspection setup.

Shown here are a variety of parts that can be inspected with CT scanning technology. The CTG scanner captures all the surface data, both inside and out. There are no line of sight issues, as the X-ray beam penetrates completely through the material.

The beauty of this is that the 3D characterization of the part surfaces is very complete. Anyone who had ever worked with various 3D scanning systems for inspection or reverse engineering has run into situations where it’s just not possible to completely scan the surface of the part (let alone the inner surfaces).

In the case of reverse engineering the gaps in the surface data need to be recreated and filled in with software to create a “closed” file (data set) with no gaps in the surface if you want to re-create the part with a rapid prototyping system. Often the data gaps are near intersections of features and complex surfaces that the laser or white light scanners miss due to line of sight issues. So “filling in” these gaps in the surface can create large errors, with “invented” surface areas that may not accurately represent then true surface.

So how does a CT scanner work? There is an X-Ray generator that sends out a cone-shaped or a line of X-Ray beam. The part is positioned on a rotary table between the X-Ray beam generator and detector with a pixel array (kind of like your digital camera). A series of individual 2D images called “radiographs” are taken. The part is rotated capturing (typically 360 -1500 images), then this series of radiographs are processed and reconstructed to create a 3D data set. In the 2D vision the systems work with pixels. In the CT scanning world the 3D data set is comprised of “voxels”, which can be thought of as 3D cubic pixels. Zeiss holds a patent on making measurements from the raw voxel data. The images can be interpolated to calculate the location of a feature edge, providing a resolution of 1/10th of a voxel.

CT scanning technology continues to advance, with an ever-increasing set of good applications being discovered and developed. Over time these systems will also become more affordable. Eventually this technology will replace other less effective measurement technologies.

written by: Dan Smith

Leave a Reply