CMM measures the 3 dimensions of the object

How does a coordinate measuring machine work?

A traditional coordinate measuring machine (CMM) is a machine used to measure 3-dimensional objects. They have many different sizes, styles and accuracy ranges. They can be purchased for thousands of dollars (beware of these) for well over $1,000,000. However, their core is to operate according to the same principle.

Figure 1: Gantry CMM in 3D Engineering Solutions

Figure 2: CMM with major components highlighted. Please note that the X and Y axes of most CMMs are not shown in the figure, but have swapped positions with the CMM. This large-scale CMM follows automotive conventions, with the Y-axis in the crossover position and the longer X-axis in the front/rear position.

As shown in Figure 1 and Figure 2, a typical contact probe CMM has 3 motion axes (X, Y, and Z). These axes are measured using linear scales, which can accurately determine the position of each axis (in contrast, Arm-based CMMs use rotary encoders to provide more flexibility, but generally have lower accuracy). These linear scales are basically strips of material, with many very fine marks on them, corresponding to distances (Figure 3). When the machine axis moves, the encoder reads these marks and calculates how far the machine actually moved.

Figure 3: Part of the scale. Note the larger vertical mark near the top of the golden background, and the smaller mark in the middle that is directly read by the encoder.

The Z axis (vertical axis) is located on the machine spindle (Figure 4). At the end of the sleeve is a probe head (Figure 5), which allows the probe placed on the probe head to be rotated. These probes have a sphere at the end, usually made of industrial ruby, ceramic, steel or similar materials. The material for the probe tip was chosen because of their wear characteristics and ability to be made into a very spherical shape. These spheres may be damaged due to wear, and then no longer be as accurate.

When the CMM moves, the sphere will touch a surface, which will cause the probe to deflect after reaching a certain distance/force. Many different contact probes can be purchased to change this distance/force for different applications. This contact generates a signal that tells the CMM the location of a point. This point has an XYZ position and an IJK vector (determined by the compensation described below). All the information about each point obtained by the CMM is captured by the CMM controller-(the computer is shown in Figure 6). The controller is connected to a PC running software that interprets the controller information. The collection of points collected by the CMM is called a point cloud. The point cloud examined may have only a few points for small programs, and thousands of points for larger programs. Using scanning probes or optical scanners, the point cloud can reach millions.

Figure 4: Image of the sleeve with the probe head and the probe attached near the bottom. This forms the Z axis of the machine.

Figure 5: Images of the probe (black component), contact probe (dark silver component), and probe (think component with a ruby ​​sphere).

Figure 6: The CMM controller acts as the interface between the PC and the CMM.

The CMM knows exactly where the center of the sphere is. Because the surface of the sphere is in contact with the measured part instead of the center of the sphere, the CMM itself does not know the location of the accurate recording point (Figure 7). It must use compensation to determine this.

Compensation is the process of determining how to adjust the XYZ value of the point taken by the coordinate measuring machine so that it is related to the actual position of the surface. For example, if you want to collect multiple points to capture a circle or cylinder, the CMM software is smart enough to know whether you are collecting points from the inside or outside of the cylinder (Figure 8). Then it can quickly determine the position of the center of the circle, and then compensate the center position of each point to the surface where the probe ball contacts the part. The basic principles for determining aircraft compensation are the same. When CMM takes a point for a plane feature, the original point is taken at the center of the probe ball. The software knows from which direction (for example, from the top) it takes these points, and can then know which side to "compensate" to determine the precise location of the real plane being detected.

Figure 7: The probe details highlight the center point of the sphere captured by the CMM and the actual part contact point reached by compensation.

Figure 8: Features detected by CMM. Since the CMM knows the direction the probe is coming from and the radius of the sphere, it can quickly determine where to place the actual point on the surface by using information from other points and knowing that the cylinder or plane is being acquired.

The user tells the CMM software what type of probe (the length of the probe handle, the diameter of the probe sphere, and any other accessories). Before the inspection begins, the user will calibrate the probe against a known workpiece (usually a well-polished and calibrated sphere, as shown in Figure 9). This probe calibration process is achieved by allowing the probe to contact the workpiece from multiple points in all aspects. This allows the CMM software to understand the characteristics of the sphere (its roundness or whether it has any defects) so that it can use this information in the compensation process.

Figure 9: Calibration sphere used to calibrate/characterize the probe.

Once the probe configuration is qualified (characterized), the inspection can begin. The first thing to do is to tell the coordinate measuring machine where the part you are measuring is relative to the machine. This is done through initial alignment. If you have a CAD model of the part, this is a useful step because it allows the software to know the location of the part in the space associated with the CAD model. You can also measure without a CAD model. However, it is just not that convenient.

In addition to the traditional CMM, there are other devices and CMM accessories that can capture very large point clouds. Some CMMs have "scanning" probes (meaning they drag the probe along the surface and continuously acquire points). Others have laser scanners, structured light scanners, and optical scanners, which connect to the probe and capture a large number of points at once. There may be 10,000 points in the point cloud of a large traditional CMM program. These other scanning devices can easily capture millions of points!

Although they may have more information, they are always less accurate than single-point detection CMM. In my opinion, these other scanners combine non-contact measurement, a large number of points collected, and the excellent positioning accuracy of a traditional CMM, which is the best of both worlds.

CMM hardware and software are continuously improved every year and provide new functions and higher accuracy. But at their core, they all operate on the same principles. For more information about CMM hardware and software, see https://www.3d-engineering.net/Dimension-inspection/.

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