Coordinate Measuring Machines

Coordinate Measuring Machine (CMM) Accessories

Stylus Calibration (Probe Calibration)

Calibration on the stylus (probe tip) that comes into contact with the object must be performed in order to accurately begin measurement for two reasons. The first is to recognize the spherical center coordinates of the stylus. The second is to set the stylus sphere diameter. By setting the diameter, it is possible to calculate by offsetting the radius from the point actually touching (outside the sphere) to the sphere center coordinates.
For calibration, a sphere with a known sphericity, known as a reference sphere, is generally used.

Handling Precautions

Although some models can perform measurements on the order of 0.1 μm, correct usage and management are vital for measurement accuracy.
Verify that the moving parts move horizontally and vertically during use. Also, use a measurement standard or a similar object to check for indication errors.
In order to perform accurate measurements, allowing the temperature of the target to adjust to room temperature in the metrology lab is critical. Alternatively, the measurement parameters must be set to correct for any temperature difference.
For touch probes, it is important to ensure that the probe contacts the target at a constant speed during measurement.

Maintenance and Calibration

Regular CMMs require regular maintenance and inspection in order to carry out highly accurate measurements continuously. Especially in the case of a bridge-type CMM that are mechanically driven with sliding parts, it is necessary to regularly replace worn parts, lubricate, and clean the system for optimal performance.

Coordinate Measuring Machine (CMM) Operator Training

Careful handling of coordinate measuring machines generally requires advanced operator skills. Typically, CMM programmers are highly-trained metrology professionals.
CMM programmers are required not only for proper inspection, but a CMM may be damaged resulting in high repair costs if not used properly. For this reason, full-time inspectors are needed, and significant training is a pre-requisite for operation.

Coordinates and Alignment

CMMs generally have device coordinate system that is set in the object.
The device coordinate system is defined by the device, for example, the direction of the axis that moves in the lateral direction is the X axis, and the direction perpendicular to the stage surface is the Z axis. Therefore, depending on the orientation of the object to be measured, it may differ from the reference plane or reference line of the object itself. Since it is difficult and inaccurate to physically place this on the machine coordinates, the work coordinate system is set according to the reference plane or reference line of the object.
This way, aligning the orientation of the workpiece with the orientation of the reference coordinates is called alignment.

How to set coordinates

There are three pieces of information required to set the work coordinate system.
The first is the plane that is the reference plane, and the direction perpendicular to this plane is the Z axis.
The second line is the reference line, which is generally the X-axis and the vertical direction is the Y-axis. The straight line may be measured directly from the object, or it may be a straight line connecting two different points (such as two holes) with a virtual line.
The third point is the origin. This origin is the 0 point of each coordinate value of X, Y and Z. It is also possible to specify a specific point (for example, the center point of a specific hole) as the origin, or a virtual point (intersection point) where two straight lines intersect.

Measuring Dimensions and 3D features

Generally, a user would select a measurement target called "element" such as a plane via a software menu and begin measurement. In the case of a contact type coordinate measuring machine, the stylus tip is brought into contact with the object to be measured, and the measurement point is taken. The element is measured by measuring the minimum number of measurement points specified for each element. If the number of measurement points is further increased, it is often calculated by the least squares method.
In addition to planes, measurement elements include lines, points, circles, cylinders, cones, and spheres.
Dimensions and 3D shapes are measured by calculating the distances and angles between measured elements.

Virtual Figures (Projection)

Some elements have three-dimensional shapes such as cylinders and cones, but some elements do not have three-dimensional shapes such as lines and circles. These elements are generally projected on a plane (moved perpendicular to the plane direction) so that they can be measured correctly. The plane that is projected is called the reference plane or projection plane.

Measuring Virtual Figures

Coordinate measuring machines can also measure using virtual lines and points.
Various examples of virtual elements are used, such as intersections between straight lines, tolerances between planes, intersections between planes, and circles between cones and planes.
It can be said that measurement using these virtual elements, which is difficult to measure with hand tools such as calipers, is unique to 3D measurement.

Measuring GD&T

Geometric tolerance measurements are measured in the same way as regular measuring elements.
See the geometric tolerances page for more details.

Measurement Stability

To set and measure correctly, specialized knowledge and skills are required.
It is required to maintain an appropriate temperature in the measurement room and stabilize the temperature of the object.

Responsiveness

Since it is necessary to calibrate every time various settings and probe angles are changed, it is not easy to support frequent product changes.
Since a measurement room is required, it is difficult to perform frequent measurements while processing the object.

Cost and Effort

The installation requires a large space and the construction of an environmentally-controlled quality lab which is extremely expensive.
Maintenance costs for the measurement environment and measuring equipment can be a burden.
Significant time is required to program the CMM for a few reasons. The required time run the part to the quality lab, acquiring the appropriate temperature for the part, fixturing, calibration for each probe tip and the time needed to complete the measurement.