Traditionally, measurements are performed visually using hand tools or optical comparators. However, these tools are time-consuming and have limited accuracy. On the other hand, a Coordinate Measuring Machine (CMM) utilizes coordinate processing techniques to measure the height, width, and depth of parts. Moreover, such machines can automatically measure targets, record measurement data, and obtain GD&T measurements. How to use Coordinate Measuring Machines, how they are constructed, and what are their advantages? We write about this below!
What is a Coordinate Measuring Machine (CMM)?
A Coordinate Measuring Machine (CMM) is a contact model that uses a touch probe, a spherical object for measurement, or a non-contact model that uses other methods, such as cameras and lasers. Some models designed for the automotive industry can even measure objects larger than 10 m.
Coordinate Measuring Machines - advantages
One of the advantages of a coordinate measuring machine (CMM) is that it can measure high-precision elements that are difficult to measure with other measuring machines. For example, it is difficult to measure the 3D coordinates of a specific point (hole, etc.) from a virtual starting point using hand tools such as calipers or micrometers. Moreover, measurements using virtual points and virtual lines, as well as geometric tolerances, are difficult to perform on other measuring machines, but can be performed on 3D coordinate measuring machines.
Construction of a Coordinate Measuring Machine (CMM)
Below we present the construction of a Coordinate Measuring Machine CMM:
Controller
Usually, most coordinate measuring machines are bridge or gantry machines. Apply the spherical contact point attached to the probe tip to the object on the platform, determine and measure the coordinate values in three dimensions (xyz axis). The controller is mainly used for measuring 3D forms, such as car parts and various mechanical parts, measuring 3D objects, such as prototypes, and measuring differences from drawings.
Coordinate Measuring Machine (CMM) Accessories
The coordinate measuring machine has the following accessories:
- Measuring pin / probe tip
- The measuring pin of the contact type coordinate measuring machine has a spherical diameter. The probe tip is often made of hard materials such as zirconium or rubidium.
Granite measuring table
In order to ensure high precision of measurement, the surface of the CMM is usually made of a stone plate. The stone top plate changes very slightly over time and is not easy to scratch, which has the advantage of providing long-term and stable use.
Fittings
One of the most important tools when working with a coordinate measuring machine is a holder used to keep the measuring disc in the correct position. If the measuring unit is clamped, it will not move during the operation of the coordinate measuring machine, as movement of the parts can cause errors. Common tools include mounting plates, clips, and magnets.
Compressors and air dryers
Mechanical coordinate machines require an air compressor with a dryer. They can be found on standard bridge or gantry coordinate machines.
Coordinate Measuring Machine (CMM) Software
Currently, we are dealing with two types of software for coordinate measuring machines.
- Software for our own measuring machines, which we have developed independently for each measuring machine manufacturer.
- Software developed by an external company, which can be used by measuring devices from multiple manufacturers.
How to use a Coordinate Measuring Machine (CMM)
Wait until the measuring shield acclimates to room temperature (usually 68 °F) in the metrology laboratory for at least 5 hours before taking measurements. This will help avoid measurement errors and discrepancies caused by thermal expansion. Measurements are taken by manually directing the probe to the desired measurement location or using a computer control. The CMM will record the X, Y, Z coordinates of the probe's position. As more points are taken, the system software will calculate specific dimensions, such as:
- diameter,
- length,
- angle and other critical dimensions.
Calibration of the measuring spindle (probe calibration)
To start an accurate measurement, the needle (probe tip) that contacts the object needs to be calibrated for two reasons:
- The first one is to determine the coordinates of the center of the spherical stylus.
- The second one is to set the diameter of the needle ball.
By setting the diameter, you can calculate it by moving the radius from the actual point of contact (outside the sphere) to the coordinates of the sphere's center.
Precautions when operating CMM
While some models may measure with accuracy up to 0.1 μm, proper usage and handling are crucial for measurement accuracy. Check if the moving parts move horizontally and vertically during operation. You should also use a measurement standard or similar to check for reading errors. To get accurate measurements, set the target temperature in the metrology laboratory to room temperature. Alternatively, measurement parameters must be set to correct temperature differences. For touch probes, it is important that the contact speed between the probe and the object is constant during measurement.
Maintenance and Calibration of CMM Machine
Regular coordinate measuring machines require regular maintenance and inspection to continuously perform very precise measurements. Especially in the case of bridge coordinate measuring machines with mechanically driven sliding parts, regular replacement of worn parts, lubrication and cleaning of the system is required to achieve optimal performance.
Measurements on the Coordinate Measuring Machine (CMM)
The coordinates and alignment of the CMM Machine usually have a device coordinate system set on the object. The device coordinate system is defined by the device, for example, the axis moves transversely as the X-axis, and the direction perpendicular to the table top is the Z-axis. Since physical positioning in machine coordinates is difficult and inaccurate, the working coordinate system is aligned with the reference plane or reference line of the object. This method of aligning the orientation of the workpiece with the orientation of the reference coordinates is called alignment.
How to set coordinates?
Setting the working coordinate system requires three pieces of information:
- The first plane is the reference plane, and the direction perpendicular to this plane is the Z axis.
- The second is the reference line, usually the X and Y axes vertically. The straight line can be measured directly from the object or it can be a straight line connecting two different points (e.g. two holes). a dashed line.
- The third point is the beginning. This beginning is point 0 for every X, Y, Z coordinate value.
You can also define a specific point (for example, the center of a specific hole) as the beginning or a virtual point (intersection), where two lines intersect.
Measurement of dimensions and 3D features
Usually, the user selects the measurement target using the software menu and starts the measurement. In the case of a contact coordinate measuring machine, the needle tip touches the object being measured and determines the measurement point. Objects are measured by measuring the minimum number of measurement points specified for each object. If the number of measurement points is larger, it is usually calculated using the least squares method. In addition to planes, measurement elements include:
- lines,
- points,
- circles,
- cylinders,
- cones and spheres.
Measure the size and shape of 3D, calculating distances and angles between measured elements.
Virtual shapes (projection)
Some elements have three-dimensional shapes, such as cylinders and cones, while others do not have three-dimensional shapes, such as lines and circles. These elements are typically projected onto a plane (moved perpendicular to the plane) so they can be properly measured. The projected plane is referred to as the reference plane or projection plane.
Measurement of virtual figures
Coordinate machines can also perform measurements using virtual lines and points. Various examples of virtual solids are used, such as line intersections, plane tolerances, intersections between planes, and circles between cones and planes. It can be said that measurement using these virtual elements, which are difficult to measure with manual tools such as calipers, is unique to 3D measurements.
Problems with Coordinate Measuring Machines (CMM)
Like any equipment, Coordinate Measuring Machines (CMM) can have their advantages and disadvantages, which we discuss below:
- Measurement stability - Proper setup and execution of measurements require specialized knowledge and skills. It is necessary to maintain the appropriate temperature in the measuring chamber in order to stabilize the object's temperature.
- Responsiveness - It's difficult to handle frequent product changes due to the need for calibration every time another probe setting and angle is changed. Due to the necessity of having a measuring chamber, it's hard to perform frequent measurements while manipulating objects.
- Costs and effort - Installation requires a lot of space, and in addition, a quality control laboratory must be built, which is extremely costly. The cost of maintaining the measuring environment and measuring equipment can be prohibitive. Programming CMM takes a lot of time for many reasons. The time needed to send the item to the quality lab, achieve the correct item temperature, repair, calibrate each probe tip, and perform the measurement.
Optical Coordinate Measuring Machine
Optical coordinate measuring machines are portable proximity devices. These coordinate machines use an armless system with an optical triangulation method for scanning and measuring objects in 3D. Thanks to advanced image processing technology, the optical coordinate measuring machine is very fast and guarantees metrological level accuracy. Optical 3D scanners are particularly beneficial for the development of Industry 4.0. Although optical coordinate measuring machines are slightly less accurate, they are still used in a wide range of applications. In fact, optical coordinate measuring machines are used in conjunction with conventional coordinate measuring machines to eliminate production bottlenecks. Therefore, parts that require a significant level of accuracy are checked using traditional coordinate measuring machines. All other components can be assessed using a more economical optical coordinate measuring machine, which provides satisfactory:
- accuracy,
- portability,
- flexibility and speed.
Thanks to coordinate measuring machines, we can precisely use coordinate processing techniques to measure the height, width, and depth of parts. This is extremely important, as this action ensures that our machined material is made accurately and precisely. Furthermore, such machines can automatically measure targets, record measurement data, and obtain GD&T measurements. For this reason, it is worth using the above equipment! Coordinate measuring machines provide quality assurance for product machining. They allow for the following to be examined:
- physical objects,
- three-dimensional,
- sheet metal.
The machine itself can have different types and depend on the type of bridge or gantry type. Regardless of which equipment we choose, it is worth implementing it to improve our production.