Coordinate Measuring Machines (CMM) Information
Coordinate measuring machines (CMMs) are mechanical systems designed to move a measuring probe to determine coordinates of points on a workpiece surface. They provide precise measurements of objects for design, testing, assessment, profiling, and reverse engineering of parts. Machines are available in a wide range of sizes and designs. They use a variety of different interfaces and modeling software.
How Do Coordinate Measuring Machines Work?
CMMs are comprised of three main components: the machine itself, the measuring probe, and the control or computing system with appropriate measuring software. After placing a workpiece on the machine table, a probe is used to measure different points on it by mapping the x, y, z coordinates. The probe operates either manually via an operator or automatically via a control system. These points are then uploaded to a computer interface where they can be analyzed using modeling software (e.g. CAD) and regression algorithms for further development.
Video credit: HexagonMetrology / CC BY-SS 4.0
While all CMMs perform the same functions, each type of machine is built and designed differently. When selecting a CMM, an industrial buyer should consider its various specifications, the design of the equipment, and its inherent features.
The specifications of the machine itself are important for correctly accommodating a workpiece and meeting required measuring capabilities.
- Measuring length is the total distance a probe can move for measuring in the x, y, or z direction.
- Capacity is the maximum size of the object or workpiece that the machine can accommodate. A CMM must have a capacity sufficient to fit the size of objects the user needs to measure.
- Resolution is the smallest increment that the device can measure to. A higher resolution denotes a more specific measurement.
Design Tip: Resolution differs from repeatability and accuracy. Repeatability is how well a CMM repeats a given dimension over multiple measurements. Accuracy is the deviation of a measurement from its actual or accepted value.
- Measurement speed is the rate at which a CMM can read positions and take measurements. It may refer to the imaging speed of the probe, or to the overall measuring process, which is also a function of control type (CNC being faster than manual control).
- Weight capacity is the maximum or standard weight of the workpiece that the machine can accommodate. A CMM must have a weight capacity sufficient to hold the weight of objects the user needs to measure.
Design Tip: When considering the size of a CMM, keep in mind that size is directly proportional to cost in most cases. Therefore, a machine should only be big enough to accommodate the measure of its largest part.
CMMs can be designed to perform different types of measurement. These include dimensional, profile, angularity, depth mapping, digitizing/imaging, and shaft measurements.
- Dimensional measurements are sizing measurements made in the x, y, and z directions.
- Profile measurements are made to capture information about the form or profile of an object. These measurements may be 2-D or 3-D, depending on the machine capabilities.
- Angularity or orientation measurements are made to capture angle information between points on an object.
- Depth mapping is constructed by measuring the difference between two stereo images. Stereoimages aresuccessive images of the same scene taken at slightly different angles. The objects further away will move relatively little from one image to the next, whereas objects closer to the viewer will move by a greater degree. A depth map is then created, resulting in a single image using different intensities to represent the different depths.
- Digitizing or imaging provides a digital format or image to visually capture the geometry of the workpiece from the measurements made by the CMM.
- Shaft measurements are application-specific designations for measurements made by CMMs designed specifically for inspecting shafts.
Equipment design includes a CMM system's control mechanism, method of operation, mounting style, and probe type.
CMM probes are designed to be controlled either manually or via CNC. Selection is largely a function of part quantity, complexity, and cost.
- CNC (Computer Numerical Control) or DCC (Direct Computer Control) is a control system built in the CMM to control probe movement. CNC- CMMs are best-suited for production environments requiring a higher volume of measurements, and also in applications requiring complex and small measurements with fine features. They tend to be more expensive than manually controlled machines.
- Manual or operator-controlled devices require an operator that physically moves the probe along the axis to make contact and record measurements. Manual CMMs generally cost less than CNC-CMMs of the same size, and are better suited for prototype shops with smaller quantities of measurements.
Operation describes a CMM's method or style of measuring, which is usually dependent on the design or orientation of the probe arm(s).
- Bridge-style machines incorporate an arm suspended vertically from a horizontal beam that is supported by two vertical posts in a bridge arrangement. Themachine x-axis carries the bridge, which spans the object to be measured. The bridge supports the guide rail (the machine y-axis), the bearings, and the machine's z-axis bar. These are considered the most popular type of machine style.
- Gantry-style machines have a frame structure raised on side supports so as to span over the object to be measured or scanned. Gantrymachines are similar in construction to bridge style designs.
Figure 2 - Gantry CMM. Image Credit: Hexagon Metrology
- Horizontal arm machines incorporate an arm that supports the probe horizontally cantilevered from a movable vertical support. They are known for robust construction and low power consumption. They are capable of measuring large envelopes at acceptable accuracies and are mainly used for large workpieces such as auto body parts and weldments.
- Articulated arm machines incorporate an articulated or multi-axis arm. This allows the probe to be placed in many different directions. This design is very common for portable machines.
- Cantilever-style machines incorporate a vertical arm which is supported by a cantilevered support structure.
Sometimes a CMM requires a specific mounting style when incorporated into a system or method of operation. These mounting options include benchtop, freestanding, handheld, and portable.
- Benchtop machines mount on a benchtop or desk. This is typical of most CMMs.
- Freestanding machines can support themselves and do not require mounting.
- Portable machines are devices that may be moved freely and are not designed to be bolted or hardwired in place. Handheld machines are a subset of portable devices describing those designed to be operated by hand.
Probes and Sensors
CMMs can incorporate different types of probes and sensors that detect and record position differently.
- Touch or discrete point probes are the most common probe type and also the least expensive. Discrete point probes actually touch the surface of the workpiece. Upon contact, a signal containing the coordinates of that point is sent to the CMM.The probe is then backed off and moved to the next location where the process is repeated.Types of touch probes include kinematic (switching), strain-sensing,and piezoelectric.
Design Tip: Probes with detachable stylus modules are a good investment, allowing users to implement a stylus rack that acts as a tool changer. This makes switching between styli for different measuring tasks much easier.
- Laser triangulation probes aregenerally referred to as scanning probes.This method generally involves passing the probe over a target surface at its working range.As the probe scans the surface, it transmits a continuous flow of data to the measurement system.
- Line lasers provide the fastest way to inspect non-linear surfaces and contours. The accuracies range from + 0.001" to 0.00025". These devices are popular for reverse engineering.
- Camera probes have a still camera installed as the probe head, which takes pictures in order to find and measure points.
- Video camera probes have a moving camera installed as the probe head, which inspects workpieces through video imaging. These are best-suited for flat parts such as sheet metal stampings.
CMMs can include a number of design features which may be important to users for certain applications.
- Crash or impact protection is important for protecting sensitive components in the event of an unanticipated system crash.
- Offline programmable CMMs have software which supports offline programming using a CAD model.
- Reverse engineering CMMs have software capable of performing reversed engineering. Reverse engineeringcaptures the geometry of existing physical objects and uses this dataas a foundation for designing something new as in a CAD file.
- SPC software is incorporated in CMMs for statistical analysis of measurements.
- Temperature compensation is incorporated in CMMs for a change in the environmental temperature.
When selecting a CMM, the user may want to consider the suitability of its design for the application. For shop floor CMMs, robustness of equipment is an important consideration, since these environments are more prone to inducing mechanical failures either from dust or impact. For inspection room CMMs, the machine must be of fair size with a flexible probe and advanced software to cover all possible variations or eventualities.
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