A Coordinate Measuring Machine (CMM) is a device used to measure the geometry of physical objects. It employs a probing system to detect discrete points on the surface of an object, providing precise measurements.
The first CMM appeared in the early 1960s. Originally developed by the Ferranti Company in Scotland in the 1950s, this two-axis machine used a 3D tracing device with a basic digital readout to display XYZ positions. Ferranti utilized the CMM to measure components for military products. By the late 1960s, three-axis models were introduced.
CMMs are primarily used to test parts and assemblies, ensuring they meet the original design specifications. Integrated into quality assurance (QA) or quality control (QC) workflows, CMMs help inspect the dimensions of manufactured components, identifying or preventing quality issues.
CMMs offer significant advantages over manual inspections using traditional tools like micrometers and height gauges, including increased accuracy, speed, and a reduction in human error.
There are various types of CMMs, typically categorized by their structures, with each offering specific advantages and drawbacks. Let’s explore the different CMM types in more detail.
Types of CMMs
1. Bridge CMM
Bridge CMMs use a probing system that moves along three orthogonal axes (X, Y, and Z) in a Cartesian coordinate system. The machine detects points on an object’s surface, creating a point cloud that represents the area of interest. Bridge CMMs are often divided into two sub-types: moveable-table and moveable-bridge systems.
Pros of Bridge CMMs
- Extremely accurate
- Ideal for measuring parts with high precision tolerances
- Great for small to medium-sized components
- Capable of multi-sensor measurements, such as probing and scanning
Cons of Bridge CMMs
- Expensive
- Fixed measurement volume
- Lack of portability (the part must be brought to the machine or moved with machinery)
- Sensitive to vibrations, requiring use in a metrology lab
- Complex to operate and requiring skilled operators
- Requires rigid setups for each inspected part
2. Gantry CMM
Similar to bridge CMMs, gantry CMMs are typically much larger and designed to handle heavier or larger parts. They are mounted on a solid foundation, directly on the floor.
Pros of Gantry CMMs
- High accuracy
- Large measurement volume, suitable for large or heavy parts
- Easier to load and unload components compared to bridge CMMs
Cons of Gantry CMMs
- Expensive
- Fixed measurement volume
- Lack of portability
- Large footprint and takes up significant floor space
- Sensitive to vibrations and needs to be used in a metrology lab
- Complex operation requiring skilled operators
3. Cantilever CMM
Cantilever CMMs differ from bridge CMMs by having a measuring head attached only to one side of a rigid base, providing easier access for operators on all three sides.
Pros of Cantilever CMMs
- High accuracy
- Ideal for smaller parts
- Open access on three sides makes loading/unloading easier
Cons of Cantilever CMMs
- Expensive
- Fixed measurement volume
- Lack of portability
- Sensitive to vibrations and must be used in a controlled environment
- Requires rigid setups for each part
- Complex operation requiring skilled personnel
4. Horizontal Arm CMM
Horizontal arm CMMs feature horizontally mounted probes, designed to measure long and thin objects, such as sheet metal. These are useful for geometries that are challenging to access with vertical CMMs.
Pros of Horizontal Arm CMMs
- Suitable for long, thin parts
- Good for parts with low tolerances
- Does not require an extensive foundation system
- Easy to install with a smaller footprint
Cons of Horizontal Arm CMMs
- Less accurate than other CMMs
- Fixed measurement volume
- Lack of portability
- Sensitive to vibrations and should be used in a metrology lab
- Requires rigid setups and skilled operators
5. Portable Measuring Arm CMM
Portable measuring arm CMMs are designed for on-site measurement, allowing inspectors to take measurements on the shop floor, providing quick, real-time data. These CMMs can be articulated (6- or 7-axis systems) to measure components directly in their assemblies.
Pros of Portable Measuring Arm CMMs
- Portable and lightweight
- Can measure parts wherever they are located
- Extendable measurement volume
- Multi-sensor capabilities (probing and scanning)
- Cost-effective and easy to operate
Cons of Portable Measuring Arm CMMs
- Less accurate than other CMM types
- Sensitive to environmental vibrations
- Requires rigid setups
6. Optical CMM
Optical CMMs are non-contact devices that use optical triangulation methods to scan and obtain 3D measurements of objects. They are fast and provide metrology-grade accuracy, making them suitable for Industry 4.0 manufacturing.
Pros of Optical CMMs
- Portable and lightweight
- Can measure components directly at their location
- Extendable measurement volume
- Quick measurement acquisition times
- Cost-effective and easy to operate
Cons of Optical CMMs
- Slightly less accurate compared to conventional CMMs, depending on the application
Why CMM Speed Matters
In today’s manufacturing environment, speed and efficiency are critical to meeting tight delivery schedules, increasing throughput, and reducing costs. When bottlenecks occur at CMMs, inspection times extend, driving up costs.
The need for faster CMMs has become apparent due to the high volume of work managed by a limited number of qualified metrologists. Traditional CMMs with probes tend to be slow, especially when measuring complex shapes. Meanwhile, newer CMMs, equipped with sensors, accelerate the process but still require expert handling.
Manufacturers are increasingly turning to advanced technologies like optical CMMs, which can keep pace with demanding production environments while meeting rigorous QA and QC standards. These systems help streamline inspections and ensure faster, more efficient workflows.