Coordinate-measuring machine

Coordinate-measuring machine
Name	Coordinate-measuring machine
Caption	A modern bridge-type CMM
Classification	Metrology
Manufacturer	Hexagon AB, Carl Zeiss AG, Mitutoyo, Nikon
Related	Optical comparator, Laser tracker, Articulated arm

Coordinate-measuring machine. A coordinate-measuring machine is a device used in manufacturing and metrology to precisely measure the physical geometrical characteristics of an object. It functions by using a probing system to detect discrete points on the surfaces of a workpiece, which are then referenced to a Cartesian coordinate system. These machines are fundamental to quality control processes, ensuring components meet specified design tolerances as defined in Computer-aided design models and Geometric dimensioning and tolerancing standards.

Overview

The primary function is to collect dimensional data from physical objects for comparison against a digital blueprint. This process is critical in industries like aerospace engineering, automotive industry, and medical device manufacturing, where dimensional tolerances are extremely tight. By providing quantitative data, they play a key role in statistical process control and reverse engineering. The development of these systems has been closely tied to advancements in computer numerical control and precision engineering.

Types

Several distinct configurations exist, each suited to different measurement volumes and applications. The common **bridge-type** design features a moving bridge structure that carries the probe along three orthogonal axes, offering high rigidity and accuracy for laboratory environments. A **cantilever** design has the probe mounted on a vertical arm that extends from a single column, allowing easy access to the workpiece. For measuring very large objects, such as aircraft fuselages, **gantry-type** machines are used, where the entire structure moves over the part. Portable systems include **articulated arms**, which use rotary joints, and **laser trackers**, which employ interferometry for long-range measurement.

Components

A typical stationary machine is constructed on a massive **granite** or ceramic base plate, which provides exceptional thermal stability and vibration damping. The core mechanical system consists of three axes of motion, each equipped with a precision **linear scale**, often made from glass or a rare-earth material, to track position. The **probe head**, mounted on the Z-axis, is a critical component; modern systems use **touch-trigger probes** pioneered by David McMurtry of Renishaw plc, or scanning probes that maintain continuous contact. The system is controlled by a dedicated computer and **controller** unit.

Operation and software

Operation begins by securing the workpiece, often on a fixture, within the machine's volume. The probe is then moved manually or via computer control to touch specific points on the part's surface. Dedicated software, such as PC-DMIS from Hexagon AB or CALYPSO from Carl Zeiss AG, processes the point cloud data. This software performs best-fit alignment of the measured data to the CAD model, calculates form and position errors, and generates detailed inspection reports. Advanced software enables automation through Direct Computer Control and path planning to avoid collisions.

Applications

These machines are indispensable in high-precision manufacturing sectors. In the automotive industry, they are used to inspect engine blocks, transmissions, and body panels. Aerospace applications include verifying the contours of turbine blades and the alignment of wing assemblies. They are also crucial in the production of semiconductor wafers and the molds for injection molding of consumer products. Furthermore, they are employed in archaeology for digitizing artifacts and in forensic science for analyzing evidence.

Accuracy and calibration

The accuracy is influenced by factors including machine geometry, thermal expansion, probe performance, and vibration. Key performance metrics are defined by standards like ISO 10360. Regular **calibration** is essential, performed using calibrated artifacts such as gauge blocks or **ball bars**. Environmental control in a **temperature-controlled** room is often necessary to mitigate errors from thermal expansion. The machines themselves are calibrated by specialized organizations like the National Institute of Standards and Technology to ensure traceability to international standards.

History and development

The first commercially successful machine was developed in the late 1950s by Ferranti in Scotland, based on earlier concepts for digitizing large contours. The 1970s saw the introduction of the touch-trigger probe, a revolutionary invention by David McMurtry for inspecting Rolls-Royce Olympus jet engine components, which led to the founding of Renishaw plc. The integration of computers and CAD technology in the 1980s transformed them from simple measuring tools into comprehensive inspection systems. Continued evolution includes the use of white light scanners and integration with industrial robots for automated production lines.

Category:Metrology Category:Dimensional instruments Category:Manufacturing