ARTICLE: Coordinate Measuring Machines

An object, for example, an engine part coming off an production line, is placed on a table and the CMM takes measurements by use of a probe to determine the limits of the object inside the machine's envelope along X, Y and Z axes. The probe is commonly mechanical for industrial purposes but can also be optical or laser or white light.

The first CMMs were 2 axis machines developed in the 1950s in response to the requirements of military specifications. By the 1960s, Z-axis machines had appeared and the next decade saw the advent of computer controlled devices.

The object to be measured is placed on a granite table. The touch probe is suspended above, usually on a moveable gantry. The gantry moves along, say, the X axis. A moveable carriage containing the probe moves along the Y axis and the probe itself moves up or down along the Z axis. Advanced features such as moveable tables or probes mounted in rotating quills facilitate ease of operation but do not change the basic function of the machine. Portable CMMs utilize a swinging arm, and angular positions to determine the probe position, which enables measurements to be taken on complex parts with holes or cavities not reachable by conventional gantry-model machines. The basic CMM structure is either mounted on a table, free-standing (for large pieces such as auto bodies) or can be portable.

Early machine probes were steel balls soldered to the end of a shaft. The operator touched the probe to obtain the coordinates. Electronic touch trigger probes greatly increased accuracy by eliminating the operator's variations in the pressure applied to take each measurement. So did direct computer controlled (DCC) machines, which appeared with the advent of CNC. Newer probes utilize light or lasers to scan across the surface of the object, which give highly accurate measurements for specialized needs. Conventional touch trigger probes are sufficiently accurate for the greatest percentage of industrial uses.

The third important part of a CMM is a system for collecting and recording the measurements. This usually consists of a machine controller, a computer and application software.

Numerous considerations determine which kind of machine is best suited for a particular company.

Foremost among them is the size, type and tolerance ranges of the objects to be measured.

For a prototype shop, a manual CMM should be adequate but would be too costly in terms of time for a production facility. For those a DCC CMM is a wiser choice.

If the machine will be out on a shop floor as opposed to an inspection room, sturdiness should be a prime factor. Mechanical bearings will tolerate conditions in the production environment far better than a machine with air bearings, which would require covers and bellows to keep it clean and operational.

An inspection room CMM should be able to cover any eventuality and be equipped with a selection of styluses and the best OEM software available. Software should be an extremely important consideration. There can be quite a variation in the ease of use among software programs and the one that obtains a measurement with the fewest number of mouse clicks is the one that will save the most production time. When evaluating software it is also important to inquire about future upgrades and technical support.

When selecting a manufacturing facility for your project, inquire about the inspection tools they use and realize that a CMM is the metalworking industry standard for any type of precision work.