Coordinate measuring machine accuracy on the shop floor
Coordinate measuring machine (CMM) accuracy is dependent upon the ambient thermal environment in which it operates.
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Though often compensated for in a variety of ways, these thermally induced changes can lead to significant measurement uncertainty, particularly in the context of measurement on the shop floor, where temperature can be difficult to control. With the ever-continuing trend of moving dimensional inspection from thermally controlled metrology labs to the shop floor, understanding how temperature affects CMM accuracy is more important than ever before.
Traditionally, the thermal dependence of a CMM’s accuracy has been specified using broad temperature bands centered about 20°C (68°F). For instance, a manufacturer might specify a hypothetical CMM’s maximum permissible error of indication for size measurement, MPEE, according to ISO 10360-2 over a temperature band of 18-22°C (64-72°F) as:
MPEE = 3.0 + 3.0 * L / 1000
Where MPEE is in microns, and L is the measurement length in millimeters.
While this is a logical way (for both the CMM manufacturer and the customer) to specify the temperature dependence of CMM accuracy for a machine in a lab environment, the logic falls apart for machines installed and used in an environment where the temperature is not well controlled over both long and short time scales.
While a single temperature band specification (whether it is wide or narrow) is convenient for the CMM manufacturer, the customer is left with only the manufacturer’s conservative, but not terribly detailed estimate of how machine accuracy changes with temperature. After all, a primary reason customers purchase a shop-floor machine is to position it in a location where the ambient temperature will, in all likelihood, affect the CMM’s measurement accuracy.
Stair-step specifications
MPEE = 3.0 + 3.0 * L / 1000 (18-22°C)
MPEE = 3.3 + 4.2 * L / 1000 (16-26°C)
MPEE = 3.5 + 5.0 * L / 1000 (15-30°C)
With a measuring length L of 500 mm, this is portrayed graphically as a step function of ambient temperature.

Continuous thermal specifications
Clearly, a more detailed description of CMM accuracy under varying ambient temperature conditions is needed in environments that lack sufficient thermal controls. A more physically realistic alternative is the specification of accuracy as a continuous function of ambient temperature. And from the pragmatic viewpoint of the metrologist or quality engineer, it is eminently more useful.
To illustrate the point, let’s take a look at an actual CMM. Consider the accuracy statement of the 4.5.4 SF shop floor CMM:
MPEE = 3.1 + 0.05 * ∆T + (3.0 + 0.2 * ∆T) * L / 1000 (15-40°C)
Where ∆T is the departure of ambient temperature from 20°C.
Again, let’s consider a measuring length of 500 mm and plot MPEE as a function of ambient temperature. This time we find a more physically intuitive result and one that is much more useful to the metrology practitioner.
The usefulness is further demonstrated when we display on the same graph the previous hypothetical example where performance was specified over a series of broad thermal ranges.

In addition to providing the user with a more precise picture of the CMM’s accuracy at varying temperatures, a continuous specification is a compact and elegant way of defining machine accuracy specifications and is particularly well suited to being incorporated into automated reporting of measurement results.
Temperature variation over time
Practical advice for shop-floor deployment of a CMM
The use of a CMM with a continuous thermal specification on the shop floor equips the CMM user with a more complete picture of a machine’s expected performance in an uncontrolled environment. This enables better decision making and more confidence in measurement results. One such CMM is the 4.5.4SF shop floor machine.