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Temperature fluctuations are one of the most common causes of measurement errors on parts with tight tolerances. As production tolerances become ever tighter and the error margin for measurement results constantly shrinks, this is an important issue that users may need to consider in their inspection process.

The background to this is a natural physical phenomenon: Most materials expand when heated and contract again when cooled. The aim of a test procedure in production metrology is to determine the actual size of a workpiece. While a basis for standardizing temperature measurement for calibration has been around since the 1930s, in a real-world manufacturing setting, this often does not happen. Companies specializing in high-accuracy precision work make an effort to create environmentally controlled rooms or even plants for this purpose. However, in some areas, this has not yet been considered. The other way to start correcting these errors is to consider implementing measuring systems to monitor the temperature of the workpiece and correct the measured values.

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Many quality managers assume that any thermally induced size deviation of the workpiece to be measured is compensated for by a corresponding expansion of the measuring device and the setting standards. All components expand or contract to the same extent so that, in the end, the result is correct.

In reality, this may not be the case. The measuring device, setting master and workpiece — the three hardware components of a measuring system — can be made of different materials, so they behave differently when exposed to heat, even if they all have the same temperature. However, the temperature of the individual components can actually differ from one another:

• Workpieces that have just come out of a dry machining process can be several degrees warmer and remain so for hours.
• Components that have been processed with coolant can be cooler.
• The measuring device or the setting master can stand on a workbench in direct sunlight or under a heating or cooling valve and, therefore, be warmer or cooler.
• Temperature stratification in a room can lead to temperature differences between components near the floor and those on a high shelf.
• The relative mass of the components can also make a difference: for example, a motor block takes longer than a bore plug gage to equalize to the ambient temperature.
• In certain cases, the thermal fluctuations of the measuring device and the workpiece can also have the opposite effect, increasing the measurement error rather than compensating for it. For example, high temperatures cause the contacts of bore gages to become longer. This, in turn, causes the inside diameter to be smaller than actually measured. On the other hand, the inside diameter of a thin-walled part becomes larger at higher temperatures.
• Using setting masters that have just come from the calibration room and not stabilized to the manufacturing area temperature can produce offsets.

As noted, some manufacturing companies try to solve this problem by controlling the room environment. This includes, for example, installing sophisticated heating, ventilation and air conditioning controls or making structural changes. These measures are effective in calibration rooms and measuring laboratories but not on the shop floor. These rooms or buildings are too large and contain too many heat-generating devices or machines and, therefore, too many variables overall.

A better approach is to measure the temperature of the part, master and workpiece and compensate for thermal variation based on the known coefficients of expansion. 

With today’s user-programmable gaging software and the availability of small thermal sensors that are easy to interface with, it is simpler to combine these components and compensate for temperature right at the point of inspection.

The system can be programmed for different coefficients of expansion of the various components, and the results are fed into an algorithm that generates a temperature-compensated measurement result on the gage readout (additional compensation factors may be built into the algorithm to correct for unusual elements in gage geometry, differences between a workpiece’s surface and interior temperatures, and similar variables). Such a system will typically reduce thermally induced errors by 90 to 95%, a figure most shops can live with.

Many software-based measuring systems allow for configuration of the measuring task and input of measurement temperature data from the gage and the part to compensate for part size at a standard temperature.