Quality Test & Inspection: The Heat Is On
Tactile and multisensor coordinate measuring machines (CMMs) are no longer used exclusively in the inspection lab. More and more, they are used directly in production. This means that operators must sometimes measure at temperatures that deviate greatly from calibration temperature. Measurement errors resulting from this temperature differential are often underestimated and neglected and, therefore, cause errors in measurement results.
In addition to other effects, such as thermally induced deformation of the measuring device and changes in probe lengths, the thermally induced linear measurement deviations, due to temperature of the workpiece and the scales, have been found to be a significant source of errors. Thus, appropriate corrective measures must be taken when installing measuring devices in production.
Thermally Induced Measurement DeviationTemperatures that deviate from 20 C cause the scales in the CMM, and the part to be measured, to expand by different amounts because of their different materials. The influence of different temperatures compounds this problem. Because these expansions have the same direction in length- measuring machines, the only effect is the difference in expansion of the physical scale and the workpiece. The influence of thermal expansion is largely linear. A simple linear correction in each measurement axis can be implemented at a low cost. This is sufficient for small measured lengths.
Thermally induced change in length is principally calculated as follows:
ΔL = α • L0 • Δt
ΔL = thermally induced change in length
α = thermal expansion coefficient
L0 = Reference length; in practice, the measured length
Δt = Temperature deviation Δt = t – 20° C
Expansion of the scale:
ΔLM = α M • L0 • ΔtM
Expansion of the workpiece:
ΔLW = α W • L0 • ΔtW
The thermally induced length measurement deviation (Fig. 1) is the difference between ΔLM and ΔLW :
ΔL = L0 (α W • ΔtW - α M • ΔtM )
As seen in the equation, with the same temperature deviation Dt at the workpiece and the scale assumed for the simplest case, only the difference in expansion coefficients determines the measured length deviation DL.
On a summer day at 35 C on the production floor, when measuring a 100-millimeter component made of PVDC without correction, an additional measurement error of 208 microns is expected (See Table 1). In practice, slightly different deviations are expected because the scale temperature and workpiece temperature are usually different. This does not change the fact, however, that the deviations are unacceptably large.
Lengthwise expansions of the scale and workpiece can compensate for one another if the scale and workpiece are made of the same material, such as steel-to-steel. In practice, this is usually not the case, so the expansion coefficients are different. One can conclude that an appropriate correction for the thermal influence is needed.
Integrated Temperature CompensationA new series of CMMs provides a solid design base, made of granite with encapsulated bearings. The devices are modular and can therefore be equipped to meet exact operator requirements. The equipment includes image-processing sensors, touch probes and laser probes. The machines allow simple manual measurements as well as CNC-controlled measurement with 2-D and 3-D CAD data.
All CMMs in the new series are designed to meet requirements of production floor use. They can calculate corrections based on expansion coefficients of the device and workpiece, workpiece temperature and associated temperatures of the scales. Sensors measure these temperatures, and thermal expansion of the scales is calculated automatically.
Requirements are the entry of a precise value for expansion coefficients of the workpiece and the maintenance of spatial (DK/m) and time (DK/h) temperature gradients, in accordance with the manufacturer’s specifications. If the expansion coefficient of the workpiece is not precisely known, an estimate of the value will reduce the measured length deviation. This applies especially to large measu