By Dan Strain
Hexagon Metrology Inc.
North Kingstown, RI
Web site: www.HexagonMetrology.us
Changes in ambient temperature and the temperature of the parts we need to measure have an observable effect on many common industrial materials—for example, steel and aluminum. It’s for this very reason that for the most accurate measurements with large automated CMMs a temperature-controlled room and even part-temperature sensors are essential if you wish to achieve the best possible results.
With portable measuring systems like laser trackers and portable arms, however, the measuring systems are designed to be moved to the part to be measured, rather than the other way around. Most manufacturing environments are not well temperature controlled, if they are controlled at all. This can raise the question as to how, when, or if we should be concerned with thermal expansion of our parts when we are doing portable metrology.
Portable measurement systems and software packages may offer different tools and methods for dealing with temperature changes. These may include:
- Reference scale bars made of the same material as the part to be measured (this technique is a holdover from theodolite measurement systems).
- Measuring two points on the tool and telling the measurement system the "known" distance between those two points. This is a variation of the scale-bar method.
- Material temperature can be measured at different points during a measuring cycle and recorded in the system software with a tool that compensates for the coefficient of thermal expansion (CTE) through a calculated change in the scale of their measurement data.
- A number of points can be measured for which nominal measurement data already exist. Through the process of a best-fit transformation, the system software calculates a change in the scale of the measurement data.
All of these methods have limitations. If we were only measuring solid blocks of a specific material, any method of temperature compensation would work well. In this case, dimensional changes would be linear, and therefore would yield good CTE calculations that can compensate for thermal changes in the workpiece.
In the real world, as it happens, we are normally not measuring homogeneous blocks of material. Particularly with portable metrology, we are often measuring large workpieces that are shaped, welded, bolted, bonded, or fastened to other pieces of the same or some other material. Through these combinations of materials or orientations of materials, we change the direction of movement caused by material expansion or contraction. In the real world, objects don’t expand and contract linearly, they twist or bow, or distort in some other way. Therefore, we cannot automatically assume that compensating for CTE is the best way to characterize what really happens during thermal cycling.
That said, every method of temperature compensation has inherent weaknesses because of the real world’s complexity. In fact, when we compensate for temperature changes, it’s theoretically possible to add error. Some operators, realizing this, simply ignore object temperature and make no attempt to correct for thermal change.
Apart from physically controlling the temperature of the measurement environment, there is no perfect solution to temperature compensation. In most cases, allowing scale to be calculated through best-fit transformation will yield the best results that can be achieved. This is not true in all cases, however, and can have other consequences that the operator must be aware of. A skilled portable metrology operator will keep thermal properties in mind as he or she develops a measurement plan for a particular part by evaluating the size, materials, and construction of the object to be measured. The operator should also evaluate the characteristics of ambient environment such as location near heat sources, and possible changes in air and part temperature over the length of the measurement process.
A series of measurements can be taken and compared to nominal data, or to previous measurements. Trying multiple methods of temperature compensation to determine the best method for that specific measurement task is always a good practice. Most importantly, the skilled operator will carefully document the experimental methodology and final inspection procedures, to ensure that those who interpret the measurement results will have all the information they need. ME
This article was first published in the April 2011 edition of Manufacturing Engineering magazine. Click here for PDF.