Albion Devices, Inc.
Case study: Temperature compensated snap gage.
Hand held dimensional measurement tools such as two point micrometers are unsteady and prone to operator influence. Development of the three point snap gage with an additional rest pad reduced these errors in OD measurements. However, when measuring to very tight tolerances even snap gages still faced errors caused by the effects of temperature on larger diameters.
By adding temperature compensation capability to the snap gage a significant additional improvement in Repeatability and Reproducibity was achieved. Railroad axle journals are examples of large diameter shafts with tight tolerances on their journals. For the last 30 plus years railroad axle manufactures and wheel shops have benefitted from using these tools. Other producers of large forged and machined parts have also used them successfully, sometimes on outside diameters ranging up to as much as 30 inches (100 centimeters) or more in size.
The temperature compensation approach is to build sensors into the snap gage. One of them measures the temperature of the master and the workpiece during calibration and measurement, respectively. Another measures the temperature of the gage frame itself. These three components are the “elements” of the measuring system and each is subject to temperature changes.
An on board user-programmable controller monitors a) the measurement obtained by the snap gage measuring probe plus b) the temperatures of the three elements during the calibration and the measuring process. The controller uses this data and the pre-programmed coefficients of expansion of the three elements (workpiece, master and gage) to calculate and display in real time the temperature compensated dimensional measurement at the international reference temperature, which is set at 20 degrees C/68 degrees F (ISO 1). This is also the temperature that is usually used by design engineers when they specify dimensions and tolerances.
The chart illustrates the effect of the application of real-time temperature compensation. It shows the results of measuring a 6.190 inch diameter axle journal artifact at room temperature then heating and cooling that artifact while taking repeated measurements. Note that the uncompensated dimension varies dramatically, while the temperature compensated measurement corrects for over 95% of the thermally induced variations.
