Overcoming temperature effects in diameter measurement
06 March 2013
New technology from dimensional non-contact measurement specialist, Scantron Industrial Products has enabled a new level of accuracy in diameter measurement to be attained in the production environment.
Laser measurement of product diameter has always been highly accurate. However, in non-thermally controlled environments, such as a workshop or production line, there was always the chance of incorrect readings due to thermal expansion. Small fluctuations in the ambient temperature could affect both the laser gauge and the part, and therefore the measurement.
Now, thanks to Scantron’s ‘NO-VAR’ (No-VARiation) technology, this problem can be overcome, even in non-thermally controlled environments. The thermal co-efficient of the gauge and the part are determined and then stored in the instrument's memory. Sensors installed in the gauge then measure the ambient temperature and smart software compensates for temperature changes and adjusts the reading accordingly.
With high precision manufacturing requiring very tight tolerances, gauge repeatability and reproducibility (R&R) to a tenth of the tolerances or better is a requirement when demonstrating process capability (Cpk). A 10mm diameter steel part will change by more than a tenth of a micron per degree change in temperature of the part (thermal expansion coefficient of steel is around12 microns per metre per degree C). The 40mm high speed Xactum gauge offers R&R of ±0.1 microns (±3s over 1 second) and, with a resolution of 10 nanometres, can accurately measure changes in diameter of this order.
Operations such as grinding can alter the temperature of parts. In this situation, the diameter of the part is simply measured immediately after machining and again after it has returned to room temperature. An offset value is then programmed into the gauge, so it can compensate for this size variation with temperature.
The gauges take measurements by emitting a curtain of light from a transmitter into a receiver. Any object that is placed between the two casts a shadow, from which it is possible to calculate the diameter.
The gauges are available as either single axis or dual axis units. The single axis units are used for measuring outside diameter only. The dual axis gauges, because they take measurements in two planes, can also detect ovality and other irregularities in the product.
Dedicated software packages available with the gauge provide additional features, such as diameter display, tolerance checking, part sorting, statistical analysis and reporting and interfacing with a computer.
What's more, the gauge can be set up to communicate with existing machinery to enable ‘closed loop feedback’ control. This will maintain product within tighter tolerances than is possible during an uncontrolled production run, allowing manufacturers to commit to meeting higher customer specifications.
Data collection software also provides detailed SPC information and reports. All this leads to a reduction in scrap and minimises production downtime whilst any issues are dealt with.
Off-line gauges are also available and provide easy on-site testing for product samples. The ability to accurately check product diameter is an important part of many companies' standards room.
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