Critical internal dimension inspection measurements on cylinder bores were being rendered inaccurate by thermal influences on a production line in one of the biggest automobile engine plants owned by the largest American auto manufacturer. Aluminium blocks were being bored before the insertion of steel cylinder liners. The inability to hold an accurate size could lead to liners subsequently cracking during engine operation.
Ambient conditions in the plant, airflow from the air gauge onto the workpiece, and the boring machine process before block inspection were each influencing variations from the reference temperature. The resulting temperature gradients in the engine block and the gauge head led to significant fluctuations in measured process control data. Out-of-tolerance deviations from the specified dimensions were being reported routinely.
Measurement inaccuracies due to temperature fluctuations
The first standard issued by the International Organisation for Standardisation, ISO 1, specifies the standard reference temperature for geometrical product specification and verification. That standard reference temperature was set at 20°C / 68°F. Clearly, given the conditions above, measurements were not being taken at this temperature.
Albion Devices
Varying temperature ranges.
When certain tight tolerance features on machined parts are measured after precision machining or cleaning operations, or after otherwise being exposed to temperatures other than the reference temperature, their dimensions can be significantly altered by thermal expansion or contraction. In addition, gauge fixtures and masters may also not be at the reference temperature. The result will be that erroneous measurements will be taken unless this is taken into consideration. Normalisation to 20°C/68°F before measurement can take precious time. The quickest and most economical option in mass production situations is to compensate electronically for those errors in real time while taking measurements.
Obtaining the best results from an electronic temperature compensation system requires several inputs. Coefficients of Expansion (COE) of the workpiece, master and gauge fixture or frame (the elements of a measuring system) are user-programmed into a controller, along with relevant dimensional data relating to the workpiece. Live temperatures of each of the elements are collected from purpose-designed industrial sensors and transmitted to the microcontroller during operation. The device computes a net correction for thermal errors in real time and this solution is then added to or subtracted from the gauged dimension in the software to arrive at the temperature-corrected size of the workpiece.
Albion Devices recommend temperature probe addition
At the recommendation of Albion Devices, Inc., a gauge head was designed to include a temperature probe embedded in the gauge head. The probe used Albion’s proven, durable temperature-sensing technology in a package that would sense the temperature of the gauge head and the inner wall of the cylinder, without surface wear or scratching. It monitored temperatures within the cylinder bore while the air gauge took dimensional measurements in x and y orientations. The data obtained was used in one of Albion’s compensating controllers to calculate the necessary correction and transmit it to the host gauge computer to be added or subtracted to the measured dimension.
Albion Devices
Example of test data from the cylinder bore gage.
The plant engineer responsible for this job reported that tests of the completed, temperature-compensated gaging system gave “outstanding results”. They were better than any he had seen previously from an air gauge. Measurements were repeatable to within 1 micron.
Gauge Repeatability and Reproducibility (R&R) studies, which always inevitably show at least some variation, now produced greatly improved results. Reliable, accurate and repeatable process control information (Histograms, Cp and Cpk computations illustrated here) could now be obtained for the operation.
The automotive manufacturer experiences production line acceleration
Further and perhaps most importantly, in this case, the engineer was able to accelerate the production line in response to current demand. Previously, the boring machine had operated at 2,000 rpm, limiting cycle time to 44 seconds. Any faster operation caused excessive heat build-up in the cylinder block and resulting increases in measurement errors. With the temperature compensation system installed the bore speed was raised to 3,000 rpm without any deterioration in dimensional control.
This was possible because the gaging system feedback loop continued to report dimensions, after applying automatic correction, as if temperatures were being held steady at reference temperature: 68°F / 20°C. Production output increased by 20 cylinder blocks per hour (an increase of approx. 25%) and cycle time dropped to 36 seconds. Further tests at a maximum boring machine speed of 4,000 rpm, which reduced cycle time to 29 seconds, were equally successful, with the result that a total of over 50% production time improvement became achievable.