Metrological Support for Tolerance Control of Hole-Type Parts

Quality control is always based on certain physical principles and measurement methods. The parameter that affects any inspection is error produced by measuring instruments and methods. A measurement error may result in good parts being erroneously classified as defective and some defective parts being accepted. When inspecting main bearing diameters of a vehicle cylinder block, it is important to minimize the chance of inspection errors. Erroneous rejection of the main bearing support to the left or to the right of the tolerance limit will result in the need to replace the cylinder block. Erroneous acceptance of the support according to the smallest dimension can result in loose fit of the cap to the bearing and probable break. Erroneous acceptance of the main bearing support according to the largest size can lead to a gap between bearing shells and their subsequent play and rotation. In order to reduce the occurrence probability of these errors during tolerance control, the authors estimated erroneously accepted and erroneously rejected main bearings in detecting defects using NI 100-160 0.01; NI-PT160-0.001; and NIC-PT 160-0.002 inside calipers. Twenty cylinder blocks were analyzed and five main bearings were inspected in each of them. The use of NI-PT160-0.001 in repairs of 1000 YaMZ engines per year is estimated to reduce loss from erroneous rejection by 2.13 times and loss from erroneous acceptance by 1.88 times as compared with NI 100-160 0.01. The use of NIC-PT 160-0.002 is unpractical because of the possibility of increasing loss due to the difference in accuracy readability. The proposed methodology can be applied to detecting defects in other engine parts, for example, for monitoring the inner diameter of the cylinder liners and the big end of the connecting rod.