Measuring

The use of measuring instruments can ensure the quality of machined products. To measure quality, it is necessary to choose high-quality measurements in the selection of measuring instruments and ensure that they are properly maintained and calibrated. Measurement uncertainty is a factor associated with each measurement taken. For the measurement result to be considered reliable, measurement uncertainty must also be considered in the reporting of measurement results.

Common measuring instruments used in the machining industry include calipers, micrometers, dial gauges, and surface roughness testers. Calipers are considered a universal measuring tool for machinists, typically meeting the requirements set by the general tolerance specifications in the SFS-EN 22768-1 standard. With calipers, you can measure external and internal dimensions as well as depths. More accurate ISO tolerance system tolerances according to the SFS-EN ISO 286 standard are generally measured with micrometers, which can measure external dimensions (outside micrometers), internal dimensions (three-point micrometers), and with specialized micrometers, dimensions such as thread pitch diameters (thread micrometers) or gear tooth spacing (gear tooth micrometers). Today, the most common handheld measuring tools are available in both traditional vernier scales and digital versions with advanced functions such as the tolerance function found in Mahr micrometers or wireless data transfer that allows measurement results to be transferred wirelessly to a computer’s measurement report. Surface roughness testers are used to measure the surface quality of a machined part to ensure it meets the specified surface roughness requirements. The surface roughness tester is also an important tool to ensure that costly over-quality is not produced during manufacturing.

The calibration of measuring instruments is a part of quality assurance and is used to check the condition and measurement accuracy of measuring instruments. A measuring instrument must be calibrated when it reaches the last usage month stated on the calibration sticker or if the condition of the instrument is in doubt. A measuring instrument cannot be used if it does not have a calibration sticker or if its period has expired.

The ISO 9001 of measuring instruments is an international standard that specifies quality management system requirements. ISO 9001 requires that the measuring instruments used in monitoring and measurements must be calibrated or verified, or both calibrated and verified. Calibration should be done periodically or before use by comparing the measurement to measurement standards that are traceable to international or national measurement standards. Generally, the calibration validity of measuring devices is indicated by a color-coded sticker attached to the instrument. The color of the sticker indicates the next calibration year, and the number on the sticker indicates the next calibration month. A red color-coded sticker with a zero for the month indicates measurements that must be calibrated before use. Properly calibrated measuring instruments reduce the number of measurement error components and therefore also lower manufacturing costs.

Measuring instruments should always be kept clean and stored in their own cases. The instruments should be checked against a test block before being put into use. If a measuring instrument drops to the floor or is bumped, its condition and accuracy should be checked against a test block.