Dimensional metrology - Tutorial

The type-approval of a piece is a prerequisite to ensure its validity. It is at this stage is required when a dimensional report in relation to its design specifications, which can be classified into three groups of importance. The first includes those that have a functional significance; in the second which are more susceptible to changes in the manufacturing process; and in the third that have no functional significance and serve only for the geometry definition. In the process of manufacture of one-piece factors directly affecting the stability of their dimensions such as climate, material, machine, the operator and tool wear. The sum of this variability and which generates the measurement system itself creates considerable uncertainty when measuring a single sample, since there may be levels within the field of tolerance in other samples could be outside the, or vice versa.

Measure several samples to reduce the uncertainty

To face up to this problematic poses the need to measure several samples and do use of statistical tools for the calculation of values like the C_p or the C_pk, that give an idea of the capacity of the process. What elder was the number of samples to measure, lower will be the uncertainty on the capacity of the process. But the correct is to find a balance between the criticidad of the specifications and the number of samples to measure, not to put up the price of the process of measurement.

In this process of measurement has to try control to the maximum the factors that can increase the uncertainty of the measure. For this the measures will realizar in a venue conditioned to temperature and constant humidity, will automate the measurements with means of control CNC whenever it was possible, will employ instruments of measure calibrated and trazables with an uncertainty like minimum six inferior times to the field of tolerance, and will devote efforts in matter of the subjection of the piece to proceed to measure it, especially if it treats of a piece of plastic.

In the best of the cases, beside the piece will have designed an useful of fixation specify that it will ensure the repetibilidad in the position of all the samples. But when it does not have of an useful of fixation, have to be the metrólogos the managers to contrive a system that attach the piece with the sufficient strength so that it do not move during the measurement - but without arriving to deform his structure - that allow an easy access to all the characteristics to measure and that ensure a good repetibilidad in the positioning of all the samples.

 

The correct is to find a balance between the criticidad of the specifications and the number of samples to measure

A comprobación recommended beforel start of the essay is to measure an alone sample repeated times